methane hydrate estimates from the chloride and oxygen isotopic anomalies: examples from the blake...

39

Methane Hydrate Estimates from the Chloride and Oxygen Isotopic Anomalies

Examples from the Blake Ridge and Nankai Trough Sediments

RYO MATSUMOTO

a

Department of Earth and Planetary Science, School of Science, University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan

A

BSTRACT

: Oxygen isotopic fractionation between gas hydrate and ambientwater is determined as

�

GH

−

IW

= 1.0037 at 12–16

°

C and 31 Mpa, on the basisof direct measurements of gas hydrate-derived waters and ambient porewaters recovered from the Blake Ridge during ODP Leg 164. Oxygen isotopicanomalies give us the amount of gas hydrate of 7 to 9% (pore filling), which isalmost twice as much as estimates from chloride anomalies. The difference isprobably due to uncertainties in determining base-line profiles of the

in situ

pristine pore waters, and partially due to the effects of selective filtration/ad-sorption during pore water extraction. Two 250 meter-deep holes were drilledin the eastern Nankai trough off central Japan at a water depth of 950 m,where strong BSRs occur at about 300 mbsf. Massive hydrates were not recov-ered during this drilling but a number of soupy horizons suggest the existenceof subsurface gas hydrate. Chloride concentration and

�

18

O of interstitial wa-ters are observed to vary in a remarkable zigzag pattern with spiky anomalies,reflecting hydrate dissociation during core-recovery and water extraction. Theconcentration of gas hydrate in sediments is estimated to be about 3–7% witha spiky maximum value of 30% from chloride anomalies and between 5 and30% from

�

18

O anomalies. Significant difference in vertical distributionbetween nearby two holes in Nankai Trough probably reflect heterogeneousfluid migration through particular conduits in an accretionary wedge.

INTRODUCTION

The most important question in the study of methane hydrate is: How much meth-ane hydrate is stored in marine sediments? The environmental impact of gas hydratedepends primarily on the total amount of methane trapped in gas hydrate of a shallowgeosphere. Resource assessment of gas hydrates requires refinement of the distribu-tion, amounts, occurrence, and reserves of gas hydrate deposits. The amount of sub-surface gas hydrate is given by the areal distribution and concentration of gas hydratewithin the host sediments. The areal extent of gas hydrate distribution was thoughtto be nearly identical to the distribution of BSRs. Ocean Drilling Program (ODP)Leg164 has revealed that gas hydrate can occur in sediments without BSRs,

1

but

a

Telecommunication. Voice: +81-3-5841-4522; fax [email protected]

40 ANNALS NEW YORK ACADEMY OF SCIENCES

BSRs are still useful tools for estimating the minimum areal extent of subsurface gashydrate. On the other hand, the concentration of gas hydrate in sediments are notreadily obtained, requiring sophisticated remote sensing techniques, such as VSPand well-logging or direct measurement of sediment cores. Because of the ephemer-al nature of gas hydrate, several proxy analyses have been employed in estimatingthe amount of gas hydrate in core samples; among which the chloride anomaly tech-nique provides the most reliable proxy of methane hydrate amounts. However,recently possible effects of selective filtration/adsorption during mechanical squeez-ing has been discussed,

2

requiring reassessment of the chloride anomaly technique.Formation and dissociation of gas hydrate in marine sediments modify the oxy-

gen isotopic composition of ambient waters as well as the chloride concentration,because gas hydrate concentrates isotopically heavier oxygen (

18

O) in its cage struc-tures.

3,4

Matsumoto

5

has identified isotopically heavy oxygen containing siderites inthe Blake Ridge sediments, suggesting a close relationship between the formation ofsiderites and the dissociation of gas hydrate during burial diagenesis.

The interstitial waters squeezed from gas hydrate-bearing sediments are variablyenriched in

18

O, depending on the amounts of gas hydrate contained in the sedi-ments. Given the isotopic fractionation between gas hydrate and ambient water(

α

GH

−

IW

), pore saturation of gas hydrate is easily estimated from the

18

O

anomaly.

6

ODP Leg164 recovered a number of massive gas hydrate samples and collected hun-dreds of interstitial waters from Blake Ridge sediments. Matsumoto

7

measured

δ

18

Ovalues of gas hydrate and a number of interstitial waters in an attempt to determinethe fractionation factor (

α

GH

−

IW

) between methane hydrate and ambient waters.In this paper, a short summary of

δ

18

O variations on Blake ridge and isotopic frac-tionation between gas hydrate and water are presented. This information is appliedto determine the amount of subsurface gas hydrate in Blake Ridge and NankaiTrough sediments.

