Gas hydrates are ice-like solids that form inrigid cage structures under specific conditionsof pressure, temperature, and gas and waterconcentration.Marine gas hydrates are stable inpore spaces of sediments in water depths greaterthan ~300 m beneath the slopes of active andpassive continental margins [Kvenvolden,1988].The lower limit of hydrate occurrence in marinesediments is determined by the geo-thermalgradient, so that the zone of hydrate stability isgenerally contained within the first few hundredmeters of sediment. Continental hydrates occurin polar permafrost regions in the Arctic andSiberia. Most of the hydrates that have beendiscovered contain methane derived frommicrobial processes.

Other hydrocarbons can also form hydrates,but in different structures of the surroundingwater cages. Structure I,the most prevalent form,contains mostly (>99%) microbial methane, asmall amount of ethane,and traces of C2+ hydro-carbons [Sassen et al., 2001]. Structure II andstructure H hydrates contain significant quan-tities of thermogenic methane and larger,morecomplex hydrocarbons formed at high tem-peratures from fossil organic matter (i.e., kero-gen) or oil [Sassen and MacDonald,1994].Thegas origin is inferred from measurements of thecarbon-13 isotopic ratio (δ13);microbial methaneis depleted in 13C (δ13 < -60‰) relative to thermo-genic methane (δ13 from -20‰ to -50‰).

Hydrates are of considerable interest inter-nationally,due to the recognition that the largequantity of gas stored in the hydrates consti-tutes a significant fraction of the global organiccarbon reservoir [Kvenvolden, 1988]. Conse-quently, hydrates may constitute an importantfuture energy resource when the technologyfor extracting the gas becomes available andeconomically viable.

At present, the primary interest to the oilindustry is the role of hydrates as a geohazard.For instance, slope failures are a concern foroffshore drilling operations.Hydrates may alsobe a factor in global climate change, althoughthe flux of methane from natural marine andpolar hydrates into the atmosphere is not wellconstrained.Some researchers have suggested

that sudden, widespread dissociation of sub-marine gas hydrates may have had a signifi-cant impact on past climate [Kennett et al.,2003]. However, the hypothesis remains specu-lative, because the quantity of gas stored inthe hydrates, and its response to changingenvironmental conditions, are issues that arenot well understood.

Massive hydrate has been recovered at manysites in shallow cores within a few meters ofthe seafloor and at deeper depths in severalocean drilling legs.However,evidence of marinegas hydrates is conventionally inferred usingseismic reflection methods that image the strongsignal from the bottom simulating reflector(BSR) at the base of the hydrate stability zone.The BSR arises from the strong negative imped-ance contrast at the interface between high-velocity, hydrate-bearing sediments overlyinglower-velocity sediments that contain free gasand water.The signal is easily recognized inseismic data as an oppositely polarized reflec-tion (compared with the polarization of theseafloor reflection) that follows the seafloortopography. The continental slope in the northernCascadia Margin has been studied in severalrecent multi-channel seismic surveys [Riedelet al., 2002], and BSRs are found throughoutthe shaded hydrate region in Figure 1.

Although hydrates are known to exist world-wide, the occurrences of hydrates containinghydrocarbons of thermogenic origin on theseafloor are rare. Only a few sites have beenreported previously, such as the northern Gulfof Mexico [Sassen and MacDonald, 1994] andthe Caspian Sea [Ginsburg and Soloviev,1998].The occurrences are generally associated withfaults in areas of oil seeps,and the hydrates usu-ally contain quantities of oil [Roberts, 2001].However, the conditions for the formation ofthermogenic hydrates are poorly understood[Chen and Cathles, 2003].

This article reports the discovery of massiveoutcrops of thermogenic gas hydrate on theseafloor during a survey with the Canadianremotely operated submersible ROPOS (RemotelyOperated Platform for Ocean Science) inBarkley Canyon, about 80 km off the westcoast of Vancouver Island (Figure 1).The surveywas carried out in August 2002 to follow up theremarkable report of an accidental recoveryof hydrate by a commercial fishing boat thatwas dragging a trawl net on the canyon bottom[Spence et al., 2001].

