Gas Hydrates in the GeosystemThe German National ResearchProgramme on Gas Hydrates

Report on the First Funding Period(2000 - 2004)

GEOTECHNOLOGIENScience Report

No. 7

Gas Hydrates in the Geosystem

ISSN: 1619-7399

In Germany a National Gas Hydrate Programme has been initiated in 2001 as partof the R&D-Programme GEOTECHNOLOGIEN. Between 2001 and 2004, 15 jointprojects have been funded with 15 Million Euros by the Federal Ministry ofEducation and Research. All projects were carried out in close cooperation withvarious national and international partners from academia and industry.

This report highlights the scientific results of the first funding period addressing thefollowing objectives:

- Characterization of the chemical and physical properties of methane hydrates- Interaction of gas hydrates with the natural environment including seafloor

stability and global climate- Characterization of the unique biological communities dependent on methane

hydrate occurrences- Technologies for an improved survey of methane hydrates in both the

laboratory and the field - Technologies for the safe and commercial production of methane from

hydrates

The papers published in this report offer a comprehensive insight into the presentstatus of gas hydrate research in Germany and reflects the multidisciplinary appro-ach of the programme.

No.

7G

asHy

drat

esin

the

Geo

syst

em(2

00-2

004)

GEO

TECH

NO

LOG

IEN

Scie

nce

Repo

rt

The GEOTECHNOLOGIES programme is financed by the Federal Ministry

for Education and Research (BMBF) and the German Research Council (DFG)

GEOTECHNOLOGIENScience Report

Gas Hydrates in the Geosystem

The German National ResearchProgramme on Gas Hydrates

Results from the First Funding Period(2001 - 2004)

No. 7

Number 1

Impressum

SchriftleitungDr. Ludwig Stroink

© Koordinierungsbüro GEOTECHNOLOGIEN, Potsdam 2006ISSN 1619-7399

The Editors and the Publisher can not be held responsible for the opinions expressed and the statements made in the articles published, such responsibility resting with the author.

Die Deutsche Bibliothek – CIP Einheitsaufnahme

GEOTECHNOLOGIEN; Gas Hydrates in the Geosystem, The German National Research Programme on Gas Hydrates Report on the First Funding Period (2001 - 2004) – Potsdam: Koordinierungsbüro GEOTECHNOLOGIEN, 2006(GEOTECHNOLOGIEN Science Report No. 7)ISSN 1619-7399

Bezug / DistributionKoordinierungsbüro GEOTECHNOLOGIENTelegrafenberg 14473 Potsdam, GermanyFon +49 0331-620 14 800Fax +49 0331-620 14 801www.geotechnologien.degeotech@gfz-potsdam.de

Bildnachweis Titel / Copyright Cover Picture:G. Bohrmann, Universität Bremen / RCOM Bremen (Februar 2006)

Preface

In Germany, the National Research Programmeon Gas Hydrates »Gas Hyrates in the Geo-system« has been initiated in 2001 as part ofthe R&D-Programme GEOTECHNOLOGIEN.After a public call, more than 40 project propo-sals have been evaluated in an internationaltwo-step review procedure, involving 14 expertsfrom five countries. Finally 15 joint projects wererecommended and funded by the FederalMinistry of Education and Research (BMBF) withmore than 15 Million Euro for a three year fun-ding period (2001 – 2004). The research pro-jects involving 15 institutional partners fromacademia and industry covered a balanced port-folio of laboratory and field studies, tool designand testing and computer model development.

Funding by the government and the private sec-tors has strongly accelerated the progress of gashydrate research in Germany. Rapid progresshas been made concerning an improved cha-racterization of the chemical and physical pro-perties of gas hydrates and how they interactwith the unique biological communities depen-dent on hydrate occurrences. A scientific break-through was the successful characterisation ofbiogeochemical and microbial processes ofmethane turnover in hydrate-bearing sedi-ments. Beside the scientific results, which recei-ved worldwide recognition, a number of noveltechnologies to improve the investigations of

hydrates in both the laboratory and the fieldhave been developed. A new autoclave techno-logy allows sampling and investigating gashydrates under in-situ conditions: an importantprecondition to understand the formation andstructure of marine gas hydrates. Landersystems as long term seafloor observatorieswere developed for a wide spectrum of oceano-graphic applications. New software programsfor numerical simulation for controlled extrac-tion of methane by thermal destabilisation ofgas hydrates, could be a first step towards a safeproduction of gas from hydrates.

To maintain the momentum of gas hydrate re-search in Germany and to enlarge the existingscientific and technological Know-how, theinternational review committee recommended asecond funding period. Recently four joint pro-jects are funded by the Federal Ministry ofEducation and Research with 7.6 Million Euros.Like in the first funding period all projects arecarried out in close cooperation betweenvarious national and international partners. Allwho are interested in these activities – fromGermany, Europe or overseas – are welcome toshare their ideas and results.

Ludwig StroinkDetlev Leythaeuser

1

Table of Contents

Shallow Marine Gas Hydrates: Dynamics of a Sensitive Methane Reservoir (OMEGA)Bohrmann G., Abegg F., Amann H., Brückmann W., Drews M., Gust G., Hohnberg H.-J.,

Kipfstuhl J., Klaucke I., Rehder G., Suess E., Wallmann K., Weinrebe W. . . . . . . . . . . . . . . . . . . 4 - 19

Long-term Observatory for the Study of Control Mechanisms for the Formation and Destabilisation of Gas Hydrates (LOTUS)Linke P., Abegg C., Eisenhauer A., Gubsch S., Gust G., Greinert J., Keir R., Liebetrau V.,

Luff R., Pfannkuche O., Sommer S., Spiess .V, Wallmann K. . . . . . . . . . . . . . . . . . . . . . . . . . 20 - 39

Gas Hydrates: Occurrence, Stability, Transformation, Dynamics, and Biology in the Black Sea (GHOSTDABS)Michaelis W., Seifert R., Blumenberg M., Pape T., Lüdmann T., Wong H.K., Konerding P.,

Zillmer M., Petersen J., Flüh E. ,Reitner J., Reimer A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 - 57

Microbial Methane Turnover at Marine Methane Seeps (MUMM – SPI)Treude T., Niemann H., Orcutt B., Joye S., Witte U., Jørgensen B. B., Boetius A. . . . . . . . . . . . . 58 - 63

Microsensor Measurements in Gas Hydrate Bearing Sediments (MUMM – SPII)De Beer D. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 - 67

Biomarker Signatures of the Anaerobic Oxidation of Methane (MUMM – SPIII)Elvert M., Niemann N., Orcutt B., Jørgensen B.B. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 - 73

Distribution and Diversity of Microorganisms in Gas Hydrate Bearing Sediments (MUMM – SP IV)Knittel K., Lösekann T., Amann R. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 - 79

Physiology of Microorganisms in Gas Hydrate Bearing and other Methane-Rich Marine Sediments (MUMM – SP V)Krüger M., Nauhaus K., Meyerdierks A., Widdel F. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 - 85

High resolution imaging and physical properties of hydrate and gas-bearing sediments within the INGGAS projectReston T.J., Bialas J., Breitzke M., Flueh E.R., Kläschen D., Klein G., Talukder A., Zillmer M. . . . . . 86 - 97

3

An in-situ laboratory to study terrestrial, permafrost related gas hydrates (Mallik 2002) Weber M., Bauer K., Kulenkampff J., Henninges J., Huenges E.,

Wiersberg T., Erzinger J., Löwner, R. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 - 117

Gas hydrate induced submarine slides - An engineering geological approachGrupe B., Kreiter S., Feeser V., Hoffmann K., Becker H. J., Savidis S., Rackwitz F., Schupp J. . . . . 118 - 133

Microstructure, thermodynamics, formation- and decomposition- kinetics of gas hydratesItoh H., Klapproth A., Goreshnik E., Techmer K., Kuhs W.F. . . . . . . . . . . . . . . . . . . . . . . . . 134 - 137

New perspectives for the extraction of oceanic gas hydrates Schultz H.J., Deerberg G., Fahlenkamp H. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 - 151

Experimental determination of the petrophysical and thermodynamic properties of gas hydrates and hydrate bearing sedimentsSchicks J., Spangenberg E., Naumann R., Kulenkampff J., Erzinger J. . . . . . . . . . . . . . . . . . . 152 - 165

GASHYDRATES – Paleoatmospheric archiveReconstruction of paleoclimatic changes in the source strength of potential methane sources using the methane isotopic signature in bubble enclosures in polar ice coresFischer H. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 - 169

Gas Hydrates in Hemipelagic Sediments – CONGOSpieß V., Zühlsdorff L., Villinger H., Flueh E., Bialas J., Kasten S.,

Schneider R., Bohrmann G., Sahling H. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 - 185

Techniques and Instruments for Gas Hydrates Exploration and Research (TIGER)Degenhardt A., Hanken T., Helmke J., Jaguttis J., Masson M., Poppen B. . . . . . . . . . . . . . . . 186 - 197

Detailed seismic study of a gas hydrate deposit at the convergent continental margin off Costa Rica – DEGASMüller C., Bönnemann C., Neben S. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 - 217

Table of Contents

4

Shallow Marine Gas Hydrates: Dynamics of aSensitive Methane Reservoir (OMEGA)

1. IntroductionMethane as a greenhouse gas is twenty timesmore effective than CO2, but its concentrationwithin the atmosphere is smaller. In contrast,methane generated by microbial decay andthermogenic breakdown of organic matterseems to be a large pool in geological reservo-irs. Numerous features such as shallow gasaccumulations, pockmarks, seeps, and mudvolcanoes are present in a wide variety of oce-anographic and geological environments. Suchmethane sources may provide positive andnegative feedback to global warming and/orcooling and are therefore focal points of cur-rent research. Studying methane emission siteswill elucidate how stable these reservoirs areand how the pathways to the atmosphere areworking. Because of their high methane densi-ty, gas hydrates are of special interest whenoccurring close to the seafloor. Previous inve-stigations have shown that such hydratesgenerate extremely high and variable fluxes ofmethane to the overlying water column due totheir exposed position close to the sediment/water interface. They do not only influencetheir immediate environment, but they mayalso contribute substantially to the transfer ofmethane to the atmosphere.

2. ObjectivesThe objective within the framework of theOMEGA project was to investigate near-surfa-ce methane and methane hydrates in the BlackSea, on Hydrate Ridge (Cascadia Margin) andthe Gulf of Mexico in order to understand theirorigin, structure, and behavior as well as theirinteraction with the sedimentary and oceanicenvironment. This is crucial for evaluating andquantifying their importance in the global car-bon cycle. Past studies on these known occur-rences were limited because of the lack ofappropriate pressurized sampling techniques.Since gas hydrates react rapidly to changes inpressure and temperature, pressurized autocla-ve sampling technology as well as investiga-tions and experiments under in situ conditionsare essential. The technical development ofthese capabilities and their application to theimproved understanding of gas hydrate dyna-mics was the main focus of our collaborativeresearch project. This new autoclave technolo-gy has the following objectives:

- To quantify gas, gas hydrate and pore waterin the grain framework in sediment coresusing autoclave sampling and detailed com-puter-based tomographic imaging. A 3-dimensional density model will be deve-loped to distinguish the components of thegas-hydrate/sediment/pore-fluid system.

Bohrmann G. (1), Abegg F. (1), Amann H. (2), Brückmann W. (3), Drews M. (3), Gust G. (4), Hohnberg H.-J.

(1), Kipfstuhl J. (5), Klaucke I. (3), Rehder G. (3), Suess E. (3), Wallmann K. (3), Weinrebe W. (3)

(1) RCOM Forschungszentrum Ozeanränder an der Universität Bremen, Klagenfurter Str., GEO-Gebäude,

D-28359 Bremen. Email: gbohrmann@uni-bremen.de

(2) Technische Universität Berlin; Maritime Technik, Müller-Breslau Str., D-10623 Berlin

(3) Leibniz-Institut für Meereswissenschaften (IFM-GEOMAR), Wischhoftsr. 1-3, D-24148 Kiel

(4) Technische Universität Hamburg-Harburg, Arbeitsbereich Meerestechnik 1, Lämmersieth 72, D-22305 Hamburg

(5) Alfred-Wegener-Institut für Polar- und Meeresforschung, Bürgermeister-Smidt-Straße 20, D-27568 Bremerhaven

- To study gas hydrate formation and dissoci-ation by using chemical, physical and isoto-pe data derived from the solid hydratephase, the pore water and from the hostsediment on samples acquired with theautoclave technology.

- To examine and quantify the areal extent ofmethane and methane hydrate deposits, aswell as associated carbonates using a varietyof mapping techniques and systems.

- To conduct controlled pressurized laboratorystudies of methane-unsaturated fluids andmethane fluxes at the sediment-water inter-face during gas hydrate decomposition.Experiments will be run with pressurized gashydrate-bearing sediment samples.

3. Scientific and technical backgroundThe OMEGA project was funded by the FederalMinistry of Education and Research (BMBF) inthe frame of the special programme »GEO-TECHHNOLOGIEN«. The proposal was submit-ted in response to the call for proposals on»Gas hydrates in the geosystem – a researchstrategy« by the Ministry of Education andResearch of the Federal Republic of Germany.The project was structured in 5 subprojects:

Coordination: Prof. Dr. G. Bohrmann, S. Schenk

SP 1:Autoclave sampling and in situ preservationsystem, ASAPProf. Dr. H. Amann, Dipl.-Ing. H.-J. Hohnberg (TU Berlin)

SP 2:Formation and quantification of gas hydrates:structural analyses of gas hydrates and hostsedimentsProf. Dr. G. Bohrmann, Dr. J. Kipfstuhl, Dr. W. Brueckmann, Dr. F. Abegg (GEOMAR)

SP 3:Tracking mechanisms of gas hydrate formationand dissociation through chemical and isotopicstudies on hydrates and associated fluids.Prof. Dr. E. Suess, PD Dr. K. Wallmann, Dr. M. Drews (GEOMAR)

SP 4:Mapping and quantification of surface gashydrates and related carbonatesDr. W. Weinrebe, Prof. Dr. G. Bohrmann, Dr. I. Klaucke (GEOMAR)

SP 5:Gas hydrate pressure laboratoryProf. Dr. G. Gust (TU-Hamburg-Harburg), Dr. G. Rehder (GEOMAR)The total running time of the OMEGA projectwas 39 month (Fig.1). Workshops, meetingsand cruises have been conducted in close coo-peration with other projects (MUMM, LOTUS,GHOSTDABS, INGGAS). Specific cooperationoccurred in joint research cruises (Table 2).

5

6

4. Results of the subprojects

4.1 Autoclave sampling and in situ preserva-tion systems (SP1)

Prerequisites for the investigation of gas hydra-te containing gas rich sediments are samplingtools which preserve the in situ pressure andtemperature. This is necessary to keep thehydrates within their in situ stability field andavoid sediment structure damage throughgeneration and ebullition of free gas.To fulfill these requirements two different toolshave been developed. The first is the MultiAutoclave Corer (MAC) which is in functionand size like a multiple corer and operated onthe deep sea cable of the ship (Fig. 2). Thisallows the use of a video system for high pre-cision deployments. In general the MAC con-sists of a frame which can hold up to fourremovable pressure vessels (Laboratory Trans-fer Chambers, LTC). Below the LTCs is the corecutting unit. When deployed on the seafloorthe core cutting unit with the LTCs on top pe-netrates the seafloor, driven by gravity but

damped in speed. Upon retrieval the cores,now in a core liner, are moved into the LTCsand sealed. After this process the whole devi-ce is recovered. The core length is limited to550 mm with a core diameter of 100 mm. Topreserve the temperature each of the LTCs isenclosed by a mantel tube. This tube containsseawater which prevents heating during retrie-val, supported by the glass-fiber reinforced pla-stic material of the pressure chamber. Back ondeck the mantel tube can be filled with ice tokeep the cores cool. When completely recove-red and disconnected from the deep sea cablethe LTCs are dismantled and either directlyinvestigated or stored in an appropriate coldstorage. To preserve the pressure inside theLTCs over longer time or to counteract smallleakage, each LTC is equipped with a pressureaccumulator. Up to now the pressure vessel isdesigned for a pressure of 140 bar (1400 mwater depth). Use of the MAC in greater depthis possible but the pressure will not be stableuntil 140 bar. This keeps gas hydrates withintheir stability filed but free gas bubbles insidethe sediment would expand and dissolved gas

Figure 1: Time table of the project documenting variousmilestones, workshops and research cruises.

Table 1: Overview of research cruises

would exsolve. The weight of the MAC is 800kg, the diameter of the frame is 2.55 m andthe height is 3 m. The system has been testedby the German TÜV and the safety certificate ison hand. Finally it is possible to connect theLTCs to the pressure laboratory (s. descriptionbelow) and to transfer cores into that labora-tory. The special design and choice of materialsof the LTCs allows the MAC cores to be direct-ly investigated with computed X-ray tomogra-phy (CT, see below).

The second autoclave tool developed here isthe Dynamic Autoclave Piston Corer (DAPC).This tool consists of one pressure chamberwith attached core cutting-tube (Fig. 3).Deployment and release is analogue to a con-ventional piston corer. The core cutting tube isspecially designed to penetrate gas hydratebearing sediments in a free fall mode. Whenthe device is released by the trigger weight thepressure chamber and the core cutting tubewith the liner penetrate and sample the sedi-ment. Similar to the MAC, the first step ofrecovery is the transfer of the core liner intothe pressure chamber by lifting the deep seacable. The next step, before pulling the wholedevice out of the seafloor, is the sealing of thepressure chamber by closing a ball valve. Backon deck of the vessel, the core cutting unit isdismantled to reduce the size of the system forfurther work on the core. The DAPC also has amantel tube. In order to increase the stabilityof the pressure chamber it is made out of steel.Similar to the MAC, the mantel tube is filledwith cold water or ice or a mixture of both.This autoclave coring device is likewise equip-ped with a pressure accumulator using a largervolume than the MAC system. The maximumcore length to be achieved with the DAPC is230 cm with a core diameter of 84 mm. Thesafety approval by the German TÜV is availa-ble. The certified pressure is also 140 bar, butas with the MAC, deployments in greaterdepth are possible with the already mentionedrestraints. The weight of the DAPC amounts to500 kg and the outer diameter is 40 cm. Dueto the design and handling of the DAPC, avideo controlled deployment is not possible.

7

Figure 2: Multi Autoclave Corer on deck of RV SONNE.

Figure 3: Dynamic Autoclave Pisten Corer (DAPC) duringdeployment from RV SONNE.

8

4.2 Structure and quantification of gas hydra-te and host sediment (SP2)

The tools described in the previous chapter,especially the MAC, together with Computer-ized Tomography (CT) are forming a milestonein the investigation of natural shallow gashydrate. For the first time it becomes possibleto sample, recover and investigate sedimentscontaining gas hydrate while in situ conditionsof pressure and temperature are preserved.This means that hydrate is kept in its stabilityfield and free gas bubbles will keep their size.One prerequisite was that the LTCs were desi-gned to be translucent for X-ray (Fig. 4) beams.This has been achieved with a combination ofaluminum and glass fiber reinforced plastic.One other aspect also was important: the weightof the LTC containing the core and some waterhad to be limited to less than about 200 kg. Thisallows using medical CT scanners, which areavailable in almost any hospital and can also behired as a mobile system. After dismantling ofthe LTCs, one after the other is placed on thetable of the CT scanner.The MAC has been used during the RV SONNEcruises 165 and 174 (SO165, SO174) toHydrate Ridge and the Gulf of Mexico, respec-

tively. The cruise SO165 was the first deploy-ment of the MAC system and after severalimprovements finally two pressurized coreswere taken and shipped to port. These coreswere CT-investigated four days after recoveryin a clinic close to San Francisco. First thingdone when a LTC is on the table of the CT is toscan an overview to determine the location ofthe core within the LTC and to control corelength and quality. Based on this overview thecore is virtually sliced up perpendicular to thecore axis to generate a 3-dimensional datasetof the density variation of the material insidethe liner. Slice thickness was set to 1 mm.

The two pressurized cores, taken with onedeployment of the MAC system during thecruise SO165 to the southern summit ofHydrate Ridge, show distinct variations in eit-her horizontal and vertical distribution of gashydrate. In both cores the main compound ofthe sedimentary matrix consists of mud withseveral clasts of carbonate. One core (LTC 3)hardly reaches a maximum hydrate volume of5 vol % and most of the hydrate is located inthe uppermost 13 cm. In the other core (LTC 4)a distinct gas hydrate horizon at a depth ran-ging between 25 and 31 cm below seafloorwas detected. The highest measured gashydrate in one slice is 47 vol %. An importantoutcome of this study is the direct proof forfree gas inside the gas hydrate layer of thiscore. The free gas volume reaches up to 2.4 vol% in one slice. Considering the depth intervalfrom 25 to 31 cm of LTC 4 the gas hydratevolume amounts to 19 vol % and the free gasreaches 0.8 vol %.

Figure 4: Image of a CT slice from an autoclave core.White colors are dense material (carbonate), light greyindicates mud, dark grey represents gas hydrae and blackdisplays free gas. It is easily to be recognized that the gashydrate in this core contains a lot of free gas while stillunder pressure.

The existence of free gas is a hint on its sourceto be in greater depth below the BSR and thatfree gas percolates through the subsurface andreaches the seafloor as observed by acousticmeasurements and video observation(Heeschen, 2003) without being convertedinto gas hydrate.This result is supported by investigations car-ried out in the Gulf of Mexico during RVSONNE cruise 174. At the site Green canyonBlock 415 at the Louisiana Slope, free gas lea-ving the seafloor was also detected by videoobservation in a water depth of 1000 m. AMAC deployment (MAC 07) and the corre-sponding CT-analysis with a mobile CT scannerrevealed free gas but no gas hydrate. Porewater analyses revealed a pore water salinitythat is several times higher than normal sea-water concentrations (Bohrmann & Schenk,2004). Calculation of the thermodynamicbalances shows that gas hydrate formation isinhibited at these high pore water salinities(Heeschen et al., subm.)

4.3 Tracking mechanism of gas hydrate forma-tion and dissociation through chemical andisotopic studies (SP3)

The samples for the geochemical analyses wit-hin the OMEGA research project have beencollected during three cruses: the RV METEORcruise 52/1 to the Black Sea, the already men-tioned cruises SO165 to Hydrate Ridge and theSO174 to the Gulf of Mexico. During METEORcruise M52/1 (MARGASCH, January 2002)mud volcanoes (MV) from the central part ofthe Black Sea and the Sorokin Trough weresampled and investigated, from which theDvurechenskii MV was sampled in more detail(Bohrmann and Schenck 2002; Bohrmann etal., 2003; Blinova et al. 2003; Krastel et al.2003; Aloisi et al. 2004a and 2004b)Mud of higher temperature (Fig. 5) and fluidsenriched in chloride and other chemical consti-tuents appear to ascend from deeper stratigra-phic levels and most probably from the LateOligocene to Miocene Maikopian Formation.

9

Figure 5: Mud volcanoes in the Black Sea. Shaded relief map of mud volca-noes in the Sorokin Trough, SE of the Crimean Peninsula. Most mud volca-noes lie on the crest of an ENE-WSW morphological ridge which is the bathy-metric expression of diapiric ridges formed in the compressional regime bet-ween the Tetyaev and Shatskii Rises (Bohrmann et al., 2003).

10

Although temperatures as high as 16.5°C arereached in sediments close to the surface, gashydrates are within their stability field (Fig. 7)and their presence was proved by gas hydratesampling. Sediments from Odessa MV, YaltaMV and an unnamed mud volcano in theSorokin Trough also revealed gas hydrates. Thiswas documented either by direct gas hydratesampling or by gas hydrate proxies like coldtemperatures measured on opened sedimentcores, or by negative chloride anomalies in thepore water profiles. High methane concentra-tions in the sediments lead to a methane flux

into the bottom water, which provokes anaer-obic oxidation of methane (AOM). Evidencefor AOM was shown by pore water data andcarbonate precipitation.

Five gravity cores (TGC-2, -3, -5, -7 and -8) andthree short cores from multicorers (MIC-3, MIC-4 and MIC-5) were obtained from the summitof Dvurechenskii MV during the M52/1 cruise.All cores are composed of very fluid, dark greymud which contains mm to cm-sized roundedrock clasts. Most clasts are mudstones probablyoriginating from the Maikopian formation.

Figure 6: Location map of seafloor temperature measurements taken on the DMV with recordedmud temperatures (left). Down core temperature gradients from in-situ temperature measure-ments (TGC-2, -3, -5, -7, and -8 stations are from Dvurechenskii MV; Bohrmann et al. 2003).

Figure 7: Hydrate stability field calculatedaccording to Sloan (1998) for pure methaneand Black Sea water chlorinity of 355 mM (1)and pore water chlorinity of 900 mM in sedi-ments from Dvurenchenskii MV (2). Temper-ature measurements were taken during theM52/1 cruise. Thickness of the hydrate stabi-lity zone at 2,000 m water depth in theSorokin Trough as graphically inferred fromthe bottom water temperature of 9°C and aconstant temperature gradient in sedimentsof 29°C km-1.The findings document that Dvurechenskiimud volcano is presently active.

The fluids expelled from Dvurechenskii MV areparticularly rich in dissolved Cl and Na. Severalprocesses including seawater evaporation, gashydrate formation, ash diagenesis, and disso-lution of halite (NaCl) can result in the forma-tion of hypersaline pore fluids. Three of these(evaporation, gas hydrate formation and ashdiagenesis) enhance fluid salinity by consu-ming water, while only halite dissolution incre-ases salinity by addition of dissolved ions. Thelater process is probably not responsible for theobserved fluid chemistry because the Na/Clratio of the fluids is close to the seawater ratiobut significantly smaller than unity. Becausethe water-consuming processes mentionedabove produce changes in the oxygen stableisotope composition of water, a further discri-mination can be made based on the δ18O ofthe expelled fluids. Aloisi et al. (2004a) haveplotted the data from Dvurechenskii MV on aδ18O-Cl- diagram and compared them with cal-culated fluid δ18O-Cl- compositions producedby the above processes of water consumption,considering a starting fluid with δ18O and Cl-

similar to present day Black Sea bottom waters(Fig. 8a). The evolution of fluid δ18O and Cl-

was modeled assuming a closed systemRaleigh fractionation behavior. In addition,they plotted the δ18O-Cl- path of a fluid expe-riencing smectite-to-illite transformation, assu-ming that the interlayer water has a δ18O of 17 ‰ and applying a two end member mixingmodel. Amongst the processes that increasesalinity evaporation leads to an increase in theδ18O of fluids while gas hydrate formation andash alteration lead to a decrease in fluid δ18O.The smectite-to-illite transformation on theother hand, freshens pore water and results inan increase in δ18O.

11

Figure 8: Origin of gas hydrates and the Cl--δ18O signa-ture of the Dvurechenskii fluids. (a) Comparison of Cl--δ18O data with the model closed system evolution of Cl--δ18O values during silicate alteration processes (path 1),smectite-illite transformation (path 2), gas hydrate forma-tion ( path 3) and evaporation (path 4); (b) Model evolu-tion of fluid Cl--δ18O values during silicate alteration (path1) and smectite – illite transformation (path 2); (c) Modelevolution of fluid Cl--δ18O values during silicate alteration(path 1), smectite – illite transformation (path 2) and gashydrate formation (path 3). The starting fluid in all calcu-lations is supposed to be similar to modern Black Seawater. The final fluid in simulations (b) and (c) is theDvurechenskii fluid. BS – Black Sea water; SW – Seawater;DV – Dvurechenskii fluid (Aloisi et al., 2004a).

12

The Dvurechenskii δ18O-Cl- data do not plot onany one of the model curves, but lie betweenthe evaporation and the ash diagenesis andgas hydrate formation curves (Fig. 8a). Thissuggests that a combination of processesresults in a net increase in salinity and in δ18Oof pore fluids, producing the δ18O-Cl- signatu-re of the Dvurechenskii fluids. We have appliedthe closed system Raleigh fractionation modeland the two end-member mixing model toreproduce two possible scenarios of Cl-- δ18Oevolution, one which considers silicate altera-tion and smectite-to-illite transformation (Fig.8b) and one which considers silicate alteration,smectite-to-illite transformation and gashydrate formation (Fig. 8c). In the scenario ofFig. 8b only one possible Cl--δ18O path thatjoins modern Black Sea waters to theDvurechenskii fluids exists for the chosen oxy-gen isotope fractionation factor and smectiteoxygen isotope composition. In the scenario ofFig. 8c on the other hand, an infinite numberof Cl--δ18O paths are possible, depending onthe relative importance of the three consideredprocesses. For clarity, only one of the many Cl--δ18O paths has been shown in Fig. 8c. It isnot possible to discriminate between the twoscenarios of Figs. 8b and 8c. Recent resultsfrom ODP Leg 204, however, show that an ele-vated pore water chlorinity is associated torapidly forming gas hydrates, if methane istransported upwards as gas bubbles (Haeckel etal., 2004). Acoustic flares attributed to intensemethane venting at Dvurechenskii MV hasbeen observed recently during echo-soundersurveys, making the scenario of Fig. 8c likely.

4.4 Mapping and Quantification of surface gashydrates and related carbonates (SP4)

Besides small-scale quantification of gas hydra-tes by sampling at discrete sites, remote sen-sing technologies reveal the distribution ofhydrates and other vent-indicators as vesico-myid clams or carbonates on a larger scale. Avital part of this approach within the OMEGAproject is the deep-towed sidescan sonarsystem DTS-1. The system contains a dual-fre-

quency sidescan sonar (chirp system with 75 or410 kHz center frequencies) and a subbottomprofiler (chirp system, 2-15 kHz). The 75 kHzallows for a 1500 m wide swath and is routi-nely processed with a 1 m pixel size. In thiscase objects that are several meters across canbe detected. The 410 kHz sidescan sonarallows for about 300 m of coverage and dataare processed with a 0.25 cm pixel size. 410kHz sidescan sonar is difficult to handle,because it requires towing the instrument ide-ally 20 m above the seafloor with several km ofcable behind the ship. Precise control of thenavigation of the tow-fish is essential. Sidescansonar systems register the backscattered acou-stic signal of the seafloor.

The following factors contribute to the backs-cattering strength in decreasing order: theregional slope, the microroughness of the sea-floor and the physical properties of the materi-al on the seafloor. In areas of constant regionalslope the microroughness of the seafloor beco-mes the dominant factor contributing to thebackscattering strength. Microroughness isstrongly related to the lithology and sidescansonar consequently allows imaging fluid esca-pe structures or gas content of the uppermostsediments, even and in particular if thosestructures that do not have a bathymetricexpression.

All geoacoustic data such as multibeam bathy-metry and in particular sidescan sonar requireground-truthing in order to achieve meaning-ful geologic interpretation. Ground-truthinghas been carried out in part through TV-grabsand coring but mainly through towed videomapping using the Ocean Floor ObservationSystem (OFOS). In general, methane seeps canbe readily detected by the occurrence of authi-genic carbonates or chemosynthetic communi-ties on the seafloor

The DTS-1 has been utilized during the RVSONNE cruise 165 on Hydrate Ridge/Oregonand three different facies could be identified inthis region. The first facies consists of an asso-ciation of gas hydrates and carbonate crusts

(Bohrmann et al., 2002). The second facies isbuild up of chemoherms (large blocks of carbo-nate, Fig. 9) and the third is identified throughpockmarks. The hydrate-carbonate associationis mostly present on top of summits of Hydrateridge, which is also confirmed by the OFOSrecords. The hydrate-carbonate associations areflanked by the blocks of carbonate. Most pro-bably they are generated from strong outflowof methane-containing fluids (Johnson et al.,2003). The pockmarks concentrate on the sou-thern flank of Hydrate Ridge and have not beenobserved previously.

Based on this facies differentiation and theirregional distribution as well as the quantitativeanalyses of the methane fluxes the first time aregional calculation of the methane flux rate ispossible (Klaucke et al., subm.).During the RV METEOR cruise 52/1 in the BlackSea different acoustic facies have been recor-ded. Due to closing of the Thetys many mudvolcanoes have been formed. High backscattersignatures at the flanks of the mud volcanoesare not only caused from the relief but also arecaused by mud flows and carbonates, formedat the top of the mud volcanoes. The mud-flows are connected to the methane seeps(Bohrmann et al., 2003).

13

Figure 9: Illustration showing the "zoom-in" approach using multibeam bathymetry, side-scan sonar and video observation. For example, at the northern summit of Hydrate Ridgelarge authigenic carbonates (chemoherms) cause typical backscatter signals. By combiningthe sidescan sonar image with the visual observations a geological interpretation of thearea can be derived.

14

4.5 Design of a mobile pressure laboratory (SP5)

One of the main tasks of the pressure labora-tory was to allow the transfer of cores, contai-ning sediment samples with gas hydrate takenwith the MAC, without pressure loss. Based onthis demand many design-engineering basiccomponents had to be considered.The pressure laboratory consists of several dif-ferent parts. The main part is the pressure ves-sel which holds the sample material and is theplace to conduct experiments. Connected tothe pressure vessel is a tank with a volume of40 l. The water inside this tank can be mixedwith substances, for instance with methane,and can be fed into the pressure vessel.Additionally there is a cylinder of water whichallows a singular exchange of the fluids insidethe pressure vessel. Together with a tempera-ture control this combination allows adjust-ment of different chemical and thermodynami-cal settings.

Transfer of sediment cores taken with theMAC is possible through a pressurized lock.The lock is mounted on top of the pressurevessel and the LTC is mounted on top of thelock. When the pressure is equilibrated, theLTC is opened mechanically with an axle-driveshaft and the core will move into the pressurevessel. After the core transfer the pressure ves-sel is closed and LTC and lock are dismounted.

The whole pressure laboratory has been deve-loped for the use at sea. It is mounted in a 20-

feet container. Because of the height of thelaboratory including the base plate it is a HighCube container. The roof of the container canpartially be opened for mounting a crane. Thecrane is used to safely handle all heavy parts ofthe pressure laboratory. Last but not least thecontainer is equipped with air conditioning toallow work also in low latitudes.The whole pressure laboratory is certified bythe German TÜV and the container has beenapproved for sea transportation according tothe regulations of the ‘Germanischer Lloyd’.

5. ConclusionsMultidisciplinary research within the frame-work of the OMEGA project lead to manyresults in near-surface gas hydrate deposits.Major highlights are:- Construction and operation of two autocla-

ve systems that sampled gas hydratesunder in-situ pressure.

- The co-existence of free-gas and gas hydra-tes have been documented in autoclavecores taken on Hydrate Ridge. Gas bubblesare surrounded by gas hydrate, which seemto be encapsulated. By this formation thehydrate separates the pore water from thefree gas phase and explains how free gascan stay in the gas hydrate stability zone.

- A deep-tow sidescan sonar system operatingin two frequencies (75 kHZ and 410 kHz waspurchased, installed with other tools andsuccessfully used in hydrate-hosting areassuch as Hydrate Ridge, the Black Sea andalong the Pacific Continental Margin.

Table 2: Technical data of the pressure laboratory.

Operating pressure 0-55 MpaTemperature -2°C bis 30°CVolume 99 lInner diamter 300 mmFree inner height 1400 mmMain hole diameter 110 mmInterconnections to pressure vessel Hydraulics, electrical, mechanically

(axle drive shaft), video, view glassesWeight 1500 kg

- Deeply altered fluids have been collectedand investigated from mud volcanoes inthe Sorokin Trough of the Black Sea andhave been compared to model data inorder to interpret the origin of the fluids.

- The presence of gas hydrate has beendocumented from various mud volcanoesin the Sorokin Trough of the Black Sea.

- In a mud flow on Odessa mud volcano acarbonate crust currently forms in associa-tion with anaerobic methane oxidation bymicrobial colonies.

- At Dvurechenskii mud volcano in the BlackSea high sediment temperatures of up to16.5°C in close contact to the ambient bot-tom water of 9°C suggest strong mud vol-canic activity.

- In seismic records over the mud volcanoesbottom simulating reflectors are not pre-sent, but pronounced lateral amplitudevariations and bright spots in the approxi-mate depth of the gas hydrate stabilityzone may indicate the occurrence of gashydrates and free gas.

- Hydrocarbon gases were determined in sedi-ments from mud volcanoes in the SorokinThrough. In comparison to a reference sta-tion outside the mud volcano area, thedeposits are characterized by an enrich-ment of high-molecular hydrocarbons (C2 –C4), an absence of unsaturated homolo-gues, a predominance of isobutane in com-parison with n-butane and the presence ofgas hydrate.

- The pressure laboratory has been success-fully developed and the transfer from sedi-ment cores to the laboratory was realizedtrough a pressure lock.

AcknowledgementsThe project and the cruises were financed bythe German Federal Ministry of Education andScience (Bundesministerium für Bildung undForschung; grants 03G0165A; 03G0174A;03G0566A) and the German ResearchFoundation (Deutsche Forschungsgemein-schaft; grant Su 114/11-1). Special thanks goto our technicians and engineers.

ReferencesAbegg F, Freitag J and Bohrmann G (submit-ted) Free gas within the gas hydrate stabilityzone of Hydrate Ridge – signs of water exhaus-tion due to rapid hydrate formation.

Aloisi G, Drews M, Wallmann K, and Bohr-mann G (2004a) Fluid expulsion from Dvu-rechenskii mud volcano (Black Sea) Part I: fluidsources and relevance to Li, B, Sr and dissolvedinorganic nitrogen cycles. Earth and PlanetaryScience Letters, 225: 347-363. GEOTECH-72

Aloisi G, Wallmann K, Drews M, and Bohr-mann G (2004b) Evidence for the submarineweathering of silicate minerals in Black Seasediments: possible implications for the marineLi and B cycles. Geochemistry, Geophysics,Geosystem 5 (1) doi: 10.1029/ 2003GC000639.GEOTECH-71

Blinova V, Ivanov M, Bohrmann G (2003) Hy-drocarbon gases in deposits from mud volca-noes in the Sorokin Trough, north-easternBlack Sea. Geo-Marine Letters 23 (3-4) 250-257. GEOTECH-26

Boetius A, Suess E (2004) Hydrate Ridge: anatural laboratory for the study of microbiallife fueled by methane from near-surface gashydrates. Chemical Geology 205: 291-310.GEOTECH-28

Bohrmann G and Torres M (in press) Gashydrates in marine sediments. In Schulz HDand Zabel M., Marine Geochemistry. 481- 512,Springer. GEOTECH-194

Bohrmann G, (2005) Gashydrate der Ozeane –feste Verbindungen aus Gas und Wasser. PdN-Chemie, 54 (6): 2-8.

Bohrmann G, Schenck S (2004) GEOMARCruise Report SO 174, OTEGA II, RV »SONNE«.GEOMAR Report 117, Kiel.

Bohrmann G, Suess E (2004) Gashydrate derMeeresböden: Ein dynamischer Methanspei-cher im Ozean. AKE-Tagungsband: 16 Seiten.

15

16

Bohrmann G, Tréhu A, Rack F, Torres M andODP Leg 204 Shipboard Scientific Party (2003).Drilling gas hydrates on Hydrate Ridge,Cascadia Continental Margin. Energy, explora-tion and exploitation 21 (4): 333-334

Bohrmann G, Ivanov M, Foucher JP, Spiess V,Bialas J, Weinrebe W, Abegg F, Aloisi G,Artemov Y, Blinova V, Drews M, Greinert J,Heidersdorf F, Krastel S, Krabbenhöft A,Polikarpov I, Saburova M, Schmale O,Seifert R,Volkonskaya A, Zillmer, M (2003) Mud volca-noes and gas hydrates in the Black Sea – newdata from Dvurechenskii and Odessa mud vol-canoes. Geo-Marine Letters 23 (3-4) 239-249.GEOTECH-25

Bohrmann, G., Suess, E., Greinert, J., Teichert,B., and Naehr, T., 2002. Gas hydrate carbona-tes from Hydrate Ridge, Cascadia ConvergentMargin: indicators of near-seafloor clathratedeposits. Fourth Int. Conf. Gas Hydrates,Yokohama, Japan: 102–107.

Bohrmann, G., Suess, E., Greinert, J., Teichert, B.Nähr, T., 2002. Gas Hydrate Carbonates fromHydrate Ridge, Cascadia Convergent Margin.Indicators of near-seafloor clathrate deposits.Proceedings of the Fourth InternationalConference on Gas Hydrates, Yokohama: 18-21.

Bohrmann G, Schenck S (2002) GEOMARCruise Report M52/1, MARGASCH, RV »Mete-or«, Marine Gas Hydrates of the Black Sea.GEOMAR Report 108, Kiel.

Bohrmann G, Greinert J, and Suess E, 2001,Methanhydrate, Enzyklopädie Naturwissen-schaften und Technik, 7. Erg.-Lfg.10/01, p. 1-8.

Drews M., Wallmann K., Aloisi G., andBohrmann G. (submitted) Fluid expulsion fromthe Dvurechenskii mud volcano (Black Sea).Part II: Methane fluxes and their relevance tothe Black Sea methane cycle. Earth andPlanetary Science Letters.

Greinert J, Bohrmann G and Suess E (2001) Gashydrate-associated carbonates and methan-ven-ting at Hydrate Ridge (Cascadia): Their classifica-tion, distribution and origin. In: Paull C (Editor),AGU Monograph 124, 99-113.

Gutt C, Press W, Bohrmann G, Greinert J, andHüller A, 2001, Brennendes Eis: Methanhydrat- Energiequelle der Zukunft oder Gefahr fürsKlima: Physikalische Blätter, v. 59: p. 1-6.

Hovland, M., MacDonald, I.R., Rueslåtten, H.,Johnsen, H.K., Naehr, T., and Bohrmann, G.,2005, Chapopote asphalt volcano, may havebeen generated by supercritical water. EOS 86(42): 397, 402 GEOTECH-182

Haeckel, M., Suess, E., Wallmann, K., andRickert, D. (2004) Rising methane gas bubblesform massive hydrate layers at the seafloor.Geochimica et Cosmochimica Acta 68 (21),4335-4345.

Heeschen KU, Tréhu AM, Collier RW, Suess E,Rehder G (2003) Distribution and height ofmethane bubble plumes on the Cascadia Mar-gin offshore Oregon from acoustic imaging.Geophysical Research Letters, 30(12), 1643,doi:10.1029/2003GL016974.

Heeschen KU, RW. Collier, MA de Angelis, ESuess, G Rehder, P Linke and Klinkhammer GP(2005) Methane sources, distributions, and flu-xes from cold vent sites at Hydrate Ridge,Cascadia Margin. Global BiogeochemicalCycles 19, GB2016, doi:10.1029/2004GB002266 GEOTECH-40

Heeschen KU, Hohnberg J, Drews M, AbeggF, and Bohrmann G (submitted) In-situ hydro-carbon inventory from pressurized cores insurface sediments, Northern Gulf of Mexico.Marine Geology.

Johnson JE, Goldfinger C, Suess E (2003)Geophysical constraints on the surface distri-bution of authigenic carbonates across theHydrate Ridge region, Cascadia Margin.Marine Geology 202: 79-120.

Klaucke I, Bohrmann G, Weinrebe W (submit-ted) Estimation of regional methane efflux onHydrate Ridge, Oregon. Geochemistry, Geo-physics, Geosystems.

Klaucke I, Sahling H, Bürk D, Weinrebe W, Bohr-mann G (2005) Mapping deep-water gas emis-sions with high-resolution sidscan sonar. EOS,86 (38): 341, 346. GEOTECH-189

Krastel S, Spiess V, Ivanov M, Weinrebe W,Bohrmann G, Shaskin P (2003) Acoustic imagesof mud volcanoes in the Sorokin Trough. Geo-Marine Letters 23 (3-4) 230-238. GEOTECH-30

Kuhs, WF, Genov GY, Goreshnik E, Zeller A,Techmer K, Bohrmann G (2004), The impact ofporous microstructures of gas hydrates ontheir macroscopic properties. Proceedings ofthe Fourteenth International Offshore andPolar Engineering Conference Toulon, France,May 23-28, 2004, 31-35. GEOTECH-100

Luff R. and Wallmann K. (2003) Fluid flow,methane fluxes, carbonate precipitation andbiogeochemical turnover in gas hydrate-bea-ring sediments at Hydrate Ridge, CascadiaMargin: Numerical modeling and mass balan-ces. Geochimica et Cosmochimica Acta 67(18),3403-3421. GEOTECH-8

Luff R., Wallmann K., and Aloisi G. (in press)Physical and biogeochemical constraints on car-bonate crust formation at cold vent sites: signi-ficance for fluid flow and methane budgets andchemosynthetic biological communities. Earthand Planetary Science Letters. GEOTECH-32

Luff R, Greinert J, Wallmann K, Klaucke I andSuess E (2005) Simulation of long-term feed-backs from authigenic carbonate crust forma-tion at cold vent sites. Chemical Geology 216,157-174. GEOTECH-99

MacDonald IR, Bohrmann G, Escobar E, AbeggF, Blanchon P, Blinova V, Brückmann W, DrewsM, Eisenhauer A, Han X, Heeschen K, Meier F,Mortera C, Naehr T, Orcutt B, Bernard B, BrooksJ, de Farágo M, (2004) Ashalt volcanism andchemosynthetic life in the Campeche Knolls.Science, 304, p.999-1002. GEOTECH-62

Pfannkuche O und Fahrtteilnehmer (2001)Cruise report ALKOR No. 192: Test of novelinstrumentation for gas hydrate research at me-thane seeps in the Skagerrak online: http://www.geomar.de /~jgreiner/web_LOTUS/ index.htm.

Pfannkuche O, Eisenhauer A, Linke P, Utecht C(2003) RV SO165 cruise report OTEGA-I, GEO-MAR Report 112, GEOMAR Forschungszentrum.

Rehder G, Kirby S, Durham B, Stern L, PeltzerET, Pinkston J, Brewer PG, (2003) Dissolutionrates of pure methane and hydrate and car-bon-dioxide hydrate in undersaturated sea-water at 1000-m depth. GeochimicaCosmochimica Acta, 68 (2): 285-292.

Reed A, Abegg F, Grader A, Winters W, (2004)Gas Hydrates in Detail. NETL Newsletter Fire inthe Ice, Spring 2004, 6-9.

Suess E, Torres M, Bohrmann G, Collier RW,Rickert D, Goldfinger C, Linke P, Heuser A, Sah-ling H, Jung C, Nakamura K, Greinert J, Pfann-kuche O, Trehu A, Klinkhammer G, Whiticar MJ,Eisenhauer A, Teichert B, Elvert M, (2001) Dy-namics of sea floor hydrate at Hydrate Ridge. In:Paull C (Editor), AGU Monograph 124, 87-98.

17

18

Shoji H., Hachikubo A., Miyamoto A., Hyaku-take K., Abe K., Bohrmann G., Kipfstuhl S.,(2002) Construction of a Pressure Cell forVisual Observations on formation Processes ofGlobular Gas Hydrate. Proceedings of theFourth International Conference on GasHydrates, Yokohama: 320-323.

Suess E., Bohrmann G., Rickert D., Kuhs W., Tor-res M., Trehu A., Linke P., (2002) Physical Pro-perties and Fabric of Near-surface MethaneHydrates at Hydrate Ridge, Cascadia Margin. Pro-ceedings of the Fourth International Conferenceon Gas Hydrates, Yokohama: 185-188.

Shipboard Scientific Party, 2002. Leg 204Preliminary Report. ODP Prelim. Rpt. (Online).Available from World Wise Web: http://www-odp. tamu.edu/pub l i cat ions /pre-lim/204_prel/204PREL.PDF

Tishchenko P., Hensen C., Wallmann K. WongC.S. (2005) Calculation of the stability andsolubility of methane hydrate in seawater.Chemical Geology. 219: 37-52.

Torres M.E., Wallmann K., Tréhu A.M., Bohr-mann G., Borowski W.S., Tomaru H. (2004) Gashydrate growth, methane transport, and chlori-de enrichment at the southern summit ofHydrate Ridge, Cascadia Margin. Earth andPlanetary Science Letters, 226:225-241.

Tréhu A, Long PE, Torres ME, Bohrmann G,Rack F, Collett TS, Goldberg DS, Milkov A,Reidel M, Schultheiss P, Bangs NL, Barr SR,Borowski WS, Claypool GE, Delwiche ME,Dickens GR, Gracia E, Guerin G, Holland M,Johnson JE, Lee Y-J, Liu C-S, Su X, Teichert B,Tomaru H, Vanneste M, Watanabe M,Watanabe M, Weinberg JL (2004) Three-dimensional distribution of gas hydrate bene-ath southern Hydrate Ridge: constraints fromODP Leg 204. Earth and Planetary ScienceLetter.222, 845-862.

Tréhu A.M., Bangs N.L., Arsenault M.A.,Bohrmann G., Goldfinger C., Johnson J.E.,Nakamura Y., Torres M.E., (2002) Complex sub-surface plumbing beneath southern HydrateRidge, Oregon continental margin, from high-resolution 3-D seismic reflection and OBS Data.Fourth Int. Conf. Gas Hydrates, Yokohama,Japan: 90–96.

Tréhu A, Bohrmann G, Rack F, Torres M and ODPLeg 204 Shipboard Scientific Party (2003)Drilling gas hydrates on Hydrate Ridge, CascadiaContinental Margin. ODP Initial Rpt., 204.

Tréhu A, Bohrmann G, Rack F, Torres M andODP Leg 204 shipboard Scientific Party (2003)Gas hydrate distribution and dynamics bene-ath Southern Hydrate Ridge. JOIDES Journalvol. 29 (2): 5-8.

Teichert BM, Eisenhauer A, Bohrmann G,Haase-Schramm G, Bock B, Linke P (2003)U/Th systematics and ages of authigenic car-bonates from Hydrate Ridge, CascadiaMargin: Recorders of fluid flow variations.Geochimica et Cosmochimica Acta 67 (20)3845-3857. GEOTECH-2

Teichert B, Gussone N, Eisenhauer A, BohrmannG (2005) Clathrites – Archives of near-seafloorpore water evolutions (δ44Ca, δ13C,δ18O) in seepenvironments. Geology, 33(3): 213-216.

Teichert BM, Bohrmann G, Suess E (2005)Microbially mediated carbonate build ups incold seep environments. Palaeogeography, Pa-laeoclimatology, Palaeoecology, 227: 67-85.

Treude T, Boetius A, Knittel K, Wallmann K,Jørgensen BB (2003) Anaerobic oxidation of me-thane above gas hydrates (Hydrate Ridge, OR).Marine Ecology Series 264, 1-14. GEOTECH-20

Winckler G, Aeschebach-Hertig W, HoloucherJ, Kipfer R, Levin I, Poss C, Rehder G, Suess E,Schlosser P (2002) Noble gases and radiocar-bon in natural gas hydrates. GeophysicalResearch Letters, 29(10), 0.1029/2001, correc-tion printed in 29 (15), 1029/2002GL01573.

19

20

Long-term Observatory for the Study ofControl Mechanisms for the Formation andDestabilisation of Gas Hydrates (LOTUS)

1. IntroductionIt is generally recognized that destabilization ofgas hydrates and the resulting release ofmethane may be one of the most powerfulinfluences on past abrupt climatic changes ofthe earth system. However, in climate researchthe release of methane from gas hydrates hashardly been considered in model calculationssince little information exists concerning thegeochemical cycle of methane from marinehydrate deposits. It is not clear whether theentire amount of methane released at the sedi-ment-water interface reaches the atmosphereor whether the exit of large amounts ofmethane to the atmosphere is prevented byoxidization in the benthic boundary layer or inthe overlying water column by methane-oxidi-zing bacteria. Stimulated by the injection ofreduced chemical species, a significant auto-trophic production with an enormous oxygenconsumption develops at vent sites, i.e. bymethane from gas hydrates and hydrogen sul-fide and ammonia from vent fluids. The effi-ciency and relevance of this »benthic filter orreactor« in comparison to »normal« heterotro-phic systems dependent on pelagic POC fluxesneeds to be determined and quantified. Fur-thermore it is not known whether the exhala-tion of methane from the sediments into the

water column represents a constant flux or ifvariations occur that are controlled by environ-mental factors. In addition, little informationexists concerning the lifetime and temporalactivity of gas hydrate deposits and methanevents, and therefore no quantitative evaluationof temporal oscillations in gas hydrate sourcestrengths has been possible to date. Even theresidence time of methane in the form of gashydrate is totally unknown. These complexdynamics must be understood before mecha-nisms responsible for hydrate formation anddestruction at the sea floor can be quantifiedand modelled.

2. ObjectivesIn view of these gaps in our knowledge, quan-tification and modelling of methane, forma-tion and release can only be achieved by in situobservatories. This approach of LOTUS willenable fundamental new insights in the long-term temporal variability of the controllingphysico-chemical and biogeochemical parame-ters in the sediment and in the water columnas well as their impact on the temporal andspatial variability of venting. Previous measure-ments with short-term sampling intervals haveshown differences on the orders of magnitudewhich indicate a significant variability in fluid

Linke P. (1), Abegg C. (2), Eisenhauer A. (1), Gubsch S. (3), Gust G. (3), Greinert J. (1), Keir R. (1),

Liebetrau V. (1), Luff R. (1), Pfannkuche O. (1), Sommer S. (1), Spiess V. (4), Wallmann K. (1)

(1) Leibniz-Institut für Meereswissenschaften (IFM-GEOMAR), Wischhofstr. 1-3, 24148 Kiel,

Germany, E-Mail: plinke@ifm-geomar.de

(2) L3-Communications ELAC-Nautik GmbH, Kiel, Germany

(3) Technische Universität Hamburg-Harburg, Arbeitsbereich Meerestechnik 1, Lämmersieth

72, 22305 Hamburg, Germany

(4) Fachbereich Geowissenschaften, Universität Bremen, Klagenfurterstraße, 28334 Bremen, Germany

and gas fluxes. A long-term observation ofmethane fluxes and their control mechanismsis necessary for a better resolution of the tem-poral variability of fluxes and their relation tothe benthic autotrophic communities. Theobjective of the multi-disciplinary programmeLOTUS was to monitor in situ the complex trig-ger mechanisms of formation and destabilisa-tion of gas hydrates on different time andspace scales and to contribute to improvedmass balances and diagenetic and prognosticmodelling. This task included the interrelationbetween fluxes associated with gas hydratesand biogeochemical reactions at the benthicboundary layer. This was realised by novellong-term observatories for the sedimentwater interface and the water column, bydating and interpretation of the natural geo-archives as well as by process-oriented model-ling of the benthic processes.

3. Scientific and technical background ofthe project The LOTUS project was proposed and fundedwithin the first funding period of the specialprogramme »GEOTECHNOLOGIEN« funded bythe BMBF. Scientifically the project is based onthe results and experiences, which have beenobtained during two RV SONNE cruises toHydrate Ridge within the TECFLUX project(BMBF). The project was composed of fourscientific subprojects with the participation offour companies (Tab. 1).

LOTUS was integrated into a network of natio-nal projects of the »BMBF Gashydrat Initiative«and international EU-projects (Fig. 2). The pro-jects LOTUS and OMEGA in combination withthe MPI project »MUMM« represent a corewithin this network with intensive scientificand technical exchange. For example, the insitu measurements of bacterial turnover rates

21

Co-ordination: Dr. P. Linke (GEOMAR)

SP 1: In situ long-term observatories for the study of control mechanisms of fluid and methane flux during formation and destabilisation of near-surfacemarine gas hydrates.Dr. P. Linke & Dr. O. Pfannkuche (GEOMAR), Prof. Dr. G. Gust (TUHH) with participation of the Fa. Oktopus

SP 2: The fate of methane in the water columnDr. R. S. Keir (GEOMAR) Prof. Dr. V. Spiess (Universität Bremen)with participation of the Fa. CAPSUM, ELAC-Nautik, STN Atlas

SP 3: Chronology and geochemical dynamics of near-surface gas hydrate depositsProf. Dr. A. Eisenhauer, Dr. G. Bohrmann (GEOMAR)Dr. J. Scholten (Institut für Geowissenschaften, Kiel)

SP 4: Modeling methane fluxes and biogeochemical processes in hydrate-bearingsurface sedimentsDr. K. Wallmann (GEOMAR)

Table 1: Structure of LOTUS

22

in the sediment (sulphate reduction) have beeninvestigated by the Max-Planck-Institute forMarine Microbiology in Bremen withinMUMM. Joint planning and accomplishmentof expeditions as well as an exchange of sam-ples and data were performed. Data from theSONNE cruises were transferred to the PAN-GAEA data center. Furthermore, in the field ofsensor development we conducted a close co-operation with the company Unisense, Aarhus.

Scientific and technical input was gained fromEU programmes (ESONET, CRIMEA) and fromprevious BMBF projects (BIGSET, TECFLUX).With our experience in long-term observato-ries LOTUS will provide a substantial input tothe planned ESONET and Neptune Network.Technical developments within LOTUS willimpact the innovation and competitiveness ofsmall companies which partly co-operate aspartners in LOTUS.

The project started in February 2001 with thetechnical development of the various in-situobservatories and instrumentation, which wastested on two cruises with RV ALKOR inOctober 2001 and May 2002. In parallel tothis, the laboratory and computing facilitieswere established (Fig. 1).

Major stepping stones for LOTUS were the twoexpeditions with RV SONNE, SO165 – OTEGA-I(Pfannkuche et al., 2002) to the Cascadia con-tinental margin (Fig. 2) and SO174 – OTEGA-II(Bohrmann and Schenck, 2003) to the Gulf ofMexico (Fig. 4). Furthermore, members ofLOTUS participated on the METEOR cruiseM52/1 - MARGASCH and two cruises with RVProf. Vodyanitsky (EU project CRIMEA) to theBlack Sea.

Research on cold seeps of the Cascadia conti-nental margin began with the discovery offluid venting and associated chemosyntheticlife (Suess et al. 1985; Kulm et al. 1986) andwas propelled forward by the findings of deep(Kastner et al. 1998) and shallow gas hydratedeposits (Suess et al. 1997; 1999). After morethan a decade of research at Hydrate Ridge byinternational and interdisciplinary scientificexpeditions, this is probably the best-studiedconvergent margin with intense fluid flow andlarge-scale gas hydrate deposits (Boetius andSuess 2004). Active venting of fluids and gases(Linke et al. 1994; Torres et al., 2002; Tryon etal., 1999, 2002; Heeschen et al., 2005), expo-sure of methane hydrates at the seafloor(Suess et al. 2001; Haeckel et al. 2004), distri-bution, composition and activity of chemosyn-

NO DJ J JF FM MA AM MJ JS N NO OD DJ F M A M J J A SJ A SJ A2001 2002 2003

Concluding synthesis

Final Report

Laboratory- and shallow water test trials

Lab evaluation:ALKOR cruise

Evaluation Sonne cruise

E EE

EPlaning and evaluation seminar

Evaluation seminar

Technical development

SONNE cruiseALKORTest cruise ALKOR

Test cruise

P/E

P/E P/E P/E P/E P/E

SONNE cruise

Project phase

Figure 1: Schedule of the LOTUS project displaying the different working phases, cruise and planing and evaluation seminars of the project.

thetic communities (Boetius et al. 2000; Knittelet al. 2003; Tryon and Brown 2001; Sahling etal. 2002; Sommer et al. 2003; Heinz et al., inpress), authigenic carbonates forming chemo-herms (Bohrmann et al. 1998; Greinert et al.2001; Teichert et al. 2003), and gas plumes inthe water column have been well documentedduring high-resolution seismic and side-scansonar surveys (Tréhu et al., 1995; Johnson etal., 2003; Heeschen et al., 2003), supplemen-ted by submersible and ROV dives (Torres et al.,2002; Linke 2003), camera surveys, as well asby video-guided sediment sampling anddeployment of benthic landers. The vent sitesand related topographic features and the tec-tonic convergence setting of the mid-slopemargin are easily accessible from the Pacificcoast of the U.S. (Fig. 2). Furthermore, thislocation straddles the stability limit of gashydrates and thus makes Hydrate Ridge an

ideal natural laboratory for the study of activefluid flow, gas hydrate formation and dissocia-tion, and its impact on the environment(Boetius and Suess 2004). Most of the data onHydrate Ridge so far were obtained during theTECFLUX programmes (TECtonically inducedFLUXes) in 1999-2000, followed by the pro-grammes LOTUS, OMEGA and MUMM in2001-2003, and the international OceanDrilling Programme (e.g. Schlüter et al. 1998;Carson et al. 2003; Tréhu et al. 2004).

In the Gulf of Mexico some of the best-docu-mented gas hydrate occurrences in the worldare situated (Fig. 3). In the northern part of theGulf gas hydrate has been found at more than50 locations in combination with the dischargeof hydrocarbons from the seafloor (Brooks etal. 1984, Milkov and Sassen 2001, Sassen et al.2001). The sediments in the northern Gulf of

23

Figure 2: Shaded relief bathy-metry of the Hydrate Ridgeregion. Contour interval is100 m and bathymetric gridis 100-m pixel resolution.Inset shows Pacific Northwestbathymetry and topographyand the location of HydrateRidge region on the lowercontinental slope of theCascadia accretionary prism.Hydrate Ridge is a NE-SW-trending thrust ridge withnorthern and southern sum-mits; (NHR) Northern HydrateRidge; (SHR) SouthernHydrate Ridge. The ridge islocated V10 km from thedeformation front (DF) andbordered on the west andeast by slope basins (HRB-W)Hydrate Ridge Basin-Westand (HRB-E) Hydrate RidgeBasin-East. ODP (OceanDrilling Programme) site 891on the crest of the first accre-tionary ridge (FAR) and site892 on NHR are shown. DaisyBank (DB) is also shown(Johnson et al. 2003).

24

Mexico overlie enormous reservoirs of liquidand gaseous hydrocarbons that rest uponJurassic-age salt deposits (Kennicutt et al. 1988,Roberts et al. 1999). These allochthonous saltbodies and sediment-filled minibasins are themost obvious features of the regional geology.Their geometry, distribution, and structural evo-lution through time influence the occurrence ofgas hydrate in the nothern Gulf of Mexico.Milkov and Sassen (2001) have developed aconceptual model of gas hydrate occurrencewith two types of gas hydrate accumulations inthe NW GOM: (1) structurally focused thermo-genic and bacterial gas hydrate on the rims ofminibasins, and (2) disseminated bacterialmethane hydrates (~100% CH4) that resideswithin minibasins. Thermogenic structure IIhydrates are containing methane (44%), ethane(11%), propane (32%), iso-butane (9.5%),butane (3%) and pentane (0.5%) (Sassen et al.1998, Orcutt et al. 2003).

Salt-driven tectonics creates fault networks thatserve as conduits for the rapid transfer of oil,gas and brines from deep reservoirs through theoverlying sediments and ultimately into thewater column (Kennicutt et al. 1988a and1988b, Aharon 1994, Roberts and Carney1997). On the seafloor, such conduits give riseto gas vents and seeps, subsurface and sedi-ment surface-breaching gas hydrates, brinepools, and mud volcanoes (Roberts and Aharon1994, Sassen et al. 1994).

The oil and gas escaping at seeps rises throughthe water column and form long linear layers onthe ocean surface (MacDonald et al. 1996).These layers of floating oil can be detected insatellite images and provide a means for findingseeps (De Beukelaer, MacDonald et al. 2003).Oil slicks that form over seeps are typically long,linear features, broadest at the point of originwhere the oil drops reach the surface, and tape-

Figure 3: Map of the northern Gulf of Mexico with documented locations of gas hydrates, oil and gas seeps(taken from Milkov and Sassen 2001).

ring away in the direction of prevailing wind andcurrent. By comparing the locations of slicks inmultiple images, it is possible to predict the sea-floor location of a seep. On the other hand, thesenatural oil slicks provide a dramatic demonstra-tion that some fraction of seeping hydrocarbonsescapes the water column and reaches theatmosphere (Leifer and MacDonald, 2003).

In-situ instrumentation of shallow or exposeddeposits of gas hydrates indicate that they alter-nately form and decompose as bottom watertemperature fluctuates (MacDonald et al.1994). Gas hydrate deposits have also beenfound to generate irregular bathymetry (Mac-Donald et al. 2003) and to support colonies ofunusual annelid worms (Fisher, MacDonald etal. 2000). Several authors have suggested thatpresence of gas hydrate enables or facilitatesformation and maintenance of tube worm ag-gregations (Carney 1994; Sassen et al. 1999). Inany event, mounded and irregular bathymetryand chemosynthetic communities have beenaccepted as reliable indicators of active hydro-carbon seepage (MacDonald et al. 1996; Ro-berts and Carney 1997).

Sediments in and around areas of active seepa-ge are characterized by elevated concentrationsof simple (C1-C5) and complex (oils) hydrocar-bons and hydrogen sulfide (H2S). Complex che-mosynthetic communities comprised of a varie-ty of microorganisms and bacteria-metazoansymbioses thrive around hydrocarbon seeps inthe Gulf of Mexico (Kennicutt et al. 1985, Mac-Donald et al. 1989, 1990 and 1996, Fisher 1990,Ferrel and Aharon 1994, Larkin et al. 1994).These communities proliferate in a cold, high-pressure environment by exploiting the abun-dance of energy rich reduced substrates, such asmethane and H2S. While the diversity and distri-bution of seep macrofauna has been the focusof intense study, the activity of free-living bacte-ria in seep sediments and around gas hydrateshas received little attention so far (Joye et al.2004). This lack of information is surprisinggiven that microbial activity may impact the fluxand composition of both liquid and gaseous

hydrocarbons and oils as they transit the seepecosystem (Kennicutt et al. 1988, Sassen et al.1998) and may even be responsible for the for-mation of seep deposits, such as carbonatereefs, chimneys, and mounds (Ferrel and Aharon1994, Suess et al. 1999, Michaelis et al. 2002).

In a recent study on anaerobic oxidation ofmethane (AOM) and sulfate reduction (SR) insediments from Gulf of Mexico cold seeps onlya weak coupling between AOM and SR wasobserved and that AOM accounts for only asmall fraction of SR activity in the methane-richsediments (Joye et al. 2004). In this system, CH4

is just one of a diverse suite of seep-derivedorganic substrates that could fuel sulfate reduc-tion (Brooks et al. 1984, Aharon 2000). A varie-ty of long-chain alkanes, complex aliphatic andaromatic compounds, and oils can be consum-ed by sulfate reducing bacteria in this system.The lack of strong (1:1) coupling between AOMand SR means that SR in Gulf of Mexico seepsediments must be driven by the oxidation ofother organic compounds more so than byAOM (Joye et al. 2004).

The magnitude of spatial and temporal variationin fluid flow at Gulf of Mexico seeps is present-ly unknown. Fluid flow data from Hydrate Ridgeshow that temporal and spatial variations in see-page occur (Torres et al. 2002, Tyron et al. 2002)and similarly variable flow rates might be expec-ted at the Gulf of Mexico cold seeps. The inhe-rent variability in seepage rates, and hence insubstrate supply, may select for a metabolicallyplastic microbial community that is adept atconsuming a variety of organic substrates whenCH4 is limiting but one that is also poised to takeadvantage of periods when CH4 is available.Furthermore, the presence of other, more ener-getically favorable substrates could generatecompetition for available sulfate betweenmicrobes involved in AOM and microbes oxidi-zing other hydrocarbons and oil. Such competi-tion could serve to structure the microbial com-munity at mixed substrate (gas- and petroleum-rich) cold seeps. The capacity for AOM in suchsediments and the impact of other reduced car-

25

26

bon substrates on AOM rates should be exami-ned carefully to better elucidate the biogeoche-mical and microbiological controls on AOM insitu. Despite the presence of alternate reducedcarbon substrates, Gulf of Mexico seep sedi-ments harbor anaerobic methane consumingmicroorganisms that act as an efficient bio-filterpreventing methane emission to the hydrosphe-re, except at a few sites where hydrate dissocia-tion and fluid flow creates so much gas pressu-re that free gas emanates from the sediments inthe form of gas bubbles (Joye et al. 2004).

The working area of the second SONNE expe-dition (Fig. 4) has been chosen out of logisticreasons, since the cruise schedule of the vesselmade it impossible to reach Cascadia a secondtime in 2003. These logistic constrains shiftedthe second cruise into the Gulf of Mexicotowards the end of the funding period ofLOTUS. This is the main reason that some of

the analytical procedures, evaluation andmodeling of data as well as publication ofresults could not be finished within the fun-ding period. Most of this work is done at pre-sent and will be finalised with the next fundingperiod.

Nevertheless, most of the working programmewhich was scheduled in the LOTUS proposal iscompleted.

4. Results of the subprojectsMeasurement of in-situ benthic fluxes by 2 novel benthic observatoriesMicrobial methanotrophy in surface sedimentsrepresents an important sink for methane andacts as a biological barrier for methane beforeit enters the water column. These microbialprocesses embedded within a complicated net-work of biogeochemical reactions, control theemission of methane across the sediment

Figure 4: Cruise track of R/V SONNE during SO 174 (OTEGA-II) with the major working area of theLOTUS project in the northern Gulf of Mexico during Leg 1. Detailed investigations were perfor-med at Bush Hill and two sites in the Green Canyon (GC 234 and GC 415).

water boundary layer and contribute towardsthe regulation of the susceptible balance ofthe greenhouse gas contents in the earth’satmosphere.Direct in situ measurements of emission ratesof dissolved methane from gas hydrate contai-ning sediments are very scarce, which is due toa limited availability of appropriate in situ tech-nology in marine sciences.Within subproject 1 two novel observatories,BIGO and FLUFO have been developed asscientific modules which fit into the basis ofthe GEOMAR Lander System. Latter has beensuccessfully deployed during several projects(ALIPOR, EU; BIGSET, BMBF; TECFLUX, BMBF)for the measurement of benthic turnover andto conduct in situ experiments (Pfannkucheand Linke 2003). As to the modules, the FLUFO has been deve-loped by TUHH/MT1, while the BIGO module isbased on a combination of contributions fromGEOMAR, TUHH and Hydro Data Inc. whichholds the patent. The FLUFO module was developed on the basisof an assessment of the performance of exi-sting devices. In particular, improvements wereenacted related to:1. test of leakage during deployment2. identification of emanating gas-liquid mixtures3. identification of direction (inflow/outflow)4. calibration in a range of exchange velocities

from 10 cm/yr to 74 km/yr.

The BIGO contains a particularly adapted ver-sion of the patented microcosm (Hydro DataInc.) with an added control system to generatethe hydrodynamic inside the chamber as pre-vailing in the boundary layer flow (TUHH). Thechamber was connected with a gas exchangesystem which permits control of the oxygencontents and additional fluid sampling units(GEOMAR). Both units are mounted into theGEOMAR Lander System and are unique ascombined tools.

During both research-cruises both observato-ries have been deployed successfully. Theirdeployment enabled to obtain a unique data-

set of in situ benthic material fluxes from sedi-ments with shallow gas hydrates at HydrateRidge, Cascadia subduction zone and in theGulf of Mexico. The presented measurementsare so far the only existing direct measure-ments of the seabed methane emission undernatural conditions.

Particularly at Hydrate Ridge, which belongs toan extensive oxygen minimum zone with bot-tom water concentrations around 50 µmol l-1,the novel gas exchange system enabled measu-rements under natural in situ conditions bymaintaining ambient oxygen concentrationsinside the benthic chambers. By use of thissystem we detected that seabed methane emis-sion apparently is sensitive for oxygen availabili-ty. Under natural oxic conditions averagemethane emission from sediments covered withmicrobial mats overlying shallow gas hydrateswas low with 5.7 mmol m-2 d-1. When inside thebenthic chambers oxygen became depletedmethane emissions increased 27.5 fold reachingmaximum values of 156.7 mmol m-2 d-1. Thus,apart from anaerobic methane oxidation aero-bic oxidation of methane, which takes placedown to oxygen concentrations of 6.3 µmol l-1

(Heyer 1990) appears to be an important pro-cess in the methane cycle within the sedimentwater boundary layer. Availability of oxygenmight further indirectly affect other biogeoche-mical processes involved in benthic methaneturnover and contribute towards the efficiencyof the benthic filter (Sommer et al. in review).

From the amount of methane, entering at thebottom of the modelled sediment column in20 cm depth and the measured methaneemission rates across the sediment waterinterface an estimate of the efficiency of thebenthic filter at the different sites can be cal-culated. At clam field sites about 83 % of theincoming methane is consumed by methaneoxidation. In sediments covered with bacteri-al mats 66 % of all methane is consumed. Sofar the efficiency of the benthic filter hasbeen estimated indirectly based on the ex situdetermination of the anaerobic oxidation of

27

28

methane, which is believed to be the majormethane consuming process in these sedi-ments. Apparently, the investigated sites atHydrate Ridge, which are characterised withlow pore water velocities (10 – 20 cm yr-1), areunder the present environmental conditions ina quiescent state. There are, however, strongimplications that under altered environmentalconditions such as enhanced surface waterproductivity and warming of bottom waterenhancing overall oxygen uptake in the sedi-ment water boundary layer this benthic filtermight loose its efficiency and high concentra-tions of methane might be injected into thewater column. There it will be oxidised aerobi-cally when oxygen is present and affect theoverall oxygen inventory of regional watermasses as it was also postulated by Hinrichs etal. (2003). Environmental conditions and theirthreshold levels beyond which the benthic fil-ter fails remain to be investigated. With theFluid-Flux-Observatory, aqueous fluid flows invertical direction up to 150 m per year (mini-mum threshold 10 cm/yr) were detected atHydrate Ridge. The distribution pattern ofthese flows reveals in addition to temporalperiodicity high variations in intensity, directionand phase (relative to the tidal signal). Allaqueous fluid flow crossing the sediment-water boundary were strongest with increasedbottom currents. Gas flows were not detected.The origin of the flow pattern is traced not tothe venting from deep sources but to a shallowcirculation pattern of fluid flow in the surface-near sediment. It is driven by a combination of(tidal) bottom currents, topography and deviceexposure (Gubsch et al., in review).

At the Gulf of Mexico total oxygen uptake (upto 56.7 mmol m-2 d-1) and seabed methaneemission (up to 7.0 mmol m-2 d-1) was similar tothat measured at Hydrate Ridge. The porewa-ter profiles have not been modelled so far, thusthe efficiency of the benthic filter cannot beestimated presently. At the Gulf of Mexico wealso discerned oxygen sensitivity of methaneefflux but not to such a strong extent as atHydrate Ridge, which was due to the higherbottom water oxygen concentrations. In both

regions oxygen availability in the sedimentwater boundary layer apparently represents animportant mechanism controlling the dissipa-tion of methane from sediment and indirectlyaffects formation and degradation kinetics ofshallow gas hydrates. The newly developed observatories representan important step from static to dynamicsystems enabling measurements of materialfluxes of the natural environmental system.Their experimental possibilities are urgentlyneeded for process-orientated studies and totest and validate prognostic models.

The fate of methane in the water columnSubproject 2 of LOTUS studied the amount offree gas released from cold vents, its distribu-tion in the water column as well as the isoto-pic signature of the methane carbon. Manytechnical developments have been part of theproject which are: the enhancement of theMETS methane sensor, the development of aWINDOWS-based software for the data stora-ge of the hydroacoustic water column data ofthe PARASOUND system, the development of alander based system for the monitoring ofbubble release (GasQuant), and the setup ofan isotope laboratory for the tracking of themethane source and its isotopic fractionationduring decomposition. All technological deve-lopments were completely implemented,tested successfully and used during severalcruises at Hydrate Ridge, the Gulf of Mexicoand the Black Sea (Bohrmann et al. 2003).

The METS sensors detected during two cruisesin the Black Sea extreme variations at one spot(either in a mooring or fixed to the FLUFO lan-der) which are caused by varying current direc-tions and the additional non-continuousmethane release form vents. This show in adramatic way that single geochemical metha-ne analyses from water casts close to vent siteshave to be interpreted very carefully and can-not be used for larger budget modelling pur-poses. The variations sometimes exceed morethan a magnitude.

Of great use is the hydroacoustic detection offree gas in the water column for the mappingof active vent sites. The new software for thePARASOUND system and the general possibili-ty of the digital data recording of the 18 kHzsignal enhance the accuracy to detect activevents and allow the post processing and 3Dvisualization of flares together with other data(e.g. bathymetry, side scan sonar, geochemicaldata). Flare Imaging was conducted duringSO165, SO174 and provided essential informa-tion for cruise and station planning. It furthershows that the relation between a disappea-ring hydroacoustic flare and assumed increa-sing methane concentrations is not as simpleas supposed at the beginning of the project. Astrong oxidation of methane in the watercolumn, the current driven distribution of dis-solved gas and additional stripping effects ofthe gas phases have a strong impact in the fateof methane in the water column. Thus, a detai-led monitoring of the environmental parame-ters such as currents with low frequencyADCPs is of special importance.

Another complication is the non continuousrelease of free gas from vent sites. The suc-cessful development of the GasQuant systemcan be used now to monitor this release overextended periods of time. Furthermore, it pro-vides a spatial view of the bubble-site distribu-tion within a larger area (1900 m2) which cannot be monitored simultaneously by ROVs orsmaller video observation systems. The use ofmore specific echosounders which can detectthe real target strength (e.g. via split beamtechnique) allows the determination of bubblesizes and their variations during the bubble rise(shrinking rate: important for the methane fluxfrom the free gas phase into the dissolved gasphase) and finally enable flux estimates of alarger area (Greinert and Nützel 2004). This isof great importance as deployments in theBlack Sea showed that the temporal variabilityis extreme, varying between »bursts« of seve-ral minutes followed by hours or »silence« to amore continues bubbling which nevertheless isnot active the entire day (e.g. the most activebubble spots in the studied seep area of the

Black Sea are only active for 25% of the day).

Despite the temporal delay of nearly one yearfor the setup of the isotopic laboratory, we arenow able to analyse a greater quantity of sam-ples in a standard procedure of very high accu-racy per day. Measurements of samples fromHydrate Ridge show in combination with thegeochemically determined methane concen-trations that high methane concentrationscoincide with low δ13C values of -40‰. Unfor-tunately the shelf area itself is also a verystrong methane source with similar isotopicsignals as those detected next to vents. Thuswe cannot for sure define the source of ratherhigh methane concentrations above 500 mwater depth observed between the shelf andthe area west of Hydrate Ridge. Neverthelessour data indicate that extensive methane oxi-dation occurs close to vents sites with a strongenrichment of 13C in the residual methane (δ13Cvalues up to -10‰). The extended water sam-pling grid over the Hydrate Ridge area furthershows that even the very active vent site at thenorthern summit in 'only' 600 m water depthhave no impact on the water surface concen-tration of methane. This demonstrates thegreat filter and dilution potential of the waterbody which has to be studied in more detail inthe course of the COMET project.

The chronology and geochemical dyna-mics of near surface gas hydrate depositsAlthough occasional gas and fluid venting canbe seen at Hydrate Ridge the massive chemo-herms are predominantly fossil relicts of pastextensive fluid flow during glacials when sealevel was much lower than today. For the inve-stigated glacial periods fluid advection rateswere calculated in the order about 40 cm/aand up to more than 150 cm/a, respectively.

The coincidence of massive carbonate build-upand glacial climatic intervals point to the possi-bility that the formation of the chemohermcarbonates and, hence, the activity of the coldseep vent sites are directly related to the heightof sea level via the pressure difference bet-

29

30

ween the height of the seawater column andthe hydraulic head and buoyancy of theupward advecting fluids in the plumbingsystem of the sediments. If the fluid control ofthe Hydrate Ridge vent sites via the height ofthe sea level is a general phenomena then theiractive gas and fluid venting did not contributeto the increasing concentrations of greenhou-se gases at glacial/interglacial transitionsduring the Late Quaternary. As a base for amore detailed deciphering of geochemicalarchives from cold seeps an improved U/Th-measurement technique for geochronologicalpurpose has been developed (MIC-ICP-MS)resulting in a five time higher precision (Fietzkeet al. 2005).

The combination of high precision U/Th-geo-chronology on smallest sample amounts withlaser ablation element profiles opened recentlynew perspectives for the reconstruction ofpaleo-activity and compositional changes ofcold seeps in high resolution. A detailed inter-pretation is topic of the current project progress.

During the course of the LOTUS project tem-perature proxies (Ba/Ca, Sr/Ca, etc) have beencalibrated as chemical and isotopic indicatorsfor fluid temperature changes and to indicatewhether chemoherm are of pure inorganic ori-gin or their trace element budget was activelyaltered by microbial activity. First measure-ments indicate an active control of microbialactivity on the trace element distribution inthese carbonates. Important steps towards theuse of Ca- and Sr-isotope proxies for cold seepenvironments were the development of the so-called cool plasma MC-ICP-MS measurementtechnique (Fietzke et al. 2004) for Ca isotopesand the highly innovative temperature calibra-tion of the δ88Sr isotope ratio of carbonates.Although the chemoherm carbonates origina-te from the anaerobe activity of sulphate redu-cing and methane oxidizing microbes theyextend well into oxygenated water. Latterobservation may indicate that the massive che-moherm build-ups provide a micro-niche forhousing of anaerobe microbial communitieswithin a well oxygenated marine environment.

In order to contribute to the determination ofrecent fluid flux in cold seep environments theimprovement of radiochemical methods (Purkland Eisenhauer 2004) is further in progress.

Modelling methane fluxes and biogeoche-mical processes in hydrate-bearing surfacesedimentsThe numerical model C. CANDI was applied toinvestigate and to quantify biogeochemicalprocesses and methane turnover in gas hydra-te-bearing surface sediments from a cold ventsite situated at Hydrate Ridge, CascadiaMargin (Luff and Wallmann 2003). Steadystate as well as non steady state simulations,based on measurements from the center of anactive vent site were carried out to obtain acomprehensive overview on the activity inthese sediments which are covered with a bac-terial mat and are affected by strong fluid flowfrom below. A fit of the model to the datasetallowed the determination of different un-known parameters. With this method the fluidflow velocitiy could be isolated to valuesaround 10 cm a-1. The turnover rate ofAnaerobic Methane Oxidation (AOM) in thesesediments is tremendously high, all methanethat reaches the surface sediment from belowis oxidized in the surface sediments. Thus,AOM is the major process, proceeding at adepth-integrated rate of 872 µmol cm-2 a-1. Asignificant fraction (14 %) of bicarbonate pro-duced by anaerobic methane oxidation isremoved from the fluids by precipitation ofauthigenic aragonite and calcite. The total rateof carbonate precipitation (120 µmol cm-2 a-1)allows for the build-up of a massive carbonatelayer with a thickness of 1 m over a period of20,000 years. Aragonite is the major carbona-te mineral formed by anaerobic methane oxi-dation if the flow velocity of methane-chargedfluids is high enough (≥ 10 cm a-1) to maintainsuper-saturation with respect to this highlysoluble carbonate phase. Non-steady statesimulations using measured fluid flow veloci-ties as forcing demonstrate a rapid respond ofthe sediments within a few days to changes inadvective flow. At flow rates exceeding appro-

ximately 100 cm a-1, dissolved methane breakthrough the sediment surface to induce largefluxes of up to 5000 µmol CH4 cm2 a-1 into theoverlying bottom water.

To investigate the conditions that induce car-bonate crust formation in the sediment andthe effect of crust formation on sedimentporosity and fluid flow rate the porosity hasbeen formulated as an inert component, defi-ned by the amount of terrigenous matter anda temporal variable component reflecting theamount of authigenic CaCO3 (Luff et al. 2004).Starting with the conditions prevailing at a pre-viously investigated reference site located onHydrate Ridge, off Oregon, several parametersare systematically varied in a number of nume-rical experiments. The simulations show thatcarbonate crusts in the sediments only form ifthe fluids contain sufficient dissolved methane(>50 mM) and if bioturbation coefficients arelow (<0.05 cm2 a-1). Moreover, high sedimenta-tion rates (>50 cm ka-1) inhibit crust formation.Bioirrigation induces a downward displace-ment of the precipitation zone and acceleratesthe formation of a solid crust. Crusts only formover a rather narrow range of upward fluidflow velocities (20 - 60 cm a-1), which is some-what enlarged (up to 90 cm a-1) if the overlyingbottom waters are supersaturated with respectto calcite. Moreover, using a non steady statemodel approach we simulate the aragoniteand calcite precipitation and dissolution in a 2 m long sediment column (Luff et al. 2005).Assuming constant conditions for 7,000 years,fluid flow, anaerobic oxidation of methane(AOM) rates and carbonate precipitation/disso-lution rates show strong oscillations evoked bythe changes in permeability and fluid flow overtime and depth. The simulation predicts cyclesof crust formation and dissolution with a dura-tion of 2,000 to 2,700 years resulting in seve-ral distinct carbonate layers. The oscillationsare dampened so that fluid flow and biogeo-chemical turnover slowly approach a steadystate after about 7,000 years towards the endof the simulation period.

Other models have been developed to simula-te the formation of gas hydrates at HydrateRidge. These simulations show that near-surfa-ce gas hydrate deposits can only be formed bythe ascent of gas bubbles (Torres et al. 2004).Moreover the models also has been used toderive AOM rates from sulfate pore water pro-files (Treude et al. 2003), fluid flow velocitiesfrom dissolved chloride concentrations(Hensen et al. 2004), rates of barite and car-bonate precipitation in the Derugin Basin(Aloisi et al. 2004a), fluid release at a mud vol-cano located in the Black Sea (Aloisi et al.2004b), the uptake of 14C in carbonate crustsformed at mud volcanoes in the MediterraneanSea (Aloisi et al. 2004c) and the methane andsulfide turnover in benthic chambers placed onactive vent sites located in the central Americansubduction zone (Linke et al. 2005).

5. Conclusions and outlookMost of the technology and expertise has beendeveloped and successfully been used withinthe first funding period of the GEOTECHNO-LOGIEN Programme. The engineering and con-struction of the complex deep-sea instrumen-tation for long-term measurements and speci-fic experiments has been conducted in closeco-operation with numerous small and medium-sized companies in northern Germany. As visi-ble in the LOTUS project, some of these com-panies have realised that this project offeredan ideal scenario for technology transfer andfirst user applications and were willing toinvest a significant share in the developmentcosts of the specific instrumentation. Further-more, the transfer of expertise in the use ofthese and other instruments and hence thequalification of leased personnel within thisproject has strengthened the internationalposition of the SMEs in marine technology innorthern Germany. IFM-GEOMAR and a num-ber of SMEs close to the institute have gaineda leading role in lander technology, which wasrecently presented on several internationalworkshops and on the largest marine techno-logy exhibition »Oceanology 2004« in London.In the future, landers will be also incorporated

31

32

as modules into glass-fibre optical cablesystems. Autonomous lander clusters connec-ted by optical cable and with data transmissionto the surface and further on by satellite link tothe shore are envisioned as an important con-tribution to future sea floor observatories.Other highlights with technological, economicand scientific outreach are:

- The development of a sensor that respondsrapidly and quantitatively to the dissolvedmethane concentration in seawater wouldbe of considerable use in the explorationfor oil and gas offshore, especially off theshelf margin. The oil and gas industry usesso-called »sniffers« for this purpose, but inthis system the ambient water must bepumped on board the ship, and thereforesniffers have a limited depth utility. The suc-cessful development of the methane sensorwould likely open up a new market in theoffshore exploration industry and LOTUSprovided the framework for the improve-ment of this sensor.

- Software and system development may leadto a system that can be implemented infuture version of a Parasound system, whichwill include digital data acquisition. Ongoingcommunication with the Parasound manu-facturer assured this mutual benefit.

- The upgrade of the ParaDigMA is now avai-lable on all three large German researchvessels (METEOR, SONNE, POLARSTERN).

- The visualisation and /or an easy-to-handlemethod for quantifying gas ebullitions byvertical echosounders can now be used inthe exploration of marine gas fields, pipeli-ne observation surveys and the successcontrol of closing submarine gas field bore-holes (STN-Atlas, ELAC Nautik).

- Hardware and software developments canbe introduced into the market or used forother products.

- Participating companies may now markettheir products with the label 'tested in rese-arch' and may cite scientific institutes forreference (ELAC-Nautik, STN-Atlas, K.U.M.,Oktopus, Capsum).

- The combination of hydrogen and carbonisotope measurement capability in a singleIRMS mass spectrometer system is a newdevelopment in the industry. This system ismarketable to several industrial and rese-arch sectors, including the oil and gas indu-stry and other laboratories concerned withisotopic analysis of hydrocarbons.

- As a base for the deciphering of geochemi-cal archives from cold seeps the U/Th-mea-surement technique has been improvedresulting in a five time higher precision.

- A combination of high precision U/Th-geo-chronology on smallest sample amountswith laser ablation element profiles onlaminated chemoherm carbonates allowedrecently the reconstruction of paleo-activityand compositional changes of cold seeps inhigh resolution.

- The overall age distribution implies a closecorrelation of increasing fluid flux with peri-ods of sea level low stands.

- For Ca-isotope measurements an alternati-ve MC-ICP-MS technique (so-called coolplasma) was developed for future studiesabout fluid source, mixing processes andthe understanding of temperature depen-dent isotope fractionation processes.

- Also one highly innovative isotopic proxie(δ88Sr) has been calibrated for fluid tempe-rature reconstructions.

- During the course of the LOTUS projectnew methods for chemical purification ofradioisotopes have been developed in orderto determine disperse and advective fluidflow out of the sediment.

- The modeling approaches developed withinLOTUS will enhance our understanding ofmethane and carbon fluxes at the seafloorin continental margin settings with gas-and hydrate-bearing sediments. It predictsorders of magnitude in the turnover atthese specific locations for comparison withother environments to determine theimportance in the global C-budget. Morespecifically, the formation of hydrate indu-ced carbonates through ascending fluidsand gases as well as the dissolution rates

have be illuminated and quantified.Moreover, the anoxic oxidation of methane(AOM) rates and the release of the by-pro-ducts into the water column have beendetermined and the complex network ofinduced biogeochemical processes hasbeen deciphered.

The existing technology will now be refinedand applied to fill the gaps in our scientificknowledge within the COMET project(Controls on methane fluxes and their climaticrelevance in marine gas hydrate-bearing envi-ronments). The results and their modeling willenhance our understanding of methane andcarbon fluxes at the seafloor in continentalmargin settings with gas- and hydrate-bearingsediments. More specifically, the formation ofhydrates through ascending fluids and gases aswell as the dissolution rates of exposed andburied hydrates will be illuminated and quanti-fied. Moreover, the methane oxidation andrelease in the water column will be determinedand the complex net of induced biogeochemi-cal processes will be deciphered.

AcknowledgementsWe are indebted to the expertise and enthusi-asm of our engineers (M. Pieper, M. Poser, M.Türk, T. Viergutz), technicians (B. Bannert, A.Bleyer, B. Domeyer, A. Gerriets, A. Kähler, A.Petersen, W. Queisser, R. Suhrberg) and stu-dents (D. Hägele, S. Kriwanek, B. Mählich, P.Orlinsky, M. Rohleder, M. Treitschke) who wor-ked in the workshops and laboratories, athome and at sea and made the project a suc-cess. Special thanks to Christine Utecht whosupported the coordinator and the subprojectswhenever 2 helping hands were required. Theproject and the SONNE cruises (SO165 andSO174) were financed by the Federal Ministryof Education and Research (BMBF) (grants no.03G0565A-E; 03G0165A; 03G0174A) withinthe programme GEOTECHNOLOGIEN. Projectreviews and scheduling of the SONNE Cruiseswas handled efficiently by the ProjektträgerJülich-Warnemünde. On behalf of all partici-

pants we wish to thank these agencies,departments, and staff for their support. TheReedereigemeinschaft ForschungsschifffahrtBremen provided technical support on the ves-sel in order to accommodate the variety oftechnological, electronic, and navigationalchallenges required for the complex sea-goingoperations. We would like to especiallyacknowledge the vessel’s master H. Andresen(SO165 and SO174) and his crews for theircontinued interest, flexibility, patience, andtheir contribution to provide an always plea-sant and professional atmosphere aboard.

ReferencesAharon P (1994) Geology and biology ofmodern and ancient submarine hydrocarbonseeps and vents: An Introduction. Geo-MarineLetters 14, 69-73.

Aharon P (2000) Microbial processes and pro-ducts fueled by hydrocarbons at submarineseeps. In: Riding, R. E., Awramik, S. M. (Eds.)Microbial Sediments, Springer-Verlag, Berlin,pp. 270-281.

Aloisi G, Wallmann K, Bollwerk SM, DerkachevA, Bohrmann G, Suess E (2004a) The effect ofdissolved barium on biogeochemical processesat cold seeps. Geochimica et CosmochimicaActa 68(8) , 1735-1748, doi:10.1016/j.gca.2003.10.010.

Aloisi G, Drews M, Wallmann K and BohrmannG (2004b). Fluid expulsion from the Dvu-rechenskii mud volcano (Black Sea) Part I. Fluidsources and relevance to Li, B, Sr, I and dissol-ved inorganic nitrogen cycles. Earth andPlanetary Science Letters 225: 347-363.

Aloisi G, Wallmann K, Haese R, Saliège JF(2004c) Chemical, biological and hydrologicalcontrols on the 14C content of cold seep car-bonate crusts: numerical modeling and impli-cations for convection at cold seeps. ChemicalGeology, 213: 359-383.

33

34

Boetius A, Ravenschlag K, Schubert CJ, RickertD, Widdel F, Gieseke A, Amann R, JørgensenBB, Witte U, Pfannkuche O (2000) A marinemicrobial consortium apparently mediatinganaerobic oxidation of methane. Nature 407:623-626

Boetius A, Suess E (2004) Hydrate Ridge: Anatural laboratory for the study of microbial lifefueled by methane from near-surface gas hydra-tes. Chemical Geology 205(3-4), 291-310.

Bohrmann G, Greinert J, Suess E and Torres M(1998). »Authigenic carbonates from theCascadia subduction zone and their relation togas hydrate stability.« Geology 26(7): 647-650.Bohrmann G, Schenck S (eds) (2003) RV SonneCruise Report SO174 OTEGA II, LOTUS OMEGAMUMM, Investigations within the BMBF specialprogramm »Gashydrate im Geosystem« GEO-MAR Report 117, 117pp.

Brooks JM, Kennicutt III MC, Fay RR, McDonaldTJ, Sassen R (1984) Thermogenic gas hydratesin the Gulf of Mexico. Science 223, 696-698.

Carney RS (1994) Consideration of the oasisanalogy for chemosynthetic communities atGulf of Mexico hydrocarbon vents. Geo-Mar.Let. 14, 149-159.

Carson B, Kastner M, Bartlett D, Jaeger J,Jannasch H, Weinstein Y (2003) Implications ofcarbon flux from the Cascadia accretionaryprism: results from long-term, in situ measure-ments at ODP SITE 892B. Marine Geology 198:159-180

De Beukelaer SM, MacDonald IR and MurrayJA (2003). »Distinct side-scan sonar, RADAR-SAT SAR, and Chirp signitures of gas and oilseeps on the Gulf of Mexico slope.« Geo-Marine Letters 23: 177-186, doi: 10.1007/s00367-003-0139-9.

Ferrell RE, Aharon P (1994) Mineral assemblagesoccurring around hydrocarbon vents in the nor-thern Gulf of Mexico. Geo-Mar. Lett. 14, 74-80.

Fietzke J, Eisenhauer A, Gussone N, Bock B,Liebetrau V, Nägler ThF, Spero HJ, Bijma J, DulloCh (2004): Direct measurement of 44Ca/40Caratios by MC-ICP-MS using the cool plasmatechnique. Chem. Geo. 206, 11-20

Fietzke J, Liebetrau V, Eisenhauer A, Dullo Ch(2005) Determination of uranium isotoperatios by multi-static MIC-ICP-MS: Method andimplementation for precise U- and Th-seriesisotope measurements. JAAS 20, 395-401

Fink CR, Spence GD (1999) Hydrate distribu-tion off Vancouver Island from multifrequencysingle-channel seismic reflection data. J. ofGeophys. Res.-Solid Earth. 104, 2909-2922.

Fisher CR (1990) Chemoautotrophic andmethanotrophic symbioses in marine inverte-brates. Crit. Rev. Aq. Sci. 2, 399-436.

Fisher CR, MacDonald IR, Sassen R, Young CM,Macko, SA, Hourdez S, Carney RS, Joye S,McMullin E. (2000) Methane Ice Worms:Hesiocaeca methanicola colonising fossil fuelreserves. Naturwissenschaften 87: 184-187.

Greinert J, Bohrmann G and Suess E (2001) Gashydrate-associated carbonates and methane-venting at Hydrate Ridge: Classification, distribu-tion, and origin of authigenic lithologies. NaturalGas Hydrates: Occurence, Distribution, and De-tection, Geophysical Monograph. 124: 99-113.

Greinert J, Bohrmann G, Suess E (2002) Gashydrate-associated carbonates and methane-venting at Hydrate Ridge: Classification, distri-bution, and origin of authigenic lithologies. In:Natural Gas Hydrates - Occurrence, Distribu-tion, and Detection, American GeophysicalUnion Vol. 124 (ed. C. K. Paull and W. P.Dillon), pp. 99-113.

Greinert J and Nützel B (2004). »Hydroacousticexperiments to establish a method for the deter-mination of methane bubble fluxes at coldseeps.« Geo-Marine Letters 24(doi10.007/s00367-003-0165-7): 75-85.

Gubsch S, Gust G, Wallmann K, Linke P (2005)The Role of bottom current on fluid releaseand measurement strategy at cold vent sites,Deep-Sea Research I, in review.

Haeckel M, Suess E, Wallmann K, Rickert D(2004) Rising methane gas bubbles form mas-sive hydrate layers at the seafloor. Geochimicaet Cosmochimica Acta 68(21) 4335–4345,doi:10.1016/j.gca.2004.01.018.

Heeschen KU, Tréhu AM, Collier RW, Suess E,Rehder G (2003) Distribution and height ofmethane bubble plumes on the Cascadia mar-gin characterized by acoustic imaging. Geo-physical Research Letters 30 (12), DOI:10.1029/2003GL016974.

Heeschen KU, Collier RW, de Angelis MA,Linke P, Suess E, Rehder G, Klinkhammer GP,Whiticar MJ (2005) Methane sources, distribu-tion, and fluxes from cold vent sites at HydrateRidge, Cascadia Margin. Global Biogeoche-mical Cycles 19(GB2016): doi:10.1029/2004GB002266.

Heinz P, Sommer S, Pfannkuche O, HemlebenC. Living benthic foraminifera in gas hydrateinfluenced sediments at the Cascadia conver-gent margin, NE Pacific. Marine EcologyProgress Series (in press).

Hensen C, Wallmann K, Schmidt M, Ranero C,Suess E (2004) Fluid expulsion related to mudextrusion off Costa Rica? A window to thesubduction zone. Geology 32 (2) 201-204.

Heyer J (1990) Der Kreislauf des Methans.Akademie-Verlag Berlin, pp. 250.

Hinrichs KU, Hmelo LR, Sylva SP (2003)Molecular fossil record of elevated methanelevels in late Pleistocen coastal waters. Science299: 1214-1217.

Johnson JE, Goldfinger C and Suess E (2003).Geophysical constraints on the surface distri-bution of authigenic carbonates across the

Hydrate Ridge region, Cascadia margin.Marine Geology 202: 79-120.

Joye SB, Boetius A, Orcutt BN, Montoya JP,Schulz HN, Erickson MJ, Lugo SK (2004) Theanaerobic oxidation and sulfate reduction insediments from Gulf of Mexico cold seeps.Chemical Geology 205(3-4), 219-238.

Kastner M, Kvenvolden KA, Lorenson T (1998)Chemistry, isotopic composition, and origin ofmethane hydrogen sulfide hydrate at theCascadia subduction zone. Earth Planet Sci.Lett. 156, 173-183.

Kennett JP, Cannariato KG, Hendy IL, Behl RJ(2003) Methane hydrates in Quaternary climatechange: The clathrate gun hypothesis, AmericanGeophysical Union, Washington D.C.

Kennicutt MC, Brooks JM, Bidigare RR, Fay RR,Wade TL, MacDonald TJ (1985) Vent-type taxain a hydrocarbon seep region on the Louisianaslope. Nature 317, 351-353.

Kennicutt MC, Brooks JM, Denoux GJ (1988a)Leakage of deep, reservoired petroleum to thenear surface on the Gulf of Mexico continentalslope. Mar. Chem. 24, 39-59.

Kennicutt MC, Brooks JM, Bidigare RR, DenouxGJ (1988b) Gulf of Mexico hydrocarbon seepcommunities. I. Regional distribution of hydro-carbon seepage and associated fauna. Deep-Sea Res. 35A, 1639-1651.

Knittel K, Boetius A, Lembke A, Eilers H, LochteK, Pfannkuche O, Linke P, Amann R (2003)Activity, distribution and diversity of sulfate re-ducers and other bacteria in sediments abovegas hydrate (Cascadia Margin, Oregon). Geo-microbiology Journal: 20: 269-294.

Kulm LD, Suess E, Moore JC, Carson B, Lewis BT,Ritger SD, Kadko DC, Thornburg TM, Embley BW,Rugh WD, Langseth MG, Cochran GR, SammanRL (1986) Oregon subduction zone: Venting,fauna, and carbonates, Science 231, 561-566.

35

36

Larkin J, Aharon P, Henk MC (1994) Beggiatoain microbial mats at hydrocarbon vents in theGulf of Mexico and Warm Mineral Springs,Florida. Geo-Mar. Lett. 14, 97- 103.

Leifer I, MacDonald IR (2003) Dynamics of thegas flux from shallow gas hydrate deposits:interaction between oily hydrate bubbles andthe oceanic environment. Earth and PlanetaryScience Letters 210 (2003) 411-424.

Linke P, Suess E, Torres M, Martens V, Rugh WD,Ziebis W, Kulm LD (1994) In situ measurementof fluid flow from cold seeps at active continen-tal margins. Deep Sea Res., 41, 721-739.

Linke P (2003). Hydrate vents-a window to thedeep bioshere ? Marine Scientist 3: 24-27.

Linke P, Wallmann K, Suess E, Hensen C,Rehder G (2005) In situ benthic fluxes from anintermittently active mud volcano at the CostaRica convergent margin. Earth PlanetaryScience Letters 235, 79-95.

Luff R, Wallmann K (2003) Fluid flow, metha-ne fluxes, carbonate precipitation and biogeo-chemical turnover in gas hydrate-bearingsediments at Hydrate Ridge, Cascadia margin:Numerical modelling and mass balances. Geo-chimica et Cosmochimica Acta 67: 3403-3421.

Luff R, Wallmann K, Aloisi G (2004) Numericalmodeling of carbonate crust formation at coldvent sites: significance for fluid and methanebudgets and chemosynthetic biological com-munities. Earth Planetary Science Letters, 221,337-353.

Luff R, Greinert J, Wallmann K, Klaucke I andSuess E (2005). »Simulation of long-term feed-backs from authigenic carbonate crust forma-tion at cold vent sites.« Chemical Geology216: 157-174.

MacDonald IR, Boland GS, Baker JS, BrooksJM, Kennicutt II MC, Bidigare RR (1989) Gulf ofMexico chemosynthetic communities II: spatial

distribution of seep organisms and hydrocar-bons at Bush Hill. Mar. Biol. 101, 235-247.

MacDonald IR, Guinasso NL, Reilly JF, BrooksJM, Callender WR, Gabrielle SG (1990) Gulf ofMexico Hydrocarbon Seep Communities VI:species composition and habitat characteri-stics. Geo-Mar. Lett. 10, 244-252.

MacDonald IR, Guinasso Jr NL, Sassen R,Brooks JM, Lee L, Scott KU (1994) Gas hydra-te that breaches the sea floor on the conti-nental slope of the Gulf of Mexico, Geology,22, 699-702.

MacDonald IR, Reilly Jr JF, Best SE,Venkataramaiah R, Sassen R, Amos J, GuinassoJr NL (1996) A remote-sensing inventory ofactive oil seeps and chemosynthetic communi-ties in the northern Gulf of Mexico. In: Schu-macher D, Abrams MA (Eds.) Hydrocarbon mi-gration and its near-surface expression, Amer.Assoc. Petrol. Geol., Denver, pp. 27-37.

MacDonald IR, Buthman DB, Sager WW,Peccini MB, Guinasso NL (2000) Pulsed oildischarge from a mud volcano. Geology,28(10), 907-910.

MacDonald IR, Leifer I, Sassen R, Stine P,Mitchell R, Guinasso Jr. N (2002) Transfer ofhydrocarbons from natural seeps to the watercolumn and atmosphere. Geofluids, 2, 95-107.

MacDonald IR (2002) Stability and Change inChemosynthetic Communities, Northern Gulfof Mexico. Volume II: Technical Report, Mine-rals Management Service, MMS publicationnumber 2002-036.

MacDonald IR, Sager WW, Peccini MB (2003)Gas hydrate and chemosynthetic biota inmounded bathymetry at mid-slope hydrocar-bon seeps: Northern Gulf of Mexico. MarineGeology 198: 133-158.

Michaelis W, Seifert R, Nauhaus K, Treude T,Thiel V, Blumenberg M, Knittel K, Gieseke A,

Peterknecht K, Pape T, Boetius A, Amann R,Jørgenson BB, Widdel F, Peckmann J, Pimen-kov N, Gulin MB (2002) Microbial Reefs in theBlack Sea Fueled by Anaerobic Methane Oxi-dation. Science 297, 1013-1015.

Milkov AV, Sassen R (2001) Estimate of gashydrate resource, northwestern Gulf of Mexicocontinental slope. Marine Geology, 179, 71-83.

Orcutt BN, Boetius A, Lugo SK, MacDonald IR,Samarkin V, Joye SB (2003) Life at the edge ofmethane ice: methane and sulfur cycling inGulf of Mexico gas hydrates. Chem. Geol. 205(3-4): 239-251.

Pfannkuche O, Eisenhauer A, Linke P, Utecht C(2002) RV SONNE Cruise Report SO165, OTEGAI – 2002, Geomar Report No. 112, 1-159.

Pfannkuche O, Linke P (2003). GEOMAR Lan-ders as Long-Term Deep-Sea Observatories.Sea Technology 44(9): 50-55.

Purkl S, Eisenhauer A (2004) Determination ofRadium Isotopes and 222Rn in a groundwateraffected coastal area of the Baltic Sea and theunderlying sub-sea floor aquifer. Marine Che-mistry, 87, 137-149

Roberts HH, Aharon P (1994) Hydrocarbon-deri-ved carbonate buildups of the northern Gulf ofMexico continental slope: A review of submersi-ble investigations. Geo-Mar. Lett. 14, 135-148.

Roberts HH, Carney RS (1997) Evidence of epi-sodic fluid, gas, and sediment venting on thenorthern Gulf of Mexico continental slope.Econ. Geol. 92, 863-879.

Roberts HH, McBride RA, Coleman J (1999) Outershelf and slope geology of the Gulf of Mexico: anOverview. In: Kumpf H, Steidinger K, Sherman K(Eds.) The Gulf of Mexico Large MarineEcosystems: Assessment, Sustainability, andManagement. Blackwell Science, pp. 93-112.

Sahling H, Rickert D, Linke P, Suess E, Lee RW(2002) Community structure at gas hydratedeposits at the Cascadia convergent margin,NE Pacific. Mar Ecol Prog Ser 231: 121-138.

Sassen R, MacDonald IR, Guinasso NJ, Joye SB,Requejo AG, Sweet ST, Alcala-Herrera J,DeFreitas DA, Schink DR (1998) Bacterialmethane oxidation in sea-floor gas hydrate:Significance to life in extreme environments.Geol. 26, 851- 854.

Sassen R, MacDonald IR, Requejo AG,Guinasso NL, Kennicutt MC, Sweet ST, BrooksJM (1994) Organic geochemistry of sedimentsfrom chemosynthetic communities, Gulf ofMexico slope. Geo-Mar. Lett. 14, 110-119.

Sassen R, Joye S, Sweet ST, DeFreitas DA,Milkov AV, MacDonald IR (1999) Thermogenicgas hydrates and hydrocarbon gases in com-plex chemosynthetic communities, Gulf ofMexico continental slope. Organic Geochemis-try 30: 485-497.

Sassen R, Sweet ST, Milkov AV, DeFreitas DA,Kennicutt II MC (2001) Thermogenic vent gasand gas hydrate in the Gulf of Mexico slope: Isgas hydrate decomposition significant? Geo-logy, 29(2), 107-110.

Schlüter M, Linke P, Suess E (1998). »Geo-chemistry of a sealed deep-sea borehole on theCascadia Margin.« Marine Geology 148: 9-20.

Sommer S, Gutzmann E, Ahlrichs W, Pfann-kuche O (2003) Rotifers colonizing sedimentswith shallow gas hydrates. Naturwissenschaf-ten 90: 273-276.

Sommer S, Pfannkuche O, Linke P, Luff R,Greinert J, Drews M, Gubsch S, Pieper M, PoserM, Viergutz T . The efficiency of the benthic fil-ter - Biological control of the emission of dis-solved methane from sediments hosting shal-low gas hydrates at Hydrate Ridge. GlobalBiogeochemical Cycles (in review).

37

38

Suess E, Carson B, Ritger SD, Moore JC, JonesML, Kulm LD, Cochrane GR (1985) Biologicalcommunities at vent sites along the subduc-tion zone off Oregon. Biol. Soc. Wash. Bull.,No. 6, pp. 475-484

Suess E, Bohrmann G, Greinert J, Linke P,Lammers S, Zuleger E, Wallmann K, Sahling H,Dählmann A, Rickert D, Mirbach v N (1997)Methanhydratfund von FS Sonne vor derWestküste Nordamerikas. Geowissenschaften15 (6), 194-199.

Suess E, Torres ME, Bohrmann G, Collier RW,Greinert J, Linke P, Rehder G, Trehu A,Wallmann K, Winckler G, Zuleger E (1999) Gashydrate destabilization: Enhanced dewatering,benthic material turnover and large methaneplumes at the Cascadia convergent margin.Earth Planet. Science Lett. 170: 1-15.

Suess E, Torres ME, Bohrmann G, Collier RW,Richert D, Goldfinger C, Linke P, Heuser A,Sahling H, Heeschen K, Jung C, Nakamura K,Greinert J, Pfannkuche O, Trehu A, Klinkham-mer G, Whiticar MJ, Eisenhauer A, Teichert B,Elvert M (2001) Sea floor methane hydrates atHydrate Ridge, Oregon. In: Paull CK, Dillon WP(eds) Natural Gas Hydrates: Occurance, Distri-bution, and Detection, vol 124. American Geo-physical Union, Washington, DC, pp 87-98.

Suess E, Bohrmann G, Rickert D, Kuhs W,Torres ME, Trehu A, Linke P (2002) Propertiesand fabric of near-surface methane hydrates atHydrate Ridge, Cascadia margin. Proc. 4th Intl.Conf. On Gas Hydrates, Yokohama, 740-744.

Teichert BMA, Eisenhauer A, Bohrmann G,Haase-Schramm A, Bock B, Linke P (2003)U/Th Systematics and ages of authigenic car-bonates from Haydrate Ridge, Cacadia Margin:Recorders of fluid flow variations. Geochim.Cosmochim. Acta 67, 20: 3845-385

Torres ME, McManus J, Hammond D, deAngelis MA, Heeschen K, Colbert S, Tryon MD,Brown KM, Suess E (2002) Fluid and chemicalfluxes in and out of sediments hosting metha-ne hydrate deposits on Hydrate Ridge, OR, I:Hydrological Provinces, Earth Planet. Sci. Lett.201, 525-540.

Torres ME, Wallmann K, Tréhu A;, BohrmannG, Borowski WS, Tomaru H (2004) Gas hydra-te growth, methane transport, and chlorideenrichment at the southern summit of HydrateRidge, Cascadia margin off Oregon, EarthPlanetary Science Letters, 226, 225-241.

Tréhu AM, Lin GB, Maxwell E, Goldfinger C(1995) A seismic subduction zone offshorecentral Oregon – New constraints on methanedistribution and crustal structure. J. Geophys.Res. 100(B8), 15101-15116.

Tréhu A, Long PE, Torres ME, Bohrmann G,Rack F, Collett TS, Goldberg DS, Milkov A,Reidel M, Schultheiss P, Bangs NL, Barr SR,Borowski WS, Claypool GE, Delwiche ME,Dickens GR, Gracia E, Guerin G, Holland M,Johnson JE, Lee Y-J, Liu C-S, Su X, Teichert B,Tomaru H, Vanneste M, Watanabe M,Watanabe M, Weinberg JL (2004) Three-dimensional distribution of gas hydrate beneath southern Hydrate Ridge: constraintsfrom ODP Leg 204. Earth Planet. Sci. Lett.222, 845-862.

Treude T, Boetius A, Knittel K, Wallmann K,Jørgensen BB (2003) Anaerobic oxidation ofmethane above gas hydrates at Hydrate Ridge,NE Pacific Ocean. Mar Ecol Prog Ser 264: 1-14.Tryon MD, Brown KM, Torres ME, Trehu AM,McManus J, Collier RW (1999) Measurementsof transience and downward fluid flow nearepisodic methane gas vents, Hydrate Ridge,Cascadia. Geology, 27, 1075-1078.

Tryon MD, Brown KM (2001) Complex flowpatterns through Hydrate Ridge and theirimpact on seep biota. Geophysical ResearchLetters 28: 2863-2867.

Tryon MD, Brown KM, Torres ME (2002) Fluidand chemical flux in and out of sedimentshosting methane hydrate deposits on HydrateRidge, OR, II: Hydrological processes. EarthPlanet Sci Lett. 201: 541-557.

39

40

Gas Hydrates: Occurrence, Stability, Transfor-mation, Dynamics, and Biology in the BlackSea (GHOSTDABS)

IntroductionMethane has attracted high attention sincelong time as an energy source and a maingreenhouse gas. This interest was even intensi-fied by recent discoveries. Vast amounts ofmethane were found in gas hydrates presentwithin marine sediments and permafrost soils(Kvenvolden et al., 1993; Kvenvolden &Lorenson, 2001), indications for large-scalemethane releases substantially effecting theglobal atmosphere and hydrosphere wereobserved in the geological record (Dickens etal., 1995; Haq, 1998; Beerling & Berner, 2002;de Wit et al., 2002), and the anaerobic oxida-tion of methane was found to be a highly rele-vant process in marine sediments (Barnes &Goldberg, 1976; Reeburgh, 1976). Thoughconsiderable progress could be achievedduring the last 30 years in elucidating the geo-chemistry and microbiology of methane dyna-mics on earth, many questions remainedunanswered including those concerning anoxicmarine habitats.The secluded ecosystem of the Black Sea repre-sents an extraordinary research area for studiesof methane related processes. It comprises theworld’s largest anoxic marine water body withstrongly elevated concentrations of hydrocar-bons. Moreover, multiple seeps of methanerich gas occur within a large water depth

range and gas hydrates are found at the outercontinental margin. We here report on theresults obtained within the project GHOST-DABS that was funded as part of the GEO-TECHNLOGIEN program to study the biogeo-chemistry of methane in the north-westernBlack Sea. With a multidisciplinary approachcombining geophysical, organic-geochemical,geological, and microbiological componentsthe project was dedicated to a comprehensiveinvestigation of gas hydrates and associatedgas/fluid seeps in the northern Black Sea withthe following topics:- Inventory of gas hydrate occurrences- Localisation and detailed investigation of

escape structures of gases and fluids at theseafloor

- Biogeochemical conversion and turnover ofmethane released from the sediment in eit-her an oxic or an anoxic bottom water envi-ronment. Studies on seep carbonates andsediment samples.

- Characterisation of the fauna (meio- andmicro-fauna) in the vicinity of seeps andvents. Relation between the biological com-munity and the physical and chemical milieuat the sediment/water boundary.

Between June 30th and July 22nd, 2001 a rese-arch cruise was undertaken using the Russian

Michaelis W. (1), Seifert R (1), Blumenberg M. (1), Pape T. (1), Lüdmann T. (1), Wong H.K. (1), Konerding P.

(1), Zillmer M. (2), Petersen J. (2), Flüh E. (2) Reitner J. (3) Reimer A. (3)

(1) University of Hamburg, Institute of Biogeochemistry and Marine Chemistry, Bundesstrasse 55,

20146 Hamburg, Germany

(2) IFM-GEOMAR, Leibniz-Institut für Meereswissenschaften, Wischhofstrasse 1-3, 24148 Kiel, Germany

(3) University of Göttingen, Section Geobiology, Goldschmidtstrasse 3, 37077 Göttingen, Germany

RV »Professor Logachev« and the German sub-mersible »JAGO«. The work program includedreflection seismic studies, investigations of thewater column and of sediment cores, anddetailed biogeochemical and microbiologicalsurveys at locations of active gas seepage. Afirst impressive glimpse on huge methane seepassociated chimney structures at the seafloorwas captured during a TV-Grab station alreadyin the late night of July 2nd, at about 230mwater depth. Surveys by the submersible JAGOat that area displayed a fantastic view on afield of active gas seeps covered by a forest ofstructures built by microbial communities thri-ving at methane gas seeps that protrude up to4m into the anoxic waters (Michaelis et al.,2002). Moreover, a comprehensive set of sam-ples could be obtained also with keeping themicrobial mats active for in vitro experiments.The excellent samples and data that could begained have already given rise to numerous perreviewed publications (Michaelis et al., 2002;Gulin et al., 2003; Krüger et al., 2003; 2005;

Thiel et al., 2003; Blumenberg et al., 2004;2005; Lüdmann et al., 2004; Nauhaus et al.,2005; Kube et al., 2005; Meyerdierks et al.,2005; Pape et al., 2005; Pimenov & Ivanova,2005; Reitner et al., 2005; Seifert et al., 2005;Treude et al., 2005; Zillmer et al., 2005). Thispaper gives a comprehensive overview onselected aspects of the results achieved.

The GHOSTDABS fieldLocated at 44°46´N 32°00´E the GHOSTDABSfield still harbours the most impressive seeprelated microbial structures so far observed inthe Black Sea (Fig. 1).

The extraordinary size of the numerous pillarsextending up to about 4m from the seafloorthat are erected by methane consuming micro-bial consortia indicates specific conditions forthat area. Looking at the low growth rate to beassumed for these organisms living on anaer-obic methanotrophy, a process that deliverssmall amounts of free energy, it must have

41

Figure1: Image of microbial reef structures (as seen from the submersible). A) Tip of a chimney-likestructure. Free gas emanates in constant streams from the microbial structures into the anoxic seawater. B) Broken structure of approx. 1 m height. The surface of the structure consists of grey-blackcoloured microbial mat, the interior of the massive mat is pink. The greenish-greyish inner part of thestructure consists of porous carbonate which encloses microbial mats and forms irregular cavities.

42

needed a long time period of more or lessundisturbed growth with permanent supply ofmethane gas from the sediments to establishsuch huge structures. The upper age limit isgiven by the transition of the Black Sea fromlimnic to marine conditions at about 7.5 kyago and the subsequent development of per-manent anoxic water body. Dating of carbona-tes by 14C (Gulin et al., 2003) resulted in agesfrom 2.9 to 5.3 ky. However, the GHOSTDABSfield is protected from slumping that is com-mon along the shelf by its position morpholo-gical ridge. Moreover, it is located directly ona fault that channelled gas supply from dee-per sediments most probably during thou-sands of years.

Water and gas chemistryIn addition to the determination of dissolvedorganic carbon (DOC) and dissolved inorganiccarbon (DIC) measurements of concentrations,δ13C and δ2H of methane in the water columnand sediments were performed (Fig. 2).

Figure 2 shows an increase of concentrationsof dissolved methane with increasing waterdepth and concentrations of up to 18 µmol l-1

in a depth of 1800 m. The deep water CTD-profiles shown in Figure 2 observed at a stationabove a mud volcano in the central Black Sea.Therefore, the concentration maximum ofmethane at 1800m water depth may be inter-preted as a plume of water masses flowing outof the tectonic ridge. δ13Cmethane valuesshow large variations between –35 to –55 d(vs. VPDB) indicating different microbial pro-duction and consumption processes within thewater column. Additionally, the biogeochemi-cal work within GHOSTDABS focused on themajor seep-area found during the cruise(GHOSTDABS-field). On the right side of Figure 1 concentrations and δ13C of dissolvedmethane within this area were shown. Thesteepest gradient of 13C was observed withinthe turbidity maximum at a water depth of120 m pointing to the maximum of methaneconsumption at that depth. In addition to dis-solved methane, concentrations and δ13C ofemanating gas from sediments and microbialformations were analysed. The sampling wasaccomplished with the use of the submersibleand a special gas sampling device. Methaneconcentrations vary between 95 and 99%(δ13C from –62.4 to –68.3 ‰). No trends were

Figure 2: left: Concentrations of dissolved organic and inorganic carbon at a deep water station, middle:Concentrations of methane and δ13CH4 at a deep water station, right: turbidity, concentration of methane andδ13CH4 above the GHOSTDABS-field, transition zone between oxic and anoxic water body at a water depth of120 m accompanied by a turbidity maximum and maximum of methane consumption (AOM)

43

Figure 3: left: δ13C-Werte of emanating gas sampled fromthe sediment and microbial structures at the GHOSTDABS-field, right: differences in δ13C of dissolved and free gas atdifferent water depths. (filled circles = dissolved methane;open boxes = emanating gas)

Figure 4: Concentrations of methane and δ13CMethane obtained from a push-core within theGHOSTDABS-field (Treude et al., 2005)

44

observed concerning the sampling location(seep, sediment, microbial constructions; Fig. 3).

However, significant differences exist bet-ween dissolved and free gas at the same sam-pling area indicating either a different gassource for both compartments or a largerdegree of consumption of the water dissolvedmethane. Sediment push-cores have alsobeen analysed for methane dynamics (Fig. 4).The clear trends observed in concentrationand δ13C allowed for a first estimation of thefractionation factor αC for the anaerobic oxidation of methane in Black Sea sediments(αC = 1.009; Seifert et al., 2005).

Microbial communities and biomarkersGeochemical investigations on microbial matswere performed, which had been sampledwith a TV grab. Detailed lipid analyses haveshown high concentrations of strongly 13C-depleted biomarkers of AOM-performingmicroorganisms. These associates comprise ofanaerobic methanotrophic archaea and sulfa-te-reducing bacteria. Characteristic archaealbiomarkers at AOM-sites are irregular isopre-noid hydrocarbons (crocetane and crocetenes,2,6,10,15,19-pentamethylicosane and -cose-nes), isoprenoid dialkylglyceroldiethers (archa-

eaol, sn-2-hydroxyarchaeol) and glyceroldial-kylglyceroltetraethers (GDGT). Lipids of the sul-fate-reducing bacterial partners are terminalbranched fatty acids (e.g. 12-methyltetradeca-noic acid, ai-C15:0) and uncommon non-iso-prenoidal monoalkylglycerolethers (MAGE)and dialkylglyceroldiether (DAGE). These bio-markers have been found within the Black Seamat sample accompanied with strong deple-tions in δ13C-values showing an involvement inthe methane-cycling (see first and secondcolumn of Table 1).

Additionally, a very uncommon componentwas observed within the massive pink mats(α,β‚-bishomohopanoic acid, δ13C = –78.4‰).Although the source organism is still unclear,this is the first proof of hopanoids produced byorganism thriving in an anaerobic environ-ment. Moreover, the configuration of the hop-anoic acid points to the direct microbial bio-synthesis of the so-called geological α,β‚-form(Thiel et al., 2003). In order to characterise variances in the micro-bial populations within the large bioherms,macroscopically different areas were sampledand separately analysed for their biomarkerdistributions and microbial interior by molecu-lar microbiology. Interesting results of this

start month 3 month 7 month 12Bulk mat -72.2 -65.5 -62.6 n.a.BiomarkerArchaeacrocetane -94.7 -88.8 -88.4 -89.5archaeol -87.9 -78.8 -62.2 -63.3sn2-hydroxyarchaeol -90.0 -87.0 -89.9 -88.3biphytane (C40:0) -91.3 n.a. n.a. -72.2Bacteriaai-15-DAGE -89.5 -85.1 -83.7 -80.9ai-pentadecanoic acid -83.9 -64.2 -47.2 +3.8hexadecanoic acid -82.6 -30.2 -26.8 +33.711-hexadecenoic acid -82.9 -31.5 +69.9 +84.511-octadecanoic acid -86.2 -49.9 +25.4 +48.2

Table 1: Biomarkers found and the respective δ13C-values of a Black Sea mat and likely source organisms (the 2ndcolumn gives the original stable carbon isotope signatures (Michaelis et al., 2002). The 3rd to 5th column give theresults of an incorporation experiment using 13C enriched methane (discussion see below; Blumenberg et al., 2005).

45

approach have shown, that – in addition to theANME-1 dominated massive pink mats(Michaelis et al., 2002) – specific areas areruled by members of the ANME-2 cluster,accompanied by obviously also different sulfa-te-reducing bacteria. Although, their biomar-kers show strong discrepancies (Fig. 5) bothbacterial associates belong to the Desulfosar-cina/Desulfococcus-group.

The ANME-2/SRB associates appear to domi-nate in the outer parts of the chimney-like con-structions, whereas ANME-1/SRB comprise themajority of micro organisms in the massivepink mats. The appearance of highly diversebut structured microbial populations withinthe Black Sea structures lead to the identifica-tion of group specific biomarkers for archaeaof the ANME-1 and the ANME-2 cluster. Thus,ANME-1 archaea are the exclusive producersof specific GDGT, whereas members of theANME-2 contain high concentrations of sn-2-hydroxyarchaeol and crocetane (Blumenberget al., 2004). These results underline the phy-logenetic separation of both groups and hintto a closer relation of the ANME-1 to metha-nogens of the Methanomicrobiales as pre-viously thought. Moreover, ANME-1 associatedSRB mainly produce ether-bound lipids (MAGEand DAGE), which are minor compound clas-ses in the ANME-2 associated SRB.

These first results of the parallel approaches ofbiomarker and molecular microbiological ana-

lyses point to high diversities of AOM-perfor-ming micro organisms in the Black Sea, leadingto large amounts of microbial biomass.However, further detailed work is needed for abetter understanding of the microbial ecolo-gies and processes regulating the populationsin the bioherms.

Moreover, for studies of methane uptake andmetabolic and biosynthetic processes occur-ring in the Black Sea mats in vitro-experimentswith 13CH4 were performed. The one-year las-ting experiments have shown methane-uptakeunder laboratory conditions (bulk mat changed~10‰). Very interesting are differences in theuptake of 13C into individual lipids. Table 1gives the changes in δ13C of selected archaealand bacterial biomarkers. In general, uptakerates of bacteria are higher than archaeal com-ponents. A maximum uptake of ~160‰ wasobserved for the ω5-hexadecenoic acid. Re-markable are the strong differences of the lipidcompounds indicating different biosynthesisrates of the individual biomarkers or a highheterogenity of micro organisms growing inthe mat (e.g. differences between ANME-1and ANME-2; Blumenberg et al., 2005).TEM analyses of fixed mat samples covering alenticular carbonate concretion revealed abun-dant cylinder-shaped micro organisms havingexternal sheaths. The sheaths seem to consistof a resistant biopolymer, as empty sheathstend to become enriched and make up a majorportion of the mats (> 80 vol.% in some sec-

Figure 5: Differences infatty acid distributions bet-ween ANME-1 and ANME-2 dominated mat samplesnormalised to i-C15:0 FA(=1), (Blumenberg et al.,2004)

46

tions). The cylindrical to rod-like shape of sin-gle filaments was visualised by FE-SEM. Singlecells have a diameter between 0.6 and 1 µm,and are variable in length (mostly about 3 µm).They form multicellular filaments in which thesingle cells are separated by distinct invagina-tions. FISH analyses of these samples revealedstrong signals from Archaea of the ANME-1cluster, which has been previously related toAOM (Hinrichs et al., 1999). We assume thatthese ANME-1 Archaea are identical to thosereported from this seep area using microscopy(Pimenov et al., 1997), FISH (Michaelis et al.,2002), and a 16S rRNA survey (Tourova et al.,2002). The ANME-1 probe also responded toextremely long multicellular chains, sometimesexceeding 100 µm in length. Similar filamen-tous forms were previously recognized micro-scopically by Pimenov et al. (1997), and inter-preted as different growth stages of theshorter, cylinder-shaped microbes. ANME-1Archaea with a like morphology were also visu-alized by FISH of microbial consortia frommethane-rich sediments of the Eel River Basin(Orphan et al., 2002).

Intriguingly, the cylinder-shaped ANME-1 cellscontain complex arrays of stacked internalmembranes, a feature which has so far notbeen reported from Archaea. Similar stacks ofintracytoplasmic membranes are the site of theC1-metabolism in Type I and Type X methano-trophic bacteria (for instance in the genusMethylococcus), and it has recently been repor-ted that common gene coding for C1-transferenzymes does exist among methanogenicArchaea and aerobic, methanotrophic bacteria(Chistoserdova et al., 1998). We thus suggesteda function of the membranes in the methanemetabolism of ANME-1 Archaea, which awaitsverification in forthcoming studies.

Among other yet unidentified microbiota, afurther component of the mat population aremicrobes forming large, localized colonies ofsome tenths of µm in diameter. FISH analysesrevealed a strong response to the DSS 658probe, suggesting that the colony-building

organisms are SRB belonging to the Desul-fosarcina/Desulfococcus (DSS) group. DSS havepreviously been reported as the predominantSRB in a microbial mat sampled from a near-bycarbonate tower (Michaelis et al., 2002).However, these authors reported coccoidforms with an internal diameter of about 0.6µm. The bacteria reported here show similarinternal diameters (0.5 to 1 µm), but longitudi-nal cell sections in TEM, and FE-SEM on nativesamples reveal that they are vibrioform, withcell lengths of 3 µm and more. It is thereforelikely that these bacteria belong to a differentDSS-taxon as those previously described(Reitner et al., 2005b).

