Sassen-1999-Thermogenic gas hydrates and hydro(3)

ventsStructureIIgashydrateneartheori?cesoffromatfromthetheassociatedGC234siteventisassumedtohavecrystallizedgasmolecularventgastosolidstructuregas.IIThegaschangehydrateininvolvesphasemolecularfractionationinformingