Sassen-1999-Thermogenic gas hydrates and hydro(4)

1984),(WintersandinandlaboratoryWilliams,simulations1969;James(Stahl,andrelativeCO2of(Tablestohydrate-moundthermogenicventsedimentsgasbyasismuchdepletedasin1316.4-Cbound1and2).Closed-systemoxidationofhydrate-hydrate-boundmethaneFritz,COcouldthanresultobservedingreater13hereC(Barkerdepletionandof2likelyterially-deriveda1981;mixtureAlperinofthermogenicetal.,1988).vent-gasHowever,COtheCO2isconsistenthydrocarbonwithCO2andbac-anet2(Sassenetal.,1998).Ourdataare234Asampleofoxidationproductionhydrate-associatedinacomplexofCO2frombacterialsedimentnaturalsystem.bacterial(CfromGC1±Cductionhydrocarbon5=138,864ppm)oxidationshowsasimilarpatternofvent(Table2).Thed13CofmethaneandnetisCOsimilar2pro-tositeDand(à(àgas.121203However,--SMOW)SMOW),relativebysedimenttovent82-(Tablesmethanegasfromthesame1andisenriched2).TheCinsistentn-C3dationwith4hydrocarbonsthekineticareenrichedin13C,beingcon-depleted(seein13TableCby31.22).isotope-Relativee??ects(Tablesto1andventofbacterial2).gas,COoxi-2is4.5.HydrocarbonalterationintubewormsedimentscoreConcentrationsinsamplesoftubeofwormC1±C5sedimenthydrocarbonsfromGCfrom234threeareprisesthe12,960Relative73.3±85.2%to75,249oftheppmtotalrange,(Tableand2methanecom-(àà48.7-toPDB),methaneofventgasfromtheandsameFig.site8).(Tables54.2-consistent1PDB)andaretube2).Thedepletedwormdepletionin13sedimentsC(à53.1-toinby13asCmuchofmethaneas5.5-issediments.withSMOW),Relativeproductiontoventofgasbacterialmethanemethane(à203-inàThe125-SMOW)thesedimentsbyasareenrichedinD(à104-tobacterialenrichmentinDmuchisbestasexplained99-(Tablesby1extensiveand2).carbonsare(particularlyoxidationofCmethane.Straight-chainhydro-3andn-C4;seeTables1dationenrichedtheCOe??ects.in13RelativeC,alsotoconsistentand2)samplesventCOwithbacterialoxi-(à228(+2.9-to-à34.4PDB),2fromsediment-

PDB)1andisdepletedin13Cbyasmuchas37.3-(TableshydrocarbonAsample2).

ofBushHilltubewormbacterialcharacteristicsconsistentsedimentwithextremeshowshydrocarbonsalteration(90.8%)otherandCis137,631(Tableppm,2).Thebutconcentrationonlymethaneoflimitshydrocarbon2(9.2%)and(Table2).gasesareareabsentsigni?cantorbelowcomponents;detectionis2).enrichedà46.0-inPDB),Relative13Cbysedimenttoasmuchmethaneventmethaneas4.4-(à(à44.1-(Tables41.6-PDB)enrichedSimilarly,RelativementtoinventDsedimentbymethane(à125-SMOW)1andisCasmuchas75-(Tables1and2).2(àas(5.5C-(à(Tables24.7-PDB)29.7-andà30.2-PDB),sedi-21andisenriched2).in13Cbyasmuch(à22.4à4.927.3---to(TablesPDB)à5.41andis-depletedPDB),theRelative2).in13COtoventCO2C2byfromasmuchsedimentas5.Synthesisandconclusions