The concretion-associated SRB were found inassociation with ANME-1 Archaea. TheANME-1 cells are frequent close to the peri-phery of SRB-colonies, but isolated clusters ofANME-1 Archaea have been observed as well.This observation has to be checked up onother samples, but it may indicate that a spa-tial proximity to SRB is not required for thearchaeal metabolism. A remarkable feature ofthe SRB is that they contain abundant granu-les resembling internal sulphur spherulescommon in some sulphide-oxidizing bacteria.GC-MS analyses of apolar solvent extractsfrom two samples revealed elemental sulphurin its eight-membered ring structure as themain compound, with concentrations of 360and 400 µg/g-1 carbonate, respectively.Longitudinal cell sections also reveal intracel-lular aggregates of crystallites showing asomewhat blotchy diffraction contrast in theTEM. By size (~30 to 80 nm i.d.) and arrange-ment (chains, clusters), they strongly resem-ble magnetosomes formed by the so-calledmagnetotactic bacteria (Pósfai et al., 1998;Schüler, 1999). Indeed, FE-SEM/EDX analysesconfirmed that the crystallites consist of ironsulphide. Notably the accumulation of intra-cellular iron sulphides characterises anaerobicmagnetotactic bacteria, with ferrimagneticgreigite (Fe3S4) being the principal mineral(Pósfai et al., 1998; Schüler, 1999). It is there-fore very likely, though remains to be confir-

med, that the iron sulphide aggregates of theBlack Sea SRB consist of greigite as well(Reitner et al., 2005a).

Our observations raise questions on the natureof the biogeochemical pathways used by themagnetotactic SRB. Possibly, elemental sulphuris generated through the oxidation of H2S(being the product of sulphate-dependantAOM) by ferric iron. This could produce ferrousiron contributing to the formation of greigitein the magnetosome chains. Iron reduction,with an energy yield strongly exceeding that ofsulphate reduction, may serve as an additionalenergy source. However, FeIII+ shows extremelylow concentrations in marine waters. Somemarine bacteria overcome iron shortage bysynthesizing extracellular organic substancesknown as »siderophores«. Some of these com-pounds consist of a peptidic headgroup with afatty acid acyl appendage, and are excreted asmicells that scavenge reactive iron in the extra-cellular environment (Martinez et al., 2000). Inthe mats associated with the carbonate con-cretions, we observed conspicious globular

structures, being 20 to 100 nm in diameterand surrounded by a lipid bilayer. Thus, theseglobules exactly match the size range andorganization of amphiphilic siderophores(marinobactin) in their vesicle stage, that werereported from laboratory experiments withMarinobacter sp. (Martinez et al. 2000). Cellwalls of concretion-associated SRB frequentlyinvaginate upon contact with the globules andit seems as if they are deliberately transferredinto the cells.

Reflection seismicThe reflection seismic studies in the frameworkof the GHOSTDABS project were dedicated tolocate possible occurrences of gas hydratesand free gas in the Black Sea and to evaluatethe amount of methane confined to theseoccurrences. Worldwide, gas hydrates havebeen found in very different environments(e.g., Kvenvolden 1988; Ginsburg & Soloviev1998; Kvenvolden & Lorenson 2001).Considering the depositional environment, thesemi-enclosed Black Sea is a potential candida-te for gas hydrate accumulation on account of

47

Figure 6: Map showingthe locations of MCS pro-files and the OBS and OBHlocations as well as themapped fault system. OBSprofiles are numbered.Inset shows the positionof the study area (dashedframe).

48

its anoxic water regime (the chemocline lies at110-140 m depth, CTD measurements of thiscruise) which favours the preservation of orga-nic matter in the sediments. Regions outsidethe stability field of gas hydrates, namely theshelf and upper slope, show a high gas con-tent in the sediments. According to our obser-vations, these areas are characterised seismi-cally by extensive acoustic blanking as in thesoutheastern Black Sea (Ergün et al. 2002) andby numerous methane seeps (Polikarpov et al.1992; Luth et al. 1999; Kutas et al. 2002;Michaelis et al. 2002). Gas hydrates werediscovered in surface samples at different loca-tions in the Black Sea below a water depth of700 m where they are stable (Yefremova &Zhizhchenko 1974; Ginsburg et al. 1990;Limonov et al. 1994; Ivanov et al. 1998;Bohrmann et al. 2003).

Our study area is located in the northwesternBlack Sea southwest of the Crimean Peninsula(Fig. 6), where the continental shelf is excep-tionally wide (100-150 km) and the shelfbreak occurs at a water depth of approxima-tely 130 m. Our seismic profiles documentthat this structural change is marked by normalfaulting and a high slope gradient (Fig. 6).1130 km of high-resolution multi-channel seis-mic (MCS) reflection profiles as well as threewide-angle seismic transects using ocean bot-tom seismometers (OBS) and ocean bottomhydrophones (OBH) were acquired in the studyarea (Fig. 6). The MCS system used consistedof a Geo-Prakla 8-channel mini-streamer withan active length of 100 m and a SeismicSystem Inc. mini-GI gun as a seismic source(total volume 0.98 l and frequencies up to 300Hz). Data processing for identification of thebottom simulating reflector (BSR) includes filte-ring, stacking, signature deconvolution andtime domain f-k migration. To determine thevelocity structure of the uppermost sedimenta-ry column, wide-angle measurements of thereflected and refracted waves were carriedout. A Kirchhoff migration was applied toobtain an (depth) image of the subsurface and

a P-wave as well as S-wave velocity model.Seismic reflection data document for the firsttime the existence of a BSR in a limited areawest of the Dnieper Canyon in the northwe-stern Black Sea (Fig. 7) (Lüdmann et al. 2004).

The quantification of methane associated withgas hydrates by mapping of the BSR has beenapplied by many authors (e.g., Holbrook et al.1996; Bouriak et al. 2000; Tinivella & Accaino2000; Milkov & Sassen 2000; Vanneste et al.2001; Lodolo et al. 2002; Hornbach et al.2003; Lüdmann & Wong 2003). However, stu-dies on the Blake Ridge and the Hydrate Ridgeshow that the computed values, especially forthe free gas zone, differ from direct measure-ments of methane concentration in drill holes(Dickens et al. 1997; Milkov et al. 2003). Thismay be related to a large number of poorlyconstrained variables and the assumptionsimplicit in the calculations based on seismicdata. Hence, the calculations of methane car-bon presented here should be regarded only asorder-of-magnitude estimates.

The estimation of total methane relies on thefollowing equation: gas content is equal tohydrate area multiplied by hydrate thickness,sediment porosity, fractional pore fill, and thegas expansion factor for methane (164 at STP)(Sloan 1990).

Seismic wide-angle data suggest that gashydrates occupy an average of about 15±2 %of the pore space in a zone of 100 m thicknessLatest analysis of P- and S-wave velocities byZillmer et al. (2005) show a maximum gashydrate saturation of 38±10 % at the base ofthe gas hydrate zone (BGHSZ). A conservativequantification of the amount of methane asso-ciated with this gas hydrate occurrence is the-refore about 12±3 x 1011 m3 (0.6±0.2 gt ofmethane carbon).

49

Figure 7: Part of the processed MCS profile 35 which crosses the northern DnieperCanyon (see Fig. 6 for location). Above the BSR (labelled) is a zone of weak reflections(GHZ); below it the amplitudes of the reflectors are enhanced. In addition, a montage(OBS profile 3) of the OBS stations (black triangles) along MCS profile 35 is shown.

50

Additionally to the quantification of themethane, the local conductive heat flow wasestimated from the BSR depth. This was doneby assuming a linear temperature gradient andthe following simple conductive heat transportrelationship:

in which the bottom temperatures (Tbottom)were determined from CTD measurements ofthe GHOSTDABS cruise, and temperatures atthe BSR depths (TBSR) were estimated usingthe experimental thermobaric stability condi-tions for the methane-seawater system ofDickens and Quinby-Hunt (1994). The gashydrates found in sediment cores from the BlackSea contain mainly methane (99.1-99.9 %,Soloviev & Ginsburg 1994; Ginsburg &Soloviev 1998; Ivanov et al. 1998) and theirisotopic composition points to a biogenic ori-gin. The thermal conductivity (k) as a functionof depth was taken from results of DSDP Leg42B Site 379A in the central Black Sea (Ross etal. 1978). To calculate the lithostatic pressureat the BSR depth, we used the theoretical rock-physics model of Helgerud et al. (1999) andEcker et al. (2000) for gas hydrate which repla-ced the pore fluids. The input parameters forthe model are the average sediment composi-tion and our measured P-wave velocities. Theporosity as a function of depth was taken fromthe results of DSDP Leg 42B (Ross et al. 1978).For the calculations, we assumed a sedimentcomposition of 55 % clay, 30 % quartz and 15 % calcite (DSDP Leg 42B, Ross et al. 1978),with gas hydrate in the pore space. The samemodel was applied to the calculation of thegas hydrate concentration. For the determina-tion of the BSR depth (ZBSR) as well as the bulkdensity, we used a velocity function deducedfrom the wide-angle measurements and therock-physics model.

The cumulative error in BSR depth, thermalconductivity as well as bottom water salinityand temperature yields an uncertainty in the

heat flow of approximately 27 %. Conductiveheat flow deduced from the BSR depth is inthe range of 21±6 to 55±15 mW m-2.

The study documents that although gas hydra-tes theoretically are present in the entire BlackSea below a water depth of about 700 m, aBSR rarely occurs. This implies that the condi-tions in the Black Sea allow only locally a sub-stantial accumulation of free gas and the for-mation of gas hydrates in the HSZ. Figure 8shows that a BSR is absent in the distal fanbelow a water depth of about 1400 m, in thedeep basin, and on the continental slopewhere drift sediments occur. This could beattributed to intrinsically gas-poor (low Corg

content) sediments, or to the fact that gas con-centrations below the gas hydrate stabilityzone (GHSZ) are so low that the impedancecontrast is insufficient to produce a BSR. Inaddition, low sediment permeability may inhi-bit the upward migration of gas and fluids intothe GHSZ. In the upper Dnieper Canyon closeto the prograding lowstand deltas at the shelfbreak, the sediments are presumably coarser,and the fluids and gas which migrated into theGHSZ allow gas hydrates to form (Fig. 8). Theirascent is probably promoted by mud diapirismtriggered by tectonic movements along theNE-SW oriented normal faults or the NNW-SSEto NW-SE trending strike-slip system. The dia-pirism destroyed the original layering, therebycontributing to an overall higher flux rate. Acontinuation of these faults from the shelfedge into the basin is obscured by the limitedpenetration of the seismic signal due to acou-stic masking below the upper layers of themud diapirs. However, along the DnieperCanyon east of the area of mud diapirism, ourprofiles show a fault which reaches the sea-floor, suggesting that it is still active (Fig. 7).This fault follows a major tectonic element (theWest Crimean Fault, Okay 1994) which offsetsthe Alpine Crimean Mountains against theCimmerian Balkanide Kalamit Ridge (Robinson1997). Thus, we speculate that the faults onthe shelf continue into the mud diapir area andthat in addition to geological factors such as

disequilibria compaction, tectonic movementsmay have triggered the diapirism.

In general, gas hydrates are finely disseminatedin the sediments and if their concentrations arelow, they are probably of little economic inte-rest. However, areas of mud diapirism, especi-ally when they occur in medium water depthson the continental slope and when the muddiapirs penetrate high porosity strata in theGHSZ, could be future exploitation targets.

CarbonatesDuring the GHOSTDABS expedition, severaltypes of authigenic carbonates and microbialmats were sampled with the TV-guided graband the submersible JAGO. Up to now, analy-ses have been focussed on the geobiology ofintrasedimentary precipitates that appear tomake up the quantitatively most significantportion of the methane carbonates in theBlack Sea. The results obtained during

these studies are subject of two publication(Reitner et al., 2005a and b) and are brieflyoutlined below.

Among the distinctive carbonate types studiedare porous plates of lithified and semi-lithifiedsediments. These plates are cemented by auto-micritic, high Mg-calcite. Methane microsee-page is thought to have induced the formationof large elongated (»stromatactoid«) andspherical voids, with the latter probably repre-senting ‘lithified’ gas bubbles. These structuresresemble the »birds eye« structures oftenobserved in fossil tidal flat carbonates (Fig. 9).Due to rapid lithification, the cavities are notcompacted. Some of them are cemented bygranular high Mg-calcite, and most internalsurfaces are covered by biofilms. All carbona-tes are strongly depleted in 13C. δ13C valuesrange from ˜-27 to –41‰ PDB for high Mg-cal-cite and from –26 to –38‰ for aragonite pha-ses, indicating that a large portion of the car-bonate derives from AOM (Fig. 8).

51

Figure 8: Mapped seismic facies and outline of the areas of BSR occurrence,showing that the BSR is confined largely to the proximal fan.

52

Figure 9: Morphology and fabrics of seepage-related carbonates. (A) Cemented carbonate sedimentwith abundant cavities eventually representing former gas bubbles. Due to early cementation the car-bonate crust has not been affected by compaction. The coin is 19 mm in diameter. (B) Thin section ofthe crust shown in A (air dried sample), showing voids within the carbonate that formed by gas see-page. Internal surfaces of the cavities are commonly covered with biofilms. (C) UV-Fluorescencemicrograph showing bird’s eye-like voids within the concretion’s matrix formed of high Mg-calcite.The strongly fluorescent rim at the contact between sediment and cement may result from organicmatter derived from a mineralized biofilm. The coin is 19 mm in diameter. (D) Lenticular carbonateconcretions embedded in lithified, well-bedded background sediments. Note that the lenticular con-cretions in the upper left are orientated sub-vertical to bedding. Insert shows a thick microbial matsurrounding the carbonate concretion. (E) Thin section of a carbonate concretion showing that bak-kground sediment predominates in the central part of the concretion, whereas the outer regions con-sist of virtually pure authigenic carbonate. Assuming a radial growth mode, this suggests that theinitial stage of concretion formation involves the cementation of sediment, whereas further growth ischaracterized by displacement of the surrounding material. (F). Lenticular concretions are commonlysurrounded by biofilms. The fluorescence micrograph using a Zeiss no. 487709 fluorescence filter(resulting in a green fluorescence) indicates enrichment of organic matter within the enclosing sedi-ment as well as in the peripheral, newly forming portions of the concretion.

53

A further carbonate variety sampled compriseslenticular concretions forming within the sedi-ments. They consist of high Mg-calcite andrange in size from several centimetres to a fewdecimetres. Several concretions are commonlyinterconnected and then form larger aggrega-tes. The lenticular concretions consist of ca.100 µm sized, elongated rod- to dumbbell-shaped crystal aggregates of fibrous calcite.Staining techniques revealed dense popula-tions of micro organisms surrounding the cry-stallites, indicating that microbes mediate theformation of these precipitates. The aggrega-tes showed a conspicuous near-rectangularorientation. We suggest that AOM triggerscarbonate precipitation through an increasein carbonate alkalinity, whereby the regularfabric of the precipitates may be controlled by the meta-structures of organic matricesprovided by excreted EPS (extracellular poly-meric substances).

The high Mg-calcite yield δ13C values between–25.5 and –28.2‰ PDB, indicating that a sig-nificant portion of the carbonate carbon deri-ves from AOM. In organic extracts of bulk car-bonate and a microbial mat sample, we foundisoprene-based membrane lipids derived fromArchaea, and carbon skeletons of putative bac-terial origin. Strong depletions in 13C, with δ13Cvalues in the range of –70 to –100‰ allow todistinguish methane-related compounds from allochthonous marine lipids (δ13C ~ –20to –30‰), and imply that methane carbon istransferred into the biomass of the sourcebiota. Similar biomarkers have been observedin other mats covering Black Sea seep-carbo-nates (Thiel et al., 2001; Michaelis et al., 2002)and in fossil, methane-rich environments (Thielet al., 2001; Peckmann & Thiel, 2004). Due totheir structures and 13C-depletions, these com-pounds could be consistently related to contri-butions from methane-consuming Archaeaand metabolically associated SRB, althoughtheir precise taxonomic affiliation still remainsto be elucidated.

ReferencesBarnes RO, Goldberg ED (1976) Methane pro-duction and consumption in anaerobic marinesediments. Geology 4: 297-300

Beerling DJ, Berner RA (2002) Biogeochemicalconstraints on the Triassic-Jurassic boundarycarbon cycle event. Global Biogeochem Cycl16: 101-1013

Blumenberg M, Seifert R, Nauhaus K, Pape T,Michaelis W (2005) In vitro study of lipid biosynthesis in an anaerobically methane oxidizing microbial mat. Appl EnvironMicrobiol 71: 4345-4351

Blumenberg M, Seifert R, Reitner J, Pape T,Michaelis W (2004) Membrane lipid patternstypify distinct anaerobic methanotrophic con-sortia. Proc Natl Acad Sci USA 101: 11111-11116

Bohrmann G, Ivanov M, Foucher J-P, Spiess V,Bialas J, Greinert J, Weinrebe W, Abegg F,Aloisi G, Artemov Y, Blinova V, Drews M,Heidersdorf F, Krabbenho A, Klaucke I, KrastelS, Leder T, Polikarpov I, Saburova M, SchmaleO, Seifert R, Volkonskaya A, Zillmer M (2003)Mud volcanoes and gas hydrates in the BlackSea: new data from Dvurechenskii and Odessamud volcanoes. Geo-Maine Lett 23: 239-249

Bouriak S, Vanneste M, Saoutkine A (2000)Inferred gas hydrates and clay diapirs near theStoregga Slide on the southern edge of theVøring Plateau, offshore Norway. Mar Geol163:125-148

Chistoserdova L, Vorholt JA, Thauer, RK,Lidstrom ME (1998) Transfer enzymes and coen-zymes linking methylotrophic bacteria andmethanogenic Archaea. Science 281: 99-102.

54

de Wit MJ, Ghosh JG, de Villiers S, RakotosolofoN, Alexander J, Tripathi A, Cindy L (2002)Multiple Organic Carbon Isotope Reversalsacross the Permo-Triassic Boundary of TerrestrialGondwana Sequences: Clues to ExtinctionPatterns and Delayed Ecosystem Recovery. JGeol 110: 227-240

Dickens GR, Quinby-Hunt MS (1994) Methanehydrate stability in seawater. Geophys Res Lett21:2115-2118

Dickens GR, O'Neil JR, Rea DK., Owen RM(1995) Dissociation of oceanic methane hydra-te as a cause of the carbon isotope excursionat the end of the Paleocene. Paleoceanogr 10:965-972

Dickens GR (1997) Direct measurement of insitu methane quantities in a large gas-hydratereservoir. Nature 385:426-428

Ecker C, Dvorkin J, Nur AM (2000) Estimatingthe amount of gas hydrate and free gas frommarine seismic data. Geophysics 65:565-573

Ergün M, Dondurur D, Çifçi G (2002) Acousticevidence for shallow gas accumulations in thesediments of the Eastern Black Sea. Terra Nova14:313-320

Ginsburg GD, Kremlev AN, Grigor’ev MN,Larkin GV, Pavlenkin AD, Saltykova NA (1990)Filtrogenic gas hydrates in the Black Sea (twen-ty-first voyage of the research vessel»Evpatoriya«). Soviet Geology and Geophysics(geolgiya I Geofizika) 31(3), Allerton Press:8-16

Ginsburg GD, Soloviev VA (1998) SubmarineGashydrates. Translated from Russia, NormaPublishers, St. Petersburg, Russia

Gulin SB, Polikarpov GG, Egorov VN (2003)The age of microbial carbonate structuresgrown at methane seeps in the Black Sea withan implication of dating of the seepingmethane. Mar Chem 84: 67-72

Haq BU (1998) Gas Hydrates: GreenhouseNightmare?Energy Panacea or Pipe Dream?GSA Today 8: 1-6

Helgerud MB, Dvorkin J, Nur A, Sakai A,Collett T (1999) Elastic-wave velocity in marinesediments with gas hydrates: effective mediummodelling. Geophys Res Lett 26:2021-2024

Hinrichs KU, Hayes JM, Sylva SP, Brewer PG,DeLong EF (1999) Methane-consumingarchaebacteria in marine sediments. Nature398: 802-805

Holbrook WS, Hoskin H, Wood WT, StephenRA, Lizarralde D, ODP Leg 164 ShipboardScientific Party (1996) Methane hydrate andfree gas on the Blake Ridge from vertical seis-mic profiling. Science 273:1840-1843

Hornbach MJ, Holbrook WS, Gorman AR,Hackwith KL, Lizarralde D, Pecher I (2003)Direct seismic detection of methane hydrateon the Blake Ridge. Geophysics 68: 92-100

Ivanov MK, Limonov AF, Woodside JM (1998)Extensive deep fluid flux through the sea flooron the Crimean continental margin (BlackSea). In: Henriet J-P, Mienert J (eds) Gas hydra-tes: relevance to world margin stability and cli-mate change. Geological Society, SpecialPublication 137:195-213

Krüger M, Meyerdierks A, Glöckner FO,Amann R, Widdel F, Kube M, Reinhardt R,Kahnt J, Böcher R, Thauer RK, Shima S (2003)A conspicuous nickel protein in microbial matsthat oxidize methane anaerobically. Nature426: 878-881

Kube M, Beck A, Meyerdierks A, Amann R,Reinhardt R, Rabus R (2005) A catabolic genecluster for anaerobic benzoate degradation inmethoxynotrophic microbial Black Sea mats.Syst Appl Microbiol 28: 287-294

Kutas RI, Rusakov OM, Kobolev VP (2002) Gasseeps in northwestern Black Sea: geologicaland geophysical studies. Russian Geology andGeophysics 43:698-705

Kvenvolden KA., Soloviev VA. (1993) Worldwidedistribution of subaquatic gas hydrates. Geo-Marine Lett 18: 32-40

Kvenvolden KA (1988) Methane hydrate – amajor reservoir of carbon in the shallow geos-phere? Chem Geol 71:41-51

Kvenvolden KA, Lorenson TD (2001) The glo-bal occurrence of natural gas hydrate. In:Paull CK, and Dillon WP (Eds.), Natural GasHydrates: Occurrence, Distribution, andDetection. American Geological UnionGeophysical Monograph 124:3-18

Limonov AF, Woodside JM, Ivanov MK (1994)Mud volcanism in the Mediterranean andBlack Seas and shallow structure of theEratosthenes Seamount. Initial results of thegeological and geophysical investigationsduring the Third UNESCO-ESF »Training-Through-Research« cruise of RV Gelendzhik(June-July 1993), UNESCO Rep Mar Sci 64

Lodolo E, Camerlenghi A, Madrussani G,Tinivella U, Rossi G (2002) Assessment of gashydrate and free gas distribution on the SouthShetland margin (Antarctica) based on mul-tichannel seismic reflection data. Geophys J Int148:103-119

Lüdmann T, Wong HK (2003) Characteristics ofgas hydrate occurrences associated with muddiapirism and gas escape structures in thenorthwestern Sea of Okhotsk. Mar Geol201:269-286

Lüdmann T, Wong HK, Konerding P, Zillmer M,Petersen J, Flüh E (2004). Heat flow and quan-tity of methane deduced from a gas hydratefield in the vicinity of the Dnieper Canyon,northwestern Black Sea. Geo-Marine Letters,24(3), 182-193.

Luth C, Luth U, Gebruk AV, Thiel H (1999)Methane gas seeps along the oxic/anoxic gra-dient in the Black Sea: manifestations, biogenicsediment compounds and preliminary results onbenthic ecology. Mar Ecol 20:221-249

Martinez JS, Zhang GP, Holt PD, Jung HT,Carrano CJ, Haygood MG., Butler A (2000)Self-assembling amphiphilic siderophores frommarine bacteria. Science 287: 1245-1247

Meyerdierks A, Kube M, Lombardot T, KnittelK, Bauer M, Glöckner FO, Reinhardt R, AmannR (2005) Insights into the genomes of archaeamediating the anaerobic oxidation of metha-ne. Environ Microbiol (in press)

Michaelis W, Seifert R, Neuhaus K, Treude T,Thiel V, Blumenberg M, Knittel K, Gieseke A,Peterknecht K, Pape T, Boetius A, Amann R,Jørgensen BB, Widdel F, Peckmann J, PimenovNV, Gulin MB (2002) Microbial reefs in theBlack Sea fuelled by anaerobic oxidation ofmethane. Science 297:1013-1015

Milkov AV, Claypool GE, Lee Y-J, Xu W, DickensGR, Borowski WS (2003) In situ methane con-centrations at Hydrate Ridge offshore Oregon:new constrains on the global gas hydrateinventory from an active margin. Geology31:833-836

Milkov AV, Sassen R (2000) Thickness of thegas stability zone, Gulf of Mexico continentalslope. Mar Petr Geol 17:981-991

Nauhaus K, Treude T, Boetius A, Krüger M(2005) Environmental regulation of the anaer-obic oxidation of methane: comparison ofANME-I and ANME-II-communities. EnvironMicrobiol 7: 98-106

Okay AI, Sengör AMC, Görür N (1994)Kinematic history of the opening of the BlackSea and its effect on the surrounding regions.Geology 22:267-270

55

56

Orphan VJ, House CH, Hinrichs KU, McKeeganKD, DeLong EF (2002) Multiple archaealgroups mediate methane oxidation in anoxiccold seep sediments. Proc. Natl. Acad. Sci. USA99: 7663-7668

Pape T, Blumenberg M, Seifert R, Egorov VN,Gulin SB, Michaelis W (2005) Lipid geochemi-stry of methane-seep-related Black Sea carbo-nates. Palaeogeogr Palaeoclimatol Palaeoecol227: 31-47

Peckmann J, Thiel V (2004) Carbon cycling atancient methane-seeps. Chem Geol 205: 443-467

Pimenov NV, Ivanova AE (2005) Anaerobicmethane oxidation and sulfate reduction in bac-terial mats on coral-like carbonate structures inthe Black Sea. Mikrobiologiya 74: 420-429.

Pimenov NV, Rusanov II, Poglazova MN,Mityushina LL, Sorokin DY, Khmelenina VN,Trotsenko YA (1997) Bacterial mats on coral-like structures at methane seeps in the BlackSea. Mikrobiologiya 66: 354-360

Polikarpov GG, Egorov VN, Gulin SB, GulinMB, Stokozov NA (1992) Gas sseps from thebottom of the Black Sea – a new object ofmolismology. In: Polikarpov GG (ed) Molis-mology of the Black Sea. Nauka, Kiev:10-28 (inRussian)

Pósfai M, Buseck PR, Bazylinski DA, Frankel RB(1998) Iron sulfides from magnetotactic bacte-ria: Structure, composition, and phase transi-tion. American Mineralogist 83: 1469-1481

Reeburgh WS (1976) Methane consumption inCariaco Trench waters and sediments. EarthPlan Sci Lett 28: 337-344

Reitner J, Peckmann J, Reimer A, Schumann G,Thiel V (2005a) Methane-derived carbonatebuild-ups and associated microbial communi-ties at cold seeps on the lower Crimean shelf(Black Sea), Facies (in press)

Reitner J, Peckmann J, Blumenberg M,Michaelis W, Reimer A, Thiel V (2005b)Concretionary methane-seep carbonates andassociated microbial communities in Black Sea sediments. Palaeogeogr PalaeoclimatolPalaeoecol 227: 18-30

Robinson AG (1997) Introduction: tectonic ele-ments of the Black Sea region. In: RobinsonAG (Ed.), Regional and Petroleum Geology ofthe Black Sea and Surrounding Regions. AmAssoc Petr Geol Mem 68:1-6

Ross DA, Neprochnov YP and the ScientificParty of DSDP Leg 42B (1978) Initial Reports ofthe Deep Sea Drilling Project Leg 42B.Washington DC

Schüler, D (1999) Formation of magnetosomesin magnetotactic bacteria. J Mol MicrobiolBiotechnol 1: 79-86

Seifert R, Nauhaus K, Blumenberg M, KrügerM, Michaelis W (2005) Methane dynamics in amicrobial community of the Black Sea tracedby stable carbon istotopes in vitro. OrgGeochem (subm)

Sloan ED Jr (1990) Clathrate Hydrates ofNatural Gases. Marcel Dekker, Inc., New YorkSoloviev V, Ginsburg GD, (1994) Formation ofsubmarine gas hydrates. Bull Geol SocDenmark 41(1):86-94

Thiel V, Blumenberg M, Pape T, Seifert R,Michaelis W (2003) Unexpected occurrence ofhopanoids at gas seeps in the Black Sea. OrgGeochem 34: 81-87

Thiel V, Peckmann J, Richnow HH, Luth U,Reitner J, Michaelis W (2001) Molecular signalsfor anaerobic methane oxidation in Black Seaseep carbonates and a microbial mat. MarChem 73: 97-112

Tinivella U, Accaino F (2000) Compressionalvelocity structure and Poisson’s ratio in marinesediments with gas hydrate and free gas byinversion of reflected and refracted seismicdata (South Shetland Islands, Antarctica). MarGeol 164:13-27

Tourova TP, Kolganova TV, Kuznetsov BB,Pimenov NV (2002) Phylogenetic diversity of thearchaeal component in microbial mats on coral-like structures associated with methane seeps inthe Black Sea. Mikrobiologiya 71: 230-236

Treude T, Knittel K, Blumenberg M, Seifert R,Boetius A (2005) Subsurface microbial metha-notrophic mats in the Black Sea. Appl EnvironMicrobiol 71: 6375-6378

Vanneste M, De Batist M, Golmshtok A,Kremlev A, Versteeg W (2001) Multi-frequencyseismic study of gas hydrate-bearing sedimentsin Lake Baikal, Siberia. Mar Geol 172:1-21

Winguth C, Wong HK, Panin N, Dinu C,Georgescu P, Ungureanu G, Krugliakov VV,Podshuveit V (2000) Upper Quaternary waterlevel history and sedimentation in the northwe-stern Black Sea. Mar Geol 167 (1-2):127-146

Yefremova AG, Zhizhchenko BP (1974) Occur-rence of crystal hydrates of gas in sediments ofthe modern marine basins. Doklady AkademiiNauk SSSR 214:1179-1181

Zillmer M, Flueh ER, Petersen J (2005) Seismicinvestigation of a bottom simulating reflectorand quantification of gas hydrate in the BlackSea. Geophys Res Int: doi:10.1111/j.1365-246X.2005.02/2635.x

57

58

Microbial Methane Turnover at MarineMethane Seeps (MUMM – SPI)

1. IntroductionMethane is an important energy source forsociety, but it is also a greenhouse gas whichhas influenced earth’s climatic history substan-tially. Methane can be produced by three pro-cesses: abiotically through hydrothermal pro-cesses, thermogenically by alteration of orga-nic matter, and microbially as the end-productof degradation processes in various environ-ments such as natural wetlands, rice fields,ruminants and aquatic sediments. Under-standing the evolution and development ofearth’s methane budget is a relevant problemin the earth sciences. A main sink for methaneon earth is a microbial process called anaerobicoxidation of methane (AOM). In marine sedi-ments, the majority of produced methane isconsumed by AOM before it can enter thehydrosphere or the atmosphere (Reeburgh,1996). Instead of oxygen, the responsiblemicrobes use sulfate, a compound of seawater,to oxidize the methane anaerobically (Barnesand Goldberg, 1976):

According to current knowledge, the process ismediated by a syntrophic consortium ofmethanotrophic archaea and sulfate-reducingbacteria (Zehnder and Brock, 1980; Hoehler etal., 1994; Boetius et al., 2000). The first inve-stigation of AOM dates back to the year of1974, when Martens and Berner (1974) specu-lated about the cause for conspicuous metha-

ne and sulfate profiles in organic rich sedi-ments. The scientists observed that methanewas not accumulating before sulfate wasexhausted. From the decrease of methane con-centrations in the sulfate-reducing zone, theyconcluded that methane must be consumedwith sulfate. Zehnder and Brock (1979 and1980) were the first who demonstratedmethane oxidation under anoxic conditionsand hypothesized a coupled two-step mecha-nisms of AOM. They postulated that methaneis first activated by methanogenic archaea,working in reverse, leading to the formation ofintermediates, e.g. acetate or methanol. In asecond step, the intermediates are oxidized toCO2 under concurrent sulfate reduction byother non-methanogenic members of themicrobial community. Since then, the know-ledge of AOM increased substantially with bio-geochemical, microbiological, and molecularinvestigations adding one peace after theother to the big puzzle. Measurements withradiotracers enabled the first direct quantifica-tion of AOM and concurrent sulfate reductionrates in anoxic marine sediments (Reeburgh,1976; Iversen and Blackburn, 1981; Devol,1983). By this technique, traces of 14CH4 and35SO4

-2 are added to the sediment and theirconversion into 14CO2 and H2

35S is quantified.Including the total methane and sulfate con-centration of the sediment, turnover rates canbe calculated. Applying radiotracer techniques,it was then for the first time possible todemonstrate a 1:1 ratio of AOM and sulfate

Treude T. (1), Niemann H. (1), Orcutt B. (2), Joye S. (2) Witte U. (1), Jørgensen B. B. (1), Boetius A. (1, 3, 4)

(1) Max Planck Institute for Marine Microbiology, Celsiusstr. 1, 28359 Bremen, Germany, aboetius@mpi-bremen.de

(2) Department of Marine Sciences, University of Georgia, Athens, Georgia, USA

(3) Alfred Wegener Institute for Polar and Marine Research, 27515 Bremerhaven, Germany

(4) International University Bremen, 28759 Bremen, Germany

reduction in the sulfate-methane transitionzone, confirming the close coupling betweenthe two processes (Iversen and Blackburn,1981). Only few measurements of AOM ratesin marine sediments existed prior to theMUMM project (for a review see Hinrichs andBoetius, 2002). Estimates for AOM at methaneseeps were missing, although seeps generallyexhibit much higher methane fluxes comparedto systems controlled by molecular diffusion(Wallmann et al. 1997). The methane flux isdefined as the transport of methane per timethrough a given area. At methane seeps, for-ces like tectonic displacements or gas over-pressure cause an advective transport ofmethane and fluids to the sediment surface. Atthe beginning of the MUMM project only afew measurements of sulfate reduction ratesat methane seeps were published, which sug-gested extremely high methanotrophic activityin these kinds of environments (Boetius et al.,2000; Aharon and Fu, 2000). Hence, the quan-tification of methane turnover at gas seepswas one of the main goals of the project.

2. Objectives of the ProjectThe aim of TP-I within the GEOTECHNOLO-GIEN Project MUMM was the determination ofmethane turnover rates in methane seep envi-ronments applying radiotracer techniques.Methane in diffusion controlled systems isquantitatively removed by AOM before it rea-ches the sediment-water interface. Our studyinvestigated the functioning of the microbialbarrier against methane in advective systems,which are characterized by high methane flu-xes including ebullition of free gas or hydrate.In collaboration with the other TPs of theMUMM Project, turnover rates were comparedwith the abundance, diversity and physiologyof the methanotrophic community at the diffe-rent study sites. Finally, we wanted to compa-re the biogeochemistry of different methane-bearing environments such as mud volcanoes(Håkon Mosby Mud Volcano), methane seepscomprising gas hydrates (Hydrate Ridge andGulf of Mexico), methane seeps in anoxicbasins (Black Sea), gassy organic-rich sedi-

ments with actual methane production (e.g.Eckernförde Bay, German Baltic), and diffusivesystems (e.g. Chilean continental margin).

3. Present Status and ResultsA considerable AOM activity was detected atall methane-bearing environments investiga-ted during the MUMM Project. The sites inve-stigated within the MUMM project included asdiverse habitats as coastal and deep-sea sedi-ments, oceanic and brackish environments aswell as surface and subsurface sediments.Hence, our study demonstrated that AOM is aubiquitous process in marine systems. A directcomparison of the different sites revealed thatthe magnitude of AOM correlated with themethane fluxes. Accordingly, turnover rateswere found to be several orders of magnitudehigher at advective methane seeps comparedto diffusive sites (Table 1). The studies furthershowed that AOM leads to a substantial remo-val of dissolved methane before it reaches thehydrosphere even in systems controlled byadvection. The main escape route for methaneis as free gas, in the form of rising gas bubbles.Our findings therefore confirm earlier assump-tions that AOM controls methane emissionfrom the ocean over a wide range of methanefluxes. Another interesting observation was tofind the hot spots of AOM activity situated clo-ser to the sediment surface when the methaneflux is high. In diffusive systems (Chilean conti-nental margin), AOM hot spots were situateddeeper than 1 m below the sediment surface.In gassy sediments of Eckernförde Bay, thezone was closer to the surface between 20 and30 cm sediment depth. At methane-seep sites(Hydrate Ridge, Gulf of Mexico, HaakonMosby Mud Volcano), methane was consumedalready within the top 10 cm of the sediment,below which sulfate was depleted. Hence,another main factor determining the positio-ning of AOM zones thereby appears to beavailability of sulfate. In anoxic parts of theBlack Sea, this results in the formation ofmethanotrophic reefs above methane seeps.Here the methanotrophic consortia grow alonggas bubble pathways into the water column

59

60

for an optimized access to methane and sulfa-te (Fig. 1). Besides turnover rate and depth ofAOM hot spots, also the biomass of themethanotrophic community correlated withthe methane flux. The highest methanotrophicbiomass is represented by the microbial matsof Black Sea seeps. The mats comprise about1011 methanotrophic cells per cm3 of mat. Stillhigh in methanotrophic biomass are sedimentsof methane seeps such as Hydrate Ridge,where more than 90% of the total microbialbiomass was found to be methanotrophic. Cellnumbers of methanotrophs than furtherdecrease with decreasing methane flux. In sub-surface sediments we found numbers as lowas 105 methanotrophic cells per cm3 of sedi-ment, which were however still able to controlmethane diffusion.

Another characteristic of AOM in sedimentsdriven by advective processes is the extremesmall-scale variability (cm-m scale). Replicatesof rate measurements varied by up to oneorder of magnitude within a sampled area ofnot more than 0.25 m2. The biogeochemistryof methane-seep sediments appears very vari-able over space and time due to permanentchanges in methane migration pathways, bio-turbation of animals as well as geologicaldisturbances in the sediment (Fig. 2). Mainproblems for an accurate quantification are theartifacts introduced by sampling and recovery

of gas-laden sediments from deep waters.Sampling and ex situ measurement proceduresterminate natural processes such as flow andinteraction with other organisms, and mayaffect turnover rates considerably. Most likely,chemoautotrophic communities, which re-oxi-dize the sulfide produced during AOM areimportant in controlling the microenvironmentat seeps. Beggiatoa, a sulfide-oxidizing fila-mentous microbe, removes sulfide efficientlyand may also introduce sulfate into deepersediment layers via vertical migration. An evenstronger mixing effect may result from the dig-ging activity of clams like Calyptogena andAcharax, which harbor symbiotic sulfide-oxidi-zing bacteria in their gills. These large animalsmay even introduce oxygen into deeper sedi-ment layers. During our investigations ofmethane-seep sediments, we combined AOMand sulfate reduction rate measurements withvertical profiling of oxygen and sulfide, indica-ting such impacts of the chemoautotrophiccommunity. Furthermore interesting in thiscontext are correlations observed between thediversity of the methanotrophic community,whose investigation is part of the TP-IV, andthe presence of different chemoautotrophicorganisms. Small-scale variations and correla-tions of both biogeochemical gradients andmicrobial diversity will be a major objective inthe second phase of the MUMM Project(2005-2008).

Table 1: Methane turnover in different methane-bearing habitats.

4. ConclusionsWith the results gained during TP-I of theMUMM Project the knowledge about the glo-bal role of AOM in marine sediments increasedconsiderably. Turnover rates measured inmethane-seep sediments revealed an impor-tant role of methanotrophic communities inthe control of methane emission. This microbi-al »filter« seems to be able to adjust to a widescale of methane and sulfate fluxes. It will beone objective of MUMM II to characterize themicrobial habitats at methane-seeps on smallerscales and with in situ technologies.

AcknowledgementsWe thank our collaborators in the GEOTECH-NOLOGIEN program »Gashydrate im Geosys-tem«, projects GHOSTDABS, LOTUS, andOMEGA. Further thanks are due to the follo-wing institutions for cooperation in field andlab work: IFM-GEOMAR (Germany), AWI-Bre-merhaven (Germany), University of Bremen (Ger-many), RCOM (Germany), IFREMER (France),University of Georgia, Athens (USA), We alsothank the crews and shipboard scientific par-ties of the expeditions on the research vessels»Sonne«, »Meteor«, »Polarstern«, »Heincke«,»L´Atalante«, »Littorina«, »Seaward Johnson«and »Professor Logachev« as well as the crewsof the submersibles and ROVs »Jago«,

61

Figure 1: A microbial methanotrophic reef structure (ca. 1 m high) growingabove methane seeps in the Black Sea. The inner carbonated core is external-ly covered by pure microbial biomass. Very soft nodules (close-up) of microbialbiomass grow on top of the reefs and bear pure methane gas.

62

»Johnson Sealink«, »Trieste«, and »Victor6000«. The study was funded by the Bun-desministerium für Bildung und Forschung andthe Deutsche Forschungsgemeinschaft in theframe of the GEOTECHNOLOGIEN projectMUMM (FKZ 03G0554A). We thank the MaxPlanck Society for supplementary funding ofthis project.

ReferencesAharon, P. and Fu, B. (2000). Microbial sulpha-te reduction rates and sulphur and oxygen iso-tope fractionations at oil and gas seeps in deep-water Gulf of Mexico. Geochim. Cosmochim.Acta 64, 233-246

Barnes, R.O., Goldberg, E.D. (1976). Methaneproduction and consumption in anoxic marinesediments. Geology 4, 297-300.

Boetius, A., Ravenschlag, K., Schubert, C.J.,Rickert, D., Widdel, F., Giesecke, A., Amann,R., Jørgensen, B.B., Witte, U., Pfannkuche, O.(2000). A marine microbial consortium appa-

rently mediating anaerobic oxidation ofmethane. Nature 407, 623-626.

Devol, A.H. (1983). Methane oxidation rates inthe anaerobic sediments of Saanich Inlet.Limnol. Oceanogr. 28(4), 738-742.

Hinrichs, K.-U., Boetius, A. (2002). The anaerobicoxidation of methane: new insights in microbialecology and biogeochemistry. In: G. Wefer, D.Billett, D. Hebbelnet al (Eds.), Ocean MarginSystems. Springer-Verlag, Berlin, pp. 457-477.

Hoehler, T.M., Alperin, M.J., Albert, D.B.,Martens, C.S. (1994). Field and laboratory stu-dies of methane oxidation in an anoxic marinesediments: evidence for methanogen-sulphatereducer consortium. Global Biochem. Cycles8(4), 451-463.

Iversen, N., Blackburn, T.H. (1981). Seasonalrates of methane oxidation in anoxic marinesediments. Appl. Environ. Microbiol. 41(6),1295-1300.

Figure 2: Scheme illustrating heterogeneity of surface sediments at methane seeps cau-sing deviations in methane turnover rates. The sediments are covered by different che-moautotrophic organisms, i.e. Beggiatoa, Calyptogena and Acharax (see text).

Martens, C.S., Berner, R.A. (1974). Methaneproduction in the interstitial waters of sulpha-te-depleted marine sediments. Science 185,1167-1169.

Reeburgh, W.S. (1976). Methane consumptionin Cariaco Trench waters and sediments. EarthPlanet. Sci. Lett. 28, 337-344.

Reeburgh, W. S. (1996). »Soft spots« in theglobal methane budget. Microbial Growth onC1 Compounds. L. M.E. and F. R. Tabita. Inter-cept, Andover, UK, Kluwer Academic Pub-lishers: 334-342.

Wallmann, K., Linke, P., Suess, E., Bohrmann, G.,Sahling, H., Schlüter, M., Dählmann, A., Lam-mers, S., Greinert, J., Von Mirbach, N. (1997).»Quantifying fluid flow, solute mixing, and bio-geochemical turnover at cold vents of theeastern Aleutian subduction zone.« Geochim.Cosmochim. Acta 61(24): 5209-5219.

Zehnder, A.J.B., Brock, T.D. (1979). Methaneformation and methane oxidation by metha-nogenic bacteria. J. Bacteriol. 137(1), 420-432.Zehnder, A.J.B., Brock, T.D. (1980). Anaerobicmethane oxidation: occurrence and ecology.Appl. Environ. Microbiol. 39(1), 194-204.

63

64

Microsensor Measurements in Gas HydrateBearing Sediments (MUMM – SPII)

1. IntroductionMethane hydrates form transient reservoirsbetween methane seeping from deep sedi-ments and the oxidized upper sediments andoverlying water phase. The methane releasedfrom hydrates is transported upwards by diffu-sion and advection, and then oxidized bymicroorganisms. The main microbiological pro-cesses associated with gas hydrates are anaer-obic methane oxidation (AOM) coupled to sul-fate reduction (SR) (Boetius et al. 2000) andsulfide oxidation by Beggiatoa or related bac-teria (Schulz and Jørgensen 2001). As result ofthis efficient oxidative microbial filter only asmall fraction of the enormous amount ofreducing power residing in the sediments rea-ches the seawater. Instead, areas rich in gashydrates are highly diverse oases teeming withfauna (Boetius and Suess 2004). The basis ofthe food chain is the chemosynthetic produc-tion of microbial biomass by methane and sul-fide oxidation. These are exclusively prokaryo-tic processes. Consortia of Archaea and sulfatereducing bacteria (SRB) oxidize methane underthe formation of sulfide. The sulfide is furtheroxidized by sulfide oxidizers. Thus the metha-ne seeps are typically covered with veils ofBeggiatoa, i.e. giant filamentous bacteria thatoxidize sulfide with nitrate that is stored intheir vacuoles. A second common path is theoxidation of methane by aerobic symbiosesbetween methane oxidizing bacteria and tubeworms or bivalves. Also symbiotic relationswith aerobic sulfide oxidizers are found.Although anaerobic oxidation of methane isessential for the removal of methane, mostbiomass production occurs by the energetic

much more favorable sulfide and aerobicmethane oxidation. Thus these oases of life arenot autarkic: essential is the supply of electrondonors oxygen and sulfate, which formation isfinally dependent on photosynthesis. Our cen-tral hypothesis is that the transport of the elec-tron donors governs the development of diffe-rent habitats.

2. Objectives of the ProjectOur goal is to learn why so many differenthabitats are present in and around cold seeps,and which conditions leads to the develop-ment of a certain microbial and faunal com-munity. We measured (1) the process rates, (2)their stratification inside the methane rich sedi-ments and (3) the mass transfer phenomenaoccurring in these sediments with combina-tions of in situ techniques. In situ measure-ments are more difficult than ex situ analyseson retrieved sediment, but they are necessarydue to the side effects of pressure release.Retrieval of pressurized sediments with gashydrates or dissolved gasses is impossiblewithout severe disturbance due to outgassing(see Fig. 1). The work is organized in 3 experi-mental units that complement each other. Theintegration of in situ rate measurements ofAOM, characterization of the microenviron-ments in the sediments with high spatial reso-lution and the mass transfer phenomena resultin actual turnover rates of methane and giveinsight in the regulatory mechanisms.

De Beer D.

Max-Planck-Institute for Marine Microbiology, Bremen, Germany, E-Mail: dbeer@mpi-bremen.de

3. Present Status and Results / Methods & Results / ResultsThe main tools for our research are instru-ments that measure on the seafloor 1) theexchange of oxygen and methane (benthicchambers), 2) sulfate reduction rate measure-ments based on injecting 35SO4

2- into a sealedvolume of sediment, 3) microprofiles of O2,H2S, pH and temperature (see Fig. 1). We arealso working on a microsensor for CH4 measu-rements, which is a major technological chal-lenge that so far has not been solved. Technicaldevelopment is in progress on the in situ sulfa-te reduction rate experiments. We have perfor-med 2 cruises on which we were able to de-ploy the instruments in situ. Highly successfulwere the microsensor measurements, withwhich we characterized the actual microenvi-ronments of AOM. This allowed conclusionson mass transfer phenomena and rates.

First successful in situ measurements with pH,H2S and O2 microsensors were done in shallowwater in the Baltic Sea near Kiel (EckernförderBucht). As the AOM zone was located bet-ween 25-40 cm below the sediment-waterinterface, very long microsensors were used(60 cm). Indeed a sulfide peak was observed inthe zone of high AOM activity. Furthermore,the measurements showed a separation bet-ween the oxic and sulfidic zone of ca 10 cm.This zone was inhabited by gliding Beggiatoa,a common companion to be found aboveAOM sediments. Most interesting were the pHprofiles, inside the Beggiatoa inhabited zone.Two explanations for these unusual profiles arepossible: 1) sulfide oxidation occurs in two spa-tially separated steps: sulfide to sulfur and sul-fur to sulfate, 2) sulfide oxidation is coupled toiron and manganese cycling. We will furtherinvestigate both hypotheses by detailed analy-

65

Figure 1: The retrieval of cold seep sediments is often highly disturbing (see bottom photo). In situ measure-ments with microsensors (top left) result in different microprofiles than those measured on these retrievedcores, although the essential features are still present. Note that the left and right graphs are identical, butdifferently scaled, to better display the phenomena near the sediment-water interface.

66

ses of sulfur intermediates and Fe/Mn distribu-tions. In the AOM zone no increased pH wasobserved. This was speculated as AOM is oftenassociated with calcite precipitation; however,the measurements showed that AOM does notlead to a shift in the carbonate chemistry thatcan explain calcium precipitation.

A second cruise led to the Haakon Mosby MudVolcano, on the continental slope betweenNorway and Spitsbergen. Combined in situand on-board measurements were done withall microsensors available for seawater studies.All 7 deployments with the free-falling landerproduced valuable data. The profiles were thefirst recorded in this mud volcano and the firstin sediments with high AOM activity. Thusthese data represent the actual microenviron-ment of AOM. We collaborated with Dr. M.Schlüter and Dr. E. Sauter (Alfred WegenerInstitute for Polar Research), who used oursensors on their profiling unit that was deploy-ed with an ROV. Essential was that they couldposition, using the ROV, the profiling unitexactly in the middle of the volcano, an areathat was not reached by the free-falling lander.Also new mats were discovered near seepsthat could only be investigated with the ROVpositioned equipment. The combined resultsgave a good overview of the different habitatsof the volcano, the main microbial processesand the controlling factors. The volcano canroughly be divided in 3 concentric areas: Area 1) The central area consisting of grey sub-surface mud. Here no AOM was observed andno sulfide was detected. The high oxygen

uptake can be attributed to aerobic methaneoxidation. The retrieved sediments showedlower interfacial oxygen gradients, which ishighly unusual, as normally the oxygen influxstrongly increases upon retrieval. The differen-ce between in situ and ex situ profiles can onlybe explained by upward advection: the pore-water flows upward through the sedimentswith a velocity of ca 12 m per year.

Area 2) A surrounding area with extensiveBeggiatoa mats (Fig. 2). This is a site of highAOM activities and thus of sulfide production.The sulfide is quantitatively oxidized byBeggiatoa, large nitrate storing bacteria thatare typically found on sediments with high sul-fidogenic activity. Remarkable is the clear sulfi-de peak at a depth of ca 2-3 cm, which coinci-des with the AOM zone. Our in situ measure-ments showed that the AOM zone is ca 0.5 cmthick, which is supported by other observa-tions, e.g. FISH studies and fatty acid distribu-tions. Remarkable is the sulfide decrease belowthe AOM zone. This phenomenon is compati-ble with an upwards advection of ca 3 m peryear: sulfide is produced in the AOM zone,part of it diffuses downwards but is blownupwards by advection with equal flux rate. Thedifference in oxygen gradients at the sedi-ment-water interface indicated a similar pore-water flow velocity. The upflow of sulfate freeporewater limits the sulfate penetration toseveral cm. It seems most likely that the down-ward diffusion of sulfate against an upflow ofporewater determines the narrow zone ofAOM. Indeed, retrieved cores showed a sharp

Figure 2: Beggiatoa arelarge filamentous bacteriathat can form white sheetson sediments. In their va-cuoles upto 0.5 M nitrateis stored that is used tooxidize sulfide to sulfur(visible as small granules inright panel). Their energyreserves (sulfur and nitrate)and gliding motility allowsthem to commute overlarge distances (cm to m).(below)

decrease of sulfate with depth. All producedsulfide was oxidized within the sediment. Nooverlap between the oxic and sulfidic zone wasobserved. The mass balance showed that fartoo little oxygen diffused into the sediments tooxidize all sulfide. Beggiatoa seems responsiblefor the sulfide oxidation. Nitrate is activelyaccumulated into their vacuoles and transpor-ted downwards within the gliding bacteria tothe sulfidic zone, where it is used to oxidizesulfide. No remarkable pH changes were foundin the AOM zones.

Area 3) An outer area dominated by Pogono-phora. These are symbiotic tube worms, knownto oxidize methane aerobically. It can not beexcluded that also sulfide oxidation occurs. Thetemperature profile showed that at least the top5-10 cm were efficiently ventilated by the activi-ty of the worms. Due to this ventilating activity,sulfate is transported deep into the sediments.Therefore AOM can also take place very deep, atca 70 cm sediment depth.

4. ConclusionsWe found that microbial processes in gassysediments are ruled by transport of the mainreactants: methane, sulfate, sulfide and nitra-te. Three transport mechanisms were recogni-zed as important: advection, diffusion andnitrate uptake and transport inside gliding bac-teria. The high upflow of porewater in the cen-ter prevents significant penetration of sulfateinto the sediments, thus AOM is not possible.In the Beggiatoa fields upflow is lower, but sig-nificant, limiting the sulfate penetration to 2-3cm depth. We speculate that the upflow pre-vents settling of tube worms: their tubeswould form channels of preferred upflow andthey would suffocate quickly. In the Pogono-phora fields upflow is probably very low, justsufficient to bring methane upwards. HereAOM occurs at 70 cm depth, fuelled by sulfa-te pumped down by the ventilating activity ofthe worms. The cold seeps are highly intere-sting areas, with a diverse mosaic of habitats.Thus they form a natural laboratory for micro-

bial ecologists. The research must be done by insitu equipment, thus we will focus our efforts infurther technological development, especiallymodules for use as payloads of ROVs.

AcknowledgementsWe are grateful for the technical assistancewith the deployments by Gaby Eickert, Anne-Katrin Schlesier and Axel Nordhausen, as wellas for the excellent microsensors from GabyEickert, Ines Schröder, Vera Hübner, Karin Hoh-mann and Cecilia Wiegand. We thank LubosPolerecky for mathematical modelling, and BoBarker Jørgensen, Norbert Kaul and Jean-PaulFoucher for helpful comments. The team fromGENAVIR/IFREMER is acknowledged for theexpert operations with ROV »Victor«. We thankthe crew and officers of the research vessel»Polarstern« for their excellent support. Thestudy was funded by the Bundesministeriumfür Bildung und Forschung and the DeutscheForschungsgemeinschaft in the frame of the GEOTECHNOLOGIEN project MUMM (FKZ 03G0554A).

ReferencesBoetius, A., K. Ravenschlag, C. Schubert, D.Rickert, F. Widdel, A. Gieseke, R. Amann, B. B.Jørgensen, U. Witte, and O. Pfannkuche. 2000.A marine microbial consortium apparentlymediating anaerobic oxidation of methane.Nature 407: 623-626.

Boetius, A., and E. Suess. 2004. Hydrate Ridge:a natural laboratory for the study of microbiallife fuelled by methane from near-surface gashydrates. Chem. Geol. 205: 291-310.

Schulz, H. N., and B. B. Jørgensen. 2001. BigBacteria. Ann. Rev. Microbiol. 55: 105-137.

67

68

Biomarker Signatures of the AnaerobicOxidation of Methane (MUMM – SPIII)

1. Introduction and Objectives of the ProjectThis subproject of the MUMM-project focusedon the identification and quantification of ana-erobic and aerobic methanotrophs in the mari-ne environment using characteristic organicmolecules of microbial origin, so-called bio-markers, and the carbon isotopic signature ofthose biomarkers as indicators for the functionof the microorganisms involved. The study wasperformed on sediments from various marinehabitats world wide (Hydrate Ridge - Cascadiasubduction zone, Håkon Mosby mud volcano –Barents Sea, Gulf of Mexico). Repetitive pat-terns of specific microbial biomarkers in thesemethane-rich sediments were used to trace themicrobial carbon flow from methane and toidentify the zones of its oxidation in situ.Moreover, by analyzing biomarkers of microor-ganisms from enrichment studies, we investi-gated which types were specifically enrichedand whether those microbes are important inthe environments studied. The aim of thisapproach was particularly powerful in combi-nation with process measurements and cultiva-tion-independent molecular techniques perfor-med by other subprojects within MUMM.

2. Present Status and Results

2.1. Biomarker diversity Specific archaeal and bacterial biomarkers indi-cative of syntrophic AOM-consortia were pre-sent in all sediments investigated. As an exam-ple, a biomarker fraction containing high

amounts of such lipids in Hydrate Ridge sedi-ments is shown in Figure 1. The incorporationof methane carbon via AOM into microbialbiomass is indicated by very depleted carbonisotope values of the specific biomarkers deri-ved from methanotrophic archaea of -135‰PDB (Pee Dee Belemnite) and from sulfate-reducing bacteria (SRB) of -103‰ (Elvert et al.,submitted). 13C-signatures of the biomass ofanaerobic methanotrophs are always lighterthan δ13C-values of the substrate methane,resulting in heavier δ13C-values in environ-ments dominated by thermogenic methaneδ13C = -30 to -60‰) compared to environmentswhich contain biogenic methane (δ13C = -50 to -110‰) (Whiticar, 1999). Accordingly, HydrateRidge seeps and the Håkon Mosby mud volcano (HMMV) (Niemann et al., submitted a)with their high proportion of biogenic methane host biomarkers which are more 13C-depleted those from the Gulf of Mexico(Orcutt et al., in press).

All sites investigated are dominated by specificbiomarker lipids such as fatty acids (e.g., FAsC16:1ω5c, C17:1ω6c, cyC17:0ω5,6) derived from SRBand phytanyl glycerol diethers (archaeol andsn-2-or sn-3-hydroxyarchaeol) produced by themethanotrophic archaea. Other prominentbiomarkers found at the various environmentsare irregular isoprenoidal hydrocarbons (e.g.crocetane, Cr:1, PMI:3, PMI:4, PMI:5) also indi-cative of the anaerobic methanotrophs andseries of short chain n-alcohols, sn-1-mono alkylglycerol ethers (MAGEs), and sn-1/sn-2-di alkylglycerol ethers (DAGEs) presumed to be speci-

Elvert M. (1,2), Niemann N. (1), Orcutt B. (3), Jørgensen B.B. (1)

(1) Max Planck Institute for Marine Microbiology, 28359 Bremen, Germany

(2) Research Center Ocean Margins, University of Bremen, 28359 Bremen, Germany, E-Mail: melvert@uni-bremen.de

(3) Department of Marine Sciences, University of Georgia, Athens, GA, 30602, USA

fic for the SRB. Nevertheless, depending on theenvironment studied the overall abundance ofthe specific biomarkers varies according to theAOM-consortia present. At Hydrate Ridge,which is dominated by a consortium of ANME-2 archaea and SRB affiliated with theDesulfosarcina/Desulfococcus group (DSS), bio-markers include highly abundant SRB-derivedFAs C16:1ω5c and cyC17:0ω5,6 (Elvert et al., 2003)and archaeal-derived archaeol and sn-2-hydro-xyarchaeol. The same distribution has beenfound in enrichments of AOM-consortia fromthis environment (Nauhaus et al., submitted).In contrast, sediments from HMMV are indica-ted by FAs specific for SRB of the genusDesulfobulbus (C16:1ω5c and C17:1ω6c) and archa-eal-derived irregular isoprenoidal hydrocarbonsPMI:4 and PMI:5 in combination with archaeoland sn-2-hydroxyarchaeol. The exclusive abun-dance of PMI:4 and PMI:5 and the absence ofall other irregular isoprenoids usually detectedat other AOM-sites points to unknown archaeainvolved in AOM at HMMV. Indeed, this is infull accordance to results obtained by Niemannet al. (submitted b) which found aggregates atHMMV that consist of a newly discoveredarchaeal group (ANME-3) and SRB related tothe genus Desulfobulbus.

2.2. Investigation area Hydrate Ridge

Surface sediments:The spatial distribution of methanotrophicarchaea and SRB sampled from sedimentsabove outcropping methane hydrate wereinvestigated by lipid biomarkers and combinedwith independent results obtained by othergroups during the project period (Elvert et al.,2003; Elvert et al., submitted). The abundanceof the specific biomarkers indicative of AOMperformed by ANME-2/DSS-consortia seems tobe strongly correlated to methane flux andthus to the specific environmental characteri-stics of the different chemosynthetic provincesfound at Hydrate Ridge (Beggiatoa sites, Ca-lyptogena fields, Acharax fields). The abundan-ce of specific biomarkers at the Beggiatoa siteis between 5 and 9 times higher than at theCalyptogena field and two orders of magnitu-de higher than at the Acharax field. Whereasthere have been pronounced maxima observedat the Beggiatoa site (3 and 9 cm sedimentdepth) and the Calyptogena field (5 cm sedi-ment depth) no obvious concentration maxi-mum has been detected at the Acharax field,suggesting that AOM is lowest in this chemo-synthetic province in the surface sediments ofHydrate Ridge, which has also been evident

69

Figure 1: GC chromatogram of the alcohol fraction in the 2-4cm sediment horizon from theBeggiatoa site at Hydrate Ridge. Archaeal biomarkers: Ar: Archaeol, sn2-OH-Ar: sn-2-Hydroxyarchaeol; Bacterial biomarkers: short chain n-alcohols, MAGEs: sn-1-mono alkyl glycerolethers, DAGEs: sn-1/sn-2-di alkyl glycerol ethers (sum of carbon atoms of both side chains andnumber of double bonds are given); Planktonic biomarkers: Phytol, Cholesterol.

70

from rate measurements and aggregate countsat this location (Treude et al., 2003). Positive correlations of specific biomarkerswith counts of AOM-consortia at the Beggia-toa site enabled an estimation of the specificbiomarker content per viable archaeal or bac-terial cell at this high methane flux site. Cell-specific values obtained for the most dominantspecific biomarkers range between 0.62 to0.90 x 10-15 g for sn-2-hydroxyarchaeol and0.22 to 0.30 x 10-15 g for archaeol as biomar-kers indicative for the ANME-2 archaea andbetween 0.46 to 0.58 x 10-15 g for C16:1ω5c and0.10 to 0.14 x 10-15 g for cyC17:0ω5,6 as biomar-kers indicative for SRB of the DSS cluster (Elvert et al., 2003). These estimates may beused in the future to calculate the number ofANME-2 or DSS cells in AOM aggregates ob-tained from sediments or enrichment studiesby the use of biomarker analyses alone.

Deep gas hydrate layers:Depth profiles of ANME-2/DSS specific biomar-kers were analysed in a 1.2 m gravity core fromsouthern Hydrate Ridge, which contained thicklayers of methane gas hydrates, (Elvert et al.,submitted). Biomarker carbon isotope values inthe core are more 13C-depleted in sedimenthorizons just above methane hydrate layersaccompanied by an apparent concentrationincrease (Fig. 2). This finding indicates thatsome of the methane stored in the hydrates is

available for the microbial community eventhough the sediments at Hydrate Ridge are inthe hydrate stability filed (water depth 770 m;4°C bottom water temperature). The methaneavailable might be supplied by the continuousdissociation of gas hydrates due to diffusioncaused by the intense concentration gradientbetween the gas hydrates and the surroundingsediment, or might be derived from the freegas present in some sediment layers.

2.3. Investigation area Håkon Mosby mud volcano At Håkon Mosby mud volcano (HMMV), anArctic deep-sea cold seep, where warm, metha-ne-rich subsurface mud is transported to thesurface in a central conduit, the change incommunity structure in response to biologicaland physical gradients have been investigatedin detail. Aerobic methanotrophs (Methylobac-ter sp.), evident from lipid biomarker analysesby the high abundance of the FA C16:1ω8c speci-fic for type-I methanotrophs (Table 1), colonizethe surface horizon where oxygen is present atthe centre of HMMV (Niemann et al., submit-ted b). Over time, i.e. along the mud flowstowards the outer volcano, aerobic methano-trophy is outcompeted by AOM performed byaggregates of SRB (Desulfobulbus sp. indicatedby C17:1ω6c) and novel archaea (ANME-3 groupindicated by the sole abundance of PMI:4 and

Figure 2: Depth profiles of pore water sulfide and chloride, ANME-2 archaeaspecific lipids (archaeol, sn-2-hydroxyarchaeol), and lipids diagnostic for DSS(FAs C16:1ω5c and cyC17:0ω5,6) in a gravity core containing distinctive layers ofgas hydrates (gray shaded areas).

PMI:5) that develop in a shallow Beggiatoacovered sub-surface horizon. At this location,comparative analysis of sulfate concentrationmeasurements and direct counts of ANME-aggregates show a correlating vertical distribu-tion pattern with a sharp decline of sulfateconcentration and aggregate numbers belowthe AOM horizon. In the oldest sediments out-side of the HMMV center, tubeworms (Pogo-nophora sp) harbouring endosymbiontic sulfi-de-oxidizing and aerobic methane-oxidizingbacteria relocate the AOM horizon to a depthof approximately 80 cm by ventilation.2.4. Investigation area Gulf of Mexico

In accordance with molecular biological, geo-chemical and radiotracer evidence, the isotopicsignatures of lipid biomarkers extracted fromGulf of Mexico cold seep sediments demon-strate the occurrence of AOM. Archaeal bio-markers – namely, archaeaol and sn-2-hydroxy-archaeol – are strongly depleted in 13C (δ13C = -80‰ to -115‰; Fig. 3), revealing the incorpo-ration of methane-derived carbon into micro-bial biomass. Differences in the carbon isotopicsignatures of these biomarkers by approxima-tely 35‰ are correlated with different sourcesof the methane, i.e. thermogenic versus bioge-nic, at the various sampling sites (δ13C-CH4 = ~

71

Table 1: Selected biomarker δ13C-signatures obtained from surface and deep sediment as well aswhole tissue extracts. Indicated are the most likely microbial sources of the biomarkers at HMMV

Figure 3: Concentration (solid bar, scale to left) and isotopic composition (black dot, scale to right)of lipid biomarkers extracted from a core of oily Gulf of Mexico sediment (5 cm sediment depth)collected in a white Beggiatoa spp. microbial mat. »Archaeol« and »sn-2-hydroxy« refer alcohol-derivatives of archaeal biomarkers; the remaining labels indicate bacterial fatty acids.

72

-45‰ at hydrate sites, ~ -85‰ at brine sites).Additionally, lipid biomarkers suggested to beindicative of DSS involved in AOM (C16:1ω5c andcyC17:0ω5,6) also show strong 13C-depletion(δ13C = -44‰ to -90‰), supporting evidencefor the role of these bacteria in methanecycling in Gulf of Mexico seep sediments.Interestingly, the abundance of DSS-derivedbiomarkers is up to an order of magnitude hig-her than that of biomarkers derived frommethanotrophic archaea which is not evidentfrom aggregate counts of the respectivemicroorganisms (Orcutt et al., in press).

AcknowledgmentsWe thank Gabi Klockether for help with instru-ments and laboratory analyses. The study wasfunded by the Bundesministerium für Bildungund Forschung and the Deutsche Forschungs-gemeinschaft in the frame of the GEOTECH-NOLOGIEN project MUMM (FKZ 03G0554A).

ReferencesElvert M., Boetius A., Knittel K., Jørgensen B.B.(2003). Characterization of specific membranefatty acids as chemotaxonomic markers for sul-fate-reducing bacteria involved in anaerobicoxidation of methane. Geomicrobiol. J., 20,403-419.

Elvert M., Boetius A., Hopmans E. C., Suess E.(submitted) Spatial variations of methanotro-phic consortia in gas hydrate-bearing sedi-ments: Implications from a high resolutionmolecular and isotopic approach. Geobiology.Nauhaus K., Albrecht M., Elvert M., Boetius A.,

Widdel F. (submitted) In vitro growth of anaer-obic consortia of archaea and sulfate-redu-cing-bacteria with methane as the sole elec-tron donor. Environmental Microbiology.

Niemann H., Elvert M., Lösekann T., Jakob J.,Nadalig T., Boetius A. (submitted a) Distributionof methanotrophic guilds at Håkon MosbyMud Volcano, Barents Sea. Geobiology.

Niemann H., Lösekann T., de Beer D., Elvert M.,Knittel K., Amann R., Sauter E., Schlüter M.,Klages M., Foucher J. P., Boetius A. (submittedb) Microbial colonization of a submarine mudvolcano: how subsurface fluid flow structuresmethanotrophic communities. Nature.

Orcutt B., Joye S. B., Boetius A., Elvert M.,Samarkin V. (in press) Molecular biogeochemi-stry of sulfate reduction, methanogenesis andthe anaerobic oxidation of methane at Gulf ofMexico methane seeps. Geochimica et Cosmo-chimica Acta.

Treude T., Boetius A., Knittel K., Wallmann K.,Jørgensen B. B. (2003) Anaerobic oxidation ofmethane above gas hydrates at Hydrate Ridge,NE Pacific Ocean. Marine Ecology ProgressSeries 264, 1-14.

Whiticar M.J. (1999) Carbon and hydrogen iso-tope systematics of bacterial formation andoxidation of methane. Chemical Geology 161,291-314.

73

74

Distribution and Diversity of Microorganisms in Gas Hydrate Bearing Sediments (MUMM –SPIV)

1. IntroductionMassive reservoirs of natural methane are pre-sent in the deep subsurface of the oceans andare considered as a promising source of energyfor the future. But methane also endangersthe global climate: each mol entering theatmosphere contributes more than 30 times tothe greenhouse effect than carbon dioxide(CO2). However, more than 90% of the metha-ne rising from the subsurface is oxidized ana-erobically to CO2 with sulfate as electronacceptor before it reaches the oxidized layers.This process of anaerobic oxidation of metha-ne (AOM) is carried out by anaerobic metha-notrophic archaea (ANME) and sulfate redu-cing bacteria (SRB) as syntrophic partners. Formore than three decades researchers throug-hout the world have been trying to isolatethese microorganisms but neither the ANMEgroups nor their sulfate-reducing partnershave been isolated yet. In 1999, when apply-ing molecular techniques on methane seepsediments for the first time, two new groupsof archaea (ANME-1 and ANME-2) had beendiscovered as potential candidates for AOM(Hinrichs et al., 1999). In the year 2000, justbefore starting MUMM project, our groupprovided the first microscopic evidence for astructured consortium of ANME-2 archaeaand SRB (»ANME-2/DSS aggregates«;Boetius et al., 2000). The consortia oxidizemethane with sulfate, yielding equimolaramounts of carbonate and sulfide (Nauhauset al., 2002, Orphan et al., 2001).

2. Objectives of the ProjectOur group focused on the identification andquantification of anaerobic and aerobicmethanotrophic microorganisms. We investi-gated different methane seeps and a variety ofother gassy sediments, from deep subsurfacecores to the Baltic Sea. Another aim was thedevelopment of new nucleic acid probes fordominant microorganisms to identify guildslinked to the turnover of methane and sulfatein the sediment.

3. Present Status, Methods, and ResultsOnly the minority of marine bacteria can becultivated under laboratory conditions. Thus,we used cultivation-independent ribosomalRNA (rRNA) based methods for the identifica-tion of microbial communities. The rRNA of thesmall subunit, in prokaryotes the 16S rRNA, iscurrently the molecule of choice for bacterialmolecular systematic studies. The analysis ofmicrobial diversity was accordingly done byconstructing 16S rRNA gene libraries followedby sequencing and phylogenetic analysis.Another 16S rRNA based method was the flu-orescence in situ hybridization (FISH), whichmakes it possible to stain bacteria dependingon their taxonomic origin with small nucleicacid probes carrying a fluorescent dye. Nine methane-rich sites were investigated: Gashydrates and the overlaying sediments atHydrate Ridge (Oregon, USA), methane seepsin the Gulf of Mexico and the Guaymas Basin,the Haakon Mosby Mud Volcano in theBarents Sea, anoxic sediments in the WaddenSea, Baltic Sea, and the Congo Basin, and thick

Knittel K. (1), Lösekann T. (1), Amann R.(1)

(1) Max Planck Institute for Marine Microbiology, Dept. Molecular Ecology, Celsiusstr. 1, 28359 Bremen,

Germany, E-Mail: kknittel@mpi-bremen.de

microbial mats from the anoxic Black Sea.During MUMM project we built up large data-bases including more than 5000 new AOM rela-ted 16S rRNA gene sequences from our ownand other groups. We found a greater diversityof methanotrophic archaea and SRB as pre-viously assumed (Knittel et al., 2003, Knittel etal., 2005, Lemke, 2001, Lösekann, 2002). A comparison of these sequences showed theubiquitous presence of methanotrophic archaeain almost all anoxic methane environments inve-stigated so far. Furthermore, ANME/SRB consor-tia were detected in all habitats (Fig. 1, Knittel etal., 2005). However, the consortia differed inabundance, size, ratio of ANME:SRB, and theirmorphology. Two different types of consortiahad been identified: structured »shell-type«consortia (Knittel et al., 2005), consisting of aninner core of ANME archaea partially or fullysurrounded by SRB (Hydrate Ridge, HaakonMosby mud volcano, Black Sea, Guaymas Basin,Wadden Sea) and »mixed type« consortia inwhich ANME and SRB are completely mixed(Hydrate Ridge, Haakon Mosby Mud Volcano,

Gulf of Mexico). In addition, consortia consi-sting of only archaeal cells (Eckernförder Bight)or just two archaeal cells and a single SRB cell(Congo Basin) were detected.

It has been shown that the three groups ANME-1, ANME-2, and ANME-3 often co-occur, howe-ver, quantitative analysis of the distribution indi-cated dominance of particular groups in certainenvironments (Knittel et al., 2003; 2005; Micha-elis et al., 2002; Lösekann, 2002; Lösekann etal., in prep.; Treude et al., in press). In the follo-wing three sites are described which are domi-nated by one ANME group each.

Black Sea Microbial Mats- Dominance of ANME-1 During submersible dives to methane seeps inthe permanently anoxic Black Sea giant micro-bial structures were discovered, composed ofmassive microbial mats of centimeter to deci-meter thickness, producing large carbonatecolumns. The mat is streaked with a system ofmicrochannels (ca. 30% of mat volume; Knittel

75

Figure 1: Anaerobic methane oxidizers in different habitats

76

et al., in prep.), allowing an exchange with thesurrounding seawater. Epifluorescence micro-scopy of mat sections revealed that the mat iscomposed of densely aggregated archaea andbacteria (Michaelis et al., 2002). The cell num-bers ranged between 150 and 300 billions percm-3 mat, which is two orders of magnitudehigher than those in rich coastal sediments andstill one order of magnitude higher than num-bers in highly active Hydrate Ridge sedimentsabove gas hydrates (see below). Quantitativeanalysis showed that ANME-1 and ANME-2groups co-occur in the mats, however, ANME-1 dominated strongly. The most abundant bac-terial population was a group of SRB affiliatedwith Desulfosarcina/Desulfococcus. These SRBoccurred in larger clusters of 10-50 µm in dia-meter or as smaller clusters and single cellsspread throughout the ANME-1 biomass.

Hydrate Ridge Sediments Above Gas Hydrates-Dominance of ANME-2 At Hydrate Ridge a large quantity of methaneis constantly released from decomposing gashydrates. The surface sediments were domina-ted by ANME-2/DSS aggregates accounting forup to 40 billion cells per cm-3 and more than90% of total cell biomass (Boetius et al., 2000;Elvert et al., 2003; Knittel et al., 2003; Treudeet al., 2003). ANME-1 cell numbers were atleast one order of magnitude lower. The micro-bial community living in pure gas hydrates didnot differ remarkably from those in the sur-rounding sediment. However, the AOM medi-ating groups ANME-1 and ANME-2 were lessactive and occurred in much lower numbers. Chemosynthetic organisms populate the sea-floor. Giant sulfur-oxidizing bacteria (e.g.Beggiatoa species) form thick mats. Also speci-fic mussels (Calyptogena species), which har-bor symbiotic sulfur oxidizers in their gills livefrom the hydrogen sulfide formed duringAOM. Surprisingly, the relative abundance ofANME-2 subgroups ANME-2a and ANME-2cwas remarkably different at Beggiatoa (80%ANME-2a, 20% ANME-2c) and Calyptogenasites (20% ANME-2a, 80% ANME-2c) indica-ting a selection of either group by the location(Knittel et al., 2005).

Haakon Mosby Mud Volcano anoxic sediments- Dominance of ANME-3 The Haakon Mosby Mud Volcano (HMMV) isan active cold seep expelling methane enri-ched mud from a zone 2-3 km below the sea-floor. Here, methane is oxidized anaerobicallyby a new type of ANME/SRB consortium. Incontrast to all other sampling sites (see Fig. 1)the archaea in these consortia could be assig-ned to a new group, ANME-3 (Lösekann et al.,in prep.). ANME-3 is affiliated with Methan-ococcoides species and is distinct from knownphylogenetic groups involved in AOM. The sul-fate-reducing partner belongs to Desulfobul-bus (DBB) spp. The finding of the new ANME-3/DBB aggregates is supported by biomarkersignatures at the same sampling sites (Nie-mann et al., in prep.). Aggregate abundancewas highest in sediments covered with mats ofsulfide-oxidizing Beggiatoa. At this site, AOMis the dominant methane consuming process(Fig. 2). In contrast, no AOM aggregates werefound in center sediments, where methane isemitted to the hydrosphere and aerobicmethanotrophic Gammaproteobacteria domi-nate the microbial community (46% of totalcells). In sediments colonized with Pogono-phora worms AOM rates were below thedetection limit, indicating an only minor im-portance of AOM at this site. Here, endosym-biotic bacteria living in the worms are probablythe major methane-consumers.

4. ConclusionsA comparison of 16S rRNA gene sequencesshowed the ubiquitous presence of methano-trophic archaea in almost all methane environ-ments investigated so far independent of insitu temperature, depth, pressure, methaneand sulfate concentrations. ANME-1 andANME-2 co-occur at all seep systems investiga-ted, however, the microscopic analysis of thedistribution shows a dominance of certaintypes within particular niches. This is in linewith the famous statement: »Everything is eve-rywhere, the environment selects!« (Baas-Becking, 1934; Beijerinck, 1913). To single outthe selection factors environmental conditions

and geochemical gradients need to be analy-zed in situ at a high-resolution. This wouldrequire parallel investigations with microsen-sors in the field or experimental studies inflow-through microcosms.

AcknowledgementsWe thank the officers, crews and shipboardscientific parties of RV SONNE during TECFLUXcruises SO143 and SO148 (Grant 03G0143Aand 03G0148A) to Hydrate Ridge and duringGHOSTDABS cruise (Grant 03G0559A) withRV Prof. LOGACHEV in summer 2001 for ex-cellent support. We greatly acknowledge theGEOTECHNOLOGIEN projects OMEGA (Grant03G0566A), LOTUS (Grant 03G0565) andGHOSTDABS for providing access to samplesand infrastructure. This study was part of theprogram MUMM (Mikrobielle UMsatzratenvon Methan in gashydrathaltigen Sedimenten,03G0554A) supported by the Bundesministeri-um für Bildung und Forschung (BMBF, Ger-many). Further support was provided by theMax Planck Society, Germany.

ReferencesBaas-Becking, L. G. M. 1934. Geobiologie ofInleiding Tot de Milieukunde. In W. P. vanStockum and Zoon N. V. (ed.), The Hague, TheNetherlands.

Beijerinck, M. W. 1913. De infusies en de ont-dekking der backteriën, Jaarboek van de Konink-lijke Akademie v. Wetenschappen. Müller, Am-sterdam,The Netherlands.

Boetius, A., K. Ravenschlag, C. Schubert, D.Rickert, F. Widdel, A. Gieseke, R. Amann, B. B.Jørgensen, U. Witte, and O. Pfannkuche. 2000.A marine microbial consortium apparentlymediating anaerobic oxidation of methane.Nature 407:623–626.

Elvert M., Boetius A., Knittel K., Jørgensen B.B.(2003). Characterization of specific membranefatty acids as chemotaxonomic markers for sul-fate-reducing bacteria involved in anaerobicoxidation of methane. Geomicrobiology Jour-nal, 20, 403-419.

77

Figure 2: Overview of the dominant methane-consuming processes atdifferent sampling sites at Håkon Mosby mud volcano

78

Hinrichs, K. U., J. M. Hayes, S. P. Sylva, P. G.Brewer, and E. F. DeLong. 1999. Methane-con-suming archaebacteria in marine sediments.Nature 398: 802–805.

Knittel, K., Boetius, A., Lemke, A., Eilers, H.,Lochte, K., Pfannkuche, O., Linke, P., Amann,R. (2003). Activity, distribution, and diversity ofsulfate reducers and other bacteria in sedi-ments above gas hydrate (Cascadia Margin,OR). Geomicrobiol. J. 20, 269-294.

Knittel, K., Lösekann, T., Boetius, A., Kort, R.,Amann, R. Diversity and Distribution ofMethanotrophic Archaea at Cold Seeps(2005). Appl. Environ. Microbiol. 71: 467-479

Knittel, K., Treude, T., Gieseke, A., Boetius, A.,Amann, R. (in prep.) In situ quantification ofmethanotrophic communities in massivemicrobial mats (Black Sea)

Krüger, M., Meyerdierks, A., Glöckner, F.O.,Amann, R., Widdel, F., Kube, M., Reinhardt, R.,Kahnt, J., Böcher, R., Thauer, R.K. and Shima,S. (2003) A conspicuous nickel protein inmicrobial mats that oxidize methane anaerobi-cally. Nature, 426, 878-881.

Lemke, A. (2001). Mikrobielle Diversität in gas-hydrathaltigen Sedimenten. Diploma thesis,University Bremen.

Lösekann, T. (2002). MolekularbiologischeUntersuchungen der Diversität und Strukturmikrobieller Lebensgemeinschaften in methan-reichen, marinen Sedimenten (Haakon-Mosby-Schlammvulkan). Diploma thesis, UniversityBremen.

Lösekann, T., Knittel, K., Nadalig, T., Niemann,H., Boetius, A., Amann, R. (in prep.). Identifica-tion of a new cluster of anaerobic methaneoxidizers at an Arctic mud volcano (HaakonMosby Mud Volcano, Barents Sea).

Lösekann, T., Knittel., K., Boetius, A., Amann,R. (in prep.). Microbial diversity in pure gashydrates (Hydrate Ridge, OR)

Michaelis, W., Seifert, R., Nauhaus, K., Treude,T., Thiel, V., Blumenberg, M., Knittel, K., Giese-ke, A., Peterknecht, K., Pape, T., Boetius, A.,Amann, R., Jørgensen, B.B., Widdel, F., Peck-mann, J., Pimenov, N.V., Gulin, M.B.. Microbialreefs in the Black Sea fueled by anaerobic oxi-dation of methane. Science 297, 1013-1015.

Nauhaus, K., A. Boetius, M. Krüger, and F.Widdel. 2002. In vitro demonstration of anaer-obic oxidation of methane coupled to sulpha-te reduction in sediment from a marine gashydrate area. Environ. Microbiol. 4:296–305.

Niemann, H., Elvert, M., Lösekann, T., Jakob,J., Nadalig, T., Boetius, A. (in prep.). Lipid bio-marker of methanotrophic guilds at haakonMosby Mud Volcano, Barents Sea.

Orphan, V. J., C. H. House, K.-U. Hinrichs, K. D.McKeegan, and E. F. DeLong. 2001. Methane-consuming archaea revealed by directly cou-pled isotopic and phylogenetic analysis. Science293:484–487.

Treude, T., Boetius, A., Knittel, K., Wallmann,K., Jørgensen, B.B. (2003). Anaerobic oxida-tion of methane above gas hydrates at HydrateRidge, NE Pacific Ocean. Mar. Ecol. Prog. Ser.264, 1-14.

Treude, T., Knittel, K., Blumenberg, M., Seifert,R., Boetius, A. (in press). Subsurface microbialmethanotrophic mats in the Black Sea. Appl.Environ. Microbiol.

79

80

Physiology of Microorganisms in Gas Hydrate Bearing and other Methane-Rich Marine Sediments (MUMM – SPV)

1. IntroductionThe anaerobic oxidation of methane (AOM) isan important microbial process representingone of the major sinks for methane on earth,oxidising up to 90 % of the methane producedin marine sediments prior to their release intothe atmosphere (Reeburgh, 1996). Biogeoche-mical evidence for AOM was first based ondepth profiles of methane and sulfate/sulfide,13C/12C ratios in CO2 and CH4 in sediment pro-files, and 14CH4-labeling studies with sedimentsamples (e. g. Alperin and Reeburgh, 1984;Hoehler et al., 1984; Iversen and Blackburn,1981; Iversen and Jørgensen, 1985). The hypo-thesis that the biochemical process underlyingAOM might be a reversal of methanogenesishas been discussed in connection with field(Hoehler et al., 1994) and laboratory studieswith growing methanogens, in which a frac-tion of added 14C-methane was converted to14CO2 during methanogenesis (Harder, 1997;Zehnder and Brock, 1979). The finding of highly 13C-depleted isoprenoidbiomarkers and archaeal (Methanosarcinales-related) 16S rRNA-gene sequences in the zoneof anaerobic methane oxidation provided furt-her evidence for the theory of a »reverse me-thanogenesis« coupled to the reduction of sul-fate to sulfide as the terminal electron-accep-ting process (Elvert et al., 1999; Hinrichs et al.,1999; Orphan et al., 2001). Molecular studiesindicate that AOM is mediated by assemblages

of archaea (ANME-1, ANME-2, and ANME-3)related to the Methano-sarcinales, and sulfate-reducing bacteria (SRB) of the Desulfosarcina/Desulfococcus group (Hinrichs et al., 1999;Boetius et al., 2000; Orphan et al., 2001b,Knittel et al,. 2005). However, microorganismsthat grow with methane under strictly anoxicconditions have not been isolated so far. Con-sequently, a detailed mechanistic understan-ding of the biochemistry and an in vitro studyof AOM has not yet been achieved (Nauhauset al., 2002 & 2005; Sørensen et al., 2001;Hoehler et al., 1994). Therefore, a combination of molecular and bio-chemical approaches in connection with simul-taneous microbiological work such as growthof biomass was applied in this project to obtainan in-depth understanding of the globallyimportant process of AOM.

2. Objectives of the ProjectThe main objective of this project was thestudy of the physiology of methane-oxidisingmicroorganisms in gas hydrate bearing sedi-ments. This included the isolation of microrga-nisms involved in AOM in marine habitats, thecharacterisation of their metabolism, growthbehaviour and genetic function. For the studyof microorganisms involved in AOM new culti-vation techniques had to be developed, toaccount for the slow growth and unknown

Krüger M. (1,2), Nauhaus K. (2,3), Meyerdierks A. (2), Widdel F. (2)

(1) Federal Institute for Geosciences and Resources (BGR), Stilleweg 2, 30655 Hannover,

Germany, E-Mail: M.Krueger@bgr.de

(2) Max Planck Institute for Marine Microbiology, Celsiusstrasse 1, 28359 Bremen, Germany

(3) LMU Munich, Department Biology I, Maria-Ward Strasse 1A, 80638 Munich, Germany

interactions of different types of anaerobes. Inaddition, marine microcosms with methanesupply were established as a source for micro-organisms and the study of chemical gra-dients. In the future these incubations shouldprovide adequate material for subsequent bio-chemical and molecular biological investiga-tions of the mechanism of AOM.

3. Present Status and ResultsThis project aimed at an understanding of indi-vidual types of microorganisms and theirphysiological capacities in methane consump-tion and production in marine environments,with special focus on two sites: The gas hydra-te-bearing sediments from Hydrate Ridge(Pacific, Oregon; Boetius et al., 2000) andmicrobial mats from the north-western part ofthe Black Sea (Michaelis et al., 2002).

Environmental regulation of AOM at different sitesIn sediments from the marine gas hydrate areaat Hydrate Ridge (HR, NE Pacific) the anaerobicoxidation of methane (AOM) is predominantlycarried out by a consortium of ANME-II-Ar-chaea, and sulfate-reducing bacteria of theDesulfococcus/Desulfosarcina group (Boetiuset al., 2000; Knittel et al., 2003 & 2005). An invitro method to investigate AOM in the labo-ratory was established with HR samples, con-firming the 1:1 stoichiometry for methane andsulfate (Nauhaus et al., 2002). The optima ofenvironmental parameters, including tempera-ture, salinity, pH, and sulfate concentration, forAOM were found to be close to the valuesobserved in situ (Krüger et al., 2005; Nauhauset al., 2005). This demonstrates the optimaladaptation of the microorganisms to the pre-vailing conditions in the environment.Reef-forming microbial mats were collected atmethane seeps in the north-western Black Sea(BS, Michaelis et al., 2002). These microbialmats consist mainly of archaea (ANME-1 clu-ster) and sulfate-reducing bacteria (Desulfosar-cina/Desulfococcus group) (Knittel et al., 2005).

Laboratory incubations with homogenisedsubsamples of the mats revealed their abilityfor AOM. Methane oxidation is coupled to sul-fate reduction in a 1:1 stoichiometry. Elevatedmethane partial pressures (0.1 to 1.1 MPa)increased the sulfate reduction rates in theBlack Sea samples only two- fold in contrast to5-fold in HR samples. The optimal temperaturefor the BS samples was between 10 and 25 °C.

The mechanism of AOMAt Hydrate Ridge, AOM appeared to be exclu-sively coupled to the use of sulfate as terminalelectron acceptor. Other alternative electronacceptors, including nitrate, ferric iron, sulfur,fumarate, manganese oxide or AQDS, werereduced, but this was not coupled to AOM(Nauhaus et al., 2005). Oxygen-dependentmethane oxidation was restricted to the topfew millimeters of the sediment. The additionof a large number of possible intermediates ofAOM to the sediment did not result in elevatedsulfide production in the absence of methane,providing no evidence for one of these com-pounds being the intermediate exchanged inthe consortia (Figure 1). Also, the addition ofknown electron shuttles, like AQDS, humicacids or different phenazines, did not result ina stimulation of AOM (Nauhaus et al., 2005).

Isolation of axenic cultures for the elucidationof the physiology and biochemistry of AOMhas not been achieved. Nevertheless, Girguiset al. (2003) and Nauhaus et al. observed theincrease of AOM-catalyzing biomass withmethane in the laboratory (see below).However, sufficient biomass for the first bio-chemical studies was obtained directly from anatural habitat, the northwestern Black Seashelf (Michaelis et al., 2002). An abundantprotein was purified directly from these mats,which closely resembled methyl-coenzyme Mreductase, the terminal enzyme in methanoge-nesis (Krüger et al., 2003). This protein couldbe assigned to anaerobically methane oxidisingarchaea of the ANME-I group, and representsa likely candidate for the initial step in AOM.

81

82

These findings as well as in situ analyses ofgenes and enzymes (Figure 2) (Hallam et al.,2003 & 2004; Krüger et al., 2003) suggest thatAOM might be in principle a reversal of metha-nogenesis. Further support for this hypothesiswas obtained in experiments with specific inhi-bitors (Figure 3). The addition of bromoetha-nesulfonate (BES), a specific inhibitor formethanogenic archaea, completely inhibitedAOM (Nauhaus et al., 2005).

Growth of AOM-microorganismsGrowth and enrichment of anaerobicallymethane oxidizing consortia composed ofarchaea (ANME-2) and sulfate-reducing-bacte-

ria (Desulfosarcina/Desulfococcus) were obser-ved and quantified in a long-term laboratoryexperiment lasting more than 2 years. Asinoculum samples from Hydrate Ridge (HR)with high in situ activity of AOM and a highbiomass content of presumably methane oxidi-zing consortia were used. This sediment wasincubated repeatedly under elevated methanepartial pressure of 1.37 MPa. In 60 subsequentincubation periods methane-dependent sulfa-te reduction rates increased continuously from0.035 to 0.24 mmol d-1 gdw

-1. Increasing activi-ty was accompanied by increasing biomassobserved as tenfold higher numbers of consor-tia and up to 150times higher concentrationsof biomarkers specifically assigned to orga-

Figure 1: Effect of different possible intermedi-ates of AOM as electron donors onsulfate reduction rates in HydrateRidge1 and Black Sea samples (mean± sd, n= 3). SR: Sulfate reduction

Figure 2: Activities§ (marked with *) and genes$ formethanogenic enzymes found in methanotrophicarchaea (ANME). § after Krüger et al., (2003), $

Hallam et al., (2004).

Figure 3: Effect of the specific inhibitors bromoethanesul-fonate (BES) and molybdate on methane dependent sulfa-te reduction in Hydrate Ridge and Black Sea samples(mean ± sd, n = 3). SR: Sulfate reduction

nisms performing AOM. Fluorescence in situhybridization with specific oligonucleotide pro-bes revealed that the consortia grown in theenrichment were the same as those present insitu. Microscopic examination indicates thatArchaea and SRB grow together as a consor-tium from the early stage of a few cells, resul-ting from the division of large aggregates, anddeveloping again into big spherical aggregatesof several micrometers in diameter.

4. ConclusionsAOM is the most important process in the tur-nover of the greenhouse gas methane in mari-ne environments. The physiological and bio-chemical characterization as well as the culti-vation and isolation of the involved microorga-nisms are essential tasks for an in-depth under-standing of this process. Many examples ofpast environmental research have shown thata broad functional understanding of biologi-cally mediated large-scale processes needsintegrated approaches ranging from habitatstudies to investigations on the cellular andmolecular level. The results obtained in this project providedfirst insights into the physiology and themechanism of AOM. Furthermore, the success-ful enrichment of the respective microorga-nisms as well as the discovery of naturally enri-ched samples provide a promising fundamentfor further, detailed studies. The increase inknowledge should then be used to more accu-rately estimate the global significance of AOMfor the mitigation of methane emissions. Furthermore, reactions of methane as the mostabundant natural hydrocarbon are also oftechnological and chemical interest. This long-term perspective ranges from an understan-ding of possible processes in gas storagecaverns to the development of catalysts forcontrolled use of methane in chemical synthe-ses. Furthermore, the project provides case stu-dies for the advancement of protein- and RNA-based methods for the in situ-study of micro-bial communities.

AcknowledgementsThe project was carried out in collaborationwith partners in the Gas Hydrate Initiative ofthe BMBF, especially the projects GHOSTDABS(University of Hamburg), TECFLUX I and II,LOTUS and OMEGA (all IFM-GEOMAR Kiel) aswell as the GenoMik Network (Göttingen).Furthermore, Prof. R. K. Thauer and Dr. S.Shima from the Department of Biochemistry atthe MPI for Terrestrial Microbiology (Marburg)significantly contributed to the biochemicalstudies. The study was funded by the Bundes-ministerium für Bildung und Forschung andthe Deutsche Forschungsgemeinschaft in theframe of the GEOTECHNOLOGIEN projectMUMM (FKZ 03G0554A).

ReferencesAlperin, M.J., and Reeburgh, W.S. (1985). Inhi-bition experiments on anaerobic methane oxi-dation. Appl Environ Microbiol 50, 940–945.

Boetius, A., Ravenschlag, K., Schubert, C.J.,Rickert, D., Widdel, F., Giesecke, A., Amann, R.,Jørgensen, B.B., Witte, U., Pfannkuche, O.(2000). A marine microbial consortium appa-rently mediating anaerobic oxidation of metha-ne. Nature 407, 623-626.

Elvert, M., & Suess, E. (1999). Anaerobicmethane oxidation associated with marine gashydrates: superlight C-isotops from saturatedand unsaturated C20 and C25 irregular isopre-noids. Naturwissenschaften 86, 295-300.

Girguis, P.R., Orphan, V.J., Hallam, S.J. andDeLong, E.F. (2003) Growth and methane oxi-dation rates of anaerobic methanotrophicarchaea in a continuous-flow bioreactor. Appl.Environ. Microbiol. 69, 5472-5482.

Hallam, S.J., Putnam, M., Preston, C.M., Detter,J.C., Rokhsar, C., Richardson, P.M., DeLong, E.F.(2004). Reverse methanogenesis: Testing thehypothesis with environmental genomics.Science 305, 1457-1459.

83

84

Hallam, S.J., Girguis, P.R., Preston, C.M., Richard-son, P.M., DeLong, E.F. (2003). Identification ofmethyl coenzyme M reductase A (mcrA) genesassociated with methane-oxidizing archaea.Appl. Environ. Microbiol. 69, 5483-5491.

Harder, J. (1997). Anaerobic methane oxidationby bacteria employing 14C-methane uncontami-nated with 14C-carbon monoxide. Marine Geol.137, 13-23.

Hinrichs, K. U. & Boetius, A. B. (2002) The ana-erobic oxidation of methane: new insights inmicrobial ecology and biogeochemistry. InOcean Margin Systems. Wefer, G.; Billet, D.;Hebbeln, D.; Jørgensen, B. B.; Schlüter, M.; vanWeering, T. (eds.) Heidelberg: Springer-Verlag,457-477.

Hinrichs, K.-U., Hayes, J.M., Sylva, S.P., Brewer,P.G., De Long, E.F. (1999). Methane-consu-ming archaebacteria in marine sediments.Nature 398, 802-805.

Hoehler, T.M., Alperin, M.J., Albert, D.B.,Martens, C.S. (1994). Field and laboratory stu-dies of methane oxidation in an anoxic marinesediments: evidence for methanogen-sulphatereducer consortium. Global Biochem. Cycles 8,451-463.

Iversen, N., & Blackburn, & T.H. (1981).Seasonal rates of methane oxidation in anoxicmarine sediments. Appl. Environ. Microbiol.41, 1295-1300.

Iversen, N., & Jørgensen, B.B. (1985). Anaerobicmethane oxidation rates at the sulphate-metha-ne transition in marine sediments from Kattegatand Skagerrak (Denmark). Limnol. Oceanogr.30, 944-955.

Knittel, K., T. Lösekann, A. Boetius, R. Kort, &R. Amann. (2005). Diversity and distribution ofmethanotrophic archaea at cold seeps. Appl.Environ. Microbiol. 71, 467-479.

Knittel, K., Boetius, A., Lemke, A., Eilers, H.,Lochte, K., Pfannkuche, O., Linke, P. (2003).Activity, distribution, and diversity of sulfate re-ducers and other bacteria in sediments abovegas hydrates (Cascadia Margin, Oregon).Geomicrobiol. J. 20, 269-294.

Krüger M., T. Treude, H. Wolters, K. Nauhaus,and A. Boetius. (2005) Microbial methane tur-nover in different marine habitats. Palaeo-geography, Palaeoclimatology, Palaeoecology.In press.

Krüger, M.; Meyerdierks, A.; Glöckner, F. O.;Amann, R.; Widdel, F. et al. (2003) A conspicu-ous nickel protein in microbial mats that oxidizemethane anaerobically. Nature 426, 878-881.

Michaelis, W., Seifert, R., Nauhaus, K., Treude,T., Thiel, V., Blumenberg, M., Knittel, K., Giese-ke, A., Peterknecht, K., Pape, T., Boetius, A.,Amann, R., Jorgensen, B.B., Widdel, F.,Peckmann, J., Pimenov, N.V., Gulin, M.B.(2002) Microbial Reefs in the Black Sea Fueled by Anaerobic Oxidation of Methane.Science 297, 1013-1015.

Nauhaus K., T. Treude, A. Boetius, and M.Krüger*. 2005. Environmental regulation ofthe anaerobic oxidation of methane in ANME-I or –II dominated communities: A comparison.Environmental Microbiology 7, 98-106.

Nauhaus, K., A. Boetius, M. Krüger, Widdel F.2002. In vitro demonstration of anaerobic oxi-dation of methane coupled to sulfate reductionin sediment from a marine gas hydrate area.Environmental Microbiology 4 (5), 296-305.

Orphan, V.J., House, C.H., Hinrichs, K.-U.,McKeegan, K.D. and DeLong, E.F. (2001) Me-thane-consuming archaea revealed by directlycoupled isotopic and phylogenetic analysis.Science 293, 484-487.

Reeburgh, W.S. (1996). »Soft spots« in the glo-bal methane budget. In: L. M.E. and F. R. Tabita(Eds.), Microbial Growth on C1 Compounds.Kluwer Academic Publishers, Intercept, Ando-ver, UK, pp. 334-342.

Sørensen, K.B., Finster, K. and Ramsing, N.B.(2001) Thermodynamic and kinetic requirementin anaerobic methane oxidizing consortia exclu-de hydrogen, acetate, and methanol as possibleelectron shuttles. Microb. Ecol., 42, 1-10.

Zehnder, A.J.B., & Brock, T.D. (1979). Methaneformation and methane oxidation by methano-genic bacteria. J. Bacteriol. 137 (1), 420-432.

85

86

High resolution imaging and physical properties of hydrate and gas-bearing sediments within the INGGAS project

AbstractMethane hydrate has been invoked as a possi-ble climate-killer (through the release of thegreenhouse gas methane), as a natural hazard(through the influence of hydrate and of under-lying free gas on the physical properties of theslope), and as a future energy resource (due tothe large volumes of carbon sequestered withinsuch hydrates). The investigation of each ofthese scenarios requires knowledge of theamount and distribution of hydrate beneath thecontinental slopes, of the processes of hydrateformation and dissociation, and of the thicknesof the free gas zone at the base of the hydrates.However, neither the fine structure of the hydra-te- and gas-bearing sediments nor the links bet-ween hydrate formation / dissociation and fluidflow are fully understood. Addressing theseissues requires high resolution imaging techni-ques, determination of the physical propertiesof the hydrate-bearing and underlying sequen-ces, and the thermal regime. The INGGAS(Integrated Geophysical Characterization andQuantification of Gas Hydrates) project set outprimarily to develop equipment suitable todevelop such equipment for future hydrateresearch, including high frequency seismic sour-ces, a deep tow seismic system, ocean bottomseismometers and heat flow probes. In this con-tribution we concentrate on the developmentof the deep-tow streamer, referring briefly to

the complementary results from the oceanbottom seismometers and heat flow probe.

High resolution imaging of the shallowsubsurfaceThe resolution of subsurface structures inreflection seismic images depends in part onthe seismic acquisition system used. Whereasvertical resolution in fundamentally controlledby wavelength (and can be improved by a sharpwavelet - a high frequency but broad band-width source - and to a lesser extent by decon-volution), the lateral resolution is determined bythe size of the Fresnel zone, itself dependent onthe source frequency, and on the velocity anddistance between the source and streamer andthe reflector. 2D migration shrinks the Fresnelzone in the in-line direction by calculating theresponse as the source and streamer are lowe-red towards the target, but has no influence onthe cross-line resolution (Yilmaz, 2001). The lat-ter can however be improved by lowering thestreamer and/or the source towards to the seafloor. The philosophy adopted within INGGAS(Integrated Geophysical Characterization andQuantification of Gas Hydrates) was to achievethis improved resolution by combining a con-ventional marine seismic surface source (airgunor GI-gun) with a deep-tow streamer and /orwith ocean bottom seismometers.

Reston, T.J. (1), Bialas, J. (1), Breitzke, M. (2), Flüh, E. (1), Kläschen, D. (1), Klein, G. (1, 3);

Talukder, A. (1), Zillmer, M. (1)

(1) IFM-GEOMAR, Leibniz Institute of Marine Sciences, Wischhofstraße 1-3, D24148 Kiel, treston@ifm-geomar.de

(2) Alfred-Wegener Institut for Polar and Marine Research, Postfach 12 0161, D-27515 Bremerhaven,

mbreitzke@awi-bremerhaven.de

(3) Institute of Geosciences, Kiel University, Olshausenstraße, Kiel.

Ocean bottom seismometersBy deploying closely-spaced ocean bottominstruments on the seafloor, it is not only pos-sible to determine the velocity structure (bothp-wave and s-wave) but also to image thestructure beneath the instruments. WithinINGGAS, ocean bottom seismometers wereused to determine the p-wave and s-wavevelocity of gas-bearing sediments within theArkona Basin (Baltic Sea) – Klein et al., inpress – see Figure 1) and of hydrate and gas-bearing sediments at a variety of continentalmargins (e.g. Figure 2).

The work in the Arkona Basin relied on theidentification and inversion of Scholte wavestravelling just beneath the seafloor. A singleOBS (including both geophones and a hydro-phone) recorded shots from a 0.3 l airgun

towed at 8 m depth. The energy travelling inthe acoustic waveguide of the water columnshowed 6 distinct modes, controlled by thephysical properties of the immediate subsurfa-ce. Inversion of these in the tau-p domain ledto a detailed Vp and Vs velocity model of theshallow subsurface, producing an excellentmatch in terms of amplitude, slowness and fre-quency. Applied to hydrate bearing sediments,such analysis promises to reveal the physicalproperties and shear strength of the seafloorwith high resolution.

The first BSR to be detected in the Black Seawas imaged using closely-spaced ocean bot-tom seismometers (c. 300m separation) and aGI-gun source. The survey in the northwesternBlack Sea (Zillmer et al., 2005) was in conjunc-tion with the GHOSTDABS project. The BSR

87

Figure 1: Derivation of p- and s-wavevelocity structure from the dispersioncharacteristics of Scholte waves recor-ded on OBS in the Arkona Basin (Kleinet al., in press).

88

occurs at depths of 205-235 m below the sea-floor itself at 1.1-1.2 km. Kirchhoff migrationwas used to determine both the sub-surfacevelocity structure and an image. S-wave velo-cities were estimated using converted waves.A sharp drop in p-wave velocity at the BSRfrom 1850-1600 m/s is consistent wit the pre-sence of free gas beneath a 150 m thick layerwith 49% porosity and 30% hydrate satura-tion (Zillmer et al., 2005).

Deep-tow systemThe use of OBS for high resolution seafloorimaging is however time consuming and thusof necessarily limited extent. An alternativemethod is continuous profiling using a surfa-ce source and a deep-towed streamer (Figure3), although this does not deliver the comple-mentary information on physical properties.The IFM-GEOMAR deep-tow system compri-ses a multichannel digital streamer (High

Tech), towed behind either directly behind a 2ton depressor weight, which keeps the deep-towed system at depth and as close to thetowing ship as possible or behind an interve-ning towfish connected to the depressor by a30m umbilical. The towfish contains a dualfrequency sidescan sonar system (DTS-1; 75kHz and 410 kHz; Edge Tech Full Spectrum)and a chirp subbottom profiler (2-15 kHz,Edge Tech) (Figures 3, 4). The controlling elec-tronics and a USBL transponder for instru-ment location are mounted either on thedepressor directly (if the towfish is notdeployed) or on the towfish. The streameritself consists of a 50 m. lead in section andmainly acoustic nodes (containing a singlehydrophone, bandpass filter, pre-amplifierand A/D converter) and several engineeringnodes (each also containing depth andmotion sensors, and a magnetic compass).The nodes can be connected by either 1m or6.25 m cable sections, allowing a variety of

Figure 2: One of the first ever images from the Black Sea of the BSR at the base ofhydrate bearing sediments, made using closely spaced OBS (Zillmer et al., 2005).

different deployment geometries. Full detailsof the system are given by Breitzke andBialas (2004).

First test – Sonne 162The deep-tow streamer was first deployedduring RV Sonne cruise SO162 over the Yaqui-na basin off Peru in 2002 (Reston and Bialas,2002). The Yaquina basin is a forearc basin as-sociated with the subsidence of the margindue to subduction erosion at the interface bet-ween the Nazca and South American plates. Itis known to contain hydrates, as evidenced bya well-developed BSR (Bialas and Kukowski,2000a, b). The pre-existing database allowedthe choice of well-defined targets as well as acomparative seismic dataset.

The intention of this survey was to gatherexperience in the handling and operation ofthe deep-tow system. We found that in 1000m water depth, the online knowledge of the

position of towfish and streamer meant thatturns between profiles of 500–600 m spacingwere feasible. Even using an 18 mm coaxialdeep sea cable, data transfer rates were suffi-cient to allow online transmission of all seismic(and sidescan sonar) data from the bottom- tothe top-side, to be displayed as common shotand offset gathers and stored on DLT tape.

Quality control of the seismic data is throughdisplay of both shot gathers and a single chan-nel. The former shows if all the channels arefunctioning; the latter provides a first glimpseof the geology along the section (Figure 5),showing that here signal penetration is about0.4 s TWT or 300 m depth, respectively andthat imaging three chemoherms embeddedwithin a weakly reflecting sequence of hemi-pelagic or turbiditic sediments. A spatially limi-ted, weak BSR can also be observed. Some ofthe strong reflections seem to continue acrossthe chemoherms but are difficult to trace

89

Figure 3: Sketch of the deep-tow streamer and sidescan sonar system (after Breitzke and Bialas,2003). Note that the reflection points at a single channel do not form a vertical line, but rather ahyperbola, meaning that standard CMP processing will not work. Also note that the acquisitiongeometry allows the undershooting of locally hard seafloor.

90

because much of the incident signal energy isscattered at the top and trace spacing of about7.7 m provides only coarse lateral resolution.

We collected a grid of 10 closely spaced profi-les of 5 km length and 100 m spacing in anarea where the »Max«, »Moritz« and »WitweBolte« chemoherms occur at about 1000 mwater depth (Bialas and Kukowski, 2000a, b).The streamer consisted of 22 acoustic andthree engineering nodes each spaced 1 mapart to allow very high resolution imaging ofsubsurface structures by close subsurfacereflection points and had an overall length of74 m (including 50m lead-in). Source was a 1.6litre air gun, generating frequencies between50-300 Hz. Data recording parameters were3.072 s recording time and 0.25 ms sampleinterval. Shot interval was 5 s and average shipvelocity 3 kn resulting in an average shot pointspacing of 7.7 m.

Deep-tow multichannel seismic data processingThe first step in processing the data is theaccurate determination of the location of eachhydrophone node relative to the ship duringdata acquisition. This is accomplished using acombination of the USBL and engineeringnode data. The asymmetric source and recei-ver geometry of the hybrid deep-tow systemcauses subsurface reflection points to lie on ahyperbola even in the case of a plane-layeredsubsurface (cf. Figure 3), so that standard CMPmethods (NMO correction and stack) are notapplicable. Instead, the various single channelswere combined into a final image using aKirchhoff 3D-prestack depth migration, speci-fying directly the unusual acquisition geometry.The migration (Figure 6), described more tho-roughly in the Figure caption, shows a highresolution image of the structures withinhydrate bearing sediments, in particular ofthose related to the transport of fluids and theconstruction of carbonate build-ups.

Figure 4: Photograph of the deep-tow system components,prepared for deployment from the ship's sternduring RV Sonne cruise SO162. Below left: the entire system on deck before deployment, showing in theforeground the streamer nodes and cables, in the middle the 2 ton depressor weight and at the back thesidescan sonar towfish. Top: detail of the acoustic (AM) and engineering modules (EM). Right: the streamerduring deployment, showing the individual modules connected by 1m sections.

Central AmericaA second major deployment of the system wasmade during So-173 in summer 2003 offshoreNicaragua (Figure 7), where the Cocos plate issubducted beneath Central America, a classicexample of an erosive margin. ODP drilling aswell as seismic velocities and images indicatethat the »margin wedge« between the shelfand the trench consists of basement materialoverlain by 0.5-1.5 km of slope sediments.Only a small frontal prism is observed and con-sists of reworked slope sediments rather thanthose scraped off the downgoing plate; rapidsubsidence of the margin is evidence for theremoval of material from the base of the mar-gin wedge and the transport of that materialto depth.

Previous surveys had identified a widespreadand well-developed BSR and a variety of fluidexpulsion features, including mound-like struc-tures (e.g. Pecher et al., 1998; Bohrmann etal., 2002), thought to represents dominantlymud diapiric structures (Figure 7). Apart fromlocal chemosynthetic carbonate caps, the sedi-ments within the mounds are dominantly from

the slope sequence. The presence or mud dia-pirs is however anomalous considering thethinness of the slope sequence above topbasement. Chemical and isotopic data indica-te that the fluids come from dewatering at theplate boundary, leading to the suggestion thatthe mounds are formed by the remobilisationof sediments at the base of the slope sequen-ce by high pressure fluids coming from below.

Of particular interest is the relationship of themounds to the underlying BSR. On surfaceseismic, the BSR is not imaged beneath themounds, possible due to the absence of gas orthe elevation of the base of the hydrate stabi-lity zone (due to high heat flow) to the surfa-ce, although, complementary heat flow studiescarried out by Bremen (using in part equip-ment developed within the INGGAS project)showed that heat flow was not sufficiently rai-sed to expect this (Grevemeyer et al., 2004).Alternatively, the absence of the BSR beneaththe mounds may be due to signal penetrationproblems through the carbonate cap characte-ristic of such mounds. The deep-tow streamerprovides a way to test this as the long lay-back

91

Figure 5: Single-fold deep tow seismic profile recorded in the »Max & Moritz« chemoherm area off Peru(Breitzke and Bialas, 2003). Each trace represents the recordings of each shot by one channel (5), after band-pass-filtering between 55/110 - 500/1000 Hz. No depth corrections have been applied. VE = vertical exaggera-tion for a velocity of 1500 m/s. Notice the »dithered« appearance of the direct arrival, of the seafloor reflectionand of the sub-surface reflection. This is due to variations in the gun depth of ~ 1 m. and needs to be remo-ved by a residual statics correction. After such removal and after the incorporation of the correct geometry(from Posidonia and from the pressure sensors on the streamer, calibarated and checkaed using the time of the direct wave), the data can be prestack migrated.

92

distance of the streamer behind the shipallows the undershooting of the mound – ray-paths reaching reflection point beneath themound enter the subsurface one side of themound and exit the other and so are not affec-ted by cap rocks.

The source was a 1.7 l GI-gun, providing a clea-ner source than used during Sonne 162 in thefrequency range 50-300 Hz. The streamer wastowed 100m above the seafloor, consisted of 17channels (14 acoustic nodes and 3 engineeringnodes), connected by sections varying between1 (twelve sections) and 6.5 (four sections)metres, giving a total active length of 38 metres.

Occurrence of BSR and its relationship tothe mud moundsThe BSR in the area is characterized by: i) rever-se polarity relative to the sea floor reflector ii)cross-cutting relationship with the sedimentarystratigraphy and iii) roughly parallelism to theseafloor. The amplitude of the BSR is variableon the deep-tow profiles, with high to mode-rate amplitudes near the mud mounds butappearing to fade away from them. Directly

beneath the mounds, the BSR either rises ordisappears (Figures 8 and 9). The former casesseems to occur where the mounds are notassociated with an offset of the seafloor (i.e.where the mounds are not obviously fault con-trolled); the latter where such an offset (andprobable fault control) exists (Figures 8 and 9).Elsewhere, the BSR also appears to disappearwhere the seafloor is offset by normal faults.

Although it is always problematic interpretingamplitude variations in terms of subsurfaceproperties, the variations described above canbe interpreted in terms of the fluid flow regimeof the subsurface. It is generally, thought thatthe presence of the BSR indicates local con-centrations of free gas beneath the base of thehydrate stability field, and that a brighter BSRindicates increasing amounts of free gas to afew % of the water saturation. The relativeslow upward flow of warm (but not hot – heatflow at the mounds is only elevated above theregional by 10-20 mW/m2 - Grevemeyer et al.,2004) fluids beneath the mounds would resultin an upward displacement of the base of thehydrate stability field, the partial dissociationof hydrate and the increase in the concentra-

Figure 6: Prestack shot migration of Profile 12. By migrating prestack, the resolution and the signal:noiseration are improved. In particular note a small positive polarity feature ~ 20 ms (15 m) beneath the seafloorabove a steeply dipping negative polarity reflection (Arrow A). We interpret the small feature as a carbonatebuild-up (~ 10 m across) in the sediment where methane has been anaerobically oxidised and the steepreflection is a conduit along which the methane has moved upwards. The fine detail of other larger carbo-nate mounds is also apparent; these mounds occur above significant upward distortion of the underlyingreflectors, which may be related to the upward movement of warm fluids. The mounds have also formedby the anaerobic oxidation of methane. The deep-tow streamer thus has revealed the fine details of the fluidplumbing system within the hydrate stability zone.

tion of free gas. This may explain the brighterBSR in the vicinity of the mounds). Howeverincreased permeability where normal faultsoffset the seafloor allows such gas to escape,resulting in no clear BSR reflection (Holbrook etal., 1996; Gorman et al. 2002). The relatively minor deflection of the BSRwhere it is observed indicates that fluid fluxeseven at the more active vents such as MoundIguana (Figure 9 - Sahling et al., 2003) may beconsiderably less than at well-studied mud vol-canoes such as Hakon Mosby (Eldholm et al.,1999) and those off Barbados (e.g. Henry etal., 1996). In both of these other cases, thefluids are derived from the dewatering of athick sedimentary pile rather than from thedewatering of the thinner subducted sequenceinferred here (Hensen et al., 2003).

ConclusionsThe deep tow streamer developed within ING-GAS has already proved useful for characteri-zing structures related to fluid flow through

hydrate-impregnated sediment. Off Peru, thesystem imaged small fluid conduits feedinglocal carbonate build-ups, confirming the ideathat such build-ups are related to the transportof deep fluids towards the surface. Off Nicara-gua, the images show that the BSR rises bene-ath the mud mounds, implying a moderatelyelevated heat flow associated with the upwardpassage of warm fluids. A local brightening ofthe BSR near the mounds may be related toincrease in the amount of free gas beneath thehydrate zone, perhaps associated with the disso-ciation of the hydrate near the warmer conduit.

AcknowledgementsThe development of the deep-tow MCS system,and of the heat flow probe and the work in theArkona Basin were funded by the GermanMinistry of Education, Science, Research andTechnology (BMBF) within the gas hydrate initia-tive of the program GEOTECHNOLOGIEN, pro-ject INGGAS (grants 03G0564A, C, D and E).Other generous funding for work off Central

93

Figure 7. Bathymetric reliefmap (illumination from NW)showing two of the physio-graphic elements off Nicara-gua Pacific margin: submari-ne mud mounds (arrowed)and deeply incised canyons.Thick segments along tracklines refer the positions ofFigures 8 and 9. Star symbolindicates the location ofMound Iguana (Figure 9)which has only a slight topo-graphic expression.

94

Figure 8: Seismic interpretation of profile DTMCS-P05 (see the location in fig. 7). Note the moundsare associated with a vertical offset of the sea floor and sedimentary wedges (indicated by blackdots) suggesting asymmetric growth controlled by normal faults. Gray dots indicate the unconfor-mity. The strong inclined reflectors masking the SE side of the mound walls are artefacts producedby the asymmetric geometry of the deep- tow reflection system. Beneath these mounds the BSRdisappears: we infer that free gas has escaped upwards along the normal faults that both offsetthe seafloor and control the location of these mounds. After Talukder et al., (in review).

Figure 9. Seismic interpretation ofprofile DTMCS-P07 (see the locationin fig.7). The BSR is clearly imagedbeneath the mound, where it isdisplaced upwards. Note also thatthe BSR is brightest below the flanksof the mound, suggesting increasedconcentrations of free gas beneaththe hydrate zone. We infer that theslow upward flow of warm fluidsassociated with the mud diapir hasdisplaced the base of the hydratestability field upwards, perhaps cau-sing the dissociation of some hydra-te to free gas, but that no conduitsexist for such free gas to escapeupwards. After Talukder et al., (inreview).

95

America came from the DFG through SFB 574.We are indepted to Captain Papenhagen and hiscrew for their excellent support during RV Sonnecruise SO162 off Peru and to Captain Kull andhis crew for their efficient help during RV Sonnecruise SO173-1.

References citedBohrmann G, Heeschen K, Jung C, Weinrebe W,Baranov B, Cailleau B, Heath R, Huehnerbach V,Hort M, Masson D (2002): Widespread fluidexpulsion along the seafloor of the Costa Ricaconvergent margin: Terra Nova 14: 69-79.

Bialas J, Kukowski N (2000a): RV Sonne CruiseReport SO146-1&2. GEOPECO (GeophysicalExperiments at the Peruvian Continental Margin- Investigations of Tectonics, Mechanics, Gas Hy-drates and Fluid Transport). Arica - Talcahuano.March 1 - May 4, 2000. Geomar Report 96.Bialas J, Kukowski N (2000b): Peruvian cruiseprovides fresh insights into gas hydrates. FirstBreak 18(8), 360- 362.

Breitzke M, Bialas J (2003): A deep-towed mul-tichannel seismic streamer for very high-resolu-tion surveys in full ocean depth: Marine Seis-mic 21, 59-64.

DeMets C, Gordon RG, Argus DF, Stein S(1994): Effect of recent revisions to the geo-magnetic reversal time scale on estimates ofcurrent plate motions. Geophysical ResearchLetters 21, 2191-2194.

Eldholm O, Sundvor E, Vogt PR, Hjelstuen BO,Crane K, Nilsen AK, Gladczenko TP (1999):SW Barents Sea continental margin heat flowand Hakon Mosby volcano. Geo-MarineLetter 19, 29-37.

Gorman AR, Holbrook WS, Hornbach MJ,Hackwith KL, Lizarralde D and Pecher I (2002):Migration of methane gas through the hydra-te stability zone in a low-flux hydrate province:Geology 30, 327-330.

Grevemeyer I, Kopf AJ, Fekete N, Kaul N,Villiner HW, Heesemann M, Wallmann K,Spiess V, Gerrerich HH, Mueller M, WeinrebeW (2004): Fluid flow through active mud domeMound Culebra offfshore Nicoya Peninsula,Costa Rica: evidence from heat flow surveying:Marine Geology 207, 145-157.

Henry P, Le Pichon X, Lallemant S, Lance S,Martin JB, Foucher JP, Alina FM, Rostek F, Guil-haumou N, Pranal VA, Castrec M (1996): Fluidflow in and around a mud volcano field seawardof the Barbadoes accretionary wedge: Resultsfrom Manon cruise. Journal of Geophysical Re-search 101, 20,297-20,323.

Hensen C, Wallmann K, Schmidt M, RaneroCR, Sahling H, Suess E (2003): Fluid expulsionrelated to mud volcanism at Costa Rica conti-nental margin - a window to the subductingslab. Geology 32, 201-204.Holbrook WS, Hoskins H, Wood WT, StephenRA, Lizarralde D, Leg 164 Science Party (1996):Methane Hydrate and Free Gas on the BlakeRidge from Vertical Seismic Profiling. Science273 (5283), p.1840-1843.

Kimura G, Silver EE, Blum P, Shipboard scientificparty leg 170 (1997): Proceedings of the OceanDrilling Program Initial reports 170, CollegeStation, TX, Ocean Drilling Program, pp. 458.

Pecher IA, Ranero CR, von Huene R, MinshullTA, Singh SC (1998): The nature and distribu-tion of bottom simulation reflectors at the CostaRican convergent margin. Geophys. J. Int. 133,219-229.

Ranero CR, von Huene R, Flueh ER (2000): Across section of the convergent Pacific marginof Nicaragua. Tectonics 19, 335-357.

Ranero CR, von Huene R (2000): Subductionerosion along the Middle America convergentmargin. Nature 404, 748-752.

96

Reston TJ, Bialas J (2002) RV Sonne CruiseReport SO162. INGGAS Test (Integrated Geo-physical Characterisation and Quantification ofGas Hydrates - Instrument Test Cruise). Valpa-raiso - Balboa. February 21- March 12, 2002.Geomar Report 103.

Sahling H, Echeverria-Saenz S, Corrales-CorderoEM, Soeding E, Suess E (2003): Sea floor obser-vation by OFOS. Kiel, IFM-Geomar, pp. 492.

Talukder AR, Bialas J, Klaeschen D, BrueckmanW, Reston TJ, Breitze M (in review): High-resolu-tion, deep towed, multichannel seismic survey ofsubmarine mounds and associated BSR off Ni-caragua pacific margin. Submitted to Geology.

Yilmaz Ö. (2001) Seismic Data Analysis. Societyof Exploration Geophysicists, Tulsa, OK, USA. Zillmer M, Flueh, ER, Petersen J (2005): Seismicinvestigations of a bottom simulating reflectorand quantification of gas hydrate in the BlackSea. Geophys. J. Int 161, 662-678

97

98

An in-situ laboratory to study terrestrial,permafrost related gas hydrates (Mallik 2002)

1. Introduction of the Mallik research drilling projectFrom December 2001 to March 2002 field-work was conducted on a new gas hydrateresearch well program at the northeasternedge of the Mackenzie Delta, NorthwestTerritories, Canada (Fig. 1). The program wasinitiated by the Geological Survey of Canada(GSC), the Japan National Oil Corporation(JNOC/JAPEX), the United States GeologicalSurvey (USGS), and the GeoForschungsZen-trum Potsdam (GFZ). The main objective of thedrilling program was to investigate gas hydra-tes formed under permafrost conditions in oneof the most prominent occurrences of this kind(see Dallimore et al., 2002, for overall descrip-tion of the project). The major questions thatwere addressed in the research program were1) the in-situ geological, geochemical, andpetrophysical properties of the gas hydratebearing sediments, 2) the response of the gashydrates to controlled production tests, inwhich the gas hydrates were destabilized byde-pressurization and thermal stimulation, and3) the effects of these stimulation tests on the in-situ material properties of the affected regions.The experiments included a wide variety of geo-scientific investigations including coring, petro-physical analysis, downhole geophysical mea-surements, and gas geochemical logging.

After a long review process the drill site nearImperial Oil Mallik L-38, an industry explora-tion well drilled in 1972 (Bily and Dick, 1974),was selected for the location of the gas hydra-te research well program (Figure 1). The Malliksite was chosen as it offered favorable logistics

and had the thickest known occurrence of gashydrate in the region. Detailed geological, geo-physical and engineering data were availablefrom the original industry well and from theMallik 2L-38 research well program conductedin 1998 (Dallimore et al., 1999). Well log inter-pretations and core samples from the 1998research well revealed a strong lithologicalcontrol on gas hydrate occurrences at Mallik.For the most part, gas hydrate occurred withincoarse-grained sandy sediments that weretypically interbedded with non-gas-hydrate-bearing, or very low gas hydrate content, fine-grained silty sediments. On the basis of thewell-log interpretations, more than 110 m ofwell defined gas-hydrate-bearing sands andsilty sands were found between 897 and 1110m (Collett et al., 1999a). Quantitative well-log-derived estimates suggested that in situ gashydrate concentrations were very high, withgas hydrates filling more than 60% of the porespace in most gas hydrate layers, and in manycases more than 80%.

The Mallik 2002 research well program inclu-ded the drilling of a 1200 m deep main pro-duction research well (Mallik 5L-38) and, forthe first time, two 1150 m deep scientificobservation wells offset 45 m from the mainwell (Fig. 2). A wide-ranging research programwas conducted, involving extensive geophysi-cal studies, core studies, and the application ofseveral technologies to investigate in-situ for-mation conditions. Field-scale experimentswere conducted to monitor the physical beha-viour of the gas hydrate deposits and the adja-cent sediments during depressurization and

Weber M., Bauer K.*, Kulenkampff J., Henninges J., Huenges E., Wiersberg T., Erzinger J., Löwner, R.

GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany, *E-Mail: klaus@gfz-potsdam.de

thermal stimulation. The science program hasbeen substantially expanded through theacceptance of research proposals that involvedover 100 researchers from more than 30 rese-arch institutions. The GEOTECHNOLOGIENresearch initiative provided funding for severalsub-projects within the Mallik program, whichwere carried out by research groups at theGeoForschnungsZentrum Potsdam. In the fol-lowing text each of the sub-projects is descri-bed and the main results are summarized.

2. Results2.1 Crosshole seismic experimentSeismic methods are widely used to detect, cha-racterize, and eventually quantify gas hydrateaccumulations. In the Mackenzie Delta, thedistribution of gas hydrates has been mappedby evaluation of available drilling information,

and extrapolation based on 2-D and 3-D indu-stry seismic data (e.g., Bily and Dick, 1974,Collett et al., 1999b). The Mallik 2002 projectincluded a series of seismic experiments cove-ring a wide range of scales: from core studies,through downhole sonic measurements, cross-hole experiments, vertical seismic profiling(VSP), and surface seismic experiments (Fig. 1).These experiments were originally proposed inorder to improve the understanding of therelationships between the seismic observationsat different observation scales. Crosshole expe-riments, which were carried out in Mallik forthe first time in gas hydrate research, corre-spond to scales between those obtained fromborehole-sonic and VSP and surface seismicexperiments. Additional to this scaling aspect,the crosshole seismic experiments were desi-gned to image possible changes in the seismic

99

Figure 1: top) The location map showsthe site of the Mallik gas hydrate rese-arch well program. bottom) The detai-led site layout shows locations for mainwell Mallik 5L-38 and the two scientificobservation wells.

100

properties during hydrate production, potenti-ally providing data on the effects of the gashydrate dissociation tests. These changes mayprovide important post-production informa-tion for the understanding of gas hydrate dis-sociation processes and stability conditions.

The crosshole seismic measurements made useof two 1160 m deep observation wells (Mallik3L-38 and 4L-38), both located 42.5 m fromand co-planar with the 1188 m productionresearch well (5L-38). Boreholes 3L-38 and 4L-38 served as source and receiver wells, respec-tively. Four complete surveys were conductedbetween boreholes 3L-38 and 4L-38: onebaseline experiment to provide the referenceseismic structure, and three monitor, or repeatsurveys carried out after the initiation of thethermal stimulation test. Each survey requiredapproximately 24 hours of acquisition time.The baseline survey covered a depth range bet-ween 800 and 1150 m, bracketing the propo-sed gas hydrate interval between 900 and

1100 m depth, as well as portions of the sur-rounding sedimentary sequences on top ofand below the primary imaging target. It wasanticipated that these sequences might hostsignificant amounts of free gas. Such free gaszones have been encountered in many gashydrate fields, particularly in deep sea environ-ments. The repeat experiments were limited toa depth range from 800 m to 1050 m, provi-ding sufficient coverage of the target regionsof the thermal stimulation tests, between 907– 920 m depth (Fig. 2).