ROPOS was deployed at the eastern end of thefishing trawler’s track and surveyed southwestalong the line. About one quarter of the distancealong the track, the submersible encounteredsmall mounds and associated clam coloniesthat stretched south to deeper waters in thesubmarine canyon. In these deeper waters, thesurvey discovered a hydrocarbon seep siteconsisting of several outcrops of hydrate at asmall plateau on the canyon wall about 860 m

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PAGES 361–368

Eos,Vol. 85, No. 38, 21 September 2004

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PAGES 361, 365

Thermogenic Gas Hydrates in the Northern Cascadia Margin

Fig.1. Location of the Barkley Canyon hydrate site.BY R. CHAPMAN, J. POHLMAN, R. COFFIN,

J. CHANTON, AND L. LAPHAM

Eos,Vol. 85, No. 38, 21 September 2004

deep. Hydrates were exposed as sheets up to 8m long on the seafloor (Figure 2) and on theflanks of thinly sedimented mounds about2–3 m high (Figure 3).

Results from the initial survey and a subse-quent visit in 2003 indicate that the site is ahighly localized thermogenic gas and hydro-carbon seep.The Barkley Canyon hydrates areunique compared with hydrates recoveredfrom shallow (8 m) piston cores at other sitesnearby in the northern Cascadia Margin

[Riedel et al., 2002], and from Hydrate Ridgefarther south off Oregon [Suess et al., 1999],which were of microbial origin.The digitalimages acquired during the ROPOS survey atBarkley Canyon revealed yellow or brownish-yellow streaks in the exposed hydrates (Figure2),which are consistent with the presence oflight oil in the hydrate.There was no evidence ofgas venting from undisturbed sediment or bydissociation of hydrate.However, the sedimentsreleased gas bubbles,quantities of light oil,

and small hydrate fragments when probed bythe ROPOS mechanical arm.

The ROPOS surveys collected hydrate frag-ments, sediment push cores,and bottom watersamples to characterize the physical andchemical properties of the hydrates and todetermine the concentration of methane at theseafloor.Preliminary analysis of the molecularcomposition and isotope ratios of gas extractedfrom the hydrate samples confirmed that thehydrocarbons are thermogenic. Stable carbonisotope ratios of the methane from three dis-sociated gas hydrate samples were between -42.6 and -43.4‰, and the gas contained significant quantities of higher (C2-C5), morecomplex hydrocarbons.

This distribution is similar to that of thermo-genic hydrates in the Gulf of Mexico that wereclassified as structure II [Sassen and MacDonald,1994].Analysis of sediment pore water fromthe push cores showed enrichment of isopen-tane, a hydrocarbon excluded from the latticeof structure II hydrate.This result provides otherevidence for the occurrence of structure IIand suggests that the hydrate formation wasrecent or ongoing at the time of sampling[Sassen et al., 2001].Also, since isopentane isaccommodated in structure H hydrate, thepresence of isopentane in the dissociated gassuggests that structure H hydrates may co-existwith structure II hydrates at the Barkley Canyonsite.The sediment pore water was also analyzedto determine the salinity.Compared with resultsfrom hydrate sites in the Gulf of Mexico, therewas no evidence of high salinity at the BarkleyCanyon site.

Dissolved methane concentrations in thewater above the hydrate mounds were as highas 220 nM, more than an order of magnitudegreater than background levels for the deepocean.Although the flux rate of methane to orfrom the seafloor is not known,the presence ofmassive hydrate suggests an active flow regime.Since seawater temperature at the bottom isaround 3°C and stable year-round, there is notlikely to be significant hydrate dissociationover time.The source of the thermogenicmethane is not presently known. Since theother known sites of thermogenic methane inthe Gulf of Mexico and the Caspian Sea arerelated to petroleum reserves, it is intriguing tospeculate that the Barkley Canyon hydrates arealso related to conventional oil and gas depositsoff Vancouver Island.

The visual survey revealed several other dis-tinctive features of the site.There were exten-sive colonies of chemosynthetic communitiesconsisting of several species of vesicomyidclams clustered around the hydrate mounds.However,there was no evidence of tube worms,as are found at thermogenic hydrate sites inthe Gulf of Mexico [Roberts, 2001] or at sea-floor microbial hydrate sites nearby at HydrateRidge and farther north on the Cascadia Margin.Thin bacterial mats cover large portions of thehydrates and sediment on most of the mounds.The seafloor sediment is very thin in the vicinityof the mounds, with a hard layer within 10–20cm of the seafloor that could not be penetratedby push cores inserted by the ROPOS mechanicalarm.This interface could be due to underlying

Fig.2. Slab of exposed hydrate in a large glacier-like structure about 6 x 8 m in area.The hydrateis streaked with yellow-brown.The size of the slab is about 1 m across.

Fig.3.Exposed hydrate on the face of a small, thinly sedimented mound.The mound is about 3 mhigh and about 4 m long.Overall, the shape of the feature is similar to a small pingo.

hydrates or a carbonate layer. However, thereare very few outcrops of authigenic carbonatesnear the mounds and no patches of carbonatepavements.The absence of any species of tubeworms, that are generally associated with car-bonate pavements, suggests that the underlyinglayer is likely hydrate.

The Barkley Canyon site is a unique naturallaboratory for investigation of the processesand environmental conditions that control theformation and dissociation of thermogenic gashydrates. Observations indicate some signifi-cant qualitative differences in the hydrateenvironment compared with sites in the Gulfof Mexico.New surveys were carried out earlierin 2004 to obtain ocean bottom water,sedimentpore water,and hydrate samples using novelsystems for extracting and storing the samplesat in situ pressure.These experiments will allowaccurate measurements of dissolved gas con-centrations and isotopic composition in thesediments from undecompressed pore watersamples, and analysis of the types of structureand gas composition of the hydrates.The distri-bution of thermogenic hydrates in the northernCascadia Margin,the relationship of the BarkleyCanyon seep site to fluid migration paths inthe underlying geological structure, and thepossible connection with a BSR,await furtherinvestigations that combine seafloor surveyswith high-resolution multi-channel seismicsurveys.

Acknowledgments

We thank Michael Whiticar (University ofVictoria) for stable carbon isotope analysis ofthe hydrate methane samples.Funding for shiptime and the use of ROPOS was obtainedfrom a grant from the Natural Sciences andEngineering Research Council of Canada. R.Coffin acknowledges financial support fromNaval Research Laboratory and the Office ofNaval Research for field activity and laboratoryanalysis. J. Chanton and L. Lapham acknowl-edge support from the U.S. National Oceanicand Atmospheric Administration and the U.S.Department of Energy to the University of Mis-sissippi Hydrates Research Consortium.

References

Chen, D. F., and L. M. Cathles (2003),A kinetic modelfor the pattern and amounts of hydrate precipitatedfrom a gas stream: Application to the Bush Hillvent site, Green Canyon Block 185, Gulf of Mexico,J.Geophys.Res., 108(B9), 2058, doi:10.1029/2001JB001597.

Ginsburg,G.D.and V.A.Soloviev (1998),Submarine GasHydrates,VNIIOkeangeologia, St. Petersburg, Russia.

Kennett, J. P., K. G. Cannariato, I. L. Hendy, and R. J.Behl (2003), Methane Hydrates in Quaternary Climate Change:The Clathrate Gun Hypothesis,AGU,Washington, D.C.

Kvenvolden, K.A. (1988), Methane hydrates— A major reservoir of carbon in the shallowgeosphere?, Chem.Geol., 71, 41–51.

Riedel, M., G. D. Spence, N. R. Chapman, and R. D.Hyndman (2002), Seismic investigations of a vent

field associated with gas hydrates, offshoreVancouver Island, J.Geophys.Res., 107(B9), 2200,doi:10.1029/2001JB000269.