GEOLOGIC SETTINGS

The Blake Ridge, located about 150 km off east coast of north America, is a spit-like submarine rise with a relief of about 3 km (see F

IGURE

1). The Ridge is charac-terized by rapidly accumulated, moderately calcareous hemipelagic sediment depos-ited by strong contour currents.

B

ottom

s

imulating

r

eflectors (BSRs) are welldeveloped in the crest of the Ridge, covering about 26000 km

2

of the area. In 1995,ODP Leg 164 drilled 750–800 meter-deep holes at Sites 994, 995, and 997 at a waterdepth of 2800 meters and penetrated BSRs on the crestal area of the Ridge.

The Nankai Trough lies in a subduction zone between the Eurasian and PhilippineSea Plate, extending about 700 km SSW–NNE offshore SE Japan (see F

IGURE

2).The accretionary prism is largely composed of siliciclastic hemipelagic sedimentswith abundant sandstone turbidites. Strong BSRs have been recognized within thissequence, extending about 35000 km

2

. In 1997, 250 meter-deep test holes, BH-1and BH-2, were drilled by Japan National Oil Corporation (JNOC)–Japan PetroleumExploration (JAPEX) in the Eastern Nankai trough. The two holes, about 100 mapart, were drilled on a flat-topped, deep-sea terrace. Experimental data concerninghydrate stability,

in situ

temperature measurements, and seismic survey data indicatethat BSR occurs at around 290

±

10 mbsf.

41MATSUMOTO: BLAKE RIDGE AND NAKAI TROUGH SEDIMENTS

82°W 80° 78° 76° 74°

34° N

32°

30°

28°

Blake Plateau

994, 995, 997

996

Charleston

Savannah

BSR

100500

10002000

40005000

3000Blake Ridge

991, 992, 993

FIGURE 1. Location of Sites 991 to 997 of ODP Leg164 on the Blake Ridge off Eastcoast of North America.

Deformation front

BSR distribution

TOKAI

5 0 0

1 0 0 0

1 5 0 0

2 0 0 0

2 5 0 0

4 0 0 04 5 0 0

3 5 0 0

3 0 0 0

Nagoya

Osaka

BH-2

BH-P

SHIKOKU

KII PEN.

134° 136° 138°

32°

34°

FIGURE 2. Location of drill holes BH-1 and BH-2 in the eastern Nankai Trough offcentral Japan.


ISOTOPIC FRACTIONATION BETWEEN GAS HYDRATE AND WATER

Experimental Methods

Each gas hydrate sample was dissociated in a Teflon-coated dissociation chamberat room temperature. Gas pressure within the chamber steadily increased and reacheda stable value in approximately 10 minutes. Gas was transferred to a gas collectiontube for later gas analysis. The volume of residual water was measured and stored insealed glass tubes for isotopic analysis. Chloride concentration of the residual waterwas measured to estimate the mixing ratio of gas hydrate water and the pristine porewater. An accurately measured volume (1.0–1.5 ml; 28 to 42 mmol of O

2

) of the waterwas placed into a small flask, stirred, and mixed with 0.10 mmol of CO

2

of knownisotopic composition in a water bath at 25.0

°

C for 15–20 hours to attain isotopicequilibration between water and gas. Oxygen isotopic ratio

18

O/

16

O of the equilibrat-ed CO

2

gas was determined using a Finnigan Delta S mass spectrometer. The resultsare represented as delta per mil notation (

δ

18

O‰) relative to the SMOW standard.The standard deviation (2

σ

) of independent analysis was 0.01–0.05‰ and the repro-ducibility of the measurements was about 0.10‰. Chloride concentrations ofpore waters were measured by ion chromatography (Model ICA-5000, TOA DempaCompany).

FIGURE 3. The δ18O (‰ SMOW) of interstitial waters at (A) Site 994 and (B) Site997. Dotted lines represent baselines for the interstitial waters seemingly unaffected by gashydrate dissociation. Baseline for Site 994 is, Y = 0.886 − 1.20 × 10−02X + 4.23 × 10−05X2

− 8.06 × 10−08X3 + 1.01 × 10−10X4 − 5.85 × 10−14X5, (r2 = 0.9906), and for Site 997 is,Y = 0.245 − 7.43 × 10−04X − 2.44 × 10−05X2 + 7.29 × 10−08X3 − 4.82 × 10−11X4 − 6.34 ×10−15X5, (r2 = 0.9890), where Y is δ18OH2O (‰ SMOW) and X is depth (mbsf).