The four time-lapse crosswell seismic surveys inthe vicinity of the production experiment atMallik 5L-38 provide a unique opportunity toexamine the in-situ seismic properties of arcticgas hydrates, and the evolution of these pro-perties during thermal dissociation. The foursurveys are high quality with sufficient signal-to-noise ratios. The data exhibit clear direct P-wave arrivals; first arrival times were identifiedon more than 90% of the traces. The other

Figure 2: Perspective view showing thetrue 3-D location of the three boreho-les, and the major stratigraphic sequen-ces. The scaling of the horizontal axis isexaggerated by a factor of 5 against thescaling of the vertical axis. The gray sha-ded region corresponds with the cove-rage of the crosswell seismic experi-ments. The depth range of the perma-frost and the gas-hydrate-bearing inter-val are also depicted.

dominant signals present on the surveys aretube-waves and tube-wave related signals,that can be largely removed by f-k filtering ofthe data in source and receiver domains. Thefour surveys are highly repeatable, howeverthe effects of the small, experimental thermaldissociation test are expected to be very subt-le. The data analysis was done in two sequen-tial steps: The first step comprised the deter-mination of the seismic background structurebefore the thermal production test was star-ted (Bauer et al., 2005b, Pratt et al. 2005).These results were used as reference modelfor the second step, in which modelling stu-dies were used to determine the feasibility ofmonitoring the changes induced by the ther-mal production test (Bauer et al., 2005a,Watanabe et al., 2005).

The determination of the seismic backgroundstructure was based on data as shown exem-plarily in Figure 3. Ray-based tomographic me-

thods were applied to the complete data set ofthe baseline experiment to derive 2-D modelsof isotropic and anisotropic velocities, andattenuation at depths between 800 and 1150m (Fig. 4). The seismic results image the majorlitho-stratigraphic environment of the targetregion. The delta front/shallow marineMackenzie Bay Sequence is characterised bysmaller velocities and slightly smoother variabi-lity, as compared with the fluvio-deltaic Kug-mallit Sequence underneath. The gas hydratesformed preferentially within several sandy lay-ers at the lowermost base of the MackenzieBay and the upper part of the KugmallitSequence. The entire gas hydrate intervalcovers the depth range between 890 and1107 m and consists of several layers of veryhigh gas hydrate concentration (between 30to 80%). These layers are characterized byhigh P velocities (up to and slightly larger than3.5 km/s), strong transverse isotropy (between5-15% faster horizontal velocities than vertical

101

Figure 3: a) Sub-set of the data from the crosswell seismic experiment. b) One-dimensional P-wave velocities estimatedfrom horizontal crosswell seismic data. c) Logarithmic presentation of the spectral amplitudes calculated by waveletanalysis for the first-arrival signals of the horizontal crosswell seismic data. d) One-dimensional P-wave attenuation esti-mated from horizontal crosswell seismic data. e) Lithological profile from coring and gas hydrate saturation estimatedfrom the difference of density and NMR porosity at Mallik 5L-38.

102

velocities), and strong attenuation (with thesmallest Q values between 15 and 8). Lateralvariations in these properties indicate that thehydrates occur in more lense-like, rather thanstratified, structures reflecting the fluvio-del-taic deposition of the host material. The bot-tom of the hydrate interval coincides approxi-mately with the lower theoretical boundary ofthe hydrate stability field (e.g., Dallimore et al.,1999, Henninges et al., 2005). Thus, accumu-lations of gas, which migrated from the samesources as responsible for the hydrate forma-tion, could be expected below the gas-hydra-te-bearing interval. However, the tomographicresults show no indications for the presence offree gas, taking into account similar velocityand attenuation values observed in the non-hydrated sediments on top of the section, andthe low degree of transverse isotropy. This isalso in agreement with the results from themud gas logging (Wiersberg et al., 2005). Thelack of free gas could be responsible for the

difficulty to detect bottom simulating reflec-tions in the regional seismic data.

The observed characteristic values of seismicvelocity, attenuation, and anisotropy for thehydrate-bearing sediments may provide impor-tant constraints for the microscopic structureand interaction between the grain frame,hydrates and co-existing pore filling. Especially,the association of velocity with attenuation isnot intuitive for porous sediments, but theresults from the crosswell data are also confir-med by the sonic data. Based on a more ten-tative argumentation, both the cementationmodel and the no-contact-cement model(Ecker et al., 1998) partly could explain theobservations, but neither model appears to becompletely consistent. We speculate, that alsothe microporous structures (Kuhs et al., 2000)could be important in terms of petrophysicalmodelling of the internal stucture of hydratebearing sediments.

Figure 4: Two-dimensional images of the seismic P-wave velocity, anisotropy, and attenuationdistribution derived from from the crosswell seismic data. The gas-hydrate-bearing interval is indi-cated by a red box. Lithostratigraphic units and gas hydrate saturation are also shown.

Modelling studies were carried out to investi-gate the possible effects of the gas hydrate dis-sociation on the repeat crosswell data. Fromthis study we conclude that a small dissocia-tion region will introduce only very weak chan-ges in the crosswell seismic wavefields. Themost significant effects occur when the directP-wave is transmitted through the dissociationregion, resulting in a phase shift for these arri-vals. Other effects, such as diffractions andmode conversions from the top and the bot-tom of the anomaly are likely to be masked byother secondary arrivals, and would exhibitvery weak amplitudes as a result of the strongattenuation of the gas hydrates. We calculateddifference data for one example receiver gat-her at 915 m depth. Small phase changes inthe direct P-waves were identified on these dif-ference data. We ascribed these phase chan-ges to real changes in formation properties asa result of the gas hydrate dissociation. Fullwaveform inversion of the differential wave-fields, as carried out by Watanabe et al. (2005)is considered to be the most suitable methodto make use of these observed phase changes,in order to image the dissociated regions.

2.2 Petrophysical properties from laboratory studiesEstimates of the total amount of methanehydrates in the earth's crust are highly specu-lative because detection and quantificationalgorithms for gas hydrate deposits are basedon imprecise empirical observations andassumptions. Quantitative relations betweengas hydrate occurrences and geophysicallyobservable parameters have to be derived fromphysical principles and laboratory measure-ments. Up to now such relations could not beestablished, because testing of natural gashydrate bearing sediments poses experimentalchallenges: Stability conditions have to bemaintained from coring until the laboratorytest as far as possible.

In the frame of the Mallik 2002 gas hydrateproduction research well program, we succee-ded in taking into account the fragility of gashydrates as far as possible and minimizing thetime lapse between core retrieval and labora-tory investigations and the time spent at non-stability conditions.Cores were taken from the Mallik 5L-38 gashydrate research well over the complete gashydrate interval from 890 to 1100 m with awireline coring system. The mud was chilled toabout 0°C, providing gas hydrate stability con-ditions below about 300 m. From core tempe-rature records it was estimated that the gashydrate was outside of the stability zone forabout 20 minutes during retrieval. At the sur-face the cores were rapidly frozen at arcticconditions (less than -30°C), but for someminutes they stayed at conditions of anoma-lous preservation. The loss of gas hydrateduring the coring trip was estimated to be lessthan 10%.

Core sections were quickly put into transportvessels that were pressurized with methane to5 MPa. The deep frozen samples were then atstability conditions for transport to the labora-tory and for storage over a period of hours tosome days. 20 samples were prepared fromthe core sections at arctic conditions, withminimal thermal impact.

A versatile Field Laboratory Experimental CoreAnalysis System (FLECAS) for measurements atcontrolled temperature, confining and porepressure, was developed and built at the GFZin Potsdam for the investigation of gas hydra-tes under simulated in situ conditions (Fig. 5).It was installed at the field laboratory at Inuvik.

The measuring system consists of P- and S-wave transducers, temperature sensors, 6 elec-trodes for resistivity measurements, a lengthsensor, a flow sensor, and pressure transducersfor pore pressure at both ends of the sampleand for the confining pressure.

103

104

A typical test is divided into five parts (Fig. 6):1) The deep frozen (< -20 °C) samples were

placed into the main pressure vessel whichwas previously chilled to less than 10 °C.Then the confining and pore pressure wereincreased to in situ conditions.

2) Within 3-4 hours the temperature wasincreased to in situ conditions. During thisheating period resistivity and sonic veloci-ties decreased gradually. A strong decreaseof the ultrasonic amplitudes was observed,but no significant response from the lengthsensor.

3) After reaching in situ conditions resistivity,ultrasonic P- and S-wave velocities and am-plitudes were recorded.

4) Then we usually triggered gas hydratedecomposition by pore pressure release. Atthe same time, pore water was extractedfrom the sample. It was intended to mea-sure permeability during this period, butthe decomposition process started immedi-ately. This was obvious by the immediate

decrease of the sample length, the recove-ry of ultrasonic amplitudes and the end-othermal cooling of the samples, causing atemperature depression of 1 to 3 °C overabout 20 minutes. A volume of roughly 1-3 l of gas at ambient conditions was pro-duced out of the sample.

5) After gas hydrate decomposition, thesample was again flushed with N2 toextract gas hydrate water. Then N2 wasflowed through the sample at a constantrate to estimate permeability of the gashydrate-free sample. These permeabilityvalues of the host sediments were ratherhigh, in the order of 1 Darcy.

6) An optional procedure was to decomposethe gas hydrate by heating above the stabi-lity zone. Then the decomposition appearsto be less brisk. The reaction of the measu-ring parameters was not as strong, becausethe gas remained in the pore space.

Matrix densities around 2.65 g/cm3 and grainsize distribution (well sorted fine sand to silt)of the sample remnants were determinedafter the experiments. Bulk density, porosity,water saturation were determined, conside-ring volume, sample mass before and afterthe experiment, as well as the gas hydratesaturation that were determined on the pre-paration remnants.

The gas hydrate content and the physical pro-perties of all samples were quite similar, withinsignificant variations. These small variationsreflect that the gas hydrate content and thetype of the host sediment are quite uniform.Thus, an empirical universal relationship bet-ween gas hydrate content and resistivity,respectively ultrasonic parameters, can not bederived from the data set. However, someconclusions for the structure of gas hydratescan be drawn:Melting of the ice in the pores significantlychanges the mechanical and transport para-meters, although the gas hydrate remains stable. The decrease of resistivity and sonicvelocity is caused by ice melting.

Figure 5: Schematic diagram of the internal setup and measuring facilities of FLECAS

Decomposition of the gas hydrate changesthe physical properties less significantly; onlythe mechanical strength is lost completely,causing the strong decrease in length duringdepressurization and a further decrease ofultrasonic velocities.

No significant change in resistivity is observedwhen the sample is heated above the stabilitythreshold. This is because the number of con-ducting ions remains constant although theconducting pore volume becomes larger whenthe gas hydrate is replaced by water and gas.

The data provide a basis for the developmentof petrophysical models which can be used toevaluate the influence of gas hydrate on ultra-sonic and electrical rock properties. These pro-perties are strongly related to the structure andlocation of the gas hydrate in the pore space.Our observations imply that the gas hydrate isfilling larger sediment pores rather than

cementing grains. Otherwise the gas hydratetogether with the host sediment grains wouldbuild a rigid frame that would inhibit strongreactions of the elastic properties on the mel-ting of the ice. The answer of the elastic para-meters on gas hydrate decomposition is muchsmaller than on ice melting. This is more anindication for a loose contact between thesediment grains and the gas hydrate.

A strong decrease of the ultrasonic amplitudesoccurs when the ice is melting. The amplitudesusually recover when the gas hydrate decom-poses. This happens in spite of the partial gassaturation that fills the pores after gas hydratedecomposition – and not so much before.Therefore this absorption effect is a mere gashydrate effect and not primary caused by gasin the pore space. A possible reason is a»shock absorber effect« of the fluids fillingmicroporous structures.

105

Figure 6: Example of a FLECASmeasuring record (sample fromMallik 5L-38, 839 m).

106

2.3 Fiber-optic distributed temperature monitoringThe size and distribution of natural methanehydrate occurrences and the release of gase-ous methane through the dissociation ofmethane hydrate are predominantly control-led by the subsurface pressure and tempera-ture conditions. Because of the related chan-ge in enthalpy, both the formation and dis-sociation of gas hydrate in nature are inevit-ably coupled to the transport of heat withinthe surrounding formation. Knowledge

about the thermal properties of hydrate bea-ring rocks (i.e. thermal conductivity, specificheat, and latent heat of phase transition) istherefore of crucial importance.

Until now, only a very limited amount ofthermal data related to gas hydrate occur-rences exists. Analysis of the geothermalconditions and the derivation of the stabilityfield for methane hydrate are often based onthe interpolation of single, thermally distur-bed bottom-hole temperature measurements

Figure 7: The Mallik 2002 Gas Hydrate Production Research Well Program drilling rig and a schematic cross section of the field experiment. Note the permanent installation of fibre-optic distributed temperature sensing cables behind the borehole casing in the cement annulus.

and drill-stem test data from petroleumexploration wells and/or assumptions aboutthe thermal properties of the formation. Oneaim of the performed borehole temperaturemeasurements was therefore to create adetailed database for the investigation of thegeothermal field in the area of the Mallik gashydrate occurrence and during the field-scaledestabilization of methane hydrate.

Distributed temperature measurements at MallikIn the framework of the Mallik 2002 an inno-vative method for the measurement of conti-nuous temperature profiles in boreholes wasdeveloped and its applicability under extremearctic conditions was proven (Henninges et al.,2005). Three 1180 m deep wells, spaced at 40m, were equipped with permanent fiber-opticsensor cables (Fig. 7). A special feature of theexperiment design is the permanent installa-tion of the sensor cables outside the boreholecasing. After completion of the well, the sen-sor cables are located in the cement annulusbetween casing and borehole wall. The fiber-

optic cables were attached to the outer side ofthe casing at every connector, within intervals ofapprox. 12 m, using custom-built cable clamps.

After the completion of the wells, continuousmonitoring of the well temperatures was per-formed over a period of up to 61 days fromJanuary to March 2002. The DTS logging wasstarted one to two days after completion ofthe respective well. Temperature profiles wererecorded with sampling intervals of 0.25 mand 5 minutes. After completion of this initialobservation period, the surface ends of thefiber-optic sensor cables were stored in specialcontainers at the wellheads to allow for futuretemperature measurements at later times. InOctober 2002 and September 2003 repetitivemeasurements were carried out successfullyduring subsequent field trips to the Mallik site.

Effects of phase transitions and thermal propertiesThe analysis of the disturbed well temperaturesafter drilling revealed a strong effect of phasetransitions on temperature changes (Henninges

107

Figure 8: Temperature profiles (T) and 20-m average temperature gradient (dT/dz) of theMallik 3L-38 observation well for successive times after completion of the well (ts). Thebase of the ice-bearing permafrost (IBPF) and gas hydrate occurrences are respectivelymarked by a sinusoidal change of the temperature gradient, which gradually diminisheswith time. The gamma-response (GR) is affected by the casing (cased-hole log).

108

et al., 2005). For the first time, the effects ofinduced temperature changes within a gashydrate deposit were monitored in-situ. Theresulting temperature gradient anomalies couldbe successfully utilized to determine the base ofthe gas hydrate occurrences and the permafrostlayer at about 1103-1104±3.5 m and 599-604±3.5 m below ground level respectively (Fig.8). The joint interpretation of the geothermaldata and the geophysical well-log data indicatesthat variations of thermal conductivity are main-ly lithologically controlled (Wright et al., 2005).The influence of hydrate saturation is only ofminor significance for the effective thermal con-ductivity of the formation.

Thermal stimulation testThe thermal stimulation test conducted at theJapex/JNOC/GSC Mallik 5L-38 well in March of2002, was designed to increase the in-situtemperature of a well defined and constrainedgas hydrate reservoir above the gas hydratestability point, while maintaining constantpressure. During the thermal stimulationexperiment, the temperature variations alongthe Mallik 5L-38 wellbore were measured(Henninges et al., 2005). DTS logging startedone day after installation of the productioncasing, and continued for a period of 17 days during the entire thermal productiontesting program.The thermal stimulation test was successful inthat the bottomhole temperature was increa-sed and held constant in excess of 50 °C; gasfrom dissociated gas hydrate was produced,sampled, and flared at surface; and significantamounts of real-time downhole temperatureand pressure data, as well as other scientificmeasurements, were obtained (Hancock et al.,2005). Data collected during the thermal sti-mulation test, including surface and downholeinstrumentation readings, as well as advancedlogging and seismic programs, was used tocalibrate numerical gas hydrate reservoir simu-lation models, and determine the kinetic andthermodynamic properties of the in-situ gashydrate (e.g. Moridis et al., 2003).

2.4 On the geochemistry of gases at Mallikfrom real-time gas analysisWithin the Mallik 2002 Gas Hydrate Produc-tion Research Well Program, we have carriedout investigations on the geochemistry ofgases in real-time. Our on-site studies combi-ned 1) on-line mud gas monitoring during dril-ling of the JAPEX/JNOC/GSC et al. Mallik 4L-38and 5L-38 wells, 2) on-line analysis of gas rele-ased during a thermal production test in theMallik 5L-38 well, and 3) decomposition expe-riments on gas-hydrate-bearing drill cores(Wiersberg et al., 2005).

On-line mud gas monitoringThe main objectives of the on-line mud gasmonitoring at Mallik were to identify gas hy-drate-bearing horizons while drilling and todistinguish them from other gas sources suchas gas accumulations below nonpermeablestrata. For this, a gas-water separator was in-stalled directly above the mudflow line toextract the gas phase mechanically out of thereturning drill mud. The extracted gas waspumped into a laboratory trailer and analyzedin real-time with a quadrupole mass spectro-meter, a gas chromatograph, and a radondetector. A complete analysis of Ar, He, N2, O2,CO2, CH4, and H2 within detection limits of lessthan 1ppmv (parts per million by volume) wasachieved after an integration time of 12s. Theon-line mud-gas-monitoring method yields gasdepth profiles for many different gases and is,in contrast to commercial mud-gas logging,not limited to combustible gases.

From the Mallik 2L-38 well it was known thatgas hydrate in Mallik wells consists predomi-nantly of methane gas hydrate (Lorenson etal., 1999; Uchida et al., 1999). It is, however,not possible to identify gas hydrate during dril-ling based only on high methane concentra-tion in the drill mud. Additional gas data arenecessary to identify the source of gas dissol-ved in the mud during drilling. Figure 9 showsmud gas versus depth profiles of Mallik 4L-38well for the methane concentration, the heli-um concentration, and the 222Rn activity. The

principle nonatmospheric gas found in the drillmud was methane, with up to 70vol.% in themain gas hydrate section at Mallik, however,even at shallow depth the methane concentra-tion reached up to 35vol.% in some cases.Elevated methane concentration were obser-ved at about 107m, 646m, 766m, and 827mdepths and in ten distinct layers within aninterval at 890–1150m depth that encompas-ses the main gas-hydrate-bearing section(890–1110m). Some, but not all are associatedwith gas-hydrate-bearing strata. In contrast tonon-hydrate bearing strata a decrease ofhelium with increasing methane concentrationhas been observed while drilling throughhydrate intervals. Subsequently, they wereidentified by geophysical logging after drilling.For example, mud gas from the depth interval766–779 m showed increasing helium concen-tration at increasing methane content. Indeed,geophysical logging yields no evidence for gashydrate occurrence in this section. Therefore,the combination of high methane concentra-tions with low helium concentration in themud gas can be used as diagnostic tool forreal-time identification of gas hydrate. Thehelium concentration in natural gas hydrate is

very low, because helium is too small to beaccommodated in the hydrate lattice atambient pressure and temperature conditions.

On-line analysis of gas released during athermal production test During the Mallik 5L-38 gas hydrate produc-tion research well thermal test program(Takahashi et al., 2003) gases were also analy-sed in real-time. A gas-hydrate-rich depthinterval from 907–920m was selected for ther-mal stimulation. The 13m interval of the casedhole was perforated, then hot brine was injec-ted for about 124 hours through a pipe downto the base of the target zone. The hot brinedecomposed the gas hydrate, resulting in afluid flow through the perforation and upliftbetween the inlet string and the well tubing.At the surface, the gas was extracted from thewater phase with a two-stage oilfield gas-water separator. The brine was reheated andreinjected. A portion of the separated gas waspiped out to the laboratory trailer for on-linegas analysis. At 2.15 hours after circulationstarted (March 5th, 2002, 20:24h), a firstmethane response was detected. During thefollowing 6 hours, the methane concentration

109

Figure 9: Mallik 4L-38 gas versusdepth profiles for CH4 (vol.%), He(ppmv) and 222Rn (Bq/m3). Depthbelow kelly bushing (4.6 m).

110

increased to about 90vol.%. The variation inmethane concentration was relatively small±2%) after reaching maximal values, whereassignificant variations in the concentrations ofother hydrocarbons, He, and CO2, were detec-ted, sometimes on a relatively short time scale.CO2 concentrations ranged from <0.01vol.%to 1 vol.%, whereas the He concentration variedbetween 5ppmv and 7ppmv. Values ofCH4/(C2H6 +C3H8) ranged between less than100 and 2500, and sometimes changed rapid-ly within one hour.The variations in CO2 correlate positively withthe temperature of the circulating brine (Fig.10), which is due to the temperature-depen-dent solubility of CO2 in the brine. Generally,the CO2 solubility decreases with increasingtemperature, leading to higher CO2 concentra-tion in the corresponding gas phase. The tem-perature dependance of the solubility of othergases in brines is distinctly smaller, therefore,the distribution coefficient between gas andwater phase for other gases is less sensitive fortemperature variations. As shown by the mud-

gas data of Mallik 4L-38, gas hydrate is cha-racterised by elevated CH4 and CH4/He ratios.This was confirmed by data from the thermalproduction test, particularly during the secondhalf of the test, where a positive correlationbetween CH4 /He as well as CH4/(C2H6+C3H8)was indicated (Fig. 11). Gases released fromdifferent depths within the perforated intervalmight explain the heterogeneity in the gascomposition during the thermal stimulation.While drilling of the Mallik 4L-38 well an incre-ase of the CH4/He ratio was detected throug-hout the interval 907–920m.

Decomposition experiments on gas-hydrate-bearing drill coresData on the composition of gas-hydrate-deri-ved gas were obtained in the field from gashydrate decomposition of gas-hydrate-bearingsediment core samples from the depth interval891–923m. These experiments also yield infor-mation about the gas-water-solid properties,and physical conditions of gas hydrate decom-position. For this, 20–30g of gas-hydrate-bea-

Figure 10: Carbon dioxide concentration and fluid temperature ver-sus time after start of the thermal production test at Mallik 5L-38well. Fluid temperature data after Hancock et al. (2005).

ring core material was deposited in an alumi-nium sample container and placed into aTeflon vessel equipped with a thermocouple, apressure sensor, and a connector to the gasmass spectrometer. After loading, the vesselwas quickly closed and cooled with liquidnitrogen to minimise uncontrolled gas hydratedecomposition. The temperature in the vesselincreased gradually after the liquid nitrogenwas completely evaporised. At approximately130K, the vessel was evacuated to remove air.Between about 155–160K, beginning of gashydrate decomposition under pressure condi-tions less than 100 mbar was indicated by afast increase of pressure and methane concen-tration. After the temperature in the vesselexceeded 283K, a gas sample was taken, thenthe vessel was evacuated, opened, and thegas-free sample mass was determined. Afterremoving water from the residual wet sand,the dry sand was weighed.

All investigated hydrate bearing core sampleshad a relatively constant gas composition. Theprinciple gas found in the samples was metha-

ne (>99 vol.%). Minor amounts of heavierhydrocarbons, released at less than 293K, canbe explained by either the presence of structu-re II gas hydrate or heavier hydrocarbons trap-ped in structure I in trace quantities. The waterto solid matter ratio is relatively constant for allsamples, while amount of gas is more variable.The latter might be a result of gas got lostduring core recovery and handling. The begin-ning of gas hydrate decomposition is indicatedby a significant increase of the pressure andthe methane concentration at a temperatureof approximately 160K within a pressure rangeof 40–70mbar.

2.5 Mallik 2002 Data and Information SystemThe »Mallik Data and Information System«was developed in cooperation with the ICDPOperational Support Group and the GeoFor-schungsZentrum Potsdam (GFZ) Data Centre. Itwas based on concepts and tools of the ICDPInformation Network and adapted to two con-ditions specific to the Mallik project. First, thecore required special handling to preserve fea-tures relevant to formation and stability of gas

111

Figure 11: CH4 / (C2H6+C3H8) (red) and CH4/He (x10-4)(black) versus time after the start of the thermal produc-tion test at Mallik 5L-38 well.

112

hydrate in the sediment. Second, the Mallikscience team was divided into three scientificsubgroups and a group of industrial partners.Access rights had to be defined for each scien-ce team member according to the confidentia-lity rules for each of the groups (Loewner R.and Conze R., 2005).

The Mallik-DIS database structureThe database structure for the Mallik DrillingInformation System (Mallik-DIS) was designed inthe fall of 2001 in preparation for integration ofparameters acquired at the drill site and at theIRC field lab with results from participating labs. The starting-point was the basic DIS - an elec-tronic toolbox to develop on-site informationsystems for scientific drilling projects. Its cen-tral part is a set of data templates typical forscientific drilling purposes deriving from anumber of earlier ICDP projects. These templa-tes were adapted according to specificdemands of the Mallik project, e.g. the greatervariety of sampling types. New data structureswere added, for example, for the first core des-cription made in the field at the JAPEX/JNOC/

GSC et al. Mallik 5L-38 production well. Basedon the generated data model, input forms andprint reports were built automatically using theDIS Graphical-User-Interface-Builder (GUI-Builder). The Data-Pump-Builder was used tocreate data import facilities for borehole mea-surements and the online gas monitoring data.The Web-Info-Builder was used to prepare andto provide project information updates on theInternet day-by-day.

Data Acquisition and DisseminationA small local area network was set up at theInuvik Research Centre (IRC). It included theDIS server machine, one client and the ICDPcore scanner unit (Fig. 12). The Mallik-DIS wasinstalled to acquire data and scan coreswithout any loss of time, close to the drill rigand the IRC core handling operations. Primarydata were captured in February 2002 duringthe coring period of the production well(Mallik 5L-38). This dataset included informa-tion from the drill site: the initial description ofthe lithology of the recovered core from the 48core runs, drilling notes for each core run and

Figure 12: Internal network structure used at the Inuvik Research Centre.

a record of samples taken at the drill rig forpressure and/or temperature sensitive tests.Subsequently, the core was documented as itarrived at the IRC in metre lengths contained inplastic or aluminum liners. High-resolutionimages of the split surfaces of the frozen andunfrozen drill cores from each of the recovered210 liners were obtained using the ICDP corescanner. The core scanner unit manufacturedfor ICDP by DMT (Deutsche Montan Techno-logie, Essen, Germany) is capable of scanningcores ranging from 1 to 15 cm diameter up toa maximum length of one metre. Through aspecial DIS interface the resulting images wereconverted to jpg-files and stored together withthe corresponding metadata in the Mallik-DIS.

Each day, a Web Info update of all data, reports,and images generated within the last 24 hourswas produced and sent to the Mallik Web site inthe ICDP Information Network via file transferprotocol (FTP). Thereby, it was possible to provi-de worldwide information access for the partici-pating scientists on the progress of the drilling innear real-time during the active operationalphase under restricted data access.

After the drilling period ended in mid-March2002, the third phase, quality control, dataintegration, modification of access rules andacquisition of secondary data and resultsbegan. The data sets collected, prepared andincorporated into the Mallik data warehouseduring the pre-publication phase of the projectamounted to several megabytes.

Users may access and integrate lithological des-criptions, all kinds of borehole geophysical mea-surements, monitoring data and an archive of allthe core runs and samples. Users may composedata profiles according to specific interests (Fig.13). The integration of multidisciplinary data setsenables individual dynamic visualization andcomparison of different kinds of information. Developing these data acquisition andmanagement systems »on-the-job« is usual fordata management of ICDP scientific drillingprojects, because some features can only bedetermined under real on-site conditions.Therefore, the underlying data model of theMallik-DIS was adapted several times due toparticular new or changing requirements, suchas the mapping of the ICDP and GeologicalSurvey of Canada (GSC) naming conventions

113

Figure 13: Examples from the Mallik Data Warehouse:detail of the lithological profile of Mallik 5L-38.

114

for samples or depths correlation. The DIS tool-box provided the necessary means for short-term modifications of the operational databa-se model, or the user interfaces, e.g. inputforms and print reports.

The importance of the core archive and theinitial lithological description of the cores areapparent in the production of a stratigraphiccolumn, which was a very effective and usefulbasic measure of progress for the Mallik pro-ject. As it was used as a reference system forall subsequent parameters measured on thecores or downhole data obtained at the drillrig, it immediately enabled the detection of,for instance, errors in depth assignment orliner interpolation. The GFZ Mallik workinggroup used the following entities for discus-sions, talks and publications during the evalu-ation phase:

- correlation of sedimentary features with geo-physical and geochemical measurements,

- correlation between the technical parameters,the logging data, and the gas parameters

- comparison of the core scans and the litho-logical data for sub-sampling,

- correlation of sedimentary characteristicswith the gas hydrate content.

Important issues for the Mallik project werethe complexity of the science teams and thecorresponding confidentiality rules for each ofthe groups. Especially in the starting phase,there were some uncertainties concerning col-laboration and access security. This showedhow important it is to negotiate, communicate,and promote the appropriate rights and dutiesas early as possible. The standard policies usedfor ICDP scientific drilling projects can be seen athttp://www.icdp-online.de/about.

3. SummaryThe achievements of the Mallik 2005 GasHydrate Production Research Well Programnear Inuvik in the Northwest Territorries,Canada, were the first controlled in-situ desta-bilization tests of terrestrial gas hydrates,

accompanied by a challenging experimentalprogram. The research program was conduc-ted by an international collaboration, includingthe GeoForschungsZentrum Potsdam. Twoobservation wells and one production wellwere drilled in horizontally aligned configura-tion. The observation wells served for tempe-rature monitoring and seismic measurementsduring the thermal stimulation test in the pro-duction well.The GeoForschungsZentrum took part in allmajor subjects of the research program:Drilling, logging, and core analysis data fromall partners were collected and made globallyavailable to participants in the Mallik DrillingInformation System. The system had to makeallowances for the scientific complexity of theproject, its internationality, and special confi-dentiality rules.A geochemical gas analysis was conductedduring the drilling and production test phase. Inthe drilling phase, the specialized mud gas ana-lysis allowed to detect gas hydrate layers and todetermine the origin of the gas. During thethermal destabilisation test, the monitoring ofthe composition produced gas compositiongave insights into the processes during destabi-lisation and transport of the gas to the surface.All three wells were equipped with distributedtemperature sensors that served for monito-ring the temperature field during the gashydrate destabilisation test and a long equili-bration period thereafter. Data collectedduring the thermal stimulation test, includingsurface and downhole instrumentation rea-dings, as well as advanced logging and seismicprograms, was used to calibrate numerical gashydrate reservoir simulation models, anddetermine the kinetic and thermodynamic pro-perties of the in-situ gas hydrate.Similar experiments as in situ have been con-ducted on gas hydrate bearing cores from theprocuction well in the field laboratory. Coreanalysis data provide a basis for the develop-ment of petrophysical models, which can beused for calibration of simulation models andformation evaluation with well logging andgeophysical field methods.

The direct imaging of the crosshole sectionbetween the observation wells with seismicmethods gave insights into the structure of thegas hydrate occurrences and their physical pro-perties. The impact of the gas hydrate destabi-lisation experiment on seismic data was detec-table. Nevertheless, the spatial extent of theinduced changes at the production well wastoo small for a quantitative evaluation.The »Mallik Data and Information System« pro-vided a database for most project data-typesand its secure and restricted distribution duringthe pre-publishing phase. It operated as a com-munication platform between the project mem-bers through the Web portal of the ICDP In-formation Network (http://www.icdp-online.de).

ReferencesBauer, K., Pratt, R.G., Weber, M.H., Ryberg, T.,Haberland, C., and Shimizu, S., 2005a: Mallik2002 cross-well seismic experiment: projectdesign, data acquisition, and modelling studies;in Scientific Results from the Mallik 2002 GasHydrate Production Research Well Program,Mackenzie Delta, Northwest Territories, Canada,(ed.) S.R. Dallimore and T.S. Collett; GeologicalSurvey of Canada, Bulletin 585, 14 p.

Bauer, K., Haberland, C., Pratt, R.G., Hou, F.,Medioli, B.E., and Weber, M.H., 2005b: Ray-based tomography for P-wave velocity, aniso-tropy, and attenuation structure around theJAPEX/JNOC/ GSC et al. Mallik 5L-38 gashydrate production research well; in ScientificResults from the Mallik 2002 Gas HydrateProduction Research Well Program, Macken-zie Delta, Northwest Territories, Canada, (ed.)S.R. Dallimore and T.S. Collett; GeologicalSurvey of Canada, Bulletin 585, 21 p.

Bily, C., and Dick, J.W.L., 1974: Natural occur-ring gas hydrates in the Mackenzie Delta,Northwest Territories; Bulletin of CanadianPetroleum Geology, v. 22, no. 3, 340-352.

Collett, T.S., Lewis, R., Dallimore, S.R., Lee,M.W., Mroz, T.H., and Uchida, T., 1999a:

Detailed evaluation of gas hydrate reservoirproperties using JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well down hole well-log displays; in Scentific results from JAPEX/JNOC/GSC Mallik 2L-38 Gas Hydrate ResearchWell, Mackenzie Delta, Northwest Territories,Canada, (ed.) S.R. Dallimore, T. Uchida, andT.S. Collett, 295-312.

Collett, T.S., Lee, M.W., Dallimore, S.R., andAgenda, W.F., 1999b: Seismic and well-log-inferred gas hydrate accumulations on RichardIsland; in Scentific results from JAPEX/JNOC/GSC Mallik 2L-38 Gas Hydrate Research Well,Mackenzie Delta, Northwest Territories,Canada, (ed.) S.R. Dallimore, T. Uchida, andT.S. Collett, 357-376.

Dallimore, S.R., Uchida, T., and Collett, T.S.,eds., 1999: Scientific results from JAPEX/JNOC/GSC Mallik 2L-38 Gas Hydrate Research Well,Mackenzie Delta, Northwest Territories,Canada; Geological Survey of CanadaBulletin 544, 403 p.

Dallimore S.R., Collett, T.S., Weber, M.H., andUchida, T. , 2002: Drilling program investigatespermafrost gas hydrates; EOS, v. 83, p. 193/198.

Ecker, Ch., Dvorkin, J., and Nur, A., 1998 : Sedi-ments with gas hydrates: Internal structure fromseismic AVO; Geophysics, v. 63, p. 1659-1669.

Hancock, S., Collett, T.S., Dallimore, S.R., Satoh,T., Huenges, E., and Henninges, J., 2005:Overview of thermal stimulation production testresults for the Japex/JNOC/GSC Mallik 5L-38Gas Hydrate Research Well; in Scientific Resultsfrom the Mallik 2002 Gas Hydrate ProductionResearch Well Program, Mackenzie Delta, North-west Territories, Canada, (ed.) S.R. Dallimore andT.S. Collett; Geological Survey of Canada,Bulletin 585, 15 p.

Henninges, J., Schrötter, J., Erbas, K., andHuenges, E., 2005: Temperature field of theMallik gas hydrate occurrence – implications onphase changes and thermal properties; in

115

116

Scientific Results from the Mallik 2002 GasHydrate Production Research Well Program,Mackenzie Delta, Northwest Territories, Canada,(ed.) S.R. Dallimore and T.S. Collett; GeologicalSurvey of Canada, Bulletin 585, 14 p.

Kuhs, W.F., Klapproth, A., Gotthardt, F.,Techmer, K., and Heinrichs, T., 2000: The for-mation of meso- and macroporous gas hydra-tes; Geophysical Research Letters, v. 27, p.2929-2932.

Kulenkampff, J., Spangenberg, E., 2005: Physi-cal properties of cores from the Mallik 5L-38 gashydrate production research well under simula-ted in situ conditions using the Field LaboratoryExperimental Core Analysis System (FLECAS); inScientific Results from the Mallik 2002 GasHydrate Production Research Well Program,Mackenzie Delta, Northwest Territories, Cana-da, (ed.) S.R. Dallimore and T.S. Collett; Geo-logical Survey of Canada, Bulletin 585, 16 p.

Loewner, R., and Conze, R., 2005: The MallikData and Information System - Development ofa Scientific Data Exchange Platform; in ScientificResults from the Mallik 2002 Gas HydrateProduction Research Well Program, MackenzieDelta, Northwest Territories, Canada, (ed.) S.R.Dallimore and T.S. Collett; Geological Survey ofCanada, Bulletin 585, 9 p.

Lorenson, T.D., Whiticar, M.J., Waseda, A.,Dallimore, S.R., and Collett, T.S., 1999: Gas com-position and isotopic geochemistry of cuttings,core, and gas hydrate from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well; inScientific Results from the JAPEX/JNOC/GSCMallik 2L-38Gas Hydrate ResearchWell,MackenzieDelta,Northwest Territories,Canada,(ed.) S.R.Dallimore, T. Uchida, and T.S. Col-lett;Geological Survey of Canada, Bulletin 544,p. 143–163.

Moridis, G.J., Seol, Y., Collett, T.S., Dallimore,S.R., Inoue, T., Mroz, T.H., and Henninges, J.,2003: Thermal properties of hydrates from tem-perature data analysis of an isolated formation

interval in the 5L-38 Mallik Research Well; MallikInternational Symposium (Chiba, Japan),Program & Abstracts, Japan National Oil Cor-poration, Technology Research Center, p. 58.

Pratt, R.G., Hou, F., Bauer, K., and Weber, M.H.,2005: Waveform tomography images of veloci-ty and inelastic attenuation from the Mallik2002 crosshole seismic surveys; in ScientificResults from the Mallik 2002 Gas HydrateProduction Research Well Program, MackenzieDelta, Northwest Territories, Canada, (ed.) S.R.Dallimore and T.S. Collett; Geological Survey ofCanada, Bulletin 585, 14 p.

Riedel, M., Kulenkampff, J., Spangenberg, E.,and Dallimore, S.R., 2005: GeophysicalProperties of Sediments from Mallik 5L-38; inScientific Results from the Mallik 2002 GasHydrate Production Research Well Program,Mackenzie Delta, Northwest Territories, Cana-da, (ed.) S.R. Dallimore and T.S. Collett; Geo-logical Survey of Canada, Bulletin 585, 10 p.

Takahashi, H., Yanezawa, T., and Fercho, E.,2003: Operation overview of the 2002 MallikGas Hydrate Production Research WellProgram at the Mackenzie Delta in theCanadian Arctic; Offshore Technology Confe-rence, May 2003, Houston, Texas, 10 p.

Uchida, T., Matsumoto, R., Waseda, A., Okui,T., Yamada, K., Uchida, T., Okada, S., andTakano, O., 1999: Summary of physicochemi-cal properties of natural gas hydrate and asso-ciated gas hydrate-bearing sediments, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate researchwell, by the Japanese research consortium; inScientific Results from the JAPEX/JNOC/GSCMallik 2L-38 Gas Hydrate Research Well,Mackenzie Delta, Northwest Territories, Cana-da, (ed.) S.R. Dallimore, T. Uchida, and T.S.Collett; Geological Survey of Canada, Bulletin544, p. 205–228.

Watanabe, T., Shimizu, S., Asakawa, E., Kamei,R., and Matsuoka, T., 2005: Preliminary assess-ment of the waveform inversion method for

interpretation of crosswell seismic data from thethermal production test, JAPEX/JNOC/GSC et al.Mallik 5L-38 gas hydrate production researchwell; in Scientific Results from the Mallik 2002Gas Hydrate Production Research Well Program,Mackenzie Delta, Northwest Territories, Canada,(ed.) S.R. Dallimore and T.S. Collett; GeologicalSurvey of Canada, Bulletin 585, 14 p.

Wiersberg, T., Erzinger, J., Zimmer,M., Schicks,J., and Dahms, E., 2005: Real-time gas analysisat the JAPEX/JNOC/GSC et al. Mallik 5L-38 gashydrate production research well; in ScientificResults from the Mallik 2002 Gas HydrateProduction Research Well Program, MackenzieDelta, Northwest Territories, Canada, (ed.) S.R.Dallimore and T.S. Collett; Geological Survey ofCanada, Bulletin 585, 15 p.

Wright, J.F., Nixon, F.M., Dallimore, S.R.,Henninges, J., and Côté, M.M., 2005: Thermalconductivity of sediments within the gas-hydra-te-bearing interval at JAPEX/JNOC/GSC et al.Mallik 5L-38, Mackenzie Delta, Canada; inScientific Results from the Mallik 2002 GasHydrate Production Research Well Program,Mackenzie Delta, Northwest Territories, Cana-da, (ed.) S.R. Dallimore and T.S. Collett; Geo-logical Survey of Canada, Bulletin 585, 10 p.

117

118

Gas hydrate induced submarine slides - Anengineering geological approach

IntroductionSubmarine slides have been considered as amajor geohazard. They can be easily triggeredby dissociating of gas hydrates. Specificallywith regard to their impact on the release oftsunamis as well as on submarine engineeringstructures submarine slides are a highly rele-vant research topic for geotechnical engineersand marine geoscientists alike. Despite this,our knowledge about the triggering processesinvolved and the mechanical parameters con-trolling slope stability in gas hydrate bearingsediments is still very limited. To address someof the fundamental scientific and engineeringquestions in this field, researchers of theTechnical University Berlin and the UniversityKiel worked together closely within the frame-work of the research project GASSTAB: »SlopeStability and Land Slides in the Deep Sea:Influence Parameter Gas Hydrates«. Three sub-projects, »Sediment Dynamics« (Technical Uni-versity Berlin, WUM), »Sediment Mechanics«(University Kiel) and »Soil Dynamics« (TechnicalUniversity Berlin), dealt with gas hydrate indu-ced failure mechanisms of submarine slidesfrom an engineering geological point of view.

Objectives of the Sediment Dynamics sub-projectSince real laboratory tests with gas hydratesare generally cost intensive and time consu-ming a virtual laboratory was created parallelto a real test system GTS (Gashydrate Test

System) developed and built in the frame ofthe sub-project Sediment Mechanics. As anumerical tool of the virtual laboratory thedistinct element method (DEM) was employedusing the commercial software Particle FlowCode (PFC2D) from ITASCA.

Up to now the formation of gas hydrate lensescould not be observed neither in soil speci-mens nor in computer simulations. The maintask of this study was to simulate the forma-tion and the decomposition of hydrates inmarine sediments and to perform different vir-tual soil mechanical consolidation tests tostudy the mechanical reaction of the sedimentunder conditions of geostatic stresses duringgas hydrate formation.

Simulation of the formation of gas hydrate crystalsThe distinct element model was used as a basefor the simulation (Cundall 1979, ITASCA2002) because this method is the only onewhich allows the simulation of the sedimentbehavior on the grain scale, large enough tomeasure the bulk properties of such a virtualsediment. In contrast to the classical finite dif-ference or finite element simulation this simu-lation is very stable and allows large arbitrarydisplacements. The growing gas hydrate cry-stals could be simulated, and the reaction andthe properties of the sediment during hydrategrowth where measured in virtual equivalentsto classical geotechnical experiments.

Grupe B. (1), Kreiter S. (1), Feeser V. (2), Hoffmann K. (2), Becker H.J. (2), Savidis S. (3),

Rackwitz F. (3), Schupp J. (3)

(1) TU Berlin, Arbeitsgebiet WUM, Müller-Breslau-Straße/Schleuseninsel, 10623 Berlin, E-Mail: grupe@vws.tu-berlin.de

(2) Christian-Albrechts-Universität zu Kiel, Institut für Geowissenschaften, Ludewig-Meyn-Straße 10, 24118 Kiel

(3) TU Berlin, Institut für Bauingenieurwesen, Gustav-Meyer-Allee 25, 13355 Berlin

For the simulation of the growing gas hydratecrystals the focus was laid on nucleated cry-stals which have a surface energy and exertsurface forces. The very base of surface forcesis the physical interaction between multiplemolecules. The behavior of surfaces has beensuccessfully modeled by molecular dynamics(MD) simulations. The approach chosen in thissimulation was to simplify the MD models andto scale them up to the size of sediment pores.Therefore an appropriate interaction betweengas hydrate simulating balls was implemented,where the Mie potential was chosen for theattractive part and the repulsion was simulatedby a standard spring potential. The interactionpotential and the resulting force is shown inFigure 1. As can be seen in this figure the gro-wing crystal starts as one disk (PFC ball) butthis one disk is then divided into several disksdepending on the forces acting on it.

The main focus in this part of the project wasthe implementation of gas hydrate growth intothe numerical model. Therefore the chosen vir-tual sediment used in most of the experiments

consists of simple disks with the standardspring contact model and a standard friction.A material composed of such distinct elementsshows characteristics like a cohesionless gra-nular sediment.

First experiments were made to check theinfluence of the surface tension on the gashydrate crystals growth within the sediment.Figure 2 demonstrates the situation after thegas hydrate growth with growing surface ten-sion in two sediments with a different grainsize; the geometry on the left hand side is fivetimes smaller than on the right. While thehydrate crystal in the fine grained sedimentkeeps its round shape and pushes away thesurrounding sediment grains the crystal in thecoarse grained sediment intrudes into theedges of the pores and does not push the sedi-ment grains away until the connected porespace is completely filled up. This experimentillustrates the influence of surface tensionwhen gas hydrate interacts with the sedimentgrains.

119

Figure 1: Potential and force over the distance of the two centres of the in¬teractingcircles (balls) from 10 % overlap to the double of the contact distance

120

Gas hydrate growth during consolidationTo get a first impression of the influence of gashydrate on the state and the history of a sedi-ment sample, the interaction of sediment andgas hydrate growth was simulated. The gas hy-drate was growing before consolidation in a firstexperiment, and the gas hydrate was growingafter consolidation in a second experiment.

Figure 3 shows one result of the experiment:The deviatoric stress is plotted over the effecti-ve isotropic stress. The deviatoric stress of thefirst experiment from the normal consolidationis reversed by the growth of gas hydrate. In thesecond experiment the gas hydrate hinders thedeviatoric strain to build up normally. Thismeans that the stress state of gas hydrate bea-ring sediment is dependent on the history ofload application and the history of gas hydrategrowth in the sediment.

Simulation of surface near gas hydrate decayMost of the gas hydrate research is done onsurface near gas hydrates, because they areaccessible with standard research ships andbecause they are related to interesting structu-res like mud volcanoes or active faults. In such

active environments methane bubbles escapeinto the free water probably being releasedfrom gas hydrate by the warmth of an ascen-ding fluid (Wood 2002). A melting procedurewas developed to simulate the surface neardecay of gas hydrate to methane. Figure 4shows the virtual sediment during the processof hydrate decay and the resulting methanebubbles in a later stage of the experiment. Inthe later stage the methane bubbles are movingthrough the sediment and some have escapedin the meantime.

Sedimentological workFor the design of the virtual and substitute sedi-ment a study of gas hydrate host sediment fromthe Blake Ridge and the Costa Rica margin wasconducted. The microstructure, the clay minera-logy and the grain size were examined. Thesamples had been taken in regions where gashydrates were found. The results do not matchwith the study of Ginsburg (2000), who propo-sed coarser grain size as the key parameter allo-wing gas hydrate to grow. The grain size data(Fig. 5) shows coarser grain size only in theupper 100 m of the core, which correlates withthe onset of the Pleistocene glaciations.

Figure 2: Hydrate growth in fine and coarse grained sediment. Darkgray balls are hydrate, light gray balls are sediment particles. Lines inhydrate balls mark the direction of the applied force. Scale has thesame absolute length on the left hand side and the right hand side.

121

Figure 3 : Deviatoric stress over isotropic stressduring gas hydrate growth and consolidation

Figure 4: Simulation of gas hydrate decay. Light gray balls are methane, medium gray balls sediment, and dark grayballs gas hydrate. Left: early stage, right: later stage. Ambient pressure equivalents 1,000 m water depth.

SEM (Scanning Electron Microscope) studiesfound no further evidence for the assumptionthat siliceous shells provide sheltered space inwhich nucleation of gas hydrates occur prefe-rentially (Kraemer et al. 2000). As a conclusionthere were no parameters which indicate apreference for the growth of gas hydrate.Therefore it is supposed, that as the nucleationof a single gas hydrate crystal is a random pro-cess the success of the nucleation is onlydependent on the conditions in one particular

pore. SEM studies of the Blake Ridge sedimentshowed strong variability of the microstructurein mm distance in various samples, suggestingthat the possibility of hydrate nucleation cannot easily be determined from bulk sedimentproperties.

ConclusionsThe experiments have shown that the simula-tion of surface tension in the DEM is possiblewith methods of the molecular dynamic simu-

lation. The influence of gas hydrate growth onthe sediment state was modeled as well as theescape of methane bubbles into the water. It ispossible to extend the application of surfacetensed ball ensembles to other processes likethe freezing of soil or other processes in poly-phase mixtures.

Experimental data is needed to validate the waythe gas hydrate related processes are simulated.Calibrated sediment-hydrate ensemble can thenbe used for other numerical experiments, whichmeasure the shear strength of the sedimentlike biaxial tests.

Objectives of the Sediment Mechanics sub-projectQuality, plausibility, and transferability ofnumerical stability calculations of gas hydratebearing deep sea slopes are highly dependingon the significance of the input parameters

and constitutive laws describing the strengthand deformability of the sediments. Strengthand deformability are basically controlled bythe stress history that the sediments haveundergone as well as the stresses within thegrain skeleton and the pore pressure to whichthe sediments are currently subjected.

Although more and more international wor-king groups are recently engaged in mechani-cal gas hydrate research (Yang et al. 2003,Winters et al. 2004), soil mechanical parame-ters and constitutive laws of gas hydrate bea-ring sediments are widely undefined theoreti-cally as well as experimentally (Sloan 2002).But still more seriously is the fact that no infor-mation is available concerning the basic con-trolling mechanism of the mechanical soilbehaviour, i.e. the stress history and the porepressure regime of marine sediments whichfollow gas hydrate formation and decay.

122

Figure 5: Grain size datafrom the Blake Ridge.Circles indicate the grainsize of the log-normalpeak of the grain popula-tion. Circle size shows thewidth of the log-normaldistributions. Blue colorindicates evidence of gashydrate in close proximityto the sample.

To address this lack of knowledge, the mainobjective of the Sediment Mechanics sub-pro-ject was to provide a basis for understandingand quantifying the stress history, stress andpressure interaction of grain skeleton, water,gas and hydrate in sediments during formationand decomposition of gas hydrates.

Gas Hydrate Test SystemCommon soil mechanical standard experi-ments and test procedures could not respondthe conceptual formulation. As a consequenceof the experimental lack, GTS (Gas hydrate TestSystem) has been designed, constructed andinstalled within the GASSTAB research project.Thereby the following questions took centrestage of the design engineering:

i. Which mechanical reactions of the sediment(anisotropy of the in-situ stress condition,water and gas pressure regime, state of(over/under) consolidation, changing ofpore space and stiffness) will follow thegrowth and decay of gas hydrates?

ii. Which role does the factor time play in thedecomposition process as well as the static /dynamic stress condition with respect tosediment structural changes, i.e. collapse(micro seismic excitation, over/under conso-

lidation, increase and decrease of porewater and/or gas pressures, anisotropy ofthe residual stress conditions)?

Experiments to be carried out for answeringthese questions have to keep strict thermody-namic and soil mechanical boundary condi-tions as well as marine deep sea conditions.GTS conforms to these stringent require-ments. It enables the generation and decom-position of gas hydrates under real marineconditions and simultaneously allows to mea-sure the geostatic stress-strain-pressure beha-viour of the sediment.

The experimental set-up consists of a 300 kNloading frame (Fig. 6). In order to simulatemicro seismic events handling of cyclic forcesup to 10 Hz is feasible. Geostatic experimentscould be carried out with an oedometric high-pressure consolidation cell (Fig. 7) which has apressure capacity up to 200 bars, and a tem-perature range from 263 to 303 K. Integratedarrays of transducers enable the system tomeasure total vertical and lateral stresses, axialstrain, pore water, and pore gas pressure sepa-rately, as well as temperature at different loca-tions within the soil sample. Pore water pres-sure and temperature equivalent to sub-seab-ed conditions in which gas hydrates occur are

123

Figure 6: Gas hydrate Test SystemGTS. General view of theexperimental set up.

124

generated using a hydrostatic pressure systemand a heat-exchange system respectively. Theflow of gas charged water or pure gas is con-trolled and monitored (Feeser et al. 2003).

Pilot researchIn the run-up to the design of GTS a varietyof pilot tests was carried out in order toimprove and optimize the engineering layoutof the test system.

The literature notes that a significant memoryeffect has been observed during the repeatedformation of hydrates (Parent, Bishnoi 1996).This means, if hydrates are dissociated andthen reformed within hours, nucleation occursat far less pressure. Initial nucleation pressuresof about 4 MN/m2 are typical for methanehydrates whereas if methane hydrate is formedagain using the same water the pressure toinitiate hydrate drops to about 2 MN/m2. Thiseffect is traced back to substantial energysavings from initial hydrate formation equili-brium. Our investigations confirm these obser-vations, even using THF (Tetrahydrofurane) asguest molecules where the hydrate formationsolely occurs under atmospheric pressure con-ditions. Further tests under overpressure werecarried out using propane. Both, THF and pro-pane results show an explicit interrelationshipbetween the kinetics of hydrate formation and

the history of water. The tests have circumst-antiated that meta-stable clusters of wateraffect the hydrate nucleation catalytically. Thisinterpretation is consistent with neutron dif-fraction analyses of water clusters in water/gassolutions (Koh et al. 2000). Hence the memo-ry effect could not be originated physically butchemically. This insight has resulted in a sub-stantial change of the original design and pro-cess engineering of GTS. Now, a system of pre-reactors provides clustered, gas charged waterwhich is discharged into the sediment underconditions of deep sea water pressure. Thenucleation of hydrate is subsequently triggeredby lowering of the temperature. No additionalsupplying of high-pressured gas is necessary.With the implemented process technology gashydrates can be reproducibly formed in thelaboratory under natural marine soil stress andwater pressure conditions.

Planning the sensor details of GTS we initiallyassumed a mechanical analogy between iceand hydrate bearing sediments. Pilot testswere focussed on K0-stress-strain, creep andyielding behaviour (Feeser, Hoffmann 2004).The results show that the mechanical beha-viour of frozen sediments cannot be transfer-red to gas hydrate bearing sediments (Fig.8and 9). So the multitude of literature datareferring to the mechanical behaviour of fro-

Figure 7: Gas hydrate Test SystemGTS. Oedometric high-pressure consolidation cell.

125

Figure 8: Mechanical beha-viour of sand, sand ice,and sand THF-hydratecompounds. K0-stress pathsin the principle stress field.Successive loading, creep,unloading, and creep.

Figure 9: Mechanical behaviour of sand ice and sand THF-hydrate compounds.Visco-plastic yielding. Derived from oedometric loading under constant verticalstress σ1 = 4.0 MNm-2.

126

zen sediments could not be involved into thefinal sensor dimensioning of GTS. Moreover, itremains questionable whether the obtainedbehaviour of THF-hydrate is transferable tomethane hydrate. The pilot tests have revea-led a considerable gap of knowledge concer-ning the mechanical behaviour of hydratebearing sediments.

ConclusionsThe stress-strain history is the authoritativemagnitude governing sediments strength anddeformability. For the experimental research ofthe stress-strain history of hydrate bearingsediments GTS (Gas hydrate Test System) hasbeen constructed. For the first time GTS ena-bles the generation and decomposition of gashydrates in sediments under real marine condi-tions and allows to measure the oedometricstress-strain behaviour of the sediment, thewater and gas pressure as well as the tempe-rature regime simultaneously. Future applica-tion of GTS is provided for the study of thetrigger mechanisms of sediment collapse dueto the dissociation of gas hydrate.

Objectives of the Soil Dynamics sub-projectThe Soil Dynamics sub-project within GASSTABmainly refers to the laboratory investigation ofsoil mechanical and soil dynamical behavior ofmarine sediments and the understanding as

well as analyzing of the complex failure me-chanism of submarine slopes. Substantialamounts of marine sediments are necessary toperform the soil dynamical and soil mechanicallaboratory tests. Therefore sediment substitu-tes as a result of the Sediment Dynamics sub-project are used throughout all tests, as plan-ned from the beginning of the project. Thesediment substitute consists of 70 % clayminerals (50 % illite, 17 % chlorite, 33 % kao-linite) and 30 % quartz with parts of feldspar.

In a second part slope stability analyses basedon McIver’s (1982) model and by using analy-tical considerations for single block failurewere performed. A number of variations give afirst impression of the possible failure behaviorof submarine slopes.

Soil mechanical laboratory testsSoil mechanical index and element tests arenecessary to classify the material and to corre-late the resulting indices and parameters withother values from natural soil material. The fol-lowing indices and parameters tests are of spe-cial interest:

- unit weight of soil grains γS- Atterberg’s limits (liquid and plastic limit, wL

and wP) and plasticity index IP- permeability kf

- soil stiffness in one dimensional compres-sion (constraint/oedometer modulus) ES

Table 1: Results from soil mechanical index and element tests

- effective shear parameters from draineddirect shear tests (cohesion c’DS and frictionangle φ’DS)

- effective shear parameters from drained tri-axial shear tests (cohesion c’TX and frictionangle φ’TX)

All tests were performed according toGerman standards for soil mechanical labora-tory tests. The main results from these testsare given in Table 1.

The grain composition and mineral characte-ristics of the substitute material is well revea-led in the measured unit weight of soilgrains. Atterberg’s limits reflect the clay con-tent and clay type of a soil and here they cha-racterize a material of medium plasticity. Themeasured permeability reveals the largeamount of fines content. The soil stiffness inTable 1 is specified for virgin loading in oedo-metric compression. Reloading after comple-tely unloading of the specimen resulted inabout double the stiffness compared to firstloading.

The measured values of cohesion and frictionangles evaluated from direct shear as well as

drained triaxial compression tests are in closeagreement.

Soil dynamical laboratory testsThe dynamical behavior of soils is mainlyinfluenced by their stiffness at small strains ofabout 10-5 % and the damping of the materi-al. Both parameters can be measured in theResonant Column (RC) apparatus (Richart et al.1970, Hardin & Drnevich 1972). For this pur-pose a new RC device was designed and builtby GDS Ltd. (UK) and first used in the frame ofthis research project. In Figure 10 the maincomponents of the RC device are shown. TheRC apparatus consists of a fixed heavy bottomplate and a free movable top plate with fourconnected magnets in radial symmetricalarrangement and associated coils. The cylindri-cal soil sample, with 50 mm of diameter and100 mm of height, is placed on the bottomplate and the top plate construction can thenbe mounted on the top of sample. The mag-nets can free rotate within the coils, which arefixed on a supporting cylinder. Finally a triaxialchamber, which is not shown in Figure 10, isplaced to allow the application of cell pressu-res up to 1 MPa to the sample.

127

Figure 10: Resonant Column device(fixed-free type) for testing the dynamical material behavior

128

Figure 11: Shear modulus degradation (a) and damping build-up (b) with in¬creasingshear strain for RC tests subjected to an isotropic stress state of 700 kPa

a)

b)

129

In the test a very low voltage is applied to thefixed coils, which leads via the magnets tovibration torsion of the sample with very smallshear strains. A variation of the input frequen-cy with constant voltage leads to resonancemotion of the sample, which determines thecharacteristic values of the test – resonancefrequency, shear wave velocity and small strainshear modulus. Continuous increase of theinduced electro-magnetic forces increases thetorsional angle and as a result the shear strainamplitude. Associated is the transition fromelastic to plastic material behavior of the soilsample.

Figure 11 clearly indicates the softening of thesoil sample, i.e. the shear modulus degrada-tion, with increasing shear strain. Associated isa strong build-up of material damping.

Analytical calculations of slope failure mechanismsMcIver (1982) first published a model to des-cribe the complex behavior of submarine slo-pes containing gas hydrates. It assumes that a

large block of hydrated sediment breaks off andslides down the slope. The single block partlyslides on the layer of dissociated gas hydrateand partly through the hydrated zone itself.

Based on the model proposed by McIver(1982) a rigid body failure mechanism is app-lied for the analyses of slope failure (Fig. 12). Itconsists of a single block which moves on a sli-ding plane. A failure plane is assumed to existin the case of a slope collapse. Three dimen-sional effects and reaction forces in the slidingplane are neglected in this model. Effectiveshear strength parameters according to theMohr-Coulomb failure criterion are applied inthe failure plane.

The static problem can be solved using theequilibrium of vertical and horizontal actingforces and the moment as well. Among allparameters enclosed in the analysis the follo-wing of special interest: slope angle, slidingplane angle, sliding plane length, frictionangle, cohesion, unit weight of soil. Figure 13exhibits the influence of slope and sliding

Figure 12: Assumed failure mechanism for analytical calculations

130

plane angle as well on slope stability. All remai-ning parameters are set to constants in thisanalysis. The bold line in Figure 13 correspondsto a safety factor of one for slope stability.

Figure 14 represents lines with a slope stabilitysafety factor of one depending on the value offriction angle and slope angle as well as slidingplane angle. The bold lines in Figures 13 and14 respectively are associated. Regarding oneseparate line with constant friction angle, thesafe region of slope and sliding plane angles ison the left side of each line, whereas the unsa-fe region lies to the right.

A large number of additional variations withthe above mentioned parameters were done(Schupp et al. 2003).

ConclusionsA number of soil mechanical and dynamicallaboratory tests were performed to investigatethe material behavior of submarine-like sedi-ment material in detail. Along with these labo-ratory investigations slope stability analyseswere done using simple models. The analysesclearly indicate the importance of the use ofappropriate material parameters in order todetermine slope stability satisfactorily.

Further analyses using more sophisticatedmethods and more realistic modeling of thecomplex material behavior and dynamic loa-ding input still have to be done. Also the ther-modynamic processes of gas hydrate forma-tion and gas release in the surrounding sedi-ment structure should be involved into themodeling of the entire coupled multi-physicsmechanisms. It has to be clarified, whetherlocal failure on element level leads to globalfailure of continental slopes, what causes slopefailure and what are the effects.

Figure 13: Stability analysis for a slope with varia-tion of slope angle as well as sliding plane angle

ReferencesCundall, P.A. and Strack, O.D.L., 1979. ADiscrete Numerical Model for GranularAssemblies. Géotechnique, 29: 47-65.

Feeser V., Hoffmann K. (2004): Gashydrat inSedimenten. Bildung und Zerfall im Licht derIngenieurgeologie (Gas hydrate in sediments.Formation and decomposition from an engi-neering geological point of view). - Proc. 15thConference on Engineering Geology, Erlangen(Germany), 349-354

Feeser V., Becker H.J., Grupe B., Hoffmann K.,Kreiter S., Savidis S., Schupp J. (2003): GTS forsoil mechanical research of gas hydrate bearingsediments. - EGS-AGU-EUG Joint Assembly,Nice, Geophysical Research Abstracts Vol. 5,06978, 2003

Ginsburg, G., Soloviev, V.A., Matveeva, T. andAndreeva, I., 2000. SEDIMENT GRAIN-SIZECONTROL ON GAS HYDRATE PRESENCE, SITES994, 995 AND 997. In: K. Paull Charles, R.Matsumoto, J. Wallace Paul and W.P. Dillon

(Editors), Proceedings of the Ocean DrillingProgram, Scientific Results, pp. 237-245.

Hardin, B.O., Drnevich, V.P. (1972): Shear modu-lus and damping in soils: measurement and pa-rameter effects. Proc. ASCE, SM6, pp. 603-624.

Itasca Consulting Group, I., 2002. PFC2DParticle Flow Code in 2 Dimensions (Version3.0 Manual). ICG, Minneapolis.

Koh C.A., Wisbey R.P., Wu X., Westacott R.E.(2000): Water ordering around methaneduring hydrate formation. - J. Chem. Phys.113, 6390 - 6397

Kraemer, L.M., Owen, R.M. and DickensGerald, R., 2000. LITHOLOGY OF THE UPPERGAS HYDRATE ZONE, BLAKE OUTER RIDGE: ALINK BETWEEN DIATOMS, POROSITY, ANDGAS HYDRATE. In: K. Paull Charles, R.Matsumoto, J. Wallace Paul and W.P. Dillon(Editors), Proceedings of the Ocean DrillingProgram, Scientific Results, pp. 229-236.

131

Figure 14: Slope stability chart for different friction angleswith variation of slope angle as well as sliding plane angle

132

McIver, R.D. (1982): Role of naturally occurringgas hydrates in sediment transport. - AAPGBull., 66, pp. 789-792.

Parent J.S., Bishhnoi P.R. (1996): Investigationsinto the nucleation behavior of methane gashydrates. - Chemical Engineering Communica-tions, 144, 51-64

Richart, F.E., Woods, R.D., Hall, J.R. (1970):Vibrations of soils and foundations. Prentice-Hall, 414 p.

Schupp, J., Savidis, S., Grupe, B., Feeser, V.,Hoffmann, K., Becker, H.J., Kreiter, S. (2003):Gas hydrates and slope stability at continentalmargins – a mechanical approach. GeophysicalResearch Abstracts, Vol. 5, EGS-AGU-EUGJoint Assembly, Nice.

Sloan E. D. (2002): Hydrate Properties. -Seafloor Stability Workshop, Houston/Texas,March 14-15, 2002

Winters W.J., Pecher I.A., Waite W.F., MasonD.H. (2004): Physical properties and rock phy-sics models of sediment containing natural andlaboratory-formed methane gas hydrate:American Mineralogist, 89, 8-9, 1221-1227

Wood, W.T., Gettrust, J.F., Chapman, N.R.,Spence, G.D. and Hydman, R.D., 2002.Decreased stability of methane hydrates inmarine sediments owing to phase-boundaryroughness. Nature, 420: 656-660.