Roberts, H. H. (2001), Fluid and gas expulsion on thenorthern Gulf of Mexico continental slope: Mudprone to mineral-prone responses, in Natural GasHydrates: Occurrence,Distribution and Detection,Geophys.Monogr. Ser., vol. 124, edited by C. K. Paulland W.P.Dillon,pp.145–161,AGU,Washington,D.C.

Sassen, R., and I. R. MacDonald (1994), Evidence ofstructure H hydrate, Gulf of Mexico continentalslope, Org.Geochem.,23, 1029–1032.

Sassen, R., S.T. Sweet,A.V. Milkov, D.A. DeFreitas, M. C.Kennicutt II, and H. H. Roberts (2001), Stability ofthermogenic gas hydrate in the Gulf of Mexico:Constraints on models of climate change, in Natur-al Gas Hydrates: Occurrence,Distribution andDetection, Geophys.Monogr. Ser., 124, edited by C. K. Paull and W. P. Dillon, pp. 131–143,AGU,Washington, D.C.

Spence, G. D., N. R. Chapman, R. D. Hyndman, and C.Cleary (2001),Fishing trawler nets massive “catch”of methane hydrates, Eos Trans.AGU,82(50), 621.

Suess,E.,et al. (1999),Enhanced dewatering,benthicmaterial turnover and large methane plumes atthe Cascadia convergent margin, Earth Planet. Sci.Lett., 170, 1–15.

Author Information

Ross Chapman, School of Earth and Ocean Sciences, University of Victoria, Canada; JohnPohlman,Virginia Institute of Marine Science,College of William and Mary, Gloucester Point; RickCoffin, Naval Research Laboratory, Marine Biogeo-chemistry Section,Washington, D.C.; Jeff Chanton,Florida State University,Tallahassee; and LauraLapham, University of North Carolina, Chapel Hill

Eos,Vol. 85, No. 38, 21 September 2004

Data analyses and model simulations haverecently indicated that as the planet is warming,the chance for extreme events increases. Karlet al. [1995] examined precipitation recordsover the 20th century and showed that thehigh-frequency (up to interannual) variabilityhas increased. Subsequently, Tsonis [1996]showed that the low-frequency variability hasalso increased.These variability trends indicatethat the frequency of extremes (more droughtevents and more heavy precipitation events)has increased whereas the mean has remainedapproximately the same. Such a tendency isobserved with other variables and is consis-tent with model projections of a warmer planet.

A tendency for increased extremes is oftentranslated as increased randomness, simplybecause the fluctuations increase.Strictly speak-ing,however, this is incorrect.An increase in theextremes affects the probability distribution ofa random variable, but the variable is still ran-dom and thus is equally unpredictable.This is in agreement with the Chaitin-Kolmogorov-Solomonoff complexity definition of random-ness [Casti,1990]. According to this definition,the degree of randomness of a given sequence

is determined by the length of the computerprogram written to reproduce it. If the programinvolves as many steps as the length of thesequence, then the sequence is called maxi-mally random. Random sequences generatedfrom probability distributions are all equallymaximally random because their values appearwith no particular order or repetition, regard-less of the form of the distribution.As such, todescribe such sequences one must write aprogram that involves as many steps as thelength of the sequence. It follows that changesin the degree of randomness cannot be assessedby changes in the probability distribution.Changes in the degree of randomness can onlybe probed by changes in the dynamical prop-erties of a system with complex behavior. Ifthe dynamics change, the system may becomemore (less) complex, which will imply that alonger (shorter) program will be needed todescribe it.

Changes in Predictability

A common element in any definition of ran-domness is unpredictability. Simply, a processis random if we cannot predict it. If changesin global temperature affect the degree of ran-domness in the climate system,then predictability

Is Global Warming InjectingRandomness Into the Climate System?PAGES 361, 364

Fig.1.Top:The sum of the positive Lyapunovexponents (the units are months -1) along thetrajectory (i.e., as a function of time) generatedby the Southern Oscillation index.The inverseof this sum is a measure of the predictability ofthe system.Bottom:The global marine tempera-ture record.As explained in the article, thesetwo signals are coherent at all frequencies lessthan 0.25 cycles/yr.BY A.A.TSONIS