�

18

O of Gas Hydrates and Waters from the Blake Ridge

The

δ

18

O values of a gas hydrate sample recovered from 259.9 mbsf (994C-31X-7) at Site 994 is 2.67‰ SMOW and four samples from 330.15 mbsf (997A-42X-3)at Site 997 were 2.82–3.51‰ SMOW. In comparison, the

δ

18

O of 14 water samplesfrom Site 994 were between 0.32‰ and

−

0.17‰ SMOW and 21 water samples fromSite 997 range from 0.25‰ to

−

0.54‰ SMOW (see F

IGURE

3). The depth profiles atboth sites separate into an upper smooth zone (0–200 mbsf), a middle zigzag zone,which corresponds to gas hydrate zone (200–450 mbsf), and a lower smooth zone(450–750 mbsf). The zigzag pattern of the middle zone is likely to have been causedby heavy oxygen containing water from gas hydrate. Regression curves to representthe depth profile of the interstitial waters of gas hydrate free sediments were pre-pared from the data of the upper and lower zones.

Oxygen Isotopic Fractionation

The difference in

δ

18

O values between gas hydrate and the ambient water(∆δ18OGH–IW) is 3.1‰ at Site 994 and 3.3–3.8‰ (mean, 3.6‰) at Site 997(see FIGURE 4). Assuming that the gas hydrate samples were in isotopic equilibriumwith the ambient waters (T = 12−16°C and P = 31 MPa), the equilibrium isotopic

FIGURE 4. Diagram showing the δ18O of gas hydrates, δ18O of the interstitial watersand baselines at Sites 994 (open squares and broken line) and at Site 997 (open circles andthin line). The difference in δ18O values (∆δ18OGH−IW) is 3.1‰ at Site 994 and 3.3‰to 3.8‰ at Site 997. Filled symbols show positive excursions due to dissociation of gashydrate.


fractionation factor (αGH−IW) is calculated to be 1.0034–1.0040 (mean, 1.0037). Forcomparison, the oxygen isotopic fractionation between ice and water is 1.0027–1.00358–10 and that of THF hydrate–water is 1.00268 ± 0.00003 at 0–4°C.11

GAS HYDRATE AT BLAKE RIDGE

Assuming a stoichiometry of the Blake Ridge gas hydrate (CH4 ⋅5.75H2O), gashydrate density of 0.97g/cm3, and gas hydrate–water fractionation (∆δ18OGH−IW)value of 3.1‰ at Site 994 and 3.6‰ at Site 997, the pore filling fraction (X) of gashydrate is given by

(1)

For Site 997, equation (1) is

(2)

M and P denote measured δ18O values of extracted waters and δ18O values of thepristine pore waters, respectively, estimated from FIGURE 1. Gas hydrate amounts(% pore filling) are given in FIGURE 5 along with gas hydrate estimates from chlo-ride anomalies.1

At Site 994, four samples in the gas hydrate zone yield a pore filling of 7.5 ± 1%,and the average amount at Site 997 is about 9%, both of which are almost twice the

X δ18OM δ18OP–( ) 2.8 0.11δ18OP–( )⁄=

X δ18OM δ18OP–( ) 3.2 0.11δ18OP–( )⁄=

FIGURE 5. Magnitude of chloride and isotopic anomalies (lower axis) and approxi-mate amount of gas hydrate (upper axis) at Sites 994 and 997. Filled symbols for chlorideanomaly data correspond to isotopic anomaly data on the right.


estimates from chloride anomalies. At Site 997, gas hydrate tends to concentrate inthe upper and lower part of gas hydrate zone (200–450 mbsf) with minimum valuein the middle (330–370 mbsf) of the zone. This pattern is identical to that of the chlo-ride anomaly.

GAS HYDRATE IN NANKAI TROUGH

Sediment Lithology

A number of spot cores were collected at both holes and 41 m-long and 68 m-longsediment cores were recovered from BH-1 and BH-2, respectively. Significant dif-ferences were not observed between the two holes. The upper part of the holes isdominated by clay to clayey silt to siltstone with occasional thin ash layers, whereasthe deeper part contains significant amounts of sand and sandy beds of possible tur-bidite origin. Core recovery rate dramatically dropped downhole; empty core-barrelswith thin sandy films or sand grains inside the barrel suggest that unrecovered, lostintervals were dominantly sand and sandstone beds. We did not recover solidhydrate, but the existence of massive hydrate horizons were indicated by occasionalsoupy horizons and anomalies in the interstitial water chemistry.

�18O and Chloride Concentration of Waters

About forty interstitial water samples were measured for oxygen isotopic compo-sition and chloride concentration at each site. The δ18O of the interstitial wateris about −0.5‰ SMOW at the top of the holes, tends to decrease downward witha strong zigzag pattern, then reaches to −1.0 to −1.5‰ at about 250 mbsf (seeFIGURE 6). In BH-2, the amplitude of zigzag fluctuation is observed to increase in

FIGURE 6. δ18O of pore waters from drillholes BH-1 (dots) and BH-2 (open circles)in the Eastern Nankai Trough. Hypothetical baseline for pristine pore waters is indicated bythe thin broken line. Isotopic anomalies are determined as the difference between the baseline values and measured values.


the lower sandstone-rich horizons (FIG. 6). Early diagenetic processes such as freeconvection, fluid migration, and ionic diffusion within highly porous sedimentsshould produce a smooth depth profile. Thus, the observed zigzag profiles arethought to be an artifact caused by dissociation of gas hydrate during core-recoveryand water extraction. This implies that the heavier shifts and spikes reflects gashydrate dissociation, suggesting that the lighter values represent depth profile of thepristine pore waters. A broken line connecting the lighter values is taken as a hypo-thetical baseline for isotopic variations.

The chloride concentration tends to decrease downhole from about 560 mM at thecore-top, toward about 500 mM at the bottom of the holes (see FIGURE 7), but thedepth profiles show irregular zigzag patterns as in case of isotopic variation. Nega-tive spikes and excursions reflect dissociation of gas hydrate. A strong negative spikeat 180 mbsf of BH-2 corresponds to approximately 30% pore filling by gas hydrate.

Amounts of Gas Hydrates

FIGURE 8 depicts the distribution and amounts of gas hydrate at Site BH-1 andBH-2 from the Eastern Nankai Trough. The distance between the sites is only 100 mand the lithologies are quite similar to each other. However, BH-1 tends to have moregas hydrate in the upper part, whereas BH-2 has more gas hydrate in the middleand lower parts. This pattern is also reflected by oxygen isotopic profiles that depict30–40 meter-thick hydrate bearing horizons in the interval 100–250 mbsf (seeFIGURE 9).

Pore filling of gas hydrate calculated from chloride anomalies are mostly around3 to 7% with a spiky maximum value of 30%, whereas the estimates from δ18Oanomalies are between 5 and 25% (FIG. 9). These values are somewhat similar to, ora bit larger than, those of Blake Ridge sediments.

FIGURE 7. Chloride concentration of pore waters at BH-1 (dots) and BH-2 (open cir-cles) in the Eastern Nankai Trough. The hypothetical baseline for the pristine pore water isshown by a thin broken line. Chloride anomalies are determined as the difference betweenthe baseline values and measured values.


DISCUSSION

Chloride and Isotopic Anomalies

The observed discrepancy between the two estimates of gas hydrate pore fillingis not large but it is significant. The possible causes of the discrepancy are: (1) un-certainty in the determination of baselines (in situ trends) of both δ18O and chlorideconcentration (especially at the Blake Ridge, where sampling density is low); (2) dif-ferential chemical effects between 18O and chloride concentration during mechani-cal squeezing; and (3) sample deterioration, which may have occurred during storageof water samples. Among these, the baseline problem is most critical. As for chlo-ride, pristine waters may have changed only slightly or may not have changedthroughout the depth interval. If this is the case, hydrate estimates would beincreased by the factor of 1.5–2.0. Furthermore, the sampling density may partiallyexplain the apparent discrepancy for Blake Ridge data. Chloride measurements were

FIGURE 8. Depth profiles of the chloride anomalies at BH-1 (dots) and BH-2 (opencircles). Chloride anomaly values are converted to the approximate amount of gas hydrate(percent pore filling) on the upper axis.


more densely spaced than those for isotope analyses. Interstitial water samples werepreferentially taken from undisturbed sediment cores, which are usually poor in orfree of gas hydrate.

Blake Ridge and Nankai Trough

Leg 164 has revealed two distinct modes of occurrence of gas hydrate in marinesediments: (1) massive, megascopic aggregates of pure hydrate, and (2) fine crystal-line hydrate disseminated in fine grained sediments. The first type occurs as nodulesor veins filling open fractures and cavities. We did not recover massive aggregates ofgas hydrates from BH-1 and BH-2 of the Eastern Nankai trough, probably becausethe holes do not penetrate fractures or faults in the area. As for the second type ofgas hydrate, we conclude that the concentration in fine-grained sediments is less than15% (pore filling) in both the passive and active margin sites. A notable differencebetween the Blake Ridge and Eastern Nankai is the vertical distribution. In theformer, zone of gas hydrate is substantially limited between 200 and 450 mbsf, how-ever in the Eastern Nankai, significant amount of gas hydrate accumulate even in

FIGURE 9. Comparison between chloride anomaly (open circles) and isotopic anom-aly (dots) at BH-2. Approximate amount of gas hydrate is indicated on the upper axis. Notethat gas hydrate tends to concentrate in the upper horizon (20–40 mbsf) and lower horizon(150–250 mbsf).


shallow subsurface at about 20 mbsf. This may be related to active gas venting andfluid seeps that are characteristic phenomena of the Eastern Nankai.12 Active fluidmigration through particular conduits in the accretionary prism may also account forthe difference in vertical distribution pattern of gas hydrates at BH-1 and BH-2.