Yang J., Llamedo M., Tohidi B. (2003):Experimental Investigation of Gas HydrateFormation and Dissociation in UnconsolidatedPorous Media. - EGS-AGU-EUG Joint Assembly,Nice, Geophysical Research Abstracts Vol. 5,08460, 2003

AcknowledgementsThe joint project GASSTAB was funded by theGerman Federal Ministry for Education andResearch (BMBF) under (Grants 03G0560A+B).The authors are solely responsible for the con-tents of this paper.

133

134

Microstructure, thermodynamics, formation-and decomposition- kinetics of gas hydrates

1. IntroductionThe aim of this project was the determinationof unknown properties of gas hydrates concer-ning their microstructure, their thermodyna-mics and kinetics. They were investigated inlaboratory experiments at environmental con-ditions of geological interest aiming to providebasic parameters for a geophysical modellingof natural gas hydrates. In addition, it wasplanned to develop new methods for a struc-tural characterisation of natural gas hydrateson a sub-microscopic scale, which are also ofpotential interest to many aspects of naturalgas hydrates.

2. Objectives of the ProjectBased on the state of knowledge in the fieldand considering our specific methodologicalknowledge we had defined the followingobjectives for our project:

• The experimental determination of funda-mental properties of gas hydrates at variablepressure/fugacity and temperature, i.e. thedetermination of hydration number, density,and compressibility for pure and some selec-ted mixed hydrates.

• The experimental determination of the re-growth kinetics upon changing pressure/fugacity or temperature conditions.

• The experimental determination of the lowtemperature stability limits of gas hydratesin order to advice on appropriate samplestorage and handling conditions.

• The verification of the current statisticalthermodynamic model of van der Waals &Platteeuw and its modifications on the basisof our experimental data. It is expected thatsuggestions for an improvement of this verywidespread theory will emerge from thiswork, which will have applications in a geo-physical context.

• The first morphological and textural charact-erization of gas hydrates on a sub-microsco-pic scale with emphasis on the sub-micronporosity discovered by us earlier on; this workshould give important insights into themechanisms for gas hydrate formation.

• Based on the morphological characterisationwe will perform a modelling of the mechani-cal as well as the transport properties ofmicroporous hydrates, specifically concerningthe formation and decomposition processes.

3. Present Status and Results / Methods & Results / Results

ThermodynamicsA number of neutron and synchrotron scatte-ring experiments were performed to determinethe cage filling of methane and CO2 hydrate.For the first time we could determine the abso-lute cage fillings as a function of fugacity andin this way provide critical tests for checkingthe wide-spread statistical thermodynamic the-ory of van der Waals & Platteeuw. Clearly, theagreement is far from being perfect (Klapprothet al. 2003) and partly based on our resultsattempts are now under way worldwide to

Itoh H. (1), Klapproth A. (1), Goreshnik E. (1), Techmer K. (1), Kuhs W.F. (1)*

(1) GZG Abteilung Kristallographie, Universitaet Goettingen, Goldschmidtstrasse 1, 37077 Goettingen,

Germany, *E-Mail: wkuhs1@gwdg.de

improve this situation by using free-energyminimisation methods or ab initio calculatedwater-gas interaction potentials. Using neu-tron- and Raman-spectroscopy and comparingthe results with molecular dynamics simula-tions we could determine the modes of guestmolecules in the hydrate cages. The quantitati-ve agreement between experiment and com-puter simulations depends on the water-waterinteraction potential; consequently a numberof widespread potentials (SPC, TIP4P and KKY)were employed; the best agreement was foundby using the KKY water interaction potential. Itwas found that the degree of dynamic couplingof water cages and guest molecules dependstrongly on the type of guest molecule encaged(Chazallon et al. 2002, Itoh et al. 2003,Schober et al. 2003). Consequently, a variationof thermal conductivities can be expected fordifferent gas hydrates.

Bearing in mind that most natural gas hydratescontain different types of guest molecules invariable proportions and in order to shed somelight on the complex behaviour of mixedhydrate systems we have performed experi-ments on the N2-CH4 and CH4-C2H6 systemusing diffraction and Raman-spectroscopy.Clearly, the predictions from statistical thermo-dynamic theory are insufficient and additionalproblems occur due to the formation of meta-stable phases belonging to van Stackelberg’stype II structure (Staykova et al. 2003).

In cooperation with Prof.Bohrmann (GEO-MAR/RCOM) a number of natural gas hydratesamples from several locations were investiga-ted by X-ray diffraction at laboratory sources aswell as at the synchrotron source at HASYLAB/Hamburg. X-ray diffraction proved to be essen-tial to determine the structure type and thedegree of preservation of these recovered sam-ples (Kuhs et al. 2004a).

KineticsA large number of experimental runs to deter-mine the formation and decomposition kine-

tics were performed using both neutron dif-fraction at D20/ ILL(Grenoble) and synchrotronradiation at BW5/ HASYLAB (Hamburg), bothfor CH4 and CO2 hydrate. The analysis was per-formed using a newly developed multi-stagemodel (Salamatin & Kuhs 2002) in which a fastfirst reaction stage was distinguished from alater diffusion-limited stage (Staykova et al.2003). These experiments were complementedby in-house runs in which the gas consump-tion/ release was measured during formation/decomposition. Using the model we can nowdetermine the activation energy, diffusioncoefficients of the process and predict thehydrate formation and decomposition beha-viour over a large range of temperatures andpressures (Genov et al. 2004). Particular atten-tion was given to the effect of »anomalouspreservation«, also named »self-preservation«,which describes to the unexpected phenome-non of a long-term stability of gas hydratesoutside their field of stability at temperaturesbelow the ice melting point. A physical under-standing of this technologically and geologi-cally important phenomenon did not exist. Wecould quantitatively confirm the effect and in acombination of diffraction and scanning elec-tron microscopy give for the first time a fullphysical explanation of the phenomenon (Kuhset al. 2004b). The onset of self-preservationarise because of the annealing of stackingfaults in the ice layer initially formed on thehydrate surface; at higher temperatures the icereorganizes into larger crystallites in anOstwald-ripening process leading to a maxi-mum of self-preservation just a few degreesbelow the melting point of ice.

MicrostructureCryo scanning electron microscopy (SEM) hasbecome a standard tool in the microstructuralcharacterisation of gas hydrates for both natu-ral and laboratory-made samples. A largenumber of samples were investigated usingthis technique, including samples from GasHydrate Ridge, Black Sea, Congo basin and theMallik research well in N.W.T./ Canada. The

135

136

sub-micron porosity of gas hydrates has beenconfirmed to be characteristic for gas hydratesand is now used as a fast means of identifyinggas hydrates in complex samples (Staykova etal. 2003, Genov et al. 2004, Kuhs et al.2004a). The detailed physico-chemical originof this microstructure is not yet understood,however. Of particular importance were studiesof the sediment – gas hydrate contact usingcryo SEM (Techmer et al. 2005). For the Malliksamples we could show that clay mineralsform intimate contacts while quartz grainsseem to be separated from gas hydrates by aliquid layer (see Fig.1.). This has important con-sequences for the elasto-mechanical behaviourof gas hydrates and their seismic response.Systematic studies of the sediment – gas hy-drate contacts are underway in the follow-upproject in the framework of the GEOTECH-NOLGIEN program, which should lead to a firstquantitative description of the elastic responseof natural gas hydrate bearing sediments.

4. ConclusionsThis project was largely devoted to open ques-tions concerning the thermodynamics, kineticsand microstructure of gas hydrates as far asthey are of importance in a geological setting.Due to the lack of well-preserved natural sam-ples, the investigations were mainly conductedon laboratory-made material. Important in-sights were gained concerning the molecularinteractions between guest and host moleculesresponsible for the anomalously low thermalconductivity. Likewise, the first determinationof the absolute cage fillings for methanehydrate was obtained. The formation anddecomposition kinetics were established expe-rimentally and a model was constructed whichhas predictive power over a large range ofpressures and temperatures. Finally, the firstphysically sound and complete explanation ofthe self-preservation effect was given.

AcknowledgementsThe work was funded by the grant 03G0553A.We thank ILL/Grenoble for beamtime and sup-port. Likewise we thank Drs. Viorel Chihaia,Georgi Genov, Till Heinrichs, Helmut Klein andDoroteya Staykova (all Göttingen) for theirhelp in various aspects of the work.

Figure 1: Cryo scanning electron micrograph of gas hydrate - sediment con-tacts of a natural sample from sub-permafrost gas hydrates of the Mallik rese-arch well (N.W.T. Canada). Left: Quartz-gas hydrate contact with intermediate(frozen) water layer of several µm thickness. Right: Clay mineral particles withintercalated microporous gas hydrates (see arrow).

ReferencesChazallon, B., H. Itoh, M. Koza, W. F. Kuhs,and H. Schober (2002). Anharmonicity andguest-host coupling in clathrate hydrates.Phys. Chem. Chem. Phys. 4, 4809-4816.

Genov, G., W. F. Kuhs, D. K. Staykova, E.Goreshnik, and A. N. Salamatin (2004)

Experimental studies on the formation of porousgas hydrates. Am. Miner. 89, 1228-1239.

Itoh, H., B. Chazallon, H. Schober, K. Kawa-mura, and W. F. Kuhs (2003). Inelastic neutronscattering and molecular-dynamics studies onlow-frequency modes of clathrate hydrates.Can. J. Phys. 81, 493-501.

Klapproth, A., E. Goreshnik, D. K. Staykova, H.Klein, and W. F. Kuhs (2003). Structural studiesof gas hydrates. Can. J. Phys. 81, 503-518.

Kuhs, W. F., G. Y. Genov, E. Goreshnik, A. Zel-ler, K. Techmer, and G. Bohrmann (2004a). Theimpact of porous microstructures of gas hydra-tes on their macroscopic properties. J.Offshoreand Polar Engineering 14, 305-309.

Kuhs, W.F., G.Genov, D.K. Staykova and T.Hansen (2004). Ice perfection and the onset ofanomalous preservation of gas hydrates. Phys.Chem. Chem. Phys. 6, 4917-4920.

Salamatin, A. N. and W. F. Kuhs (2002). For-mation of porous gas hydrates. Proceedings ofthe Fourth International Conference on GasHydrates, Yokohama, May 19-23, 2002, Yoko-hama, 766-770.

Schober, H., H. Itoh, A. Klapproth, V. Chihaia,and W. F. Kuhs (2003). Guest-host couplingand anharmonicity in clathrate hydrates. Eur.Phys. J. E 12, 41-50.

Staykova, D. K., W. F. Kuhs, A. N. Salamatin,and T. Hansen (2003). Formation of porous gashydrates from ice powders: Diffraction experi-ments and multi-stage model. J. Phys. Chem. B107, 10299-10311.

Techmer, K., T. Heinrichs und W. F. Kuhs(2005). Cryo-electron microscopic studies onthe structures and composition of Mallik gas-hydrate-bearing samples. Scientific Resultsfrom the Mallik gas Hydrate Production Re-search Well Program, Mackenzie Delta, North-west Territories, Canada. Eds. S. R. Dallimoreund T. S. Collett, »Geological Survey of Cana-da Bulletin« 585.

137

138

New perspectives for the extraction of oceanic gas hydrates

IntroductionOver the last two decades, natural gas has gai-ned increasing importance in the energy tech-nology. Without natural gas (generally metha-ne-CH4), technical systems like peak loadpower plants, gas and steam cogenerationplants with an unachieved low emission of CO2

per kWhel., and fuel cells powered by hydro-gen (H2), yielded from natural gas would oftennot be feasible. But even in other fields oftechnology, natural gas has attained significantrelevance as reservoir of hydrogen or as hydro-gen supplier. Facing today’s population growthand the increase in agricultural productivityand the resulting sweeping demand of ammo-niac fertilizers would be hardly met withoutnatural gas as hydrogen resource, and wouldentail a manifold of carbon dioxide emissions.

These characteristics – highly valued, not onlyin ecological respect – are in contrast to the fore-cast that natural gas, as one of the fossil pri-mary energy carriers, will have only a shortexploration period of 60 years [1]. Accordingly,the detection of the so-called gas hydratematerials (briefly: gas hydrates), detected overthe lengths of the submarine area, in the rid-ges of the continental shelf, in a water depthof about 500 to 1500 m, has given rise to thehope that the dilemma forecasted above maybe prevented. However, this hope is only realis-tic if the exploitation of gas hydrates on a

technical scale will be mastered successfullywithout significant losses. As natural gas, par-ticularly methane, is characterized by an in-creased infrared activity (higher by a 23 fac-tor), these significant losses are not only aneconomical drawback, but make part of thedetrimental impacts on the climate. As a con-sequence, the above mentioned, uniquelylow specific CO2 emission of a GuD powerplant is affected.

Gas hydrates are solid, icelike (Fig.1 for exam-ple) physical inclusion compounds betweenwater and small gas molecules, as is methane,ethane, carbon dioxide, or mixtures amongthese gas molecules. Research on gas hydratesis gaining increasing importance because ofthe above outlined factors. According to con-servative estimates, the deposits contain morethan double the energy content than the fossilenergy resources coal, conventional naturalgas and crude oil together (about 10.000 Gt),[2, 3] (Fig. 2). Even if actual published and pes-simistic values are approved in future (500-2.500 Gt) [4] there is much more carbon keptin gas hydrates worldwide than is kept in theproved (state end 2002 [1]) worldwide naturalgas reserves. Vast majority of gas hydrates hasbeen detected in submarine deposits, whereasthe permafrost soils of Canada and Siberia,which are equally suitable soils for the forma-

Schultz H.J. (1), Deerberg G. (2); Fahlenkamp H. (3)

(1) Celanese Chemicals Europe GmbH, Werk Ruhrchemie, Otto-Roelen-Str. 3, D-46147 Oberhausen,

Heyko-Juergen.Schultz@celanese.de

(2) Fraunhofer Institute for Environmental, Safety, and Energy Technology UMSICHT, Osterfelder Straße 3,

D-46047 Oberhausen, Germany, de@umsicht.fhg.de

(3) Universität Dortmund, Fachbereich Bio- und Chemieingenieurwesen, Emil Figge Straße 70, D-44221 Dortmund,

H.Fahlenkamp@bci.uni-dortmund.de

tion of gas hydrates, hold only few deposits ofthese substances, as has been proven.

To develop a safe and sustainable access to theto date non-used gas hydrates, a technology forthe extraction of hydrates and a simulationmodel for the device has been developed andanalyzed for the first time in the research pro-ject. Due to the high cost for offshore pilotplants, the »dynamic simulation« was chosen asworking and research approach. On the basis ofa complete mathematical device model in inter-action with the exactly described destabilizationof the gas hydrates and the controlled extrac-tion of the natural gas emitted from the depo-sit, this approach permits the development andscientific verification of an extraction method.This novel method allows –provided a testingphase in a large-scale experiment is con-ducted– the extraction of oceanic and also ofpermafrost gas hydrates.

Properties of Gas HydratesGas hydrates belongs to the group of realclathrates [5] without chemical link betweenhost and guest molecule. They are stable atlow temperatures and high pressure condi-tions. Small guest molecules, such as methane,are embadded in a cage of water molecules

(Fig. 3). Each cage can usually take exactly oneguest molecule [2]. Through the interactionbetween the guest molecules and water mole-cules in the lattice structure, the formed cavities are strengthened, which would bethermodynamically unstable without guestmolecule [6]. This requires a minimum size ofthe guest molecules (approx. 0,35 nm, [2]) toenable a guest molecule to stabilize a hydratecage. The upper limit value for the guest mole-cule diameter determines the specific cage size.

Three different gas hydrate structures havebeen detected in natural systems (Fig. 3). Twofurther structures have been manufactured inthe laboratory [7, 8], others are predicted theo-retically. Basically, the cage structures differ insize, number and distribution of the watermolecules. The structures I and II found in naturedisplay a cubic form, structure H has a hexago-nal form. The structure formed depends stron-gly on the size of the guest molecules. Fig. 3 illu-strates the cage structures I, II and H. The cagetype with the title 512 exists in all three cagestructures. It is characterized by a cage consi-sting of 12-pentagons. The different gashydrate structures consist of different cagetypes which span the cavities, in which the gasmolecules are included.

139

Figure 1: A gas hydrate manufactured in vitroat the laboratory of Fraunhofer UMSICHT,Oberhausen (here: methane hydrate)

Figure 2: Simplified sketch of carbon resour-ces on the earth, according to Kvenvolden [3]

140

Hydrates of different gases have been detectedin nature, also in extra terrestric systems. Muchmore than 90 percent of natural gas hydratesin the terrestric system are methane hydrates,which due to their physical stabilization condi-tions exist in ocean floor sediments and in per-mafrost soils of the polar regions.

Whereas the physical parameters pressure andtemperature describe the generally possibledistribution, another limiting factor is thegeneral availability of a sufficient amount ofgas, preferably CH4. In the deep sea, major partof the methane comes from the organic com-ponents respectively from the fermentativedecomposition of organic compounds or thebacterial CO2 reduction in the sediments. Partof it also derives from the thermocatalytictransformation processes formed in deepersediments. Most part of CH4 forms in the areaof continental shelves, where, due to a highplankton productivity of the oceans and a highsedimentation rate, large amounts of organicmaterial are available in the sediment for theformation of gas. As a result, gas hydrates areto be found not only at all passive and activecontinental shelves, but also in the Caspic Sea,in the Black Sea, in the Mediterranean and inthe Baikal sea [9].

Verified and partly sampled gas hydrate depo-sits are to be found worldwide (Fig. 4). For afictive deposit in more than 1000 m of waterdepth, first the gas hydrate stability zone (HSZ)is shown in Fig. 5. The gas hydrate balancecurve of a representative natural gas is repres-ented by the continuous line. Below this line,the gas hydrate formation conditions are gene-rally given. The hydro chemical (decreasingtemperature at increasing depth) and geother-mal (increasing temperature from geothermalpower at increasing depth) temperature gra-dients with the increase of –0.5°C /100 m or+2.0°C/100 m as well as the ocean floor, limitthe hydrate stability zone (HSZ) of about 600m thickness. Above the hydrate stability zone,there is the oceanic water column, below thezone there is sediment with free circulating gas.

The verification of gas hydrates is possible bythe geophysical registration of the so-calledbottom-simulating reflector (BSR). The BSR is aseismic reflector which forms at the interfaceof hydrate containing sediments with thosecontaining free methane gas. The reflector isfound in depth up to several hundred metersbelow the ocean floor and indicates the lowerboundary of the gas hydrate stability zone.Accordingly, gas hydrates are to be expectedprincipally above the BSR, below there is freecirculating methane. The exact mechanism of

Figure 3: Cage structure of the three natural gas hydrate structures I, II and H

the BSR localization is not the subject of thisarticle. It has to be mentioned, nevertheless,that the gas hydrate sediment layers cause aweaker reflection of the seismic waves than dotypically sediment layers and gas-rich floor sec-tions. Gas hydrate floor sections without fieldswith free gas underneath are not exactly detec-table today, therefore the worldwide gas hydrateresources may be larger than assumed today.

Extraction Method and DeviceThe novel processing method for the extrac-tion of gas hydrates is based on the mam-moth-pump-principle (Fig. 6) and works like anoverdimensioned coffee machine.

The method includes the feeding of heatedocean water into the gas hydrate depositthrough a concentric double pipe system,which leads to the thermal destabilization ofthe gas hydrate and to the release of the con-

141

Figure 4: Gas hydrate reservoirs worldwide [11]

Figure 5: Gas hydrate stability zone (HSZ) in a fictive deposit

Figure 6: Gas hydrate / gas extraction device forthe oceanic extractionbased on the mammothpump system

142

tained gas (especially methane). If saltyocean water is used, the dispersion of thegas hydrates is supported by the inhibitinginfluence of the salt.

By the ascending gas bubbles (»airlift«) whichaccelerate the rising of the surrounding fluid,and the density difference between the bothtwin pipe sectors of the extraction system –downcoming sector: fluid phase (+ solidphase), upstreaming section: fluid and gasphase (+ solid phase) – a self maintaining cir-culation process is induced. The released natu-ral gas is fed into the device head via theupstreamer (external annular space in Fig. 6) –a collector (drilling ship or platform), fromwhich it may be transported to a subsequentutilization. The released gas volumes in thedeposit may be controlled via temperature andvolume of the circulating water. A part of thegained gas flow may be used for the heatingof the circulating water.

Mathematical Model of the deviceThe complex technical system of the describedmammoth loop system is described in a detai-led rigorous mathematical model for the dyna-mic simulation in order to provide significantevaluation possibilities on feasibility, operatio-nal safety, and efficiency properties over theentire extraction process. With regard to themodel set-up, the following physical pheno-mena are considered:- fluid dynamics,- thermodynamics,- hydro kinetics (dispersion

and formation kinetics),- mass transfer,- heat transfer.

The contribution of these aspects to the modelof the apparatus (Fig. 6) are formulated separa-tely as modules, implemented and unified in asuitable program structure. The modeling of thesystem is performed on the basis of a multi-sec-tor cell net model with backflow (Fig. 7, Fig. 8).

Cell net models are considered highly flexibletools for the modeling of multiphase fluidflows, and comprise the serial linking of ideal-ly backmixed volumes (»cells«) which are inmaterial and energetic interaction among eachother. By a hypothetical backflowcontrarious to the convective mass flow assu-med for each cell, the deviation from the idealplug flow is considered. The underlying multi-phase system (index m for physical phases) theNC-linked cells are passed through by the volu-me flows . The backflows circulating bet-ween the cells are described as backflow ratioin equation 1:

(1)

The practical application is performed in amanner to associate these model parameterswith those of the well known continuousdispersion model.

(2)

That means that the related backflow ratiomay be determined as a function of the disper-sion coefficient at a freely selected number ofcells NC. The backflows in this case are no phy-sically measurable units (therefore termed hypo-thetical). They have to be interpreted alwaystogether with the associated number of cells NC.The selected number of cells is only relevant forthe discretization of the flow sectors and has noimpact on the backmixing characteristics.

Due to the differing flow regimes and physicalphenomena in single device sectors (Fig. 6),the model structure developed for the extrac-tion system is primary based on the subdivisionof the plant into four separately modeled macro-scopical sectors groups. These are suitable balan-ce volumes, which are each partitioned into a netof NC,k sector-dependant, ideally mixed cells:1. upstreamer2. downcomer3. head4. bottom

143

Figure 7: Schematic layout of the used multi-sector cell net model

Figure 8: Schematic layout of the balance cell system (convective flow: dashed arrows, backflow: dotted arrows)

144

Fig. 7 shows the simplified and schematicimage of the four coupled sectors of the usedmulti-sector cell net model. In Fig. 8 the sche-matic linking of the cells by the balance cellsystem is presented. The vertical, dashedarrows indicate the main or primary convectiveflow, the vertical, dotted double arrows repre-sent the superimposed or backflows as a mea-sure for the consideration of deviations fromthe ideal plug flow. The horizontal arrowsrepresent the transfer flow crossing the inter-facial area between the phases. Additionally tothe above mentioned three flow types, alsofeed and removal as well as flows by reactions(chemical rates) may be considered, which arenot listed here for a better schematic view.

For the modeling of the thermal destabilizationof gas hydrate, the complex thermodynamicsof gas hydrates have to be investigated funda-mentally. The impact of deposit parameters,such as sediment properties, porosity andhydrate contents on the destabilization proces-ses is essential. Since the extraction processmay include the feeding of solid particles fromthe deposit site to the head of the device, thedispersion and formation kinetics are anotherfactors not to be neglected.

Possible impacts of the mass transfer among theinvolved phases, such as the absorption of thereleased gases in the liquid phase, as well as ofheat transport processes regarding heat losses,relevant for the assessment of economical wor-king conditions, are included in the model.

The balancing of the device sectors is perfor-med cell by cell. The connection of cells anddevice sectors is given by the subsystem-lin-king macroscopic component (mass flows),energy, momentum and information flows.The balance equations are complemented byalgebraic approximation equations, resulting ina differential-algebraic equation system ofthousands of equations depending on thenumber of cells NC.The solution of the system is performed simul-taneously, using numerical methods. The simu-

lation provides, among other things, pressure,concentration, temperature, and velocity profi-les /parameters over the geometry and the ent-ire process time.

Construction Study for the Extraction SystemExample details for the construction designand dimensioning of the described extractionsystem are developed for the technical evalua-tion and verification of the feasibility and safe-ty of the extraction device. Due to the basicand unique character of the extraction system,suggestions are presented which are fed intothe simulation tool as a geometric basis descri-bing the extraction process. During furtherinvestigations, system components, buildingparts and other parts will have to be subjectedto a critical review and optimization. However,the simplified layout provides a realistic basisfor the conducted evaluation of economic effi-ciency, safety and feasibility of the extractionprocess and device.

Pressure profiles, simultaneously determined,indicate that the pressure difference betweenthe two twin pipe sectors or to the ambiancedo not exceed 7 bar (6.16 bar) in the examplesystem. During various further simulations,which are not explicitly presented here, a maxi-mal excess pressure value of 15 bar was notsurpassed. Due to the high climatic effect ofthe methane exploited and the safe workingconditions at the extraction process accordingto the model suggested, a worst-case internaland external excess pressure value of pExcess = 36 bar at a 1000 m long twin pipe setmay be assumed for the estimate layout of theconstruction. This calculated excess pressuremay also be used for the dimensioning of thehead of the device. The internal diameter ofthe external pipe is calculated to 1.5 m, theinternal diameter of the internal pipe is 0.75m. The construction material for the twinpipe arrangement is considered to be fromsteel (St 37 = worst-case). In compliance with the general regulations forthe manufacturing of machinery and pipe con-

˜

struction, the wall thickness of 30 mm for theexternal pipe and 21 mm for the internal pipeare calculated.

The twin pipe may be manufactured in parts of20 m for example, as is illustrated in Fig. 9.The pipe parts are coupled via flanges, withthe internal pipe being mounted first, followedby external pipe, and finally the new pipe partconcentrically fixed through a spacer. The flan-ges of the external parts are connected with40 pieces of M-48 screws, including 11 screwsfor safety reasons. For internal pipe flanges, 30screws M-24 are sufficient, 14 screws of themfor safety reasons.

Selecting a pipe connection type, a flange con-nection that is at least partly form-adapted andremovable, is the most suitable one. This ensu-res the reusability of the twin pipe sections bysimple dismounting of the parts, after theexploitation of a deposit has been finished.The twin pipe parts do not necessarily have tobe 20 m long, but may also be adjusted to thespecific manufacturing parameters. It is inaddition theoretically possible and recommen-dable not to screw the parts together with aflange connection, since particularly the moun-ting takes more time than a conventional wel-ding connection. According to the practicalmanageability and transport features, onlyeach second or third twin pipe part may there-

fore be connected by flange connection, theothers by welding, which may be more easilydone ashore requiring lower costs. Connectionparts such as flanges, screws and gaskets, haveto be considered in the calculations always independency of each other.

The bottom section of the twin pipe system,which is protruding to the deposit, is designeddifferent. As is displayed in Fig. 10, the lengthof the last part of the external pipe equals 30m, the internal pipe may be 10 m longer forexample, in order to achieve a better circula-tion of the flow in the formed cavity and areduced short circuit flow.

A perforation of the external pipe in the bottomsector of the device is possible, for example withholes of 15 cm diameter (Fig. 10). As a result, thelateral gas collection is increased according tothe water dispersion pump principle.

The head coupling part, serving as link bet-ween head and twin pipe device, has also aspecific construction. As depicted in Fig. 11, inthe 2.5 m high head part the internal pipe isfed from the centering part to the surroundingarea, via a 90° bow which is welded to theexternal pipe. The downcomer is led laterallyfrom the head sector in horizontal direction,which permits an easy-construction, separateheating of the downstreaming fluid. The hea-

145

Figure 9: Example 20 m part of the Twin pipe system, withflange connection and spacers at the upper side

146

ted ocean water is fed into the downstreamer(here: internal pipe) to the downcomer via the90°-bow. Consequently, upstreaming and down-coming flows are separated in the head connec-tion unit, because the upstreaming multiphasemixture is fed to the head sector vertically.

For the reduction of the bending stress cau-sed by the ambient oceanic flow, the twinpipe system may be constructed elastically, tobe mounted in a certain spacing, e.g. each60 m distance, for example by compensators.Fig. 11 gives a schematic picture of an exem-plary compensator with a height of 1 m anda wall thickness of 30 mm for the externaland internal pipes.

The bending stress here separates the com-pensators in the 60 m long pipe system, sothat the extraction pipe / device may be consi-dered roughly as a rope, and not as a bendingbridge. As the ocean currents here usuallymove only in one direction (horizontally), thecompensator is flexible only in this direction.

Discussion of Selected Simulation ResultsIn the following, selected simulation results forthe extraction of oceanic gas hydrates on thebasis of an exemplary realistic system are pres-ented. The worldwide distribution of gashydrate reservoirs discloses various potentialexploitation sites, each with differing local

Figure 10: Bottom section of the twin pipe system

Figure 11: Twin pipe system with head sector coupling partand schematic of an example joint compensator for the twinpipe system

ambient conditions. One of the well investiga-ted and intensively sampled offshore depositsites of gas hydrates is the »Hydrate Ridge«alongside the U.S. west coast which is chosento be a model deposit for the simulation.

The hydrate ridge is an extended geological for-mation, extending approx. 30 km in north-southdirection, with a width of about 15 km, alongsi-de the American continental margin nearOregon and Washington. It is a submarinemountain massif in the range of the GermanHarz mountain chain and has emerged from thesubduction of the Juan de Fuca-plate under theNorth American continent to the east. Largequantities of gas hydrate samples could be takenparticularly in the southern peak of the hydrateridge, at a sea depth of approx. 780 m [9]. 30Vol.-% of the deposit consist of hydrate, 70 Vol-% are sediments. For the composition of theincluded gas, the compounds methane (98 %),ethane (1,5 %) and propane (0,5 %) are assu-med. The temperature decreases with increasingdepth from 15°C in the surrounding air (surface)to 5°C in the deposit.

The hypothetical extraction device, which issimulated, is designed to explore a gas hydra-te field in a depth of 800 m. The diameter ofthe external pipe (upstreamer) is assumed tobe 1.25 m, of the internal pipe (downcomer)to be 0.50 m. Alternatively glass fibre reinfor-ced plastic material is taken as material for theextraction device. The heat input, amountingto 2.5 MW constantly over the entire process,is fed in the first downcomer cell.

The liquid flow velocity of the feed into thedeposit via the downcomer is 0.2 m3/s. Thedevice is partitioned into 35 cells, 16 cells aretaken each by the up- and the downstreamer,2 cells by the head sector, and 1 cell is taken bybottom sector.

For the extraction process under investigation,the simulation results show a steady state afterabout 15 000 s (approx. 4 h) of the start upphase, which is owing to the thermal decom-

position of the gas hydrate. The gas productmass flow amounts to approx. 1.54 kg/s. If this value is set into relation with the related caloric value of the product gas (here

), a fictive hea-ting or product gas enthalpy flow is yielded, asshown in equation 3.

(3)

With the product enthalpy flow, a first funda-mental measure for the assessment of the eco-nomic efficiency of the extraction method isprovided. The energetic (yield) coefficient α,taking into account the heating enthalpy flowfed to the system ( equation 4), constitutesthe most important target value and measurefor a more detailed and deeper evaluation ofthe efficiency of the extraction technique.

(4)

Fig. 12 displays the progress of the energetic(yield) coefficient α for the system describedwith a gas hydrate contents of 30 Vol.-% inthe deposit, assuming different comprehensiveheat losses in the deposit. At a heat loss of 50 % in the deposit, it amounts to 33.42 W/W,at a heat loss of only 25 % it is 46.92 W/W,and at a heat loss of 75% assumed, it is still16.8 W/W. This means that the extraction ofproduct gas supplies a manifold of 17 to 47times quantity of enthalpy than is fed to thesystem as heating energy. As is expected, theheat losses in the deposit have a great impacton the gas yields. In all of the three calculatedscenarios, however, the fictive enthalpy flow,released during the heating, is significantly hig-her than the heating enthalpy flow that is fedto the system.

With high quality (stainless) steel (1.4359,1.4571) as material for the extraction system,the efficiency of the method is reduced com-pared to a GRP construction (glass fibre pla-

147

148

stics). Besides the higher specific density, theheat transfer capacity of stainless steel withroughly

has a fundamentally different dimension thanthat of GRP, amounting

This property has significant impacts on theheat transfer between the flow regimes andthe heat loss to the ambience. A second analy-sis of the defined system with a heating ener-gy fed of constantly 2.5 MW yields the result,that when using steel as material at unchan-ged conditions, the heat loss to the ambienceincreases that much, that the hydrate decom-position temperature is not reached. This isdue to the intense cooling of the downcomingfluid on the way to the deposit. Consequently,the heating energy fed in has to be increased.In the case of an increase of the heating ent-halpy flow to 5.0 MW, however, an energeticcoefficient α of 15.98 W/W is still reached withstainless steel as material for the extractiondevice. Again, a 17 fold quantity of energy is

released than is supplied to the system.

The influence of the ocean current velocity onthe efficiency of the technique is given particu-larly when using steel as material. Table 1shows the product gas enthalpy flows andenergetic coefficients in the steady state, rela-ting to the different sea current velocities as ameasure for the heat loss to the ambience ofthe twin pipe device (stainless steel as materi-al). The higher the sea current velocity, the hig-her is the heat transfer coefficient for circum-fluent pipes, according to the implementedcalculations of the VDI-Wärmeatlas [10]. As aresult, the heat transfer coefficient and theheat losses increase at constant conditions.The effects may be seen in Table 1.

The product gas enthalpy flow and the ener-getic coefficient are given for a sea currentvelocity of 0.00 m/s with 104.64 MW or maxi-mum 19.93 W/W respectively and continuallydecrease at increasing sea current velocity, thusreducing the efficiency of the extraction method. The vast impact on heat losses to theambience is thus clearly elucidated. Due to thelow material-specific heat transmission via thetwin pipe device when using glass fibre reinfor-

Figure 12: Yielding coefficients α against time for different heat loss values (HLV) in the deposit

ced materials, here the product enthalpy flowand energetic coefficient are nearly independentof the sea current velocity and nearly neglectable.

Conclusion and OutlookIn the view of the forthcoming shortage ofconventional crude oil and natural gas reservo-irs in a mid term period, the present researchstudy on the exploration of the immense, world-wide spread gas hydrate deposits and their uti-lization as a natural gas resource is a funda-mental contribution to the development of agas hydrate extraction system. Traditional fore-casts predicting a natural gas resources deple-tion within a period of 60 years, given a con-stant consumption and constant quantities [1],are hence contradicted. The chemical proper-ties of natural gas, the classical hydrogen car-rier among the carbon dioxides and minerals,justify the classification of the presented extrac-tion technique as a sustainable process deve-lopment. The implementation of this techni-que does not only permit the long term utiliza-tion of peak load plants and GuD, steam andpower cogeneration plants, but also providesthe required quantities of hydrogen, if the fuelcell technology, as for now still in the testingphase (1 MW scale), is to be scaled up to the di-mension of today’s power plants. Furthermore,gas hydrate reservoirs are expected to be rough-ly as widely spread as carbon resources, accor-ding to current estimates, which consequentlyhelps to avoid the dependency on supply mono-polies, valued detrimentally in political terms.

In this study, a new gas hydrate productionrespectively extraction device has been develo-ped theoretically. Using a complete, detailedmathematical system model the feasibility ofnew technology has been investigated. Basedon a »multi-sector cell net model with back-flow« including physical-chemical (transport)phenomena for the exploitation device of gashydrate deposits could be successfully develo-ped and implemented. The utilization of amulti-sector cell net system, proven for thesimulation of multi-phase (fluid) flows, has theadvantage of high flexibility, an easy to useunderlying balance equation system, easyscale-up, and the easy adaptation of modeldepth. Via the linking of the system-describingdifferential-algebraic equation system to anumerical equation solver, the simulation toolprovides, among others, parameters on con-centration, temperature and velocity profiles aswell as material and heat transport values overthe entire device area. As a result, practical cal-culation values for the dimensioning and veri-fication of the feasibility, economic efficiency,and safety of the method are presented.

The simulation results allow the determinationof optimum operating conditions and opera-tion modes, allowing the conclusion that themammoth pump principle may be applied forthe controlled thermal destabilization of oce-anic and permafrost gas hydrates, yielding ahigh exploitation rate of natural gas. The simu-lation tool implemented may be further usedfor the layout, dimension planning and projectplanning of an extraction system and for theoptimization and planning of field tests.

149

Ocean current velocity [m/s] 0,00 0,25 0,50 0,75 1,00 1,25 1,50Product gas enthalpy flow [MW] 104,64 87,19 84,92 78,81 75,02 73,42 68,97 Energetic (yielding) coefficient α [W/W] 19,93 16,44 15,98 14,76 14,00 13,68 12,79

Table 1: Product gas enthalpy and energetic (yield) coefficient in steady state. Dependencyon different oceanic current velocities, for stainless steel as construction material

Symbols Unit Parameter, meaning, description

[m2/s] effective dispersion coefficient

[W] product gas enthalpy flow

[W] heating enthalpy flow

[1] sector index (upstreamer, downcomer,

head, and bottom)

[kg/s] product gas mass flow

[1] phase index (gas, fluid or solid phase)

[1] total number of cells

[m3/s] volume flow

[m3/s] backflow

[1] molar percentage of a component in the gas phase

[W/W = 1] energetic yield coefficient

[J/kg] heating value = upper heating value,

product gas enthalpy

[1] backflow relation

150

However, due to the estimates for the construc-tion costs of the plant designed, coming to thecost for a drilling platform, the practical appli-cation of the concept is still a unsolved problem.The support of a partnering company from theenergy, specifically, the power plant sectorwould be very helpful in this regard.

The extraction process according to the sugge-sted, novel process technology via mammothloop device is considered to be technically feasi-ble, energetically efficient and safe for man andenvironment as a result of the principally fore-seen leakage-free extraction of natural gas.Facing the ever increasing shortage of the ener-gy carriers crude oil and natural gas, theimmense reservoirs of gas hydrates constitute a

great alternative energy resource that may beexploited in a promising and innovative wayusing the mammoth pump system. Until thedevelopment of efficient and entirely CO2 free(coal-fired) power plants will be realized, gashydrates may be used as primary energy resour-ce to be efficiently exploited using the extrac-tion system presented in this contribution.

With regard to speculative scenarios from theecological and social points of view, the preven-tive removal of unstable gas hydrate fields withthe suggested extraction device may be consi-dered as active climate protection measure.

Table 2: Symbols

Literature[1] BP; Statistical review of world energy

2003, 2003

[2] Sloan, E. D. Jr.: Clathrate Hydrates ofNatural Gases, Marcel Dekker Inc., NewYork 1998

[3] Kvenvolden, K. A.: Methane hydrate – amajor reservoir of carbon in the shallowgeosphere?, Chemical Geology, Vol. 71,pp. 41-51, 1988

[4] Milkov V. A.: Global estimates of hydrate-bound gas in marine sediments: howmuch is really out there?, Earth-ScienceReviews, Vol. 66, pp. 183-197, 2004

[5] Nixdorf, J.: Experimentelle und theoreti-sche Untersuchung der Hydratbildungvon Erdgasen unter Betriebsbedingungen,Dissertation TH Karlsruhe, 1996

[6] Parrish, W. P.; Prausnitz, J. M.: Dissociationpressures of gas hydrates formed by gasmixtures, Ind. Eng. Chem. Proc. Des.Develop. 11 (1972) 1, S. 26-35

[7] Kurnosov, A. V.; Manakov, A. Yu.;Komarov, V. Yu.; Voronin, V. I.; Teplykh, A.E.; Dyadin, Yu. A.: A new gas hydratestructure, Doklady Physical Chemistry, Vol.381, Nos. 4-6, pp. 303-305, 2001

[8] Konstantin, A.; Uchadin, K.A.; Ripmees-ter, J. A.: A complex clathrate clathratehydrate structure showing bimodal guesthydration, Nature, 397, S. 420-423, 1999

[9] Forschungszentrum für marine Geowissen-schaften (GEOMAR), Homepage der Gas-hydrate, http://www.gashydrate.de, 2003

[10] VDI-Hrsg.; VDI-Wärmeatlas, 8. erweiterteAuflage, VDI-Verlag Düsseldorf, 1997

[11] Bundesanstalt für Geowissenschaften u.Rohstoffe, Hannover/Berlin (BGR): Reser-ven, Ressourcen und Verfügbarkeit von En-ergierohstoffen 1998, 1998

[12] Takahashi, H.; Yonezawa, T.; Fercho, E.:Operation Overview of the 2002 MallikGas Hydrate Production Research wellProgram at the Mackenzie Delta in theCanadian Arctic, Offshore TechnologyConference, Houston, Texas, U.S.A., 05.-08.05.2003

151

152

Experimental determination of the petro-physical and thermodynamic properties of gas hydrates and hydrate bearing sediments

Introduction and objectives of the projectThe majority of global carbon is bound up ingas hydrate occurrences in continental perma-frost regions or below the sea floor continentalmargins. The potential exist that methane gascould be released from these occurrences, whichwould have disastrous consequences. Ther-modynamic conditions control the formation ofgas hydrates and their decomposition, howeverthe role of the properties and the influence offluid compositions are only vaguely known. Thedetermination of the physical and chemicalparameters affecting the stability of gas hydrateoccurrences provide a sound basis for:- understanding the kinetics of gas hydrate

formation and decomposition- the quantitative determination of the gas

hydrate content of sediments with help of geophysical well logging and field measurements

- understanding seismic signatures and fea-tures of gas hydrates (absorption, BSR)

The relevant parameters for stability, formationand decomposition processes of (methane) gashydrates (in sediments) were focus of thiswork. The project was divided in two sections:

1) Thermodynamic properties of (mixed) gashydrates: the aim of this work was establishprecise phase for the fundamental thermo-dynamic properties of pure methane hydra-tes and mixed gas hydrates containing CO2,C2H6, C3H8 and H2S beside methane. Thesephase diagrams establish stability fields,decompositions lines, miscibility gaps, com-position of the gas phase and the hydrate

phases. In addition, they predict specific con-ditions for the coexistence of hydrate pha-ses with different structures and phasetransition which can be caused by changingenvironmental conditions. Composition aswell as structurs have been determined byRaman spectroscopy.

2) Petrophysical properties of gas hydrate bea-ring sediments: The physical properties ofgas hydrate bearing sediments depend onthe gas hydrate content and the distribu-tion of the hydrate phase within the hostsediment. The aim of this project was tosimulate the natural process of pore spacehydrate formation in the lab and to measurethe physical sediment properties as a func-tion of hydrate content. These data providea basis both for improving the existingmodels or for developing new methods forthe characterization of naturally occurringhydrate deposits based on geophysical sur-face and borehole measurements.

Present status and results1. Determination of thermodynamic propertiesof (mixed) gas hydratesA key milestone was designing and construc-ting a pressure cell for the experimental set upwhich can be used over a temperature rangebetween -27 °C and + 80 °C. The temperatureof the sample cell is controlled by a cryostatand the temperature is determined with a pre-cision of ± 0.1 °C and the accessible pressurerange is between 0.1 and 10.0 MPa. A pressu-re controller adjusts the sample pressure with a

Schicks J., Spangenberg E., Naumann R., Kulenkampff J., Erzinger J.*

GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany, *E-Mail: erz@gfz-potsdam.de

precision of 2% rel.. The small sample volume(0.393 cm3) and the all-around cooling of thesample prevent temperature gradients. At agas flow of 1 ml/min, it takes 17 sec for theincoming gas flow to pass the cell body and toenter the inner cell space; this time is sufficientto allow the gas to attain the cell temperature.A quartz window permits the analysis of thephases by Raman spectroscopy as well as visu-al observation and the recording of microsco-pic photo-documentation of formation anddecomposition processes. The experimentalsetup with the confocal Raman spectrometer(LabRam, JobinYvon) permits the focus of thelaser beam on a precise point, e.g. the surfaceof a hydrate crystal, thus assuring that onlyselected volume is analyzed.

The experiments were carried out using thefollowing procedure: 150 µl of pure anddegassed water were placed in the sample cell.The cell was carefully sealed and flushed withthe appropriate gas before pressurization. Thesystem was cooled as rapidly as possible untilhydrates formed. After that, the system is war-med at constant pressure in order to melt most

of the hydrate. When only a few crystals areleft, the temperature is lowered about 0.5 °Cand the euhedral crystals of gas hydrate growunder steady state conditions. After the waterphase has completely transformed into hydra-te crystals Raman spectroscopic investigationat defined pressures and temperatures wereperformed on the following systems: CH4-H2O,CH4-CO2-H2O.

For testing the functionality of the experimen-tal set-up the first experiments were done withthe system CH4-H2O because numerous inve-stigations have already been performed on thissystem. The determined decomposition data cor-respond well to literature data (shown in Fig. 2)An interesting observation made by Ramanspectroscopy is that not all the analysed cry-stals show the expected structure I spectra.Some of the analysed crystals show differentspectra which indicate of structure II. Untilnow it was generally accepted that guestslike CH4 or CO2 form structure I hydrates,either individually or in combination, undermoderate conditions (P≤100 MPa and T ≤+20°C) and that structure I hydrate is the

153

Figure 1: Design of the sample cell

154

thermodynamically most stable phase forsuch guest.The experimental set-up allows the in-situ Ramanspectroscopic measurements on different cry-stals with different structures which are coexi-

sting under identical P-T conditions as is shownin Figure 3: It is known that the C-H symmetricstretch vibrational frequency for methane canbe correlated with cage size – with vibrationalfrequencies for small cavities shifted to higher

Figure 2: P-T-diagram for methane hydrate

Figure 3: Coexistence of structure I (s I) and structure II (s II) methanehydrates. The width of the image is equivalent to 450 µm.

frequencies relative to those for methanemolecules in large cavities according to thePimentel-Charles (1963) »tight-cage-loose-cage model« as recently elaborated by Sub-ramanian and Sloan (2002). Since the ratios oflarge to small cavities for structure I and struc-ture II hydrate are 3:1 and 1:2, respectively, therelative peak areas in the experimental spectrashould readily distinguish between the twostructures. It turns out that the ratio for theRaman bands of most of the crystals is close to

3:1 (Figure 4a), indicative of structure I aswould be expected for methane hydrate.Nevertheless, the Raman spectra of a few cry-stals (Figure 4b) gave spectra with peak inten-sities 1:2, indicating the presence of structureII hydrate.

It is clear that structure II methane hydrate isless stable than structure I methane hydrateunder these conditions: if the conditions arechanged, for instance, to lower temperature at

155

Figure 4: Raman spectra of structure I (a) and structure II (b) methane hydrates

156

constant pressure, or to higher pressure atconstant temperature, the structure II crystalstransform. First the crystals become less well-defined. A rounding of the crystals, remini-scent of a melting process, is followed by atransformation to a fine-grained hydrate mass.This exothermic transformation process -

beginning with the structure II crystals - indu-ces a recrystallization of the entire hydratephase until a new steady state is reached. Allof the hydrate phase now has the fine-grainedtexture before new well-defined crystals startsto grow again.

Figure 5: Raman spectra documenting the transformation of struc-ture II methane hydrates into structure I methane hydrates.

157

Raman spectroscopic measurements (shown inFigures 5a-d) on this hydrates during thechange in morphology show a loss of thestructure II character: the intensity of themethane band at 2904 cm-1 (large cavities)increases, while the intensity of the methaneband at 2916 cm-1 (small cavities) decreases.Finally, structure II methane hydrate can nolonger be detected by Raman spectroscopy.These results have been published elsewhere(Schicks and Ripmeester, 2004)

A similar process was observed for the systemCH4-CO2-H2O. Analogous to the behaviour ofthe pure methane hydrates, CH4-CO2-hydratesgrow as euhedral crystals at conditions close tothe decomposition line (Figure 6a). Decreasingthe temperature induces a change in morpho-logy of the crystals as it is shown in Figure 6b.In the end only a gas hydrate mass with a fine-grained texture remain (Figure 6c).

Raman spectroscopic measurements indicatethat a small amount of such euhedral crystalshave the unstable structure II (approx. 5-7%)whereas a large majority of the crystals havestructure I. Figure 7 presents the pressure andtemperature conditions where a coexistence ofstructure I and structure II hydrate crystals ispossible. Please note that – compared to thepure methane hydrate - the decompositionline of the mixed CH4-CO2-hydrates is shiftedto higher temperatures or lower pressures.

Hitherto no evidence has been reported forthe coexistence of structure I and structure IIhydrate phases in natural gas hydrate samples.Raman spectroscopic investigations of gashydrate samples from the Mallik 5L-38 drillingproject indicate only structure I methanehydrate (see also Figure 8). It should be notedthat during the recovery of natural sampleschanges in pressure and temperature are inevi-table, such changes could induce a transfor-mation process.

In order to study the influence of other gasessuch as H2S on the stability and composition of

Figure 6a-c: Growth and transformation processof CH4-CO2-hydrates

158

Figure 7: P-T-diagram of CH4-CO2-H2O System.

Figure 8: Raman spectrum of natural methane hydrate from Mallik 5L-38 project

gas hydrates, Raman spectroscopic investiga-tions were conducted on C-H-N-S bearing gasinclusions in fluorite. The data lead to the con-clusion that H2S is preferentially incorporatedinto gas hydrates whereas nitrogen was notdetected in the hydrate phase. Furthermore,such mixed gas hydrates show a high thermalstability; the decomposition temperature wasdetected at 300 K.

2. Determination of the petrophysical properties of gas hydrate bearing sediments:

Our experimental concept is based on a modelof the natural process of pore space hydrateformation. Methane solved in water migratesupwards into a formation which provides lo-wer pressure and temperature conditions. Inmarine environments or in arctic areas suchupward migrating, methane charged water willpass through a zone with pressure temperatu-re conditions under which methane hydrate isstable. Whether or not hydrate forms at thisdepth range will depend on the methane sup-ply and the sediment properties. To simulate this process in the lab we have

designed and built an apparatus which con-sists of a thermal insulated box with two com-partments (Figure 9).

The temperature in both compartments can becontrolled independently. The first compart-ment is kept at a temperature above the hy-drate stability. It represents the deep subsurfa-ce where methane is formed and water is char-ged with methane. It contains a methane volu-me which is separated from the methane cylin-der and the rest of the system by two valves.The volume can be charged with methane byopening valve v1. The exact amount of gas inthe volume can be calculated from the idealgas law. Via valve v2 the gas volume is connec-ted to the rest of the system which is filled withdegassed NaCl solution. By opening valve v2

the water reservoir is charged with methane.From the pressure drop in the gas volume wecan calculate how much gas was transferred tothe water reservoir where it starts to dissolve inthe aqueous solution. To speed up this processthe water is circulated trough the reservoirwith the bypass valve v3 opened until thesystem pressure is constant. Then the bypass isclosed and the regulation valve RV is opened

159

Figure 9: Experimental system used to generate methane hydrate inthe pore space of a sediment sample and which can measure thepetrophysical sediment properties.

160

so that methane charged water can flow tothe sample cell in the second compartmentrepresenting the depth range in the subsurfa-ce that is within the hydrate stability field. Thepressure and temperature is measured at thefluid inlet and outlet of the sediment cell. Theclosures of the cell are made of stainless steeland act as current electrodes A and B. Theycontain a Pt100 temperature sensor and ultra-sonic P- and S-wave transducers. The cell con-sists of three high tensile strength Plexiglasrings separated by two thin stainless steel ringswhich act as potential electrodes M and N. Theinner diameter and the length of the cell is50mm. Measurements of sonic wave velocitiesand electrical resistivity are recorded at prede-termined time intervals. The methane chargedwater coming from the first compartmentcools and enters the sample which is in thehydrate stability field. When hydrate forms inthe sediment it consumes water but excludesthe salt ions, hence the salt content of theremaining solution increases. The increasing

salt content results in an increasing electricalconductivity of the water which is measuredwith the conductivity sensor CS. The waterpassing the sample cell looses a part of the sol-ved methane due to the hydrate formationprocess. When the water enters the reservoiragain, it is recharged with methane. The metha-ne consumption results in a drop of systempressure. To prevent the pressure from decrea-sing to the stability boundary where hydrateformation stops additional methane must beperiodically feed into the system. The amountof hydrate formed can be calculated from thechange in the electrical conductivity of thewater. Because the solubility of methane is low(about 10-3mol/mol at 5MPa and 20°C) the for-mation of hydrate from a solution of methanesaturated water is a very slow process.

Due to the absence of empirical data on theinfluence of hydrate saturation on the petro-physical properties of hydrate-bearing sedi-ments, it is a challenge to estimate the hydra-

Figure 10: Resistance of the circulating water and hydratesaturation of a glass bead sample versus time.

te content of a formation based on geophysi-cal field and well log data. Of particular interestwould be the use of sonic velocities and electricresistivities because they are more stronglyaffected by the presence of gas hydrate as com-pared to other geophysical properties.To determine the amount of hydrate in thepore space from physical in-situ measurementsPearson et al (1986) suggested the use ofArchie’s Law (Archie, 1942). Archie’s Law con-sists of two equations. The first Archie equa-tion is for fully water saturated rocks with aconductivity σ0

(1)

where F0 is the formation resistivity factor ofthe fully water saturated rock, σw is the con-ductivity of the pore water, φ is the porosity ofthe rock, and a and m are the empirical Archieparameters.The second Archie equation is for partly satu-rated rocks with a conductivity of σt

(2)

where Sw is the water saturation, σt is theconductivity of the partly saturated rock, Ft isits formation resistivity factor, and n is theempirical saturation exponent.For practical applications equation (1) is oftenused with the resistivity index I,

(3)

which is the ratio of conductivities when therock is fully vs. partially saturated. In equations(1) - (3), and throughout this paper, brine is as-sumed to be the only conducting phase. Follo-wing this suggestion the fraction of the total

pore space occupied by gas hydrates has beenestimated from resistivity measurements in gashydrate research wells e.g. ODP Leg 164 site994 (Paul et al., 1996) and Mallik 2L-38(Dallimore et al., 1999). The empirical satura-tion exponent in both studies was chosen tobe n= 1.9386 (Pearson et al. 1983).The empirical saturation exponent is controlledby the distribution of the conductive brine inthe pore space, thus it depends on wettingproperties, saturation history, and the rockmicrostructure. The influence of different typesof hydrate occurrences on the resulting electri-cal properties has been studied theoretically bySpangenberg (2001). The formation of porespace hydrate was investigated based on asphere packing model. For the situation thatthe pore water is the wetting phase and thehydrate forms as non-cementing material inthe pore space, the model predicts a saturationexponent that depends on the saturation itself.Our measurements confirm this theoreticalprediction. Figure 11 shows the saturation expo-nent of the hydrate-bearing glass bead sedi-ment together with the resulting errors a con-stant saturation exponent is assumed.

Most attempts to predict hydrate contentsfrom velocity data are based on derivates ofthe time-average relation (Wyllie et al., 1958),which relates the velocity of a fluid saturatedconsolidated rock to the velocity of the solidphase, the velocity of the fluid phase and thevolume fractions of both phases. To apply thisapproach to ice- or hydrate-bearing formationsa three phase time average relation version hasbeen used (Timur, 1968; Pearson et al., 1986)shown by the following equation:

(4)

where Vtar is the p-wave velocity of the hydra-te-bearing sediment, Vh is the p-wave velocityof pure hydrate, VW is the compressional wavevelocity of the pore fluid, Sh is the hydrate satu-ration , and φ is the porosity containing the

161

hydrate and pore fluid. A drawback of the timeaverage approach is that its predictions can bewildly inaccurate if the rock is unconsolidated(Wyllie et al., 1958). In such a situation an arti-ficially low matrix velocity can be used (Hoyeret. al, 1975) to adjust for the unconsolidatedstate of the porous medium. For marine sedi-ments sometimes the equation of Wood(1941) is used, which can also be adjusted forhydrate-bearing sediments to

(5)

where ρ is the bulk density of the sediment inthe form

(6)

ρw is the density of the pore water, ρh is thedensity of pure hydrate, and ρm is the densityof the matrix material. This equation pertainsto particles in suspension and can sometimesunderestimate the true velocity porosity rela-tionship in marine sediments. Lee et al. (1996)use a weighted combination of the time avera-ge relation (4) and Wood’s equation (5) to pre-dict the velocity of hydrate-bearing sediments:

(7)

A comparison of the time average relationwith an adjusted matrix velocity, Lee’s weigh-ted 3-phase equation and our measurementsof ultrasonic p-wave velocities are shown inFigure 12.With increasing hydrate content we observedan increase in signal attenuation. At a hydra-te saturation of about 40% we detected anew first arrival which is generally weak andonly just above the noise level. This observa-tion explains the sudden increase of velocityseen in Figure 12.

By increasing hydrate saturation still further thisfirst arrival becomes more pronounced. Thisbehaviour is related to the special situation ofwave propagation within a medium which iscomposed of two frameworks, a grain frame-work and a hydrate framework. Certainly, furt-her investigations are necessary to understandthe peculiarities of wave propagation in hydratebearing sediments.

162

Figure 11: Measured and predicted saturation exponentas a function of water saturation and the Archie predic-tion for the hydrate content with constant n.

ConclusionUntil now the reason for the formation of struc-ture II CH4-(CO2)-hydrates under the conditionswhere only structure I CH4-(CO2)-hydrate wasthought to be stable is not clear. Subramanianand Sloan (1999) observed during real-timeRaman spectra monitoring a transformation ofdissolved CH4 to CH4 in the large and small cagesof structure I methane hydrates. In the initialstages of hydrate formation the large to smallcages ratio was 0.5, which correspond well withthe large to small cages ratio of structure IIclathrate hydrates. They assumed that the for-mation of the large cavity is the rate-limitingfactor in the structure I methane hydrate forma-tion. Staykova et al. (2003) observed the tran-sient formation of structure II CO2-hydratesduring the growth of pure CO2-hydrates on ice-grains. The 129Xe NMR experiments of Moudra-kovski et al. (2001), who determined the cageoccupancy ratio as a function of time during the

early stages of the formation and growth ofhydrate also observed a predominance of smallcages during in the initial stages of the reaction,indicating a major role for the small cavities. Inthese studies the presence of large numbers ofguest molecules in small cages at early timesin the hydrate growth process suggests thatstructure II hydrate may be the kineticallyfavoured structure as it contains the smallcage as the major building block for the struc-ture II framework. The observation of a coexi-stence of structure I and structure II hydratesat the conditions mentioned above suggeststhat the initial product is determined by kine-tics in which structure II appears as a meta-stable phase. We quantified for the first time the relation-ship between electrical resistivity and sonic p-wave velocity on hydrate saturation. Themeasurement of electrical resistivity is in goodagreement with a theoretical prediction for

163

Figure 12: Comparison of the measured p-wave velocities with the time average relationwith an adjusted matrix velocity and Lee’s equation with W=1.51 and n=1.

164

spherical packing. However, for the sonicwave velocities, the observed data show thatthe dependence of velocity on hydrate satu-ration is more complex than what is predictedby the currently used models.

AcknowledgementThe GEOTECHNOLOGIEN programme of BMBFand DFG provided funding of this work throughthe Research Grant G0555A.Further we would like to thank John Ripmeester,National Research Council of Canada for thefruitful cooperation.

References Archie, C.E., The electrical resistivity log as anaid in determining some reservoir characteri-stics, Trans. Am. Inst. Min. Metall. Pet. Eng.,146, 54-62, 1942

Dallimore, S. R., T. Uchida, and T. S. Collett,Scientific Results from JAPEX/JNOC/GSC Mallik2L-38 Gas Hydrate Research Well, MackenzieDelta, Northwest Terretories, Canada. Geolo-gical Survey of Canada, Bulletin 544, 1999

Hoyer, W.A., S.O. Simmons, M.M. Spann, andA.T. Watson, Evaluation of permafrost withlogs, Trans. SPWLA Annu. Logging Symp.,16th paper 15pp., 1975

Lee, M.W., D.R. Hutchinson, T.S. Collett, andW.P. Dillon, Seismic velocities for hydrate-bea-ring sediments using weighted equation, J.Geophys. Res., 1001, B9, 20,347-20,358, 1996

Moudrakovski, I. L.; Sanchez, A. A.; Ratcliffe,C. I.; Ripmeester, J. A. (2001) J. Phys. Chem. B,105, 12338

Paull, C.K., R. Matsumoto, P.J. Wallace,Proceedings of the Ocean Drilling Program,Initial Reports, Vol. 164, 6. Site 994, pp 142-144, 1996

Pearson, C.F., Halleck, P.M., McGire, P.L., Her-mers, R.E., and Mathews, M.A., Natural GasHydrate Deposits, a Review of in situProperties, J. Phys. Chem., v. 87, no. 21, pp.4180-4185, 1983

Pimentel, G.C.; Charles, S.W. (1963) PureAppl. Chem., 35, 111

Schicks, J.M.; Ripmeester, J.A. (2004) Angew.Chem. Int. Ed., 43, 3310

Spangenberg, E., Modeling of the influence ofgas hydrate content on the electrical propertiesof porous sediments, J. Geophys. Res., 106,B4, 6535-6548, 2001

Staykova, D.K.; Kuhs, W.F.; Salamantin, A.N.;Hansen, T. (2003) J. Phys. Chem., 107, 10299Subramanian, S.; Sloan, E.D. (1999) Fluid PhaseEquilib., 158-160, 813

Subramanian, S.; Sloan, E.D. (2002) J. Phys.Chem. B, 106, 4348

Timur, A., Velocity of compressional waves inporous media at permafrost temperature, Geo-physics, 33, 584-594, 1968

Wood, A.B., A Text Book of Sound, 578 pp.,Macmillan, New York, 1941

Wyllie, M.R.J., A.R. Gregory, and G.H.F.Gardener, An experimental investigation offactors affecting elastic wave velocities inporous media, Geophysics, 23, 459-493, 1958

165

166

GASHYDRATES – Paleoatmospheric archiveReconstruction of paleoclimatic changes in thesource strength of potential methane sourcesusing the methane isotopic signature in bub-ble enclosures in polar ice cores

ObjectivesMethane concentration records in polar icecores [Blunier and Brook, 2001] show promi-nent variations in parallel to rapid climatevariations during the last glacial period, the socalled Dansgaard/Oeschger events [Johnsen,et al., 1992]. The origin of this additionalmethane on the one side and the trigger forthe temperature shifts on the other side is stilla matter of debate, where especially the roleof a potential methane release by destabiliza-tion of marine gas hydrates remains uncer-tain. The hydrogen and carbon isotopic signa-ture of the corresponding CH4 changes in icecores can resolve this puzzle. Accordingly thelong-term perspective of this project is thequantitative differentiation of the contribu-tion of different CH4 sources to the changesobserved in ice cores.

Such measurements, however, have not beenpossible so far because of the strong limita-tions in available sample size and the require-ments on the analytical precision. The only pre-vious ice core δ13CH4 analyses used 10 kg ofice, representing about 1.5 meters of ice core[Craig, et al., 1988]. Accordingly, the goal ofthis project has been the technical develop-ment of an efficient air extraction for air enclo-sed in clathrate and bubble ice as well as of a

high precision gas chromatography isotoperatio mass spectrometry method (GCirmMS)on small ice samples (200-400 g) to enablesuch measurements.

ResultsIn the course of this project an automated,quantitative and fractionation free method forthe extraction and isotope analysis of CH4

using an innovative GCirmMS technique couldbe established based on previous work by[Merritt, et al., 1995]. For δ13CH4 a reproduci-bility of 0.1-0.2 ‰ for a sample size of only 20ml STP of air (equivalent to approximately 200g of ice) could be established using preconcen-tration and complete combustion of CH4 toCO2 in a micro-combustion oven. This fulfilledthe required accuracy to resolve the expectedδ13CH4 changes during Dansgaard/Oeschgerevents (Figure 1). For δD(CH4) also a newGCirmMS method could be established rea-ching a reproducibility of about 6 ‰ on 100 mlSTP of air. This still falls about a factor of twoshort of the anticipated accuracy/sample requi-rements, but is already sufficient to resolve theexpected effects in δD in the atmosphere for asignificant release of methane from marine gashydrates (see Figure 3). First results on air andice core samples reproduced previously publis-

Hubertus Fischer

Alfred Wegener Institute for Polar and Marine Research, Columbusstrasse, 27568 Bremerhaven, Germany,

E-Mail: hufischer@awi-bremerhaven.de

167

Figure 1: Isotope temperature (δ18O) and CH4 profiles in theGISP2 ice core, Central Greenland [Blunier and Brook, 2001;Grootes, et al., 1993]

Figure 2: Comparison of δ13C values of recent air [Quay, et al.,1999] and preindustrial [Craig, et al., 1988] ice core samples withvalues determined in this study. Full circles indicate a bubble closeoff in the firn column at 73m, open circles at 63 m, which influen-ces the age of the air enclosed.

168

hed values very well (Figure 2). Routine appli-cation of this method on ice core samples willfollow in the future.

Model considerationsFor the interpretation of the ice core data interms of changes in CH4 concentration and iso-topic composition a simple box model of theglobal methane cycle has been developed,which represents a substantial expansion of pre-vious model calculations for CH4 concentrations[Chappellaz, et al., 1997]. The model reprodu-ced recent and preindustrial CH4 concentrationsand carbon isotopic signature very well.

To predict the influence of a methane releaseby destabilisation of marine gas hydrates ashort term CH4 source of 100 Tg CH4/a over 40years with an isotopic signature of δ13C = -60‰and δD = -180‰ [Whiticar et al., 1986] wasassumed. This leads to a short term increase inCH4 concentration to 650 ppbv, of δD to -40 ‰ and a decrease in δ13C to –52 ‰ (thinlines in Figure 3). Due to the low pass filterinduced by the bubble close off, this amplitu-de is reduced to about 460 ppbv, –100‰ and-49.2‰, respectively (thick lines).

ReferencesBlunier, T., and E. J. Brook (2001), Timing of mil-lenial-scale climate change in Antarctica andGreenland during the last glacial period, Sci-ence, 291, 109-112.

Chappellaz, J., et al. (1997), Changes in theatmospheric CH4 gradient between Greenlandand Antarctica during the Holocene, J GeophysRes, 102, 15987-15997.

Craig, H., et al. (1988), The isotopic composi-tion of methane in polar ice cores, Science,242, 1535-1539.

Grootes, P. M., et al. (1993), Comparison of oxy-gen isotope records from the GISP2 and GRIPGreenland ice cores, Nature, 366, 552-554.

Johnsen, S. J., et al. (1992), Irregular glacialinterstadials recorded in a new Greenland icecore, Nature, 359, 311-313.

Merritt, D. A., et al. (1995), Carbon isotopic ana-lysis of atmospheric methane by isotope-ratio-monitoring gas chromatography-mass spectro-metry, J Geophys Res, 100, 1317--1326.

Quay, P., et al. (1999), The isotopic compositionof atmospheric methane, Glob BiogeochemCyc, 13, 445-461.

Figure 3: Model results on a disturbance of the global methanecycle by a hypothetical release of CH4 from marine gas hydrates.

169

170

Gas Hydrates in Hemipelagic Sediments – CONGO

1. INTRODUCTIONThe nature of sedimentary structures anddeformation as well as the type of dewateringand fluid flow is significantly different betweenactive and passive continental margins. Whileactive margins are characterized by subduc-tion, compression, sediment accretion, pro-nounced faulting, and locally high pore pres-sure (Moore & Vrolijk, 1992; Hyndman et al.,1992; Carson & Screaton, 1998; Zühlsdorff &Spieß, 2004), passive margins are characteri-zed by continuous sediment accumulation andthe development of sedimentary basins, wherethe creation of vertical permeable pathwaysrequires marginal block faulting, basin subsi-dence, differential loading, slope instabilities ordiapirism of mud or salt (Graue, 2000; Hooperet al., 2002; Babonneau et al., 2002). Thus,the nature of pathways as well as the drivingforces controlling vertical fluid flow are diffe-rent at active and passive margins, and, conse-quently, the development of gas hydratesystems in the shallow sub-seafloor may besignificantly affected.

Permeability as a key parameter controls thesupply of fluids (including gas) from greaterdepth, the fluid transport within the GHSZ,and thus the presence, concentration anddistribution of gas hydrate. At active margins,permeability is enhanced by massive deforma-tion, but fine grained passive margin sedi-ments may often act as a hydraulic seal withregard to vertical fluid flow. Thus, larger gashydrate reservoirs at passive margins can only

develop if efficient migration pathways to-wards and into the GHSZ do exist, gas satura-tion of pore water is reached within or bene-ath the GHSZ, and sufficient volumes of waterare available to allow for the growth of mas-sive hydrate layers. As a consequence of limi-ted permeability, the distribution of gas hydra-tes on passive margins may be more limitedcompared to active margins, as it is indicatedby the worldwide distribution of known gashydrate reservoirs and gas hydrate BSRs(Kvenvolden, 1994; Gornitz & Fung, 1994).However, hydrocarbon systems within thickersediment packages, that are characterized by ahigh content of organic matter, as well as areasof massive slumping, provide favorable condi-tions for gas hydrate accumulation if sedimen-tary, mechanical or tectonic processes createpermeable pathways for hydrocarbon transport.

During Cruise SO 86 with R/V Sonne in 1993,locations of anomalous acoustic columnarblanking features had been identified in fewmultichannel seismic and sediment echosoun-der lines from the Lower Congo Basin at 5°S inabout 3000 m water depth, and were subse-quently studied in detail during R/V MeteorCruise M47/3 in 2000. Clear indications forfluid venting were derived from samples of car-bonate precipitates, shell fragments, livingvent fauna and massive gas hydrates, whichcould be recovered from the sea floor in thevicinity of circular depressions of a few hund-red meters diameter and a few to twentymeter depth. Beeing a typical feature found on

Spieß V. (1), Zühlsdorff L. (1), Villinger H. (1), Flueh E. (2), Bialas J. (2), Kasten S. (1),

Schneider R. (1), Bohrmann G. (1), Sahling H. (1)

(1) Department of Geosciences, Bremen University, Klagenfurter Straße, D-28334 Bremen, Germany

(2) IFM-GEOMAR, Wischhofstraße 1-3, D-24148 Kiel, Germany

some passive margins, and the only so farobserved structures on the Congo margin rela-ted to fluid venting, the pockmarks were cho-sen as a primary target for the first phase ofresearch on gas hydrates and gas and fluidventing in the Congo region.

The surface expressions of such venting sitesmay be somewhat characteristic for the drivingforces, flux rates, gas focusing mechanism andsediment deformation styles in the region, sinceonly pockmarks have been so far found in theCongo region, while other fluid venting sites arecharacterized by positive sea floor morphology,as mounds or mud volcanoes. To investigate therelationship between the deeper sources of ven-ting and the surface expressions, an integratedresearch program was designed for few suchpockmark structures, combining multichannelseismics, sediment echosounder surveys andswath bathymetry operated from the vesselwith deep tow side scan and seismics, heat flowin situ measurements, video profiling, watersampling, gravity coring and videoguided multi-corer and grab sampling.

The Lower Congo Basin is located off themouth of the Congo River (Figure 1), whichprovides significant sediment influx from thecontinent. A major portion is transportedthrough the Congo Canyon (Heezen et al.,1964; Shepard & Emery, 1973; Droz et al.,1996) directly into the deep sea. In waterdepths below 3000 m, the narrow and deepchannel opens into the Congo Cone, wheretypical fan deposits and channel/levee systemscan be found.

However, drilling of three sites during ODP Leg1075 (Wefer et al., 1998) as well as the respec-tive seismic pre-site survey data (Uenzelmann-Neben, 1998) confirmed that the upper fewhundred meters of the sediment section northof the Congo Canyon are dominated by hemi-pelagic sediments, which reveal low reflectivityand only subtle changes in physical propertieswith depth. The hemipelagic sediment inputoriginates primarily from the fine grained

suspended material of the river and high bio-logic productivity (Jansen et al., 1984), associ-ated with significant flux of organic matter tothe sea floor. The hemipelagic sequences thusrepresent the host facies for gas and gashydrate occurrences.

The overall structure of the continental marginis mainly shaped by salt diapirism and raft tec-tonics, which are closely related (Emery et al.,1975; Marton et al., 2000; Valle et al., 2001;Rouby et al., 2002). Although major differen-ces regarding sedimentation rates and tecto-nics exist regionally, pronounced faulting maylocally support hydrocarbon and gas migrationand favors conditions for shallow gas and gashydrate accumulation.

Three of the largest pockmarks called HydrateHole, Black Hole and Worm Hole (Figure 1) aswell a number of smaller features were there-fore re-visited during R/V Meteor Cruise M56(2002) for detailed seismoacoustic mapping intwo and three dimensions and an extensivevideo survey and sampling program. A secondphase was planned to further investigate keyparameters for such systems as the degree ofdeformation or the facies of host sediments forhydrates and gas.

2. OBJECTIVESFluid venting systems function on multiple sca-les, when the pathways caused by large-scalesalt-related sediment deformation, the reservoirsof free gas beneath and of gas hydrates withinthe gas hydrate stability zone are connected tothe shallow subsurface, where methane fluxesthrough focusing chimneys counteract withmethane oxidation and local biologically media-ted processes create precipitates and gas hydra-tes and support a large variety of life forms. Theoverall objective was to study a gas hydratesystem, where individual control parameters canbe distinguished and investigated by high reso-lution methods looking at potential migrationpathways, gas traps, hydrate reservoirs, seafloorfeatures and any post-sedimentary changes.

171

172

Among the various objectives of such an inter-disciplinary studies, we planned to focus on:- Characterization of seismic properties at maxi-

mum available resolution within a sedimentvolume, which extends from the sea floor tobeneath the gas hydrate stability zone.

- Integration of seismic data of different fre-quency content to optimize resolution atany given depth level both for structuralimaging with reflection seismics and tomo-graphic characterization.

- Comparison of lateral and vertical changesin seismic properties with evidence from seafloor mapping, in situ physical and chemicalmeasurements, coring transects and subse-quent studies on pore water, cores and sam-ples and study of microbiological changes.

- Reconstruction of fluid/gas fluxes throughthe sediment column as a function of struc-tural disturbance and methane supply.

- Understanding of processes related to fluid

upflow within the surface zone of steepgeochemical and physical gradients.

- Linkage of surface signals expressed in avariety of chemical, physical and biologicalparameters to seismic properties at depthand the occurrence and distribution of gashydrates and free gas.

3. PRESENT STATUS AND RESULTS

3.1 METHODSHigh resolution seismic and acoustic imaging isa major tool to study both principal geologicalparameters controlling the occurrence of gashydrate as well as the distribution of free gasand gas hydrate within shallow sediments. Thepresence of gas hydrate further reveals infor-mation about the nature and efficiency of fluidflow pathways (Zühlsdorff et al., 2000). In thehemipelagic sedimentary sequences off the

Figure 1: Location map of Congo pockmark area with Hydrate Hole, Black Hole andWorm Hole as the main targets for seismoacoustic surveys and sea floor sampling.

Congo, where layering and uniform propertiesexist at the time of deposition, modification ofsediment physical properties due to mixingbetween water, gas and gas hydrate withinpore spaces affects amplitude and phase pro-perties of seismic reflections. Furthermore,fluid flow and gas or gas hydrate accumula-tions are often associated with sediment defor-mation or faulting on different scales, partlyrelated to salt diapirism at greater depth.

While multichannel seismics in conjunctionwith ocean bottom hydrophones, sedimentechosounding and swath bathymetry wereused for first order structural mapping, deeptow side scan and seismic data allowed tozoom in onto the surface characteristics onthe meter scale providing a basis for subse-quent sampling. Video sled transects in thevicinity of the pockmarks then allowed tocalibrate the different backscatter signalsconfirming the direct relationship of highbackscatter with the massive occurrences ofbioherms or chemoherms. Dedicated videoguided sampling and densely space gravitycorer transects were the last steps in the rowof methods and techniques to collect all avai-lable information on the processes, whichwere linked to the presence and migration ofmethane in soft hemipelagic sediments.

Hydro-acoustic systemsA deep-towed DTS-1 side-scan sonar was usedto identify active seepage areas. For M56 side-scan sonar mapping, acoustic energy at 75 kHzmain frequency, which is back-scattered fromtargets on the seafloor, is recorded over 750 mwide swaths to both sides. The signal strength isrelated to both micro-topography and physicalproperties of these targets. Across-track resolu-tion is 0.1 m whereas along-track resolution is0.75 m at a typical towing speed of 2.5 knots.During operation, a sensor on the instrumentprovided information about heading, roll andpitch. Underwater navigation and depth measu-rement was provided by a responder-based tele-metry system, but not working properly.

Bathymetric data were collected with theHydrosweep swath sounder onboard R/VMeteor that is operated at 15.5 kHz and pro-duces usable data up to 2 x water depth witha horizontal resolution of the order of 100 m(at 3000 m depth and depending on ship’sspeed and other factors). To suppress refrac-tion effects on the outer beams, theHydrosweep system uses a calibration mode tocompare depth values of the central and outerbeams in order to calculate a mean soundvelocity. Using this configuration, residualdepth errors are minimized to <0.5% of thewater depth, on the order of 15 m for typicalwater depths in this area (Grant & Schreiber,1990). However, for high coverage during 3Dseismic surveying (up to 100x), noise could besignificantly suppressed and vertical resolutionappeared to be on the order of 1 meter.

The Parasound echosounder onboard R/VMeteor is a narrow-beam system that is opera-ted at 4 kHz and used to image the uppermostpart of the sediment column at very high reso-lution. Depth penetration depends on the typeof sediment and was typically on the order of80-100 m. Footprint size is only 7% of waterdepth, diffraction hyperbolae are suppressed,and both lateral and vertical resolution are sig-nificantly higher in comparison to conventionalsediment profiling systems. Both Hydrosweepand Parasound systems are hull-mounted andcompensate for heave, pitch and roll. They areoperated simultaneously during multi-channelseismic or DTS-1 data acquisition.

Multi-channel seismics and ocean bottom instrumentsThe multi-channel seismic system of theUniversity of Bremen is optimized for highresolution imaging of small-scale sedimentarystructures and closely-spaced layers on a meterto sub-meter scale. The alternating operationof a small chamber watergun (0.16 L, 200-1600 Hz, Sodera) and a Generator-InjectorGun (GI-Gun, Sodera) with chamber volumesof 2 x 1.7 L (30-200 Hz) yielded two separate-ly recorded seismic data sets along each seis-

173

174

mic line. Both data sets distinguish withrespect to depth penetration and temporalresolution and are further supplemented bysimultaneously recorded Parasound data (seeabove) in order to image each depth level atbest possible resolution and to be able to com-pare seismic signatures on different scales.

Using a Syntron multi-channel streamer with96/48 recording groups separated by 12.5 m(Cruise M47/3) and 6.25 m (Cruise M56),respectively, trace fold of 8-15 was achievedfor a CMP spacing of 10 m, that provided thebest compromise between trace density, imagequality and noise reduction. Six remotely con-trolled birds with compass units kept the strea-mer at ~3+/-0.5 m depth and provided cross-track distances for all streamer groups.Positioning was provided differential GPSrecordings. Standard data processing includedediting, bandpass filtering, correction for geo-metrical spreading, stacking and time migra-tion. During Cruise M56, an area of about 7 x2 km size was covered by parallel profiles sepa-rated by 25 m for a threedimensional imagingof detailed sediment structures and vent-rela-ted features. Individual 2-D lines were binnedand stacked for a pre-defined grid of 10 m(inline) and 25 m (crossline) cell size, leading toa cell coverage between 5 and 10 fold. Traceinterpolation could be avoided due to thesmall inline spacing.

Four ocean bottom hydrophones and fiveocean bottom seismometers were located inthe vicinity of Hydrate Hole during the 3D seis-mic survey, to collect data for a tomographicanalysis of velocity and signal attenuation nearthe columnar blanking zone and within thegas hydrate stability field. More accurate thanfrom multichannel seismics with the 600 mlong streamer, velocity models derived fromOBS measurements shall provide the basis forthe quantification of gas hydrates and gasreservoir near the migration channel.

During the side scan profiles run with the DTS-1 system, seismic acquisition was opera-

ted in parallel, collecting GI Gun shots with a24-channel deep tow streamer attached to thefish. Due to the proximity of the receiversystem to the sea floor, the footprint of seismicreflections is much smaller and lateral resolu-tion can be significantly enhanced. However,fish navigation was not available and positio-ning quality is limited. Static corrections arevery difficult, and so far processing was limitedto dealing with single channel data sets.

Sea floor observations and heat flowThe video sled OFOS, towed close to the seafloor, was used to visually image the sea floor,while slowly cruising with a speed around 0.5knots, searching for indications of fluid ven-ting, precipitates, gas hydrates or vent fauna. Itis equipped with a black-and-white videocamera, xenon lights, a still camera and flash-lights, externally powered through the coaxialcable. The video signal is transmitted throughthe cable to a realtime display and is stored ontapes. A memory CTD gives additional infor-mation about depth, which is very helpfulwhen no navigation system is available. Inaddition to this standard configuration, amethane sensor was mounted on the sledduring this cruise. Data obtained by this sensorare discussed in the chapter about CTD/rosettedeployments.

To determine the regional and local heat flux insitu, miniaturized temperature loggers (MTL)were used. 4 to 6 MTLs were mounted ontothe strength member of a 6 m long probe oronto the barrel of the gravity corer. 71 mea-surements with the 6 m probe and 4 measure-ments on a gravity corer were carried outalong several transects crossing the main pock-mark structures. Measurements took 7-10minutes for penetration and 15 to 45 minutesfor transit with gear towed just a few tens ofmeters above the sea floor. Thermal conducti-vity was measured on sediment cores for cali-bration of heat flow profiles.SamplingFor sampling, TV guided grab (TVG) and multi-

corer devices were used, combined with a con-ventional gravity corer. The TVG is equippedwith a camera system and lights that arepowered by deep-sea batteries. The TV Grab isable to sample ~1.8 square meters of the seafloor. The TV-MUC, equipped with a camerasystem and xenon lights, consists of a conven-tional multicorer with 8 cores of 10 cm in dia-meter and is designed to recover undisturbedsurface sediment sections and the overlyingbottom water. It was particularly used to takewell-defined surface samples in clam or carbo-nate fields, and also attempts were made tosample bacterial mats or areas which seemedto show gas hydrates at the sediment surface.

With the gravity corer sediment cores between3.5 and 14.0 m core length were taken at 17stations. Altogether about 165 m of sedimentswere recovered. For dedicated sampling of gashydrates, instead of a plastic liner a plastic foilwas used to lay the sediment on deck andimmediately sample all sections for pieces of gashydrates. This »crude« method allowed us togain excellent, well-shaped hydrates that sho-wed nearly in-situ morphology and structure.

3.2 PRESENT STATUSOther than most GEOTECHNOLOGIEN projectson gas hydrates, the CONGO project startedlate during the funding period in spring 2002,since ship time was only available in late 2002,and funding and time for subsequent proces-sing of data and samples was restricted to ayear or less. As a consequence, most of thework, which was designed on a long term fun-ding and research program, as was requestedby the original gas hydrate program, could notbe finished. An extension of the research fun-ding was expected, but was not provided. Thefollowing results are therefore preliminary, andpublications could not yet be finished withinthe funding period.

3.3 PRELIMINARY RESULTS

Bathymetric and side-scan dataWithin the area covered by Cruises M47/3 andM56, bathymetric charting indicated severalpockmarks with depressions on the order of 1-30 m and diameters of 50-2000 m. Work,however, was concentrated on three large fea-tures called Hydrate Hole, Black Hole andWorm Hole (Figure 1). Since these featureswere covered by a 3-D seismic grid, swathmapping data coverage in this area was >100.While Hydrate Hole and Black Hole revealednumerous bathymetric features of differentsize and at different depth, Worm Hole repre-sents the largest seafloor depression, showing>40 m depth variation (Figure 1). The pro-nounced morphology may be related to a saltdiapir underneath as indicated by seismic datafor several pockmark structures in the area (seebelow), suggesting that the side walls ofWorm Hole are not necessarily the result ofejective processes.

At all three locations, the morphology of thepockmarks is characterized by small steps rat-her than by continuous depth variation, as itcould be confirmed from depth versus timerecords obtained from deep-tow video surveys.Outside the large depressions, smaller scalefeatures were observed that reveal diametersof the order of 1-10 m.

In general, a limited correlation between side-scan and bathymetric data is observed, indica-ting that both data sets provide basically diffe-rent types of information (Figure 2). Side-scansonar data reveal high backscatter patcheswithin the pockmark structures that usually donot cover the whole pockmark area. The wide-spread occurrence of clams, shells and musselsobserved at the seafloor during video observa-tions indicated active venting at high backscat-ter locations. Thus, high amplitude patchesvery likely correlate with enhanced methaneflux supporting vent communities. At HydrateHole, different levels of morphologic and bak-kscatter anomalies are observed, with the hig-hest backscatter associated with the North-East patch, whereas the topographic depres-

175

176

sion is much more pronounced at the South-West patch (Figure 2). Different characteristicsof the different patches may suggest differentages and stages of activity. Furthermore, allgravity corers taken within high backscatterareas contained several layers of gas hydrates,while hydrates were absent in the upper ~10m near the rim of the venting area.

Parasound data The Parasound record across Hydrate Hole andBlack Hole in Figure 3 is characterized by late-ral and vertical reflection amplitude variationsand shows acoustic signatures that are typicalfor most of the Congo pockmarks. At the bot-tom of the seafloor depressions, patches ofhigh reflection amplitudes without sharpboundaries and with only a few coherent re-flector elements are observed. Beneath,

almost complete acoustic blanking appearsto overprint a layer package of high reflec-tion amplitudes, that otherwise is laterallycontinuous. All described high amplitudefeatures are located well within the GHSZ, asit is estimated from water depth and stan-dard diagrams for hydrate stability, e.g.Dickens & Quinby-Hunt (1994).

Parasound data can be analyzed in greaterdetail at Hydrate Hole, where a dedicated sur-vey with closely spaced (75 m) grid lines wascarried out during Cruise M47/3. Due to thehigh data density, amplitudes and other acou-stic attributes can be investigated in a near-3-D volume. Digital seismograms were used todetermine maximum reflection amplitudeswithin 3 m thick slices and to map amplitudeanomalies as indicator for structural elements,that are not related to the hemipelagic layering(Figure 4). Time slices reveal four units with dif-ferent characteristics: (1) a seafloor depressionassociated with a weak seafloor return and theseafloor occurrence of carbonates and gashydrates within the upper 6 m of the sedi-ments (as it was confirmed by core data), (2)high amplitude patches beneath the pock-mark within the uppermost 10 m, a blankingzone beneath the high amplitude patches,and high amplitude patches next to the blan-king zone at 30 m depth, (3) a transition ofthe blanking zone from linear to circularshape at 40-50 m depth, associated with themaximum occurrence of high amplitude pat-ches (10-20 m thick) around the blanking,and (4) most pronounced blanking along aSW-NE trending lineament.

Due to the low wet bulk density of the hemi-pelagic sediment section, normal reflectioncoefficients are very low, and high amplitudestherefore may indicate the presence of anoma-lous physical properties. High reflectivity zoneswere identified both in the vicinity of gashydrate findings at the seafloor as well asaround a chimney characterized by very lowreflection amplitudes, that may indicate thepresence of free gas. Using the detailed surfa-

Figure 2: Side scan sonar image (top) and high resolutionbathymetry (bottom) of Hydrate Hole. The SE depression isappx. 15 m deep, while the high backscatter patch in theNW reveals a depth variation of only few meters.

177

Figure 3: Digital Parasound profile across Hydrate Hole and Black Hole, revealing highamplitudes near the sea floor above columnar blanking zones. Strong reflectors appearat 50 m sub-bottom depth near the blanking zone, decreasing with distance to them.

Figure 4: Time slice throughParasound data volume aroundHydrate Hole with maximumamplitudes within 3 m thicklayers displayed as dark colors.The lowermost 4 layers revealthe spatial distribution of highamplitudes around 50 m sub-bottom depth

178

ce evidence, high reflection amplitudes maythus be attributed to the presence of gashydrates or carbonate precipitates and canlikely be used to trace surface informationdown to depth. One possible interpretationwould be that low reflection amplitudes indi-cate a potential gas supply conduit and thatgas hydrate growth occurred around its rim.

Two-dimensional seismic dataThough GI-Gun data are less sensitive for localvariations in sediment physical properties thanvery high resolution Parasound data, the seis-mic image across Hydrate Hole and Black Holeis also dominated by lateral and vertical varia-tions in reflection amplitude (Figure 5). Higherreflection and scatter amplitudes at ~250 msTWT sub-bottom depth inhibit imaging of dee-per parts of the sediment section. Since theestimated base of the GHSZ is located abovethis depth, it is likely that free gas is trappedbeneath a low permeable cover of hemipelagicsediments. However, the data suggest that theseal is broken at locations of small-scale faul-ting and beneath the pockmarks.

As for Parasound data, diffuse patches of highreflection amplitudes are observed beneath

the seafloor depressions of the pockmarks. Alayered zone at about 100 ms TWT sub-bot-tom depth reveals locally increased reflectionamplitudes in the proximity of columnar blan-king zones that are interpreted as pockmarkfeeder-channels (see below).

Figure 6 shows a close-up of Black Hole and acomparison of GI-Gun and watergun data. Wa-tergun amplitudes are very weak in this area,however, zones with locally higher amplitudecan be better identified and resolved. A highamplitude patch with positive polarity is obser-ved 50-60 ms TWT beneath Black Hole that isinterpreted as gas hydrate. A likely scenario isthat gas hydrate is clogging the vertical path-way and may subsequently grow laterally intomore permeable parts of the sediment section,associated with a local increase of reflectioncoefficients in the vicinity of the pockmark.2-D seismic lines from the regional seismic gridwere used to create isopach maps for severaltime intervals. Age information from ODP Site1075 was used by tracing eight laterally conti-nuous reflectors from the drill site location intothe pockmark study area. Sediment thicknessbetween these reflectors was subtracted byaverage thickness and the residuum was nor-malized relative to the standard deviation inorder to reveal pronounced relative variations

Figure 5: Multichannel seismic line across Hydrate Hole and Black Hole, acquired during R/V Meteor CruiseM47/3. High amplitudes at depth indicate massive free gas occurrences, and several columnar blankingzones connect to depressions at the sea floor. Also, complicated faults patterns are observed.

in sedimentation rates. For the time interval ofabout 0.5–0.88 Ma, a local positive anomaly ofthe order of three standard deviations is obser-ved (Figure 7), indicating an event of signifi-cantly increased sediment accumulation that isnot observed in all other studied time intervalsbetween 0 and 2.0 Ma. This suggests that atsome time after 0.9 Ma, a massive subsidenceevent with subsequent sediment fill may havebeen associated with gas hydrate decomposi-tion and enhanced degassing near the depres-sion, which is now marked by the occurrenceof numerous seafloor pockmarks.

Three-dimensional seismic data3-D seismic data across Hydrate Hole and BlackHole further support the assumption that typi-cally low reflection coefficients of the opal-richand water-rich sediment section are affected bythe presence of hydrocarbons. The high ampli-tude patch beneath Black Hole, interpreted as ahydrate cap that locally increases seismic impe-dance, is of limited extent (Figure 8), supportingthe assumption that the blanking zone under-neath represents a zone of focused fluid upflow,that supplies sufficiently large volumes of gas to

allow for the formation of massive gas hydrates.The blanking would then be explained by scat-tering of seismic energy at free gas bubbles.

Though free gas usually is associated with seis-mic data attenuation, trapping of free gasbeneath a low permeable seal may increasethe reflection amplitude of the seal layer byincreasing the velocity contrast at the seal-gas-boundary. The package of high amplitudereflector elements at about 250 ms TWT sub-bottom depth (Figure 8) may thus be interpre-ted as trapped gas beneath an low permeablelayer (see above). 3-D mapping indicates thatthis zone is locally elevated beneath Black Hole(Figure 8), probably indicating a deeper saltdiapir, that is associated with doming structu-res, faulting and probably higher permeabilityabove the diapir.

Faulting due to diapirism may further be super-imposed by zones of weakness within a regio-nal fault pattern that probably is of polygonalstructure. A fault that was mapped east ofBlack Hole turns out to reveal a clearly definedfault plane that is obliquely oriented to the

179

Figure 6: Comparison of GI Gun (left) and watergun seismic data (right) near Black Hole.

180

Figure 7: Anomaly in sedimentation rate forthe time interval 0.88 to 0.50 Ma, derivedfrom normalized isopach maps. The patch is located SE of the pockmark area.

Figure 8: View at the 3D seismicvolume around Hydrate Hole andBlack Hole, with shallow gas occurrences, assumed hydrate capand fault zone.

Figure 9: Time slicesthrough 3D seismicvolume revealingseveral linear orien-tations, which mightbe part of a polygo-nal fracture andfault pattern.

main strike and parallel to a connecting linebetween Black Hole and smaller pockmarks tothe Northwest (Figure 8). Time slices indicatethat other fault orientations are present as well(Figure 9), suggesting that pockmarks may inprinciple be located at the corners of faultpolygons. However, the creation (and subse-quent clogging) of pathways beneath the pok-kmarks is not yet completely understood.

Sampling and video surveyingPrimary target of the sampling campaign hadbeen three pockmarks, which were also sur-veyed in detail with all available geophysicalmethods. Accordingly, a dense set of echo-sounder lines as well as high resolution bathy-metry and side scan sonar mosaics could beused to pick interesting sampling targets.Specifically, high backscatter patches were firstsurveyed by video transects, revealing all kinds

of indications for fluid venting (Figure 10), astubeworms, clam shells or carbonate precipita-tes. The pockmarks show a distinct small scaletopography with several steps, probably faults,and steep flanks. However, the outline of themorphological pockmark structure and thelocation and shape of the backscatter patchesdo not match at all. The active area appearssmaller, and in case of Hydrate Hole, venting isno longer active within the depression, but ina shallower part NE of the main structure, in aminor depression of just a few meters only.Carbonate precipates dominate in most of thearea, with tubeworm and clam shell fieldsappearing in smaller patches.Few gravity cores were taken in Black Hole andWorm Hole, recovering gas hydrates and car-bonate precipitates. A coring transect was car-ried out in Hydrate Hole with a spacing of 100m or less, to search for systematic trends in theoccurrence of gas hydrates and other vent

181

Figure 10: Results of an OFOS survey across Hydrate Hole drawn on top ofechosounder profile. Vent indications are found, where Parasound surfaceamplitudes and side scan sonar backscatter strength are high.

182

indicators. A clear trend was found in thedepth of the shallowest gas hydrate layer,which increased from ~1 m in the centertowards ~6 m near the rim (Figure 11).

Gas hydrates did occur macroscopically in 9gravity cores and show a wide variety in distri-bution and fabric formation. Pure white hydra-te occurred in layers or veins several millimetersto centimeters thick. Gas hydrate layers occurparallel to bedding but also exist as near-verti-cal veins, some of them over a vertical range of40 centimeters. The veins have a thickness of afew millimeters. Few examples of veins andgas hydrate pieces were found with open spa-ces, indicating either channels kept open bycontinuous flow or trapped gas bubbles.3.4 DISCUSSIONSites characterized by continuous or episodicfluid venting, associated with the presence ofhydrocarbons, often reveal complex surfaceand internal structures, which are difficult to

image and cause problems to relate resultsfrom local sampling with structural reconstruc-tion and estimations of average flux rates.Detailed seismic and acoustic surveys in bothtwo and three dimensions and at sufficientlyhigh resolution are essentially required as abasis for interpretation. However, the choice ofan appropriate environment clearly is as impor-tant in order to gain a deeper understandingof flow related processes. A study area may beconsidered to be appropriate if such processesoverprint a well known and clearly definedbackground and if flow-affected and unaffec-ted areas can be distinguished. Hemipelagicsediments appear to be suitable since the typi-cal layering is associated with coherent seismicreflectors, that show minor lateral amplitudevariation and can be continuously traced overlong distances with respect to the scales offlow-related features. The Congo pockmark study area is characteri-zed by a fine-grained sediment cover that pro-

Figure 11: The sampling transect confirms gas hydrates recovered in up to 10m cores in the center of the high backscatter patch, with decreasing mini-mum depth from 6 m to 1 m.

vides an efficient seal for upward fluid and gasmigration. Shallow gas occurrence is usuallyrestricted to zones beneath the GHSZ. Thus, agas hydrate BSR is not observed in the region.Instead, zones of extremely high reflectionamplitudes follow stratigraphic boundaries,that are likely associated with higher permea-bility, e.g. due to a facies change, more pro-nounced sediment deformation or slumping.

The survey and sampling work during R/VMeteor Cruises M47/3 and M56 was focusedon locations, where sediment deformation orsmall scale faulting, likely related to salt diapi-rism at greater depth and associated with localsubsidence, created conduits between the deepgas reservoirs and the seafloor. Large scale pok-kmarks of several hundred meters diameter andup to 30 m depth are a consequence of verticalfluid transport. Several 2-D seismic lines and the3-D seismic survey in the vicinity of these pock-marks reveal a clear picture of post-sedimentaryenhancement of reflection amplitudes, that candirectly be related to the trapping of free gas(below the GHSZ) or the presence of gas hydra-te (within the GHSZ).

The characteristics of seismic and acousticdatasets presented in this study are differentwith respect to acquisition geometry, signalgeneration, lateral and vertical resolution, anddepth penetration. They provide imaging re-sults of different nature and on multiple scales,and they are highly complementary, eventhough an integrated interpretation is notstraightforward. However, the key seismic andacoustic signatures characterizing the systemare undisputed: (1) topographic and backscat-ter anomalies at the seafloor related to obser-ved fluid venting, (2) patches of high reflectionamplitudes beneath the pockmarks, that arerelated to increased reflection coefficients of alayer package within the proximity of thepockmark, (3) columnar amplitude blankingbeneath patches of higher reflection amplitu-des, and (4) a high reflectivity zone at greaterdepth, that inhibits further depth penetrationand does not reveal a sharp base.

In order to consistently interpret seismic andacoustic features, additional information can beused. On one hand, seafloor sampling, coringand video observations allow to identify activeventing areas and to relate shallow high ampli-tude reflections with the proved occurrence ofgas hydrates and carbonates (the latter of whichare not expected to occur deeper than a fewmeters sub-bottom depth). On the other hand,estimates of the expected depth of the base ofthe GHSZ help to distinguish between featureslikely related to free gas and features likely rela-ted to massive gas hydrates. Thus, constraintsare provided for the upper and lower bounda-ries of the studied system. Furthermore, seafloorobservations can likely be translated downwardinto the sediment section.

A simple conceptual model would describe freegas that is trapped beneath a low permeableboundary and beneath the GHSZ. Were the sealis broken and permeable pathways for verticalfluid transport exist, free gas rises into theGHSZ, where massive hydrates may be formed.Gas hydrates filling pore spaces may significant-ly reduce permeability and may therefore clogthe vertical pathway, forming a self-trappingintermediate reservoir. Gas may consequentlymigrate laterally into the more permeable layersof the section forming hydrate and thus increa-sing the seismic reflection coefficients of therespective layers. On the other hand, seismicenergy is scattered at free gas bubbles withinthe transport channel between the gas reservoirbelow and the hydrate cap above.

However, this concept does not explain howthe pockmarks are initially created, how gascan be transported through the GHSZ in orderto form hydrates close to the seafloor, andhow free gas can still exist within the GHSZ. Asa hypothesis, venting is mostly driven by over-pressured gas that was produced from decom-posing hydrates during a pronounced subsi-dence event probably related to sub-bottomsalt movement. During the rapid rise of fluidsthrough pre-existing zones of weakness likely

183

184

related to polygonal faulting or diapirism,more gas is released out of solution and aneruptive process is initiated. Collapsing of theseafloor may indicate predominant gas andfluid release but not much material transport.After the eruption, gas pressure decreases andventing slows down or even completely stopsfor some time. Gas transported during theeruptive event transforms into hydrate close tothe seafloor and clogs the feeder-channel ofthe pockmark. Free gas may still exist within anarrow zone beneath the pockmark if focusedfluid upflow affects the temperature gradientand thus rises the base of the GHSZ or if thepermeability of hemipelagic sediments is notsufficient to provide the required volumes ofwater in order to form gas hydrate. Self-acce-lerating upflow events may be repeated peri-odically if some mechanism exists to re-esta-blish gas over-pressure.

4. CONCLUSIONSSedimentation patterns within the study areaindicate a massive subsidence event and sub-sequent rapid sediment fill that was likely asso-ciated with gas hydrate destabilization andenhanced degassing near the depression,which is now marked by the occurrence ofnumerous seafloor pockmarks.

3-D seismic data across these pockmarks indi-cate that the typical low-amplitude signatureof opal-rich and water-rich sediments is super-imposed by zones of high reflection amplitu-des near faults and other potential fluid path-ways. Since the lateral extent of these highamplitude zones is limited, a direct relationshipto fluid and gas migration can be expected. Ahigh amplitude patch with positive polarity isobserved at 40-50 m sub-bottom depth andinterpreted as a gas hydrate cap that plugs thefeeder channel of a pockmark. The upflowzone at greater depth is characterized byamplitude blanking, indicating free gas bub-bles that scatter seismic energy. A package ofhigh amplitude reflector elements at 150-200m sub-bottom depth suggests the presence of

trapped gas beneath a low permeable layerpackage. This package is bent upwards at thevicinity of the pockmarks, probably indicating adeeper salt diapir that is associated with faul-ting and likely higher permeability above thediapir. However, the creation of pathwaysbeneath the pockmarks is not yet completelyunderstood. Preliminary results based on 3-Dmapping of fault plane orientations suggestthat faulting due to diapirism may be superim-posed by zones of weakness within a regionalfault pattern that probably is of polygonalstructure or derives from salt uplift.

ACKNOWLEDGEMENTSResearch project and Meteor cruises were par-tially funded by the »Bundesministerium fürBildung und Forschung« and »Deutsche For-schungsgemeinschaft«. The proponents great-fully acknowledge the competent support ofthe crew of R/V METEOR.

REFERENCESBabonneau N, Savoye B, Cremer M, Klein B(2002): Morphology and architecture of the pre-sent canyon and channel system of the Zairedeep-sea fan: Mar. Petrol. Geol. 19: 445-467.

Carson B, Screaton E. (1998): Fluid flow in accre-tionary prisms: Evidence for focused, time-vari-able discharge. Rev. of Geophysics 36: 329-351.

Dickens GR, Quinby-Hunt MS (1994): Methanehydrate stability in sea water. Geophys. Res.Lett. 21: 2115-2118.

Droz L, Rigaut F, Cochonat P, Tofani R. (1996):Morphology and recent evolution of the Zaireturbidite system (Gulf of Guinea). Geol. Soc.Am. Bull 108: 253-269.

Emery KO, Uchupi E, Phillips J, Bowin C,Mascle J. (1975): Continental margin offWestern Africa: Angola to Sierra Leone. Am.Ass. Petrol. Geol. Bull. 59: 2209-2265.

Gornitz V, Fung I. (1994): Potential distributionof methane hydrates in the world’s oceans.Global Biogeochem. Cycles 8: 335-347.

Grant JA, Schreiber R. (1990): Modern swathsounding and sub-bottom profiling technologyfor research applications: The Atlas Hydro-sweep and Parasound systems. Mar. Geophys.Res. 12: 9-19.

Graue K. (2000): Mudvolcanoes in deepwaterNigeria. Mar. Petrol. Geol. 17: 959-974.

Heezen BC, Menzies RJ, Schneider ED,Ewing WM, Granelli NCL (1964): Congosubmarine canyon. Am. Ass. Petrol. Geol.Bull. 48: 1126-1149.

Hooper RJ, Fitzsimmons RJ, Grant N,Vendeville BC (2002): The role of deformationin controlling depositional patterns in thesouth-central Niger Delta, West Africa. J.Struct. Geol. 24: 847-859.

Hyndman RD, Wang K, Yuan T, Spence GD(1993): Tectonic thickening, fluid expulsionand the thermal regime of subduction zoneaccretionary prisms: The Cascadia Margin offVancouver Island. J. Geophys. Res. 98:21,865-21,876.

Jansen JHF, van Weering TCE, Gieles R, vanIperen J (1984): Middle and late Quaternaryoceanography and climatology of the Zaire-Congo Fan and the adjacent eastern AngolaBasin. Neth. J. Sea Res. 17: 201-249.

Kvenvolden K (1994): Natural gas hydrateoccurrences and issues. Ann. NY Acad. Sci.715: 232-246.

Marton LG, Tari GC, Lehmann CT (2000):Evolution of the Angola passive margin, West

Africa, with emphasis on post-salt structural sty-les. In: Mohriak W, Talwani M (eds), Atlantic riftsand continental margins. AGU, Washington DC:pp 129-150.

Moore JC, Vrolijk P. (1992): Fluids in accretiona-ry prisms. Rev. of Geophysics 30: 113-135.

Rouby D, Raillard S, Guillocheau F, Bouroullec R,Nalpas T (2002): Kinematics of a growth fault/raft system on the West African margin using 3-D restoration. J. Struct. Geol. 24: 783-796.

Shepard FP, Emery KO (1973): Congo submari-ne canyon and fan valley. Am. Ass. Petrol.Geol. Bull. 57: 1679-1691.

Uenzelmann-Neben G (1998): Neogene sedi-mentation history of the Congo Fan. Mar.Petrol. Geol. 15: 635-650.

Valle PJ, Gjelberg JG, Helland-Hansen W (2001):Tectonostratigraphic development in theeastern Lower Congo Basin, offshore Angola,West Africa. Mar. Petrol. Geol. 18: 909-927.

Wefer G, Berger WH, Richter C et al.. (1998):Proc. ODP Init. Repts.; 175. College Station, TX(Ocean Drilling Program).

Zühlsdorff L, Spieß V, Hübscher C, Villinger H,Rosenberger A (2000): Implications for focusedfluid transport at the northern Cascadia accre-tionary prism from a correlation between BSRoccurrence and near-sea-floor reflectivityanomalies imaged in a multi-frequency seismicdata set. Int. J. Earth Sciences 88: 655-667.

Zühlsdorff L, Spieß V. (2004): Three-dimensio-nal seismic characterization of a venting sitesreveals compelling indications of naturalhydraulic fracturing. Geology 32: 101-104.

185

186

Techniques and Instruments for Gas HydratesExploration and Research (TIGER)

1. IntroductionThe exploration of conditions in deep sea is stillprimarily restricted to purely scientific institu-tions. The commercial use of the sea for oilproduction or fishery has led to the develop-ment of simple and effective equipment. Theseexploration techniques already existing are notsuitable for the observation of gas hydratefields. If gas hydrate fields will be used as rawmaterial or energy source in the future, thesescientific equipments have to be adapted andmanufactured for the industrial applications. InJapan and the USA the intensive investigationconcerning the future use of gas hydrates hasalready started. Therefore the development ofappropriate industrial investigation techniquescan be expected, although appropriate activitiescould not be found by a literature research.Major aim of the project was the developmentof a submarine experimental station for the col-lection of measuring data of sedimentary gashydrates in the deep sea. Emphasis was placedon long-term data collection on the release andthe behaviour of methane at the sea bottomunder natural site conditions. A further aim wasthe integration of regional enterprises, preferab-ly medium-sized companies, into this specificproblem of the deep sea exploration in the pre-liminary stage of a potential utilisation of gashydrates. From the linkage of research projects

and economic interests on the one handpotentials for the development of marketableproducts result, on the other hand availableknowledge from the industry can be used forresearch purposes. The project is divided intothe following work packages: preparation of afeasibility study, status quo acquisition / investi-gation, development and laboratory test,testing phase / preliminary planning of the mea-suring station, development of a measuring unitsuitable for the deep sea (prototype), first fieldtests and conclusion evaluation and reporting.The prototype components of the measuringprobe were developed in order to adapt it to theemployment in a deep-sea measuring station.The components were intensively tested in thelaboratory and field. The results of the tests aswell as the requirements of the individual partsamong themselves were harmonised and led tothe planning of the final prototype. After con-clusion of the planning phase the individualunits (methane sensor, data logger and pressu-re-resistant enclosure) were assembled into afully functional test unit.

2. Objectives of the ProjectMajor aim of the project is to align two aspectsof maritime research and development. On theone hand the installation of a measuring sta-

Degenhardt A. (1), Hanken T. (2), Helmke J. (3), Jaguttis J. (4), Masson M. (5), Poppen B. (6)

(1) Technologie Transfer Zentrum Bremerhaven, An der Karlstadt 10, 27568 Bremerhaven,

degenhardt@ttz-bremerhaven.de

(2) iSiTEC GmbH, Stresemannstr. 46, 27570 Bremerhaven, thanken@isitec.de

(3) Gaskatel GmbH, Holländische Str.195, 34127 Kassel, helmke@gaskatel.de

(4) de la Motte & Partner GmbH, Am Stüb 10, 21465 Reinbek, P.W.delaMotte@t-online.de

(5) Capsum Technologie GmbH, Technologiepark 24, 22946 Trittau, m.masson@capsum.com

(6) JHK Anlagenbau und Service GmbH & Co. KG, Labradorstr. 5, 27572 Bremerhaven,

Anlagenbau-und-service.poppen@jhk.de

tion on the sea bottom will provide valuabledata for the evaluation of gas hydrate fieldsand the possibilities of their economic use. Onthe other hand small and medium-sized enter-prises in North Germany shall be introduced tothe specific problems of the exploration indeep sea and gas hydrate extraction. Theknowledge collected in the basic research andthe technology used offer a potential whichshould not be underestimated for the develop-ment of marketable products. In return valua-ble know-how from the industry is made avai-lable for the basic research. Originally a projectduration of 10 years (in five subprojects) fromthe exploration to the development of a tech-nology for the use of gas hydrate sources wasplanned. Devices and equipment concentra-ting on scientific research will be transferredinto innovative products of maritime technolo-gy in global demand.In the context of the further exploration ofmarine gas hydrate sources regarding a futureuse the project TIGER I (Techniques and ofinstrument for gas of hydrate exploration andResearch, 03G0561 A), funded by the BMBF,pursues several aspects. A partner consortium of medium-sized compa-nies under the co-ordination and conceptualmanagement of Technology Transfer CenterBremerhaven, Environmental Institute, wasinvolved in this project. The enterprises involvedwere assigned to the different tasks as follows: The working group Empting - de la Motte wasconcerned with the overall feasibility, descrip-tion and investigation of the development ofequipment for the production of electricityfrom sea currents. In particular the special pro-blem areas for the design and set-up of a pro-totype with regard to relevant boundary condi-tions was discussed and results with previewon the further treatment of the research pro-ject were presented.Major task of the company iSiTEC was thedevelopment of the data collection system(data logger), which records and stores themeasuring signals of the sensors attached andif necessary offers additional control functions.The substantial requirements on the system

were: processing of analogue measurements,safe data storage with sufficient capacity, readout of the data and parameterisation by PC,low energy consumption, extended tempera-ture range and employment in the sea water-resistant pressure housing until a depth of2.000 m. In the first development stage thedata logger will be employed in a test con-struction, in order to gain first experienceswith the system by short-term measurements. In the second stage extended functions will berealised, e.g. logging of several sensors, event-controlled recording, intelligent energy mana-gement, employment duration of up to 1 year,employment depth up to 3.500 m. Capsum was responsible for the further deve-lopment of the methane sensor. The originaltasks were the reduction of the time constantand increase of responsivity. In the course ofthe project the increase of stability, the reduc-tion of the energy consumption and increaseof the employment depth, which is importantfor the long-term autonomous employment,were investigated. Task of Gaskatel was the development ofequipment for the production of electricityfrom fuel cells. This contains the search andtype selection of the possible fuel cell typeswith critical examination of the own systems»low-temperature alkaline« and »high tempe-rature phosphoric acid«, the examination ofthe existing cells on pressure strength, the exa-mination and search of the electro-chemicalreactivity of methane at the electrodes andpossibilities of the thermal transformation ofmethane in hydrogen. Moreover Gaskatel wasresponsible for production and test of the pro-totypes after the defaults of performance,temperature, fuel and disposal.JHK was responsible for the development of apressure-resistant housing for the componentsof the deep-sea station as well as the concep-tional cooperation for energy storing and self-sufficient energy distribution system. This con-tains the search, examination and representa-tion of the state of the art, the listing of arequirement catalogue for the solution of thetasks set and evaluation, interpretation and

187

188

interface planning as well as the prototypeproduction and the execution of tests. Impac Offshore carried out a feasibility studyfor a deep-sea station for measuring, taking upand collecting gas and gas hydrates. The Technology Transfer Center Bremerhavenwas the project co-ordinator, coordinating thetasks of the individual partners, merging thescientific know-how as well as securing andmonitoring of the technology transfer.

3. Present Status and Results / Methods & Results / ResultsThe work was split up in two steps; first stepwas a small test unit consisting of the methanesensor, the data acquisition unit and the pressu-re-resistant housing. In a second step the alter-native energy supply was developed and theentire deep-sea measuring station was desi-gned. The following paragraphs show the deve-lopment achieved for the different components.

Methane sensorIn the following table the obtained improve-ments are represented in comparison to the exi-sting commercial version. The improvements ofthe individual parameter (independently fromeach other) are shown in the following table.

Different field tests in the river Elbe, in Norwayand in the Mediterranean were carried outunder normal oceanic conditions and duringdifferent seasons, which showed that a stabili-ty of at least 6 months without significant driftis possible. On laboratory scale 12 monthscould be reached, although some sensors fai-

led. Therefore main emphasis of future researchis placed on the development of a fast methodfor the pre-selection of sensors for long-termemployments.

Reduction of time constantThe investigation of different detector typesand electronic configurations showed that theminimum response time, i.e. the time until thefirst signal rise after a change in concentration,is within seconds. The T90-time, i.e. the time toreach 90% of the final signal lies between 1and 5 min dependent on turbulences and/orincident flow. The decay time is between 3 and15 min. These values apply to the full measu-ring range and are independent from the den-sity gradient. By modifying the sensor head theresponse time could be improved. The gasvolume behind the membrane was reducedsignificantly. The moisture sensor could beomitted, the membrane surface and the sup-porting structure were reduced, etc. The lastinvestigations showed that the response timecould be reduced further down to 45 seconds,which will most likely be the lowest limit thatcan be obtained without external devices likeagitator or propeller.

Increase of employment depth In a first phase the employment depth couldbe increased from 2.000 to 3.500m, the corre-sponding pressure resistance was improvedfrom 250 to 400 bar by an optimisation of thesealing, the housing design and manufactureas well as by the adaptation of the membranecarrier. In a second phase the sensor headdesign was improved, which was based on the

Parameter Commercial version ImprovementTime constant 5 min 1 minResponsivity 50 Nm 10 NmElectric power consumption230 mA 60-100 mAPressure resistance250 bar 494 bar

Table 1: Improvements of the methane sensor

reduction of the membrane area that is expo-sed to pressure. Moreover the carrier structurebehind the membrane was changed. Staticand dynamic pressure tests were successfullyaccomplished up to 494 bar.

Increase of responsivityA responsivity of 10 nmol/l in some cases even3 nmol/l could be achieved by accurate adjust-ment of the heating temperature. Howeverthis strongly depends on the detector anddetector type. The best reproducibility could bereached with gallium oxide detectors compa-red to the common tin dioxide detectors.Further investigations are necessary in order tocontrol all parameters, which affect theresponsivity. E.g. the long-term interrelationsbetween the responsivity and the aging of thedetector are still not known.

Electric power consumptionThe power consumption partly depends on theelectronics, but mainly on the detector and theheating of the active layer of the semiconduc-tor, which in turn depends on the manufactu-ring quality. Depending on the manufacturerthe power consumption is different. With thebest detectors the power consumption for thecomplete sensor including electronics could bereduced to 60 mA with 12V DC power supply.Other detectors use approx. 90 to 100 mA,which is still an improvement compared to aconsumption of 230 mA by the original sensor.By a redesign the electronics could be furtherimproved. The indicated values apply to elec-tronics with analogue outputs, the digitisationconsumes additionally approx. 30 mA, where-of the A/D transducer consumes 10mA alone.

Operating modeIn order to minimise the energy consumptionand to allow an event-controlled operation adiscontinuous operation of the sensor was ass-essed. Investigations showed that this affectsthe long-term stability of the detectors; more-over the switch-off of the heating poses therisk that water vapour condenses in the detec-tor chamber. This condensation water can only

vaporise if the heating is switched on for a cer-tain time. In the long term repeated switch offwill lead to more and more condensationwater, which would destroy the detector.However this could not be approved duringthe tests. Nevertheless a complete disconnec-tion of the electronics is not recommended;the detector heating and the processor controlshould remain switched on. The switch-off ofselected components does not bring much perse (10 mA for the A/D converter), but never-theless presents more than 10% of the energyconsumption. The saving potential could beused for the event controlling of the sensor,whereby data storage capacity can be saved. Aversion of electronics was developed, whichallows the switch on and off of the A/D con-verter from the outside by specific commands.

Accuracy The solubility of methane theoretically de-pends on the salinity. Nevertheless the metha-ne sensor is usually calibrated with fresh water.Only exception is the calibration with seawaterat temperatures below 2°C. The effect of thesalinity is actually small: at temperaturesaround 15-20°C the solubility between 0 and30‰ changes by approx. 10%, i.e. under typi-cal deep-sea employment conditions the mea-suring error of the methane sensor amounts toonly few percent, which gets lost in the normalsignal noise. The general accuracy of the sensor lies bet-ween 10 to 30% of the measured value, whe-reby the worse values occur only sporadically,e.g. at a certain concentration and a certaintemperature. The accuracy depends both onthe manufacturing quality of the semiconduc-tor detectors and on the calibration procedure.The following steps have to be considered:Inaccuracy of the mass flow control, whichcontrol the gas mixture, temperature and inci-dent flow fluctuations at signal recording aswell as aging processes within the semicon-ductor during the calibration and finally fluctu-ations of the heating control of the semicon-ductor and the selection of the parameters set-up of the calibration formula. An optimisation

189

190

of the calibration is planned, which aims toincrease the accuracy to 5 - 10%, a further im-provement seems not realistic. On the one handthe semiconductors are primarily manufacturedbulk product for applications, where no highaccuracy is required. E.g. the semiconductorsused are usually not produced in clean rooms;therefore fluctuations in the chemical composi-tion and thus the characteristics emerge.

Cross sensitivityThe cross sensitivity on H2S was examined inair. A concentration of 10 and 100 ppm H2S inair causes a suppressed cross sensitivity, whichnormally gives no signal. 300 ppm H2S in airgives the same signal as for 10 ppm methane.Starting from 1% H2S in air the detector bre-aks down and can possibly be regenerated, alt-hough this is more expensive than a simpleexchange. The cross sensitivity on hydrogen isclearly visible for concentrations in the rangeof mol/l. For applications, where methane andhydrogen are coexisting in very high concen-

trations, the signal allocation is more difficult.Future research and development should tar-get this problem.

Data acquisitionIn particular the reliable function of the device(storage of the data) is relevant for the employ-ment in a long-term system. It should be ensu-red that data is not lost during a system failu-re. A further important point is the realizationof energy management functions in order toreduce the energy consumption between themeasurements (sleep mode) as well as theshut-down of parts of the system in the case oflow energy resources or a failure of the mainpower supply. Moreover the necessity of anadditional stand-by battery / power pack incase of system failure needs to be assessed.

The data acquisition unit essentially consists oftwo basic components: the controller moduleand the basis module.

First development step Second development stepEmployment duration Up to 4 weeks Up to 1 yearEmployment depth Up to 2000 m Up to 3500 mNumber of methane sensors 1 1-2 (optional more)Measurement interval 1 measurement per hour 1 measurement per hour

(variable) (variable)Measurement duration (average) 1 minute (variable) 1 minute (variable)Operation mode Stand-alone / sleep mode Stand-alone / sleep mode /

event controlledEnergy supply Internal Internal and externalPower consumption 0,3 W 0,3 WExternal sensors CTD (optional) CTD and other (optional)

Table 2: Properties of the data acquisition unit

191

Figure 1: Controller module

Figure 2: Basis module

The data acquisition unit essentially consists of twobasic components: the controller module and the basismodule.

192

Controller moduleThe controller module was manufactured inSMD execution, in order to place an inexpensi-ve, efficient space saving module on smallestarea. This module already contains the sub-stantial, for most applications sufficient, func-tions.

Basis moduleThe standard controller module is attached tothe custom-designed basis module. Here theperiphery components like power supply, plugconnections, interfaces, memory expansions,A/D converters and signal adaptation are inte-grated according to the application.Specifications of the basis module:

Power supply DC-DC converter 18 - 36V DCPlug connection Terminals, stripsInterfaces RS232 (optional: RS 485, USB, Ethernet)Data memory SD-Card (up to 512 Mbytes capacity)A/D-converter 24-Bit resolution, 6 sps (optional: 16 Bit, 1ksps)I/O functions REED-switch for proximity activation

Relay / MOSFET outputAnalogue signal range0-2,5V (optional: 0-5V,0-10V, 0-20mA, 4-20mA)

Table 4: Properties of the basis module

Frequency Up to 25 MHz (optional: 100 MHz)EPROM 64 kB FlashRAM 4 kB (optional: 8 kB RAM)I/O channelsUp to 32A/D-converter 12 Bit resolution, up to 8 channels, 100 ksps

8 Bit resolution, up to 8 channels, 500 kspsD/A-converter 12 Bit resolution, 2 channelsInterfaces 2 x RS232/TTL, SPI, SM-BUS, MCU-BUSTime / calendar RTC with calendar function and backup-batterySensors Temperature (internal)Dimensions Approx. 40 x 50 x 10 mm

Table 3: Properties of the controller module

193

Energy supply by fuel cellDue to the findings from the previous planningof the power supply by fuel cells the main partof the system would be an Alkaline Fuel Cell(AFC). The electrolyte of an AFC is usually madeof 30% caustic potash dilution. The reactionwater dilutes the electrolyte of the AFC. Thedischarge of water in a deep-sea measuring sta-tion has to be realised via diffusion. The factthat the density of caustic potash solutionincreases with its concentration can be utilised.The common working temperature range ofan AFC is 60 - 80 °C. If a caustic solution witha concentration of 30% is used the fuel cellcan work far below 0 °C, although the AFChas to be bigger at lower temperatures. TheAFC is dimensioned in such a way that it sup-

plies a minimal voltage of 5 V and a current of1 A at a temperature of 4 °C. Hydrogen andoxygen are used as primary energy sources,which are stored as high-purity gases in com-pressed gas cylinders. The system has to beplaced in a pressure resistant housing and ope-rated at atmospheric pressure. At the given performance data and an effi-ciency of the AFC of approx. 60% one needs30.000 l of hydrogen. At a cylinder pressure of300 bar this means a volume of 100 litres.15.000 l (corresponds to 50 l at 300 bar) ofoxygen are needed to oxidise this amount ofhydrogen. Thus altogether three 50 l-bottlesare needed, in order to store the appropriategas masses, which have a net weight ofapprox. 225 kg.

Power consumption Approx. 300 mWOperational temperature range -20 to 50°CDimensions Approx. 60 x 140 x 20 mm

Table 5: Properties of the entire data logging unit

This results in the following features of theentire data logging unit:

Figure 3: Fuel cell design

194

The reaction water resulting from the alkalinefuel cell is taken up by the electrolyte (KOH) anddilutes it. In order to ensure a safe operation ofthe AFC, the KOH concentration must lie bet-ween 30% and 10%. Since a discharge of thewater is not possible, an adequate amount ofKOH solution has to be carried along to preventthat the concentration falls below the mini-mum. 11 kg KOH are sufficient to take up 19,27kg of arising water. The fuel cell stack should consist of 7 single cells,which provide a minimal voltage of 5 V if serial-ly connected. The stack is placed in a containerfilled with KOH, which takes up the reactionwater, too. The container is built as a pressuretank and coupled with the gas pressure. Thecaustic solution can flow against the electrodesfreely; a pump is not needed. The reaction wateris discharged from the cell by diffusion. The stack is operated in dead-end mode on thegas side, i.e. the stack does not have a gas out-let. Thus the problem of a contamination of thegases has to be tackled. This will be solved byplacing a container for the inert gases at eachoutlet of the stack, which prevents the hydro-gen and oxygen concentration from fallingbelow 50 vol%. With a hydrogen purity of 6.0(99.9999%) and an oxygen purity of 5.5(99.9995%) a container of 21 l is required inorder to prevent the cell from damage. Pressure reducers are used on the gas input sidein order to regulate the cylinder pressure toapprox. 0,5 bar positive pressure. The systemhas been designed in a simple way as in the pastsystems, which work with pumps, and valvescaused trouble that led to system malfunction.Moreover the power requirements of thesystem itself can be reduced to 0 and thereforereduces the gas volume needed. The different components of the power supplyunit are not high pressure resistant and have tobe placed in a pressure resistant housing.

Energy supply by sea currentsThe enhancement of the efficiency is a veryimportant factor for the use of a sea currentplant as power unit for the deep sea measuringstation, i.e. existing losses have to be minimised

and the locally available current has to be usedin an efficient way.For a rotor diameter of 10 m flow rates of atleast 2-2,5 m/s are needed. Although the ener-gy demands of the station are quite low therotor diameter has to be between 3 and 9 munder prevailing flow rates of only 0,1 to 0,2m/s above the sea bottom.Single rotors supply higher energy yields com-pared to several small rotors. Furthermore sha-dowing effects occur if several rotors are used.In contrast bigger rotors have a higher inertia asthe constructions are heavier and more com-pact. An independent start-up assuming flowrates of 0,1-0,2 m/s and rotor diameters up to9 m appears technically not feasible, thereforethe optimal ratio between plant size and plantnumber has to determined in tests.

Pressure-resistant housingThe test unit was built to allow the examinationof gas hydrates by the methane sensor and thecorresponding data acquisition unit. The dataacquisition unit including the power supply wereintegrated in a pressure pipe. During the designphase emphasis was put on the weight reduc-tion. For this reason titanium was chosen as pipematerial. The pressure pipe is suitable for anemployment depth of up to 6000 m. Pressurepipe and methane sensor were fastened to a fra-mework. The framework consists of square pipe1.4571. Next step will be the development of apressure housing for additional devices. One of the main technical problems of the explo-ration and future extraction of gas hydrates is thevaporisation of the gas during outcrop. Due to ri-sing temperatures and decreasing pressure thegas discharges from the icy structure of the gashydrates. In order to prevent this, equipment hasto be developed which keeps the surrounding ofthe gas hydrates stable, especially during outcrop.In the basic set-up the material to be examined(later outcropped) is placed in a chamber of 14litres made of plexiglass, which is placed itself ina pressure housing filled with tap water. Thebottom of this housing has both feed-throughbores as well as ports. Thus the inside and theoutside chamber can be accessed from outside.

195

This is necessary since pressure and temperatu-re are controlled via the tap water and the innerchamber has to be accessed for taking samples. Many investigations, which have to be accom-plished with this equipment, are based on thesampling from the inner plexiglass chamber.Partly these samples shall be examined underpressure in a separate external chamber (bypressure-resistant sensors or optical techniques).The samples can partly be released by a throttlevalve and submitted to analysis. Main aim is todesign and build a universal device, with ena-bles both types of sampling. Hereby a multitudeof boundary conditions have to be considered.

Test system (first development step) The first development step of the data acquisi-tion unit was integrated in a test structure inorder to gain first experiences with the diffe-rent system components. This test structureconsists of the methane sensor, the data acqui-sition unit, the pressure housing and the auto-nomous power supply with battery cells. Thefollowing pictures show the test unit (firstdevelopment step) before its first employmenton the research ship Polarstern.

Field testingDuring a joined AWI-IFREMER cruise (19th ofJune to 26th of July 2003) with RV Polarstern,

Figure 4: Test system

Figure 5: Data logger and battery cell of test system

196

equipped with the Remotely Operated Vehicle(ROV) Victor, the Haakon Mosby Mud Volcanowas investigated. The HMMV is located at theBarents Sea continental margin between Nor-way and Svalbard in a water depth of ~1280 m.One of the research topics was the measure-ment of CH4 in the lower bottom water. For thispurpose the METS CH4 Sensor (Capsum,Geesthacht) and the data logger developed byISITEC (Bremerhaven) was applied.Due to the diving schedule of the ROV it wasnot possible to deploy the lander system, deve-loped in cooperation with the partners of theproject TIGER, at a single spot over a period ofseveral days, since the recovery could not beassured. Nevertheless, the deployment of theCH4 sensor during ROV surveys covering largeparts of the Mud Volcano worked successful.For this purpose the sensor was installed in thebasket of the ROV. The data logger was pre-programmed to mea-sure in 5 minute intervals over the 48 hours ROVdive. The dive covered large parts of the MudVolcano including spots where free gas wasreleased from the seafloor. The stability of theSensor system including the energy consump-tion provided a very interesting data set.The spatial pattern of the recorded CH4 signalshows, although not in every detail, a similaritywith the pattern of CH4 concentration derived byHydrocast and measurements by Gas-Chroma-tography onboard ship and visual observation ofactive sites of gas release. Improving technicalaspects as the incoming flow reaching the mem-brane of the Sensor and decreasing memoryeffects the Sensor-Data Logger System seems tobe suitable for monitoring the spatial distributionof CH4 concentrations in deep sea environmentsenriched in CH4 as mud volcanoes or seep sites.The track lines of ROV Victor during a survey of48 hours covering large parts of the HaakonMosby Mud Volcano were recorded. The signalmeasured by the METS Sensor was transfer-red to geographic coordinates via the timestep of the data logger and of the ROV (syn-chronized prior to deployment). By applica-tion of GIS the data were mapped on thebathymetric chart of HMMV.

4. ConclusionsThe different components of the prototypeunit were developed in order to adapt them tothe employment in the deep-sea measuringstation. The component units were subject tointensive laboratory and field tests. The resultsof the tests as well as the requirements of theindividual parts among each other were har-monised and led to the planning of the proto-type. After completion of the planning phasethe different components (methane sensor,data logger and pressure housing) wereassembled into a fully functional test unit. For the methane sensor the time constantcould be reduced to approx. 45 seconds by themodification of the sensor head. This will pro-bably be the lowest limit, which can be achie-ved without additional external devices likeagitators or propellers. Attempts with differentmembranes (e.g. thinner membrane, which the-oretically has a higher gas diffusion rate), didnot lead to the anticipated success. The pressu-re resistance of the sensor could be increased ina first phase from 250 to 400 bar, which corre-sponds to an increase of employment depthfrom 2000 to 3500 m. After further modifica-tions static and dynamic tests up to 494 barwere carried out successfully. Moreover the sen-sitivity was increased up to 3 nmole/l. Howeverfurther tests will be necessary to define thedetection limit.The modular data acquisition unit developed isa multifunctional data logger using the latestenergy-saving components as well as commer-cial memory cards (SD Card), which fulfils aswell intelligent control tasks. First of all the unitcan be employed in scientific-technologicsystems under harsh site conditions and atextreme temperatures (e.g. deep sea, polarareas). The hard and software integration of thememory card and its function as safe mass sto-rage offers the application for various acquisi-tion and control problems in the area of micro-controllers. Particularly good experiences weremade with the trouble-free operation of thesetechnologies under extreme conditions.A fuel cell was recommended for the powersupply of the deep-sea probe TIGER. The power

197

output will be a current of 1 A at a voltage of 5V. The operating temperature is 4 °C and theoperating lifetime is 8760 hours. The storage ofthe reaction gasses should be done in pressurevessels. They should be of good purity.Commonly used gas qualities and pressure ves-sels fit to these requirements. Different fuel cellsystems were discussed as power supply. Thesystem that fits best to the requirements of theTIGER power supply is an alkaline fuel cellsystem with simple peripherals. It was shown,how such a system could be realised. It was alsoshown that the AFC is able to work under thegiven conditions. The electrodes that were usedfor the first time under these conditions showedsufficient results. It should be possible to reducethe active area of the electrodes to 100 cm2 byoptimisation.A prototype system fitting to the power outputneeds should be built and tested in a long-termrange to prove the reliability of the system.A pressure resistant pipe for the incorporationof the methane sensor, the data logger and thepower supply was developed and manufactu-red. During the design emphasis was placed ona weight minimisation. For this reason titaniumwas used for the pipe material. The pressurepipe can be used in depths of up to 6000 m.Pressure pipe and methane sensor were faste-ned to a base frame made of steel 1.4571. In order to start with the first tests as early aspossible, the test unit was supplied by conven-tional batteries. In parallel to the developmentof the prototype a power supply by fuel cell,planned for the future deep sea measuring sta-tion was developed. Concerning the fuel supplyof the cell the further development of establis-hed processes as well as the use of alternativesources in the application area of the deep-seameasuring station were considered. The consi-deration of local fuel extraction partly led tointeresting approaches, but has not provedsatisfactory during the project duration and wastherefore rejected. Further studies concerning possible alternativesto the power supply by fuel cells using resourcesavailable in the surrounding of the deep seameasuring station (utilisation of sea currents,

salinity, etc.) could not be realised within theproject / could not guarantee an error-freeoperation of the deep sea measuring stationdue to insufficient information on the localcondition in the deep sea. However there areinteresting approaches, which could be furtherinvestigated in subsequent projects. The prototype unit was tested on a cruise ofthe research ship Polarstern during a period ofseveral days in the area of the Haakon MosbyMud Volcano (Norway). Initial results could beobtained, but further research is required toprove the deep sea measuring station and itscomponents.

AcknowledgementsThis research and development project was sup-ported by the BMBF (reference number03G0561A). Special thanks go to Prof. Schlüter from theAlfred-Wegner-Institut, who has supported theproject especially during the field testing phase.

ReferencesDuan et al. (1992) »The prediction of methanesolubility in natural waters to high ionicstrength from 0 to 250°c and from 0 to 1600bar«, Geochimica et Cosmochimica Acta, 56,pp1451-1460

Energie Perspektiven, Ausgabe 4 /2003

www.freenet.de/Freenet/Wissenschaft/Innovationen/Hightech

www.ipp.mpg.de/ippcms/de/pr/publikationen

Marine Current Turbines Ltd., Präsentationenzu Seaflow

Marine Current Turbines Ltd.– Press Release

Yamamoto et al (1976) »Solubility of methanein distilled water and seawater«, Journal ofChemical Engineering Data, 21(1), pp78-80

198

Detailed seismic study of a gas hydrate deposit at the convergent continental margin off Costa Rica – DEGAS

ABSTRACTIn previous studies, amplitude variation withoffset (AVO) analysis and waveform inversiontechniques have been applied to determinequalitative or quantitative information on gashydrates and free gas in the sediment.However, the quantitative contribution ofgas hydrates to the acoustic impedance con-trast observed at the Bottom-SimulatingReflector (BSR) and the reliability of quantita-tive AVO analyses are still topics of discus-sion. In this study, common midpoint gathersfrom multichannel wide-angle reflection seis-mic data acquired from offshore Costa Ricawere processed to preserve true amplitudeinformation at the BSR for a quantitativeAVO analysis incorporating incidence anglesup to 60°. Corrections were applied foreffects that significantly impact the observedamplitude such as source directivity. AVO androck physics modelling indicate that free gasimmediately beneath the gas hydrate stabili-ty zone can be detected and low concentra-tions can be quantified from AVO analysis,whereas the offset dependent reflectivity isnot sensitive to gas hydrate concentrationsof less than about ten percent at the base ofthe gas hydrate stability zone.Patchy BSRs are observed southeast of theNicoya Peninsula on the continental marginoffshore Costa Rica. AVO analysis indicatesthat this phenomenon is related to the pre-sence of free gas saturations less than 5%beneath the gas hydrate stability zone, pro-

bably related to focused vertical fluid flow. Inareas without BSRs, the results indicate thatno free gas is present.

INTRODUCTIONResearch on gas hydrates is of worldwidescientific and industrial interest. Gas hydratesare studied to prevent their formation in natu-ral gas pipelines. In offshore gas explorationand production, gas hydrates increase the riskfor shallow gas blowouts and submarineslumps. Paleoclimate research suggests thatmethane from gas hydrates played an impor-tant role in climate change (Kennett et al.,2003). Finally, gas hydrates are a potentialenergy resource for the future (Collett, 2000).The importance of gas hydrates to climatechange and as a potential future energyresource depends strongly on the worldwideamount of methane trapped in gas hydrates oraccumulated as free gas beneath the gashydrate stability zone (GHSZ). Although esti-mates of the global amount of methane inhydrates are speculative, there is generalagreement that the quantities are very large(Kvenvolden and Lorenson, 2001).Most oceanic occurrences of gas hydrates areinferred from observations of BSRs on marineseismic reflection profiles. The BSR coincideswith the base of the GHSZ. BSRs generallymark the interface between higher P-wavevelocity (hydrate bearing sediment) and lowerP-wave velocity (free gas-bearing sediment).

Müller C., Bönnemann C., Neben S.

Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Geozentrum Hannover, Stilleweg 2, D-30655 Hannover,

Germany, E-Mail: Boennemann@bgr.de

The quantitative relationships between para-meters derived from seismic investigation ofgas hydrates, and the in situ quantity of gashydrates and free gas in the sediment are stilluncertain and define one of the outstandingresearch problems.Under the scope of the German research anddevelopment programme GEOTECHNOLOGIENand it’s research theme »Gas Hydrates in theGeosystem« the present study focuses on quan-tification of the amount of gas hydrates andfree gas trapped inside an isolated gas hydratepatch, outlined by the areal extent of the BSR inreflection seismic sections. No wells are presentin the area under investigation, located southe-ast of the Nicoya Peninsula offshore Costa Rica(Fig. 1). Determining the gas hydrate concentra-tion and free gas saturation in the sedimentsfrom an amplitude variation with offset (AVO)analysis was a major objective of this study.There is still controversy about the contributionof gas hydrates at the base of the GHSZ to theobserved negative impedance contrast at the

BSR (Collett, 2001). The accuracy of quantitati-ve results from AVO analysis is not yet proven(Cambois, 2000), and widely used empiricalrelationships for the elastic moduli of rocks(e.g. Hamilton, 1979) are often inappropriatefor quantitative studies. Therefore, this studytreats quantitative AVO analysis with consider-able emphasis on the open question about thecontribution of the gas hydrates to the impe-dance contrast at the BSR.

NATURE OF THE BSRWhile seismic reflections generally are causedby impedance contrasts due to lithology chan-ges, the BSR is caused by a thermobaric transi-tion from solid gas hydrates in the sediment porespace within the GHSZ to free gas in the porespace beneath the GHSZ. Therefore, the BSRmimics the seafloor reflection and often cross-cuts the stratigraphy. Although scientific drillingand seismic studies agree on the dominanteffect of free gas on the formation of the BSR

199

Figure 1: Offshore Costa Rica the oceanic Cocos Plate is subducted underneath the continental CarribeanPlate at the Middle America Trench (MAT). Seismic lines from cruise SO81 (green lines) and BGR99 (blacklines) are indicated as well as the 3-D survey area of cruise BGR92-3D. An overall BSR distribution of about7700 km2 has been inferred from BGR99 data (blue areas). The area under investigation is represented bythe BGR92-3D box.

200

(Bangs et al., 1995), there is still debate aboutthe quantitative contribution of gas hydrate tothe observed negative impedance contrast(Collett, 2001; Diaconescu et al., 2001).The formation of gas hydrates in the subsurfa-ce can basically be described by two models(Hyndman and Spence, 1992): In the firstmodel (Model A), the methane that goes intoforming the hydrate is assumed to be genera-ted in situ from organic material. This modelhas free gas being generated within and bene-ath the GHSZ. It is assumed that if enoughmethane is present to form substantialamounts of hydrate within the GHSZ, thereshould also be enough for substantial amountsof free gas beneath the GHSZ, and this excessgas migrates upward and forms the BSR. In thesecond model (Model B), hydrate forms frommethane that is removed from rising pore flu-ids expelled from deeper in the sedimentarycolumn. The highest concentrations of gashydrate are expected at the base of the gashydrate stability zone. In Model B it is notnecessary that free gas is present beneath theGHSZ. The Ocean Drilling Program, usuallyaccompanied and followed up by reflectionseismic studies, found evidence for both of theabove models.ODP Leg 164 on the Outer Blake Ridge, offs-hore South Carolina, drilled two holes (site 995and 997) through a pronounced BSR in an areaof prominent acoustic blanking and stronglateral changes in BSR reflection amplitude(Matsumoto et al., 1996). Sonic logs measuredin these holes show a significant decrease incompressional wave speed beneath the BSRindicating the presence of free gas to at least250 m beneath the BSR (Dickens et al., 1997;Helgerud et al., 1999). A third hole (site 994),where a BSR is absent, shows a less pronoun-ced decrease in velocity, probably due to theabsence of significant amounts of free gasbeneath similar gas hydrate concentrations (Luand McMechan, 2002). Holbrook et al. (1996)confirm these sonic log observations from seis-mic traveltime inversion studies that indicate asignificant decrease in seismic velocities direct-ly beneath the BSR at site 995 and 997. This

study also concluded, more generally, thatboth methane hydrate and free gas exist evenwhere a clear BSR is absent.A BSR is regionally distributed throughoutmuch of the Chile Triple Junction region.Downhole temperature and logging data col-lected during ODP Leg 141 suggest that theseismic BSR is generated by low seismic veloci-ties associated with the presence of a few per-cent of free gas in a ~10 m thick zone beneaththe GHSZ (Bangs et al., 1993; Brown et al.,1996), indicating that the formation of the BSRis dominated by the presence of free gas.Offshore Peru (ODP leg 112), a prominent BSRis observed on most of a reflection seismic linealong with a significant weakening of the BSRreflection amplitude close to its landward ter-mination. Pre-stack waveform inversion sug-gests the main contribution to the strong BSRcomes from a free gas layer and low P-wavevelocities rather than elevated velocities causedby the presence of gas hydrates at the bottomof the GHSZ (Pecher et al., 1996a). However,waveform inversion also indicated that theweak BSR reflection amplitudes at the land-ward termination of the BSR were mainly cau-sed by elevated P-wave velocities at the base ofthe GHSZ suggesting very little, if any, free gasis trapped beneath the GHSZ there (Pecher etal., 1996b).Multichannel reflection seismic data were ana-lyzed from an area with a clear BSR on the nor-thern Cascadia subduction zone margin offVancouver Island. Results from AVO analysis,high-resolution velocity analysis, and modelingof vertical-incidence data indicate the forma-tion of the BSR is related to a 10 to 30 m thickhigh-velocity layer immediately above the BSR,without any seismically detectable free gasbeneath the BSR (Hyndman and Spence,1992). On the other hand, Singh et al. (1993)concluded from waveform inversion that theBSR in this region is formed by hydrate-bearingsediment overlying gas-saturated sediment.Analyses of data from two drill sites from ODPLeg 146 offshore Vancouver Island confirm themain contribution from a free gas layer to theobserved BSR (MacKay et al., 1994).

Like any conventional gas reservoir, a reservoirmodel with substantial amounts of free gasrequires an effective top seal. However, thelithology at the base of the GHSZ on continen-tal margins is often homogeneous due to wea-kly consolidated sediment without structuraltraps. In this case, only the gas hydrate in thepore space contributes to a significant reduc-tion of the vertical permeability, and thus actsas a trap under which methane can accumula-te (Kvenvolden and Barnard, 1983). However,the quantitative contribution of the gas hydra-te to the contrast in acoustic impedances atthe BSR remains unclear.Free gas in the pore space is indicated whenseismic velocities are below the backgroundvelocity trend for brine saturated sediment.AVO analysis offers a second tool to detectfree gas, if data quality enables the estimationof the contrast in Poisson’s ratio. A decrease inPoisson’s ratio across an interface can producea significant AVO anomaly. The present study addresses the influence ofgas hydrates at the base of the GHSZ to theobserved acoustic impedance contrast. For thispurpose, an AVO study on high-resolutionlong offset reflection seismic data from offsho-re Costa Rica was carried out.

THE STUDY AREAThe study area is located on the convergentcontinental margin offshore Costa Rica, wherethe oceanic Cocos Plate is subducted underne-ath the continental Caribbean Plate. The mar-gin consists of a margin wedge covered byslope sediments, underthrust by trench sedi-ments, and is fronted by a small accretionaryprism. The dominant structural feature is theburied margin wedge, which is a wedge-sha-ped unit with relatively high seismic velocitiesof more than 4 km/s. Offshore NicoyaPeninsula wide-angle seismic data indicate alandward increase of margin wedge velocitiesup to more than 6 km/s (Christeson et al.,1999). The impedance contrast between low-velocity slope sediments and the marginwedge creates a pronounced seismic reflec-

tion, often referred to as the rough surface orBOSS (bottom of slope sediments) reflector.Since ODP-Leg 170 (Kimura et al., 1997) it isgenerally agreed that the margin wedge iscomposed of older oceanic igneous and asso-ciated sedimentary rock. The structure of themargin was modified during the subduction ofoceanic crust near the Cocos Ridge and theseamount segment along the northern flank ofthe ridge as observed on deep reflection seis-mic data by Hinz et al. (1996).

Scientific drilling offshore Costa RicaThe first coring on the Costa Rica Margin wasat Site 565 on Deep Sea Drilling Project (DSDP)Leg 84 (von Huene et al., 1985). The presenceof gas hydrates and the stickiness of the mudin the upper part of the section ended drillingat this site before the primary objective (sam-pling the high-amplitude reflection below thesedimentary apron) could be accomplished.Gas hydrates were recovered at 285 and 318m below sea level. During this leg, a 1.05 mlong core of massive hydrates was recoveredoff Guatemala (Kvenvolden and McDonald,1985) and downhole well-logging suggest thepresence of a 15 m thick hydrated zone con-taining a 4 m thick nearly pure hydrate section.During Ocean Drilling Program (ODP) Leg 170in 1996, five holes were drilled in the vicinity ofDSDP site 565 (Fig. 1) to determine mass- andfluid-flow paths through a well-constrainedaccretionary complex and calculate mass andfluid balances. Shipboard results (Kimura et al.,1997) demonstrated that the Costa Rica mar-gin did not undergo frontal accretion and thatthe bulk of the small, deformed sedimentarywedge at the toe of the margin was not for-med by scraping off the incoming sedimentfrom the Cocos Plate. Gas hydrates werefound at site 1041, concentrated between~100 and 280 m below seafloor, the zone ofhighest total organic carbon (TOC) content.The primary mode of gas hydrate occurrence isdisseminated, as indicated by the almost con-stant salinity in this depth interval, with thinsheets of gas hydrate filling microfractures.

201

202

Below about 280 m below seafloor, just bene-ath a lithologic transition, the concentration ofvolatile hydrocarbons are highest and thenbegin to decrease downhole (Kimura et al.,1997). In 2002, during ODP Leg 205, anotherthree holes were drilled in this area. The scienti-fic objectives for this leg were to study seismo-genic zone and subduction factory questions.

BSRs and gas hydrates offshore Costa RicaIn addition to scientific drilling, gas hydrateshave been inferred from the presence of BSRsalong the convergent continental margin ofCosta Rica as reported by several authors(Pecher et al., 1998; Shipley et al., 1979;Yamano et al., 1982).In 1999, the German Federal Institute forGeosciences and Natural Resources (BGR) car-ried out a high-resolution reflection seismicsurvey from offshore Costa Rica. One objectiveof this cruise was to map the overall gas hydra-te distribution deduced from BSR occurrencealong the convergent margin off Costa Rica.Results show a total area of about 7700 km2

underlain by BSRs (Fig. 1) with different cha-racteristics regarding occurrence, reflectionstrength, and depth below the seafloor thatindicate a large variability of heat flow andlikely a strong variability of fluid flux throughthe sediment. These variations are related tothe different oceanic crustal segments subduc-ting under Costa Rica. In the northernmostarea NW of Nicoya Peninsula BSRs are continu-ous in water depths from 700 to 2900 m (Fig.1). In the area of ODP Leg 170, no BSRs werefound in the seismic data. The absence of BSRsmight be explained by the subduction of oce-anic crustal segments of different ages, leadingto differences in the thickness of the gashydrate stability zone. North of the paleo plateboundary (PPB) about 1 m.y. older, and thuscooler, oceanic crust is subducted(Barckhausen et al., 2001), and therefore thethickness of the GHSZ should be considerablylarger than south of the PPB. The GHSZ is pro-bably thicker than the sedimentary columnover the margin wedge and thus no free gas is

present to generate a pronounced BSR. In aprevious project, BSRs were mapped in depth inthe BGR92-3D box (Fig. 1). These BSRs show asmall-scale patchy distribution (Hinz et al.,1999). In the area of the proposed IODP-drillingsites NW of Osa Peninsula, BSRs are found atshallow subbottom depths, which can be rela-ted to the influence of the subduction of theigneous Cocos Ridge and the younger and thuswarmer oceanic crust under Osa Peninsula.

FIELD DATAThe second objective of the BGR99 cruise wasto collect high resolution profiles over the pre-viously mapped 3-D box (Fig. 1) for a detailedanalysis of the BSRs. Core data from site 1041(ODP-Leg 170) were used for rock physicsmodelling.

Reflection seismic dataFor this part of BGRs 1999 survey, the volumeof the tuned airgun array was reduced from3800 in3 (62.3 l) to 2000 in3 (32.8 l) at a towingdepth of 5 m for both the array and the strea-mer. This produced a much broader spectrumwith greater energy at higher frequencies anda center frequency of about 60 Hz (Fig. 2).The acquisition parameters were set to 25 mshotpoint interval, 1 ms sample rate and 7 srecord length. In total 420 channels, at anincrement of 12.5 m, with a maximum offsetof 5225 m were recorded on profiles BGR99-59 to BGR99-69, resulting in approximately550 km of 105-fold data.A section of the central seismic line BGR99-60in the area under investigation, with a clearBSR is shown in Fig. 4. The thickness of the gashydrate stability zone increases seaward due tohigher pressure and lower temperature at theseafloor. The landward termination of the BSRis observed near shotpoint 2400, where thedepth of the BSR below seafloor decreasesrapidly. On a parallel line five kilometres fromthis line, the BSR is very weak and difficult toidentify as shown in Fig. 5 (line BGR99-61).The weakness or absence of the BSR is anexample of the previously described patchy

occurrence. Whether gas hydrates are absentor the concentration of gas hydrate and freegas saturation is too low to be recognized as aBSR cannot be determined from stacked seis-mic sections alone. A slump is observed in thissection between shotpoint 1340 and 1380.The head scarp of the slump coincides with theminimum water depth for gas hydrate stabilityof about 580 m for a methane-seawatersystem in this area, indicating mass wastingrelated to the destabilization of gas hydrates.The weak BSR might be explained by the esca-pe of free gas from beneath the GHSZ that ledto a significant reduction in the acoustic impe-dance contrast. A similar process was sugge-sted by Delisle & Berner (2002). They observednumerous gas seeps at water depths less thanthe minimum water depth for hydrate stabilityin the Makran accretionary prism and propo-sed that the gas hydrate layers act as an effec-tive cap rock to upward-directed flow of fluidscontaining significant amounts of gas.

Well dataIn the 3-D box area, no information from wellsis available. Therefore elastic parameters andphysical rock properties determined at site1041 were used to model the AVO response of

shallow unconsolidated sediments at the BSR(Fig.6). The mineralogical composition and theporosity versus depth measurements onextracted core specimens provided parametersfor an effective medium theory (EMT), descri-bed below, to calculate the P-wave velocityand density versus depth. These modelledparameters fit laboratory measurements of thep-wave velocity (PWS3) and the density mea-surements on extracted core specimens(Kimura et al., 1997) at site 1041 reasonablywell (Fig. 6). The EMT was also used to calcu-late the S-wave velocity that was not availablefor any of the sites drilled offshore Costa Rica.

METHODS

Offset-dependent reflectivityOstrander (1984) showed that the reflectioncoefficient from gas saturated sands varies inan anomalous fashion with increasing offsetand used this anomalous behaviour as a directhydrocarbon indicator. This work popularizedthe methodology known as amplitude varia-tion with offset analysis (AVO). The anomalousbehaviour of reflections from gas saturatedsediments can be explained from Gassmann’s(1951) equations, that predict a significant de-

203

Figure 2: Amplitude spectrum calculated for five CMPs fromline BGR99-60. The centre frequency is at about 60 Hz

204

crease in P-wave velocity and a small increase inS-wave velocity (due to the decrease in density)when even a small amount of gas is present inthe pore space. This decrease changes the P-wave reflection coefficient and causes a decrea-se in Vp/Vs ratio and thus in Poisson’s ratio, thatresults in an AVO anomaly. AVO analysis provi-des information about the S-wave velocity orPoisson’s ratio contrast from P-wave data. Inexploration geophysics this method is appliedqualitatively to look for AVO anomalies thatmight indicate the presence of hydrocarbons.The angle-dependent reflection coefficient atan interface separating two semi-infinite iso-tropic elastic media is fully described by theKnott and Zoeppritz equations (Knott, 1899;Zoeppritz, 1919), which are given in concisematrix form by Aki & Richards (1980). Thereflection coefficient at any given angle of inci-dence is completely determined by the densityand the P-wave and S-wave velocities of eachmedium. Therefore, the major task in AVOanalysis is to restore relative amplitudes (AVOprocessing) in seismic pre-stack records beforeextraction and interpretation of the amplitudeinformation.

The effective medium theoryThe angle-dependent reflection and transmis-sion coefficients at an interface separating twosemi-infinite isotropic elastic media are descri-bed by the Knott and Zoeppritz equations,which are functions of the elastic properties ofthe sediment. These elastic properties dependon the physical properties of the sediment, e.g.the mineralogy, the porosity, the effective pres-sure, and the type of pore fluid. For gas hydra-tes, P-wave velocities for hydrate bearing andgas-saturated sediments that have been deter-mined from logging vary significantly withlocation, gas hydrate concentration and gas-saturation. For example, Prensky (1995) foundthat P-wave velocities for hydrate-bearing sedi-ments range between 2.1 km/s and 4.5 km/s.In many cases, reflection seismic studies lack insitu-information about the seismic velocities.When available, well logs often provide only P-wave velocities. Shear-wave velocities are usu-ally unavailable. Therefore, for AVA and wave-form inversion modelling, empirical relations-hips for calculating propagation velocities areoften required, e.g. Hamilton (1979) for P-wave velocities and Castagna et al. (1985) forS-wave velocities (Adriansyah and McMechan,

Figure 3: The area under investi-gation is located on the middleslope of the convergent continen-tal margin offshore Costa Rica,southeast of the Nicoya Penin-sula. Eight wide-angle reflectionseismic lines have been acquiredin 1999, located in a 3-D surveyarea from 1992 (15 x 30 km2) inorder to study a gas hydrate de-posit using pre-stack seismicmethods.

2001; Pecher et al., 1998; Tinivella andAccaino, 2000). These methods do not consi-der in-situ physical properties of the sediment,and thus provide only a rough estimate of gashydrate concentration and free gas saturation.Dvorkin et al. (1999) developed an effectivemedium theory that allows to calculate the ela-stic moduli of shallow unconsolidated marinesediment from mineralogy, porosity, effectivepressure, and pore fluid compressibility. Themodel assumes that the modulus-pressurebehaviour of the sediment at 36 to 40% poro-sity (critical porosity) is described by a denserandom pack of identical elastic spheres. Theeffective bulk and shear modulus of this packis then given by the Hertz-Mindlin contact the-ory (Mindlin, 1949). The calculation of theporosity-dependent effective moduli for poro-sities above critical porosity is performed usinga modification of the Hashin-Shtrikman(Hashin and Shtrikman, 1963) upper bound.Finally, the moduli of the saturated sediment iscalculated from Gassmann’s equations(Gassmann, 1951). Dvorkin et al. (1999) gavea complete description of the theory includingverification at an ODP site.For weak, highly porous rocks, porosity, effec-tive pressure and fluid properties play a majorrole in the sediment’s elastic moduli, that areconsidered in the EMT theory (e.g. Helgerud,2001). Ecker et al. (1998) found from AVAanalysis of one CMP on the Outer Blake Ridgethat the gas hydrate is located away from thegrain contacts and does not affect the stiffnessof the sediment frame.For AVO analyses, this theory allows elastic

moduli to be calculated from an analytical setof equations using in situ physical properties ofthe sediment. The physical properties usedhere represent the sediment mineralogy andporosity of shallow marine sediments offshoreCosta Rica from measurements acquired onODP Leg 170. The mineralogy is composed of5% calcite, 70% clay, and 25% quartz. Theelastic parameters for minerals, pore water,and gas hydrate used in our calculation isshown in Table 1. Hydrostatic pore pressureand a porosity of 60% were assumed.

AVO PROCESSINGThe following describes the processingsequence. Corrections were applied to ampli-tudes extracted from raw CMP gathers. Thetravel paths of the seismic signal were estima-ted by 1-D raytracing.

RaytracingSeismic reflectivity at an interface is a functionof incidence angle. A seismic trace, however, isrecorded at a fixed offset, where the reflectionangle changes with time. Source and receiverdirectivity are also a function of angle, and thespherical divergence is a function of the distan-ce between source and receiver. Therefore it isappropriate to apply all amplitude correctionsin the angle-domain. In order to determine thetravel path and the relationship between sour-ce-to-receiver offset and incidence angle at thereflecting interface, a 1-D ray tracing followingthe derivation of Dahl & Ursin (1991) has been

205

Table 1: Elastic properties of sediment solid phase components, brine, hydrate, andmethane used for the effective medium theory calculations (after Helgerud, 2001).

206

implemented. In this approach, the ray para-meter is iteratively determined. The seafloor isreasonable smooth and dips less than 3°. The1-D earth model is characterized by thickness, P-and S-wave velocities, and density of each layer.The ray path is determined obeying Snell’s law.Any of the corrections described below arebased on this ray tracing approach, where P-wave velocities and layer thicknesses are basedon semblance-based velocity analysis.

Geometrical spreadingIn a homogeneous medium, the energy trans-mitted outwards from the source is distributedover a spherical shell. Therefore the energydensity decays proportional to r2, where r is theradius of the wavefront. The wave amplitude,which is proportional to the square root of theenergy density, falls off proportional to r. Whenthe velocity increases with depth, the decay inamplitude occurs even more rapid with distancedue to ray bending. In this study, the correctionfor spherical divergence is based on the raypath,derived from the 1-D ray tracing.

AbsorptionWhen the signal propagates through the sub-surface, wave energy is also transformed intoanother form of energy. Wave motion is gradu-ally absorbed by the medium and reappears inform of heat. In absorbing media the amplitudedecay occurs exponential with r (Luh, 1993). Inthis study the absorption has been restored forthe center frequency of 60 Hz and an effectivequality factor Qp of 200 for P-waves was usedfor layers between seafloor and BSR.

Receiver directivityThe directivity of the seismic streamer is calcu-lated as the directional response of a lineararray (Keary and Brooks, 1984) implementingthe cable geometry for the BGR99 cruise. Am-plitude compensation due to receiver directivi-ty has been performed for the center frequen-cy of 60 Hz. The amplitude reduction due tothe receiver directivity amounts to about 28%at an incidence angle of 60°.

Figure 4: Time-migrated seismic section of line BGR99-60 across the middle continentalmargin of Costa Rica, southeast of Nicoya Peninsula. A clear BSR indicates the base ofthe gas hydrate stability zone with a seaward increasing thickness. The amplitude varia-tion with angle analyses are performed on CMPs from this line. CMP locations wherequantitative AVO analyses have been performed are indicated.

Source directivityIn reflection seismic surveys the source array isusually designed to focus energy downwardinto the subsurface. This implies more or lessdistinctive source directivities leading to energyemissions that are dependent on the angle ofemergence. For wide-angle AVO analyses thesource directivity is a critical factor which signi-ficantly alters the observed amplitude variationwith offset and thus may lead to misinterpre-tation. In many studies only little, if any, data-independent information about the sourcedirectivity is available (e.g. from modellingusing appropriate software packages). In manystudies, authors simply estimate the directivityfrom waveform modelling (e.g. Hyndman &Spence, 1992) or calibrate their data using theseafloor AVO response as a reference (e.g.Ecker et al., 1998).During cruise BGR99, lines BGR99-59 toBGR99-69 have been acquired using a tunedairgun array consisting of four sub-arrays. Eachsub-array consisted of seven airguns with volu-mes varying between 0.33 l and 1.64 l. A sour-ce directivity plot modelled with the NUCLEUS®

software from PGS Seres AS was available. Thesource directivity has been extracted for 40

and 60 Hz, respectively. With respect to thecenter frequency of 60 Hz, the source directivi-ty of the BGR99 array was found to be bestrepresented by a cos2-function (Fig. 7). At anangle of incidence of 60° the amplitude reduc-tion due to the source directivity amounts to75%. Compared to the receiver directivity(28%), the source has a significantly strongereffect on the amplitudes.

Transmission lossAnother type of effect that influences thewave amplitude as the signal propagatesthrough the overlying media is the loss due toenergy conversion and reflection/transmissionin the overburden. Castagna (1993) showedthat these angle dependent effects are mostsignificant when the reflectivity above the tar-get horizon is very strong. Each reflection istransmitted through the shallower interfacestwice, once on the way down and once on theway up. On the return trip the changes in Vp,Vs, and r have reversed sign. Therefore, theangle-dependent effects can be neglected in afirst order (e.g. Spratt et al.,1993). As shown inFig. 4, the BGR99 data show only low reflecti-

207

Figure 5: Time-migrated seismic section of line BGR99-61 located at a distance of five kilometresfrom line BGR99-60. A weak and discontinuous bottom simulating reflector outlines the base of thegas hydrate stability zone. The prominent feature in this section is a slump with its head scarp locatedat the minimum water depth for hydrate stability (580 m, 770 ms TWT) suggesting an origin of theslumping related to gas hydrate destabilization.

208

vity between the BSR and the seafloor reflec-tion. Regarding the seafloor AVA response, theamplitude variation with offset for the seafloorreflection is very small, only about 3.5% fromvertical-incidence to the maximum angle ofincidence. The transmission loss has been con-sidered by correcting for the vertical-incidenceseafloor reflection/transmission coefficientonly.

Reflection coefficientsAssuming that the sea bottom is horizontaland smooth, and that the sea bed is a reflectorbetween two half-spaces, the vertical-inciden-ce seafloor reflection coefficient RSeafloor can beestimated from the primary seafloor reflectionamplitude and the amplitude of the first sea-floor multiple (Warner, 1990) from:

(1),

where Am and Ap are the amplitudes of themultiple and primary seafloor reflection,respectively. The water depth must be largecompared to source-to-receiver offset and thevariation in reflection amplitude at near-verti-cal incidence must be negligible. The observedseafloor primary and multiple amplitudes werecorrected for absorption (Qp = 10000, fc = 60Hz) and spherical divergence. Angle-depen-dent transmission loss is not of concern withinthe water layer. After determining the seafloorreflection coefficient, the BSR amplitude res-ponse was corrected for spherical divergence,absorption, source and receiver directivities.The vertical incidence reflection coefficient atthe BSR RBSR can then be derived from:

(2)

where ABSR and ASeafloor are the amplitudes ofthe primary BSR and seafloor reflection,

Figure 6: Elastic moduli are calculated from physical rock properties (mineral composition and porosity) at site 1041 ofODP Leg 170 (Kimura et al., 1997) implementing the effective medium theory from Helgerud et al. (2001). The porosi-ty measurements on extracted core specimens (left, circles) are represented by a linear trend (left, solid line) decreasingfrom 65% at the seafloor to 45% at 400 mbsf. The calculated P-wave velocity and bulk density (middle and right,solid line) adequately represent the measured parameters (middle and right, circles). The mineral composition of theun-consolidated slope sediments is assumed to be constant with depth and is represented by 70% clay, 25% quartzand 5% calcite at this site. Gas hydrates were found between 100 m and 280 m below seafloor (grey area).

respectively. The angle dependent reflectivity isthen derived from scaling the BSR AVA ampli-tude response to the derived vertical-incidenceBSR reflection coefficient.

AVO MODELINGThe full Zoeppritz equations were used to cal-culate synthetic angle-dependent reflectioncoefficients versus offset (RVO). In order tostudy the effect of hydrate-saturated and free-gas saturated sediment on the RVO responseof the BSR, and to interpret the real data withrespect to gas hydrate concentration and freegas saturation, synthetic RVA responses werecalculated for a »hydrate- over brine-saturatedsediment« and a »brine- over free gas-satura-ted sediment« model of the subsurface. Thesesimplified models have been chosen in order tohave only one variable in each model, i.e. thehydrate concentration in the first model andthe free gas saturation in the second model.The gas hydrate was considered to be homo-

geneously distributed in the pore space, follo-wing Ecker et al. (1998) results for high-poro-sity hydrate-bearing sediment at the OuterBlake Ridge.Synthetic RVA curves were calculated for bulkgas hydrate concentrations and bulk free-gassaturations increasing from 1 to 10% at anincrement of 1%, respectively. The results inFig. 8 clearly show the dominant effect of thefree gas on the vertical-incidence reflectioncoefficient and on the shape of the RVAresponse. The red array of curves that repre-sents the gas-free BSR case have vertical-inci-dence reflection coefficient magnitudes of lessthan 0.05, even at bulk gas hydrate concen-trations of 10%. Furthermore, these curves arecharacterized by almost constant reflectioncoefficients at intermediate angles of inciden-ce due to almost no contrast in Poisson’s ratio(Fig. 9), while a significant increase in reflectioncoefficient magnitude is only observed at inci-dent angles greater than 60°, due to the incre-asing P-wave velocity with increasing gas

209

Figure 7: The directivity of the airgun array used on the BGR99 cruise hasbeen derived from a directivity plot created with the NUCLEUS software fromPGS Seres AS (symbols). The directivity is best represented by a cos2-function(solid line). The effect of the source directivity on the amplitudes is signifi-cantly larger than the effect of the receiver directivity (dashed line).

210

hydrate concentration (Fig. 9). In contrast, theRVA responses representing the free gas BSRcase (brine-saturated over free gas-saturatedsediment and blue curves in Fig. 8) are cha-racterized by strong vertical-incidence reflec-tion coefficients that significantly increase inmagnitude with increasing saturation of freegas. Moreover, the reflection coefficients alsosignificantly increase at intermediate angles ofincidence due to the increasing contrast inPoisson’s ratio (Fig. 10). Since we assume thatthe mineral frame does not change across theBSR, the above mentioned different shapes ofthe AVO curve represent the different pore fil-lings. Fig. 8 shows that low concentrations ofgas hydrate cannot be resolved using AVOanalysis. It is also evident that the differentia-tion of high saturations of free gas is not pos-sible because of the major drop in P-wave velo-city and Poisson’s ratio that occur at low gassaturations. However, the synthetic data indi-cate that AVO analysis using the full Zoeppritzequations and an effective medium theory

enables free gas to be detected beneath theBSR and provides estimates of free-gas satura-tion at low saturation levels. The contributionof up to 10% bulk gas hydrate concentrationto the observed RVA response is very small andcannot be resolved. Thus a model with hydra-te-bearing sediment overlying gas-saturatedsediment cannot be distinguished from amodel with brine-saturated sediment overlyinggas-saturated sediment using this method.

RESULTS

Extraction of amplitudesThe next step in AVO analysis was the extrac-tion of amplitudes from pre-stack seismic CMPgathers. From the raw pre-stack seismic CMPgathers, the main lobe of the BSR reflection(trough) and the seafloor reflection (peak) waspicked as a function of offset. Fig. 11a showsCMP 4520 from line BGR99-60 with a clearBSR reflection characterized by reversed polari-

Figure 8: Synthetic AVA responses representing the gas-free BSR model and the free gas BSRmodel. The red array of curves represent the gas-free model with hydrate-bearing over brine-satu-rated sediment, were the bulk gas hydrate concentration increases from 1 to 10%. The blue arrayof curves represent the free gas model with brine-saturated sediment over gas-saturated sedi-ment, were the bulk gas-saturation increases from 1 to 10%.

ty with respect to the seafloor reflection. Inaddition to the source-to-receiver offset, theangles of incidence for the seafloor and theBSR reflections are indicated in Fig. 11b and11c, based on 1-D raytracing. In this record,the high signal-to-noise ratio allowed amplitu-des of the BSR to be extracted up to incidenceangles of about 80°. Beyond 60°, interferencebetween the BSR reflection and other reflec-tions corrupts the amplitude information. Thisbehaviour was observed on all CMPs in thisstudy. Therefore, amplitudes beyond incidenceangles of 60° were not used for AVO analysis.Waveform modelling that includes the reflecti-vity of the overburden is needed to account forinterference.Reflection coefficient versus angle of incidenceLine BGR99-60 with a clear BSR and strongvariations of the post-stack BSR reflectionamplitude along the line (Figure 4) was selec-ted for AVO analysis. 40 CMP gathers wereextracted from the pre-stack data at locationswith weak, intermediate and strong post-stack

BSR reflection amplitudes. Unfortunately, itwas not possible to use the amplitude infor-mation up to incidence angles of 60° in mostof the gathers. The deterioration of BSR reflec-tion amplitudes was often caused by interfe-rence, probably related to faulting or conflic-ting dips between reflections from the BSR andstratigraphic interfaces. If the maximum inci-dence angle on a selected CMP for which theBSR reflection amplitude could be extractedwas much less than 60°, the CDP was disre-garded for AVO analysis.In Fig. 10, CMP 4500, CMP 4530, and CMP4720 are CMPs with high signal-to-noise ratiofrom areas of intermediate, high, and lowpost-stack BSR reflection amplitudes, respecti-vely (ref. Fig. 4). In Figure 12, the derivedreflection coefficients versus angle (RVA) areplotted with synthetic RVA responses for freegas saturations varying between 1 and 10%underneath brine saturated sediment (bluearray of curves). The CMP data match the syn-thetic free gas curves. Due to convergence of

211

Figure 9: Elastic properties calculated from the effective medium theory as a function ofbulk gas hydrate concentration. Note, the Poisson’s ratio, density, and S-wave velocityare almost unaffected by the change in gas hydrate concentration.

212

the synthetic RVA curves, determination of freegas saturation becomes less accurate withincreasing free gas saturation. AVO analysispredicted free gas concentrations of about 2%(CMP 4500, intermediate BSR reflection ampli-tude), 4 to 5% (CMP 4530, strong BSR reflec-tion amplitude), and about 1% (CMP 4720,weak BSR reflection amplitude). It is not possi-ble to accurately estimate the concentration ofgas hydrate with this method, since the reflec-tion coefficient is not sensitive to concentra-tion of gas hydrate as shown before.

DISCUSSION AND CONCLUSIONSUsing an analytical relationship between physi-cal properties and elastic moduli of the rockand the full Zoeppritz equations to model AVOresponses avoids uncertainties introduced fromempirical relationships and approximations tothe Zoeppritz equations. The use of an explicitsource directivity function is uncommon, becau-

se of lack of information. As shown here, atuned airgun array can introduce significantamplitude reduction at high angles of inciden-ce. This study benefited from a data set withhigh signal-to-noise-ratio and a broad frequen-cy spectrum. However, the results show thatquantitative predictions can only be achievedto a limited degree from AVO analysis, evenwith the information available for this study.This is mainly because of the low sensitivity ofthe AVO response to changes in the gas hydra-te concentration and to free gas saturationsabove about 5%. The measured amplitudes were limited to inci-dence angles of up to about 60°. Interferencebetween the BSR reflection and other reflec-tions led to a significant deterioration of ampli-tudes at high angles of incidence. Deconvolu-tion and multiple suppression were not appliedto the data to avoid alteration of the amplitu-des. Information from high angles of incidencecan be used with full waveform inversion,

Figure 10: Elastic properties calculated from the effective medium theory as a function of bulk freegas saturation. Note, the Poisson’s ratio, density, and S-wave velocity changes significantly withincreasing free gas saturation compared to the gas hydrate case (Fig. 9). The brine-saturated situa-tion is represented by 0% bulk free gas saturation. With increasing free gas saturation, the con-trast in Poisson’s ratio ∆σ to the brine-saturated case is increasing.

which includes AVO effects, the source signa-ture, and the reflectivity of the overburden. Butthis approach also requires calibration wells inthe area under investigation.AVO analysis on extracted amplitudes fromraw data was time consuming. Many CMPswere unusable for AVO analysis due to dete-riorated amplitudes at the seafloor or the BSRreflection. Therefore, reliable 2-D informationon the free gas distribution can not be obtai-ned. For this purpose a 2-D waveform inver-sion including wide-angle AVO effects is requi-red, which has not yet been done. Never-theless, the approach followed in this studyemphasises the advantage of AVO analysis onmultichannel seismic data compared to ocean-bottom hydrophone (OBH) and ocean-bottomseismometer (OBS) data. In the multichannelcase a huge number of CMPs are available to

select AVO responses with adequate signal-to-noise ratio, while only a few records are availa-ble in OBH/OBS studies. The consideration ofsynthetic AVO responses is important to esti-mate the sensitivity of the AVO responses tothe gas hydrate concentration and free gassaturation. This understanding prevents over-fitting the real data and drawing incorrect con-clusions. This is why no inversion of the extrac-ted AVO responses was performed.This study also indicates that the patchy BSRssoutheast of the Nicoya Peninsula, i.e. the areawith a BSR in seismic sections, are related tothe presence of free gas beneath the GHSZ.The area without BSRs can be considered asnot containing free gas beneath the GHSZ. In conclusion, the method presented in thisstudy emphasises opportunities and drawbak-ks regarding quantitative AVO analysis. With

213

Figure 11: The amplitude information for AVO analysis is extracted from pre-stack CMP gathers.(A) Raw CMP gather 4520 from line BGR99-60 showing a normal-polarity sea-floor reflection anda reversed-polarity BSR reflection. The reflectivity within the hydrate stability zone is very weakcompared to the seafloor and BSR reflectivity. The record is displayed with a reduction velocity of2.5 km/s. (B) The flattened seafloor reflection indicates an amplitude decrease without any signifi-cant alteration of the wavelet with increasing offset. Angles of incidence based on 1-D raytracingare indicated. (C) The flattened BSR reflection interferes with reflection hyperbolas from shallowerand deeper events at high offsets. Since these effects cannot be considered in AVO analysis, theuseful amplitude information is often limited to incidence angles of about 60° or less.

214

reference to the gas hydrate occurrence on thecontinental margin offshore Costa Rica andthe origin of the bottom simulating reflector inseismic sections, the following results havebeen obtained:- A strong BSR in reflection seismic sections

with vertical incidence reflection coefficientsof more than -0.1, and increasing reflectioncoefficients magnitude with increasingangle of incidence, is a clear indication offree gas beneath the GHSZ.

- A BSR can also be caused by the exclusivepresence of gas hydrate at the base of theGHSZ. In this case, the RVA response is cha-racterized by a small vertical incidence reflec-tion coefficient of less than about 0.05 forbulk hydrate concentrations of up to 10%,and a minor increase of reflection coefficientswith increasing angle of incidence.

- Quantitative AVO analysis enables free gassaturations up to 5% to be estimated.Above these concentrations, differences inreflection coefficients are small.

- Quantitative AVO analysis does not allowthe estimation of the gas hydrate concen-tration when free gas is present beneath theGHSZ, because the variation in reflectionresponse is small in comparison to the rangeof variation in gas hydrate concentration.

- Quantitative predictions from AVO analysisalone are limited, even when an analyticalrelationship between the physical proper-ties and the elastic moduli of the rock, andan explicit source directivity function areconsidered.

- This study indicates that the patchy occur-rence of the BSRs southeast of NicoyaPeninsula is caused by the presence of freegas beneath the GHSZ, which may havemigrated through deep-reaching faults andbecome trapped beneath the GHSZ. Even atlocations with a very strong BSR, the resultsindicate free gas saturations of less thanabout 5%. Outside the area of BSR occur-rences, gas hydrate may be present.

Figure 12: Reflection coefficient versus angle of incidence for three CMPs from lineBGR99-60 representing areas with intermediate (CMP 4500), strong (CMP4530), andweak (CMP4720) post-stack BSR reflection amplitudes. Synthetic curves are calculatedfor bulk free gas saturations from 1 to 10% (blue curves).

ACKNOWLEDGMENTSThe work presented in this paper has beensupported by the Federal Ministry forEducation and Research (BMBF) and theGerman Research Council (DFG) under thescope of the research and development pro-gramme GEOTECHNOLOGIES.

REFERENCESAdriansyah, and G.A. McMechan, AVA analy-sis and interpretation of a carbonate reservoir:northwest Java basin, Indonesia, Geophysics,66, 744-754, 2001.

Aki, K., and P.G. Richards, Quantitative seismo-logy, theory and methods, W.H. Freeman, NewYork, 1980.

Bangs, N.L.B., D.S. Sawyer, and X. Golovchen-ko, Free gas at the base of the gas hydratezone in the vicinity of the Chile triple junction,Geology, 21 (10), 905-908, 1993.

Barckhausen, U., C.R. Ranero, R. von Huene,S.C. Cande, and H.A. Roeser, Revised tectonicboundaries in the Cocos Plate off Costa Rica:Implications for the segmentation of the con-vergent margin and for plate tectonic models, J.Geophys. Res., 106 (B9), 19,207-19,220, 2001.

Brown, K.M., N.L. Bangs, P.N. Froelich, and K.A.Kvenvolden, The nature, distribution, and originof gas hydrate in the Chile Triple Junctionregion, EPSL, 139 (3-4), 471-483, 1996.

Cambois, G., Can P-wave AVO be quantitati-ve?, The Leading Edge, 19, 1246-1251, 2000.Castagna, J.P., AVO Analysis - Tutorial andReview, in Offset-dependent reflectivity: theoryand practice of AVO analysis, edited by J.P.Castagna, and M.M. Backus, Society ofExploration Geophysicists, 1993.

Castagna, J.P., M.L. Batzle, and R.L. Eastwood,Relationships between compressional andshear-wave velocities in clastic silicate rocks,Geophysics, 50, 571-581, 1985.

Christeson, G.L., T.H. McIntosh, T.H. Shipley, E.R.Flueh, and H. Goedde, Structure of the CostaRica convergent margin, offshore Nicoya Penin-sula, J. Geophys. Res., 104, 25443-25468, 1999.

Collett, T.S., Natural Gas Hydrate as a PotentialEnergy Resource, in Natural Gas Hydrate inOceanic and Permafrost Environments, editedby M.D. Max, pp. 123-136, Kluwer AcademicPublishers, 2000.

Collett, T.S., A Review of Well-Log AnalysisTechniques Used to Assess Gas-Hydrate-BearingReservoirs, in Natural Gas Hydrates: Occurrence,Distribution, and Detection, edited by C.K.Paull, and W.P. Dillon, American GeophysicalUnion, Washington, DC 20009, 2001.

Dahl, T., and B. Ursin, Parameter estimation ona one-dimensional anelastic medium, J. Geo-phys. Res., 96, 20217-20233, 1991.

Delisle, G., and U. Berner, Gas hydrate actingas cap rock to fluid discharge in the Makranaccretionary prism?, in The Tectonic andClimatic Evolution of the Arabian Sea Region,edited by P.D. Clift, D. Kroon, C. Gaedicke, andJ. Craig, pp. 137-146, Geological Society,London, 2002.

Diaconescu, C.C., R.M. Kieckhefer, and J.H.Knapp, Geophysical evidence for gas hydratesin the deep water of the South Caspian Basin,Azerbaijan, Marine and Petroleum Geology,18, 209-221, 2001.

Dickens, G.R., C.K. Paull, P. Wallace, and ODPLeg 164 Scientific Party, Direct measurement ofin situ methane quantities in a large gas-hydra-te reservoir, Nature, 385, 426-428, 1997.

215

216

Dvorkin, J., M. Prassad, A. Sakai, and D.Lavoie, Elasticity of marine sediments: Rockphysics modeling, Geophys. Res. Lett., 26,1781-1784, 1999.

Ecker, C., J. Dvorkin, and A. Nur, Sedimentswith gas hydrates: Internal structure from seis-mic AVO, Geophysics, 63, 1659-1669, 1998.Gassmann, F., Elasticity of porous media: Überder Elastizität poröser Medien, Vierteljahrs-schrift der Naturforschenden Gesellschaft, 96,1-23, 1951.

Hamilton, E.L., Sound velocity gradients inmarine sediments, J. Acoust. Soc. Am., 65,909-922, 1979.

Hashin, Z., and S. Shtrikman, A variationalapproach to the elastic behavior of multipha-se materials, J. Mech. Phys. Solids, 11, 127-140, 1963.

Helgerud, M.B., Wave speeds in gas hydrateand sedimets containing gas hydrate: A labo-ratory and modelling study, PhD thesis thesis,Stanford University, 2001.

Helgerud, M.B., J. Dvorkin, A. Nur, A. Sakai,and T.S. Collett, Elastic-wave velocity in marinesediments with gas hydrates: Effective mediummodelling, Geophys Res. Lett., 26 (13), 2021-2024, 1999.

Hinz, K., C. Bönnemann, C.R. Ranero, D.Ristow, J. Hauschild, and T. Gindler, Final Re-port DFG Project Hi 179/26-1 and -2: Threedimensional detailed imaging of prominentstructures in reflection seismics at the activecontinental margin off Costa Rica, pp. 1-30,BGR-Report 0119.373, 1999.

Hinz, K., R. von Huene, C.R. Ranero, and andthe PACOMAR Working Group, Tectonic struc-ture of the convergent Pacific margin offshoreCosta Rica from multichannel seismic reflec-tion data, Tectonics, 15 (1), 54-66, 1996.

Holbrook, W.S., H. Hoskins, W.T. Wood, R.A.Stephen, and D. Lizarralde, MethaneHydrate and Free Gas on the Blake Ridgefrom Vertical Seismic Profiling, Science, 273,1840-1843, 1996.

Hyndman, R.D., and G.D. Spence, A SeismicStudy of Methane Hydrate Marine BottomSimulating Reflectors, J. Geophys. Res., 97(B5), 6683-6698, 1992.

Keary, P., and M. Brooks, An Introduction toGeophysical Exploration, Blackwell ScientificPublications, 1984.

Kennett, J.P., K.G. Cannariato, I.L. Hendy, andR.J. Behl, Methane Hydrates in QuaternaryClimate Change: The Clathrate Gun Hypothe-sis, 216 pp., American Geophysical Union,Washington D.C., 2003.

Kimura, G., E. Silver, P. Blum, and et al., Proc.ODP, Init. Repts., 170, College Station, TX(Ocean Drilling Program), 1997.

Knott, C.G., Reflexion and refraction of elasticwaves with seismological applications, Phil.Mag., 48, 64-97, 1899.

Kvenvolden, K.A., and T.D. Lorenson, TheGlobal Occurrence of Natural Gas Hydrate, inNatural Gas Hydrates: Occurrence, Distribu-tion, and Detection, edited by C.K. Paull, andW.P. Dillon, pp. 3-18, American GeophysicalUnion, Washington D.C., 2001.

Kvenvolden, K.A., and T.J. McDonald, Gashydrates of the Middle America Trench, DeepSea Drilling Project Leg 84, Initial Reports,Deep Sea Drilling Project, 84, 667-682, 1985.Lu, S., and G.A. McMechan, Estimation of gashydrate and free gas saturation, concentration,and distribution from seismic data, Geo-physics, 67 (582-593), 2002.

Luh, P.C., Wavelet Attenuation and Bright-Spotdetection, in Offset-dependent reflectivity: the-ory and practice of AVO analysis, edited by J.P.Castagna, and M.M. Backus, Society of Explo-ration Geophysicists, 1993.

MacKay, M.E., R.D. Jarrard, G.K. Westbrook,and R.D. Hyndman, Origin of bottom-simula-ting reflectors: Geophysical evidence from theCascadian accretionary prism, Geology, 22,459-462, 1994.

Matsumoto, R., C. Paull, P. Wallace, and theLeg 164 Scientific party, Gas hydrate samplingon the Blake Ridge and Carolina Rise: ODP, Leg164 Preliminary Report, 1996.

Mindlin, R.D., Compliance of elastic bodies incontact, Trans. ASME, 71, A-259, 1949.

Ostrander, W.J., Plane-wave reflection coeffi-cients for gas sands at nonnormal angles of inci-dence, Geophysics, 49 (10), 1637-1648, 1984.

Pecher, I.A., T.A. Minshull, S.C. Singh, and R.von Huene, Velocity structure of a bottomsimulating reflector offshore Peru: Results fromfull waveform inversion, Earth Planet. Sci. Lett.,139, 459-469, 1996a.

Pecher, I.A., C.R. Ranero, R. von Huene, T.A.Minshull, and S.C. Singh, The nature anddistribution of bottom simulating reflectors atthe Costa Rican convergent margin, Geophys.J. Int., 133, 219-229, 1998.

Pecher, I.A., R. von Huene, C.R. Ranero, N.Kukowski, T.A. Minshull, and S.C. Singh, For-mation mechanisms of free gas beneath thehydrate stability zone at convergent margins -geophysical evidence from bottom simulatingreflectors at the Peruvian and Pacific CostaRican margins, in Proc. International conferen-ce on natural gas hydrates, pp. 593-600, Tou-louse, France, 1996b.

Prensky, S., A Review of Gas Hydrates andFormation Evaluation of Hydrate-BearingReservoirs (paper GGG), in Meeting of theSociety of Professional Well Log Analysts, Paris,France, June 26-29, 1995.

Shipley, T.H., M.H. Houston, R.T. Buffler, F.J.Shaub, K.J. McMillin, J.W. Ladd, and J.L. Worzel,Seismic evidence for widespread possible gashydrate horizons on continental slopes andrises, AAPG Bulletin, 63, 2204-2213, 1979.

Singh, S.C., T.A. Minshull, and G.D. Spence,Velocity Structure of a Gas Hydrate Reflector,Science, 260, 204-207, 1993.

Tinivella, U., and F. Accaino, Compressionalvelocity structure and Poisson's ratio in marinesediments with gas hydrate and free gas byinversion of reflected and refracted seismicdata (South Shetland Islands, Antarctica),Marine Geology, 164, 13-27, 2000.

von Huene, R., J. Aubouin, and others, Init.Repts. DSDP, 84, 967 pp., U.S. Printing Office,Washington, D.C., 1985.

Warner, M., Absolute reflection coefficientsfrom deep seismic reflections, Tectonophysics,173, 15-23, 1990.

Yamano, M., S. Uyeda, Y. Aoki, and T.H.Shipley, Estimates of Heat Flow Derived fromGas Hydrates, Geology, 10, 339-343, 1982.

Zoeppritz, K., Erdbebenwellen VII. Über Reflex-ion und Durchgang seismischer Wellen durchUnstetigkeitsflächen. Mathematisch-Physikali-sche Klasse, Nachrichten der Königlichen Ge-sellschaft der Wissenschaften zu Göttingen,66-84, 1919.

217

218

Index

AAbegg C. . . . . . . . . . . . . . . . . . . . . . 20Abegg F. . . . . . . . . . . . . . . . . . . . . . . . 4Amann H.11 . . . . . . . . . . . . . . . . . . . . 4Amann R. . . . . . . . . . . . . . . . . . . . . . 74

BBauer K. . . . . . . . . . . . . . . . . . . . . . . 98Becker H.J. . . . . . . . . . . . . . . . . . . . 118Bialas J.. . . . . . . . . . . . . . . . . . . 86, 170Blumenberg M.. . . . . . . . . . . . . . . . . 40Boetius A. . . . . . . . . . . . . . . . . . . . . . 58Bohrmann G. . . . . . . . . . . . . . . . 4, 170Bönnemann C. . . . . . . . . . . . . . . . . 198Breitzke M. . . . . . . . . . . . . . . . . . . . . 86Brückmann W. . . . . . . . . . . . . . . . . . . 4

DDe Beer D. . . . . . . . . . . . . . . . . . . . . 64Deerberg G. . . . . . . . . . . . . . . . . . . 138Degenhardt A. . . . . . . . . . . . . . . . . 186Drews M. . . . . . . . . . . . . . . . . . . . . . . 4

EEisenhauer A. . . . . . . . . . . . . . . . . . . 20Elvert M. . . . . . . . . . . . . . . . . . . . . . . 68Erzinger J. . . . . . . . . . . . . . . . . . 98, 152

FFahlenkamp, H.. . . . . . . . . . . . . . . . 138Feeser, V.. . . . . . . . . . . . . . . . . . . . . 118Fischer, H. . . . . . . . . . . . . . . . . . . . . 166Flüh, E. . . . . . . . . . . . . . . . . 40, 86, 170

GGoreshnik E . . . . . . . . . . . . . . . . . . 134Greinert J . . . . . . . . . . . . . . . . . . . . . 20Grupe B. . . . . . . . . . . . . . . . . . . . . . 118Gubsch S . . . . . . . . . . . . . . . . . . . . . 20Gust G . . . . . . . . . . . . . . . . . . . . . 4, 20

HHanken T . . . . . . . . . . . . . . . . . . . . 186Helmke J. . . . . . . . . . . . . . . . . . . . . 186Henninges J. . . . . . . . . . . . . . . . . . . . 98Hoffmann K . . . . . . . . . . . . . . . . . . 118Hohnberg H.-J. . . . . . . . . . . . . . . . . . . 4Huenges E . . . . . . . . . . . . . . . . . . . . 98

IItoh, H. . . . . . . . . . . . . . . . . . . . . . . 134

JJaguttis J. . . . . . . . . . . . . . . . . . . . . 186Jørgensen B. B. . . . . . . . . . . . . . . 58, 68Joye S. . . . . . . . . . . . . . . . . . . . . . . . 58

KKasten S . . . . . . . . . . . . . . . . . . . . . 170Keir R.. . . . . . . . . . . . . . . . . . . . . . . . 20Kipfstuhl J. . . . . . . . . . . . . . . . . . . . . . 4Klapproth A. . . . . . . . . . . . . . . . . . . 134Kläschen D.. . . . . . . . . . . . . . . . . . . . 86Klaucke I. . . . . . . . . . . . . . . . . . . . . . . 4Klein G. . . . . . . . . . . . . . . . . . . . . . . 86Knittel K. . . . . . . . . . . . . . . . . . . . . . 74Konerding P. . . . . . . . . . . . . . . . . . . . 40Kreiter S.. . . . . . . . . . . . . . . . . . . . . 118Krüger M. . . . . . . . . . . . . . . . . . . . . . 80Kuhs W.F. . . . . . . . . . . . . . . . . . . . . 134Kulenkampff J. . . . . . . . . . . . . . 98, 152

219

Index

LLiebetrau V.. . . . . . . . . . . . . . . . . . . . 20Linke P. . . . . . . . . . . . . . . . . . . . . . . . 20Lösekann T. . . . . . . . . . . . . . . . . . . . . 74Löwner, R.. . . . . . . . . . . . . . . . . . . . . 98Lüdmann T. . . . . . . . . . . . . . . . . . . . . 40Luff R.. . . . . . . . . . . . . . . . . . . . . . . . 20

MMasson M. . . . . . . . . . . . . . . . . . . . 186Meyerdierks A. . . . . . . . . . . . . . . . . . 80Michaelis W. . . . . . . . . . . . . . . . . . . . 40Müller C. . . . . . . . . . . . . . . . . . . . . 198

NNauhaus K. . . . . . . . . . . . . . . . . . . . . 80Naumann R. . . . . . . . . . . . . . . . . . . 152Neben S. . . . . . . . . . . . . . . . . . . . . . 198Niemann H. . . . . . . . . . . . . . . . . 58, 68

OOrcutt B.. . . . . . . . . . . . . . . . . . . 58, 68

PPape T. . . . . . . . . . . . . . . . . . . . . . . . 40Petersen J. . . . . . . . . . . . . . . . . . . . . 40Pfannkuche O. . . . . . . . . . . . . . . . . . 20Poppen B . . . . . . . . . . . . . . . . . . . . 186

RRackwitz F. . . . . . . . . . . . . . . . . . . . 118Rehder G. . . . . . . . . . . . . . . . . . . . . . . 4Reimer A. . . . . . . . . . . . . . . . . . . . . . 40Reitner J.. . . . . . . . . . . . . . . . . . . . . . 40Reston T.J.. . . . . . . . . . . . . . . . . . . . . 86

SSahling H. . . . . . . . . . . . . . . . . . . . . 170Savidis S.. . . . . . . . . . . . . . . . . . . . . 118Schicks J. . . . . . . . . . . . . . . . . . . . . 152Schneider R. . . . . . . . . . . . . . . . . . . 170Schultz H.J. . . . . . . . . . . . . . . . . . . . 138Schupp J. . . . . . . . . . . . . . . . . . . . . 118Seifert R. . . . . . . . . . . . . . . . . . . . . . . 40Sommer S. . . . . . . . . . . . . . . . . . . . . 20Spangenberg E. . . . . . . . . . . . . . . . 152Spiess .V . . . . . . . . . . . . . . . . . . 20,170Suess E. . . . . . . . . . . . . . . . . . . . . . . . 4

TTalukder A. . . . . . . . . . . . . . . . . . . . . 86Techmer K. . . . . . . . . . . . . . . . . . . . 134Treude T. . . . . . . . . . . . . . . . . . . . . . . 58

VVillinger H. . . . . . . . . . . . . . . . . . . . 170

WWallmann K. . . . . . . . . . . . . . . . . . 4, 20Weber M . . . . . . . . . . . . . . . . . . . . . 98Weinrebe W.. . . . . . . . . . . . . . . . . . . . 4Widdel F.. . . . . . . . . . . . . . . . . . . . . . 80Wiersberg T. . . . . . . . . . . . . . . . . . . . 98Witte U. . . . . . . . . . . . . . . . . . . . . . . 58Wong H.K. . . . . . . . . . . . . . . . . . . . . 40

ZZillmer M. . . . . . . . . . . . . . . . . . . 40, 86Zühlsdorff L. . . . . . . . . . . . . . . . . . . 170

Notes

Notes

Notes

Notes

Gas Hydrates in the GeosystemThe German National ResearchProgramme on Gas Hydrates

Report on the First Funding Period(2000 - 2004)

GEOTECHNOLOGIENScience Report

No. 7

Gas Hydrates in the Geosystem

ISSN: 1619-7399

In Germany a National Gas Hydrate Programme has been initiated in 2001 as partof the R&D-Programme GEOTECHNOLOGIEN. Between 2001 and 2004, 15 jointprojects have been funded with 15 Million Euros by the Federal Ministry ofEducation and Research. All projects were carried out in close cooperation withvarious national and international partners from academia and industry.

This report highlights the scientific results of the first funding period addressing thefollowing objectives:

- Characterization of the chemical and physical properties of methane hydrates- Interaction of gas hydrates with the natural environment including seafloor

stability and global climate- Characterization of the unique biological communities dependent on methane

hydrate occurrences- Technologies for an improved survey of methane hydrates in both the

laboratory and the field - Technologies for the safe and commercial production of methane from

hydrates

The papers published in this report offer a comprehensive insight into the presentstatus of gas hydrate research in Germany and reflects the multidisciplinary appro-ach of the programme.

No.

7G

asHy

drat

esin

the

Geo

syst

em(2

00-2

004)

GEO

TECH

NO

LOG

IEN

Scie

nce

Repo

rt

The GEOTECHNOLOGIES programme is financed by the Federal Ministry

for Education and Research (BMBF) and the German Research Council (DFG)