syntheticOurconclusionscannotbeextendeddance,e??ectgascommunitieshydratescanbut,havewhereapresenttoallinchemo-abun-cessesondiversebacterially-mediatedprofoundgeochemicallocalizedpro-gasonthehydratesinchemosyntheticcommunities.Alterationofseaˉoor.appearsOur?ndingsrelatedintothisdurationregardofareexposuremethaneCarbonlimitedandisotopicCOpropertiesofhydrate-boundnovel.2fromBushHillareconsistentofàmethanebacterialfromoxidationthermogenic(Tables1and2).Thedwith13Cà46.3-PDB)andalteredgasventhydrategas((nn=3;mean=hydrate-bound42.8-PDB)hydratemethane.isconsistent=5;mean=Thewithmeanbacterialoxidationofn(Bush=5)ismoundsediments(Bushd13HillCandofmethaneGC234:inshouldHill=àand46.1GC-234:PDBandintubewormsedimentsbetweenbeemphasizednthat=4)variabilityis=à50.7within-PDB.andItHowever,sedimenthydratemoundmethanesamplesedimentfromsiteshasoneisadsamplelarge(see13ofBushTableHill2).andCofà51.0-PDB,d13CallvaluessedimentthataresamplesdepletedfrominGC13234havemethanegenicgenicmethanethesealsosediments,ventgasfromisvents.ThissuggestsCrelativethattothermo-thermo-andnotthattheinonlysitusourcebacterialofmethanemethaneinisterialThepresent.

atedoxidationdDofmethaneofprovidesanindicatorofbac-thegasesinsedimentsthermogenic(Tablesgas1andhydrate2).Forandexample,associ-andtrendgasGChydrate234:ofdnD=3;from(Bushmean=thermogenicventgas(BushHillHill:àn200=5;-SMOW),mean=toàaltered168-

496R.Sassenetal./OrganicGeochemistry30(1999)485±497

SMOW),àmean=138-consistentàSMOW),tohydratemoundsediments(n=5;mean=118-SMOW)andtotubewormsediments(n=4;Previousmediatedstudieswithincreasingshowskineticisotopee??ectshaveoxidationofmethane.buriedBorowskimethaneprocessesgasoccuronlyhydratesinsedimentsshownthat(Craggassociatedbacterially-etal.,1996;withcatorThetrendofd13etbacterialCal.,ofCO1997).

2oxidationinsamples(Tablesprovides1anotherindi-(BushoftoHilld13andCGCofCOand2).The234:2nfrom=3;mean=thermogenicà2.5-ventPDB),gas21.1alteredmean=-PDB),gashydrate(BushHill:n=4;mean=à(à22.7-toPDB),hydrateandmoundtosediments(n=5;pletionn=4;mean=à29.8-PDB)tubeshowswormincreasingsedimentsingoxidationof13C,e??ects.

andisbroadlyconsistentwithincreas-de-andFactorsimpactratesofsuchventingasˉuctuatinghavebeentemperature,notedpressure,1997).recentlyUnalteredgashydrategashydratestabilityat(RobertstoandpotentiallyCarney,Hillexposed,butalteredgasGChydrates234appearsatthetobeGassitewhenhydratesexperiencedatbothlongerexposureattheseaˉoor.BushHillsampled.Wesuggest,sitesappearbasedontodatabedecomposingofbyand(1)site,bacterialthatgashydratestabilitycouldbetheimpactedBushin(2)bybacterialdepletiondepletionofhydrate-boundofhydrate-formingmethane,gasesexample,sedimentsBushonesampleassociatedoftubewithwormgassedimenthydrates.fromFortheHillsitecontainsonlymethaneandCthe2(Table2);turegasinlowIIabundance.

gasisthushydrateunlikelybecausetobeCinequilibriumwithstruc-3andi-C4areabsentorenvironmentTheoccurrencegenicdominatedofbacterialbygasmethanehydratesinandthisextremethermogenichydrocarbonshydrocarbonscarbonisfromsigni?cant.ventWethermo-methanehypothesizethatbacteria,faunawhichiscouldpartiallypotentiallyrecycledfavorbyandothermethanotrophicmethanogenicturesandinotherhighpressures.extremeenvironmentsatlowtempera-Acknowledgements

MacDonald,WeacknowledgethesupportprovidedtoSassen,ManagementandJoyethroughtheMinerals96-CT-30318.ServicetheWeappreciateGulfofMexicothesupportcontractprovided1435-01-byResearchJSLteamWilmingtonCenter,andUniversitytheNOAAofNorthNationalCarolinaUnderseamersible.toChuckFig.duringFisher,3iscreditedsamplecollectionbyresearchsub-atandFig.toIan5toMacDonald,IanMacDonald.

Fig.4AssistanceandcommentsAlicePichahchyinsampleiscollectionalsobyNormanGuinassoKiriakoulakisofareourappreciated.reviewers,acknowledged.B.A.CraggandThehelpfulKostasAssociateEditorDG.Wol??References

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