CONCLUSIONS

1. Oxygen isotopic fractionation between gas hydrate and ambient water isabout 3.2‰ at 12–16°C and 31 Mpa, based on field samples.

2. The amounts of gas hydrate in Blake Ridge is a few percent of pore saturation(chloride anomalies) to about 9% (isotopic anomalies), whereas in Nankai troughthe amounts range between 3 and 30% (chloride anomalies) and from 5 to 25%(isotopic anomalies).

3. The discrepancy between the chlorine and δ18O anomaly estimates is proba-bly due to uncertainties in determining the baseline profiles for the pristine intersti-tial waters, and at least partially due to filtration/adsorption effects during waterextraction.

ACKNOWLEDGMENTS

Drilling and coring of BH-1 and BH-2 were performed by Fugro Japan, beingconducted by JNOC and JAPEX. Onsite observation, description, and sampling ofsediment cores and squeezing of the interstitial waters were performed by theHydrate Research Group of Drs. Y. Hiroki (Osaka Kyoiku University), Lu Hailong(Tokyo University), T. Fujii (JNOC-TRC), T. Uchida, A. Waseda, K. Baba, M. Yagi(JAPEX), and RM.

REFERENCES

1. PAULL, C.K., R. MATSUMOTO & P.J. WALLACE. 1996. Proc. ODP, Init. Repts. 164.Ocean Drilling Program, College Station, TX.

2. CAVE, M., L. GRIFFAULT & S. REEDER. 1998. The extraction and characterization ofpore-water from lower permeability argillaceous rock samples. 23rd General Assem-bly of EGS, Nice. Abst., SE406.

3. HESSE, R. & W.E. HARRISON. 1981. Gas hydrates (clathrate) causing pore-water fresh-ening and oxygen isotope fractionation in deep-water sedimentary sections of terrig-enous continental margins. Earth and Planet. Sci. Lett. 55: 453–462.

4. HARRISON, W.E. & J.A. CURIALE. 1982, Gas hydrates in sediments of Holes 497 and498A, Deep Sea Drilling Project Leg 67. In Init. Repts., DSDP 67. J. Aubouin &R. von Huene et al.,Eds.: 591–595. U. S. Govt. Printing Office. Washington.

5. MATSUMOTO, R. 1989. Isotopically heavy oxygen-containing siderite derive from thedecomposition of methane hydrate. Geology 17: 707–710.

6. USSLER, W. III & C.K. PAULL. 1995. Effects of ion exclusion and isotopic fraction onpore water geochemistry during gas hydrate formation and decomposition. GeoMar-ine Lett. 15: 37–44.


7. MATSUMOTO, R. & W.S. BOROWSKI. 2000. Gas hydrate estimates from newly deter-mined oxygenisotopic fractionation (αGH−IW) and δ18O anomalies of the interstitialwaters. ODP Leg 164, Blake Ridge. Proc. ODP, Sci. Results, 164. Ocean DrillingProgram, College Station, TX.

8. O’NEIL, J.R. 1968. Hydrogen and oxygen isotope fractionation between ice and water.J. Phys. Chem. 72: 3682–3684.

9. CRAIG, H. & B. HOM. 1968. Relationship of deuterium, oxygen-18, and chlorinity inthe formation of sea ice. Trans. Am. Geophys. Union 49: 216–217.

10. JAKLI, G. & D. STASCHEWSKI. 1977, Vapor pressure of H2O ice (−50 to 0°C) and H2Owater (0−170°C). J. Chem. Soc. Faraday Trans. I 73: 1505–1509.

11. DAVIDSON, C.W., D.G. LEAIST & R. HESSE. 1983. Oxygen-18 enrichment in the waterof a clathrate hydrate. Geochim. Cosmochim. Acta 47: 2293–2295.

12. LE PICHON, X., K. KOBAYASHI & KAIKO-NANKAI SCIENTIFIC CREW. 1992. Fluid vent-ing activity within the Eastern Nankai trough accretionary wedge: a summary of the1989 Kaiko-Nankai results. Earth Planet. Sci. Lett. 109: 303–318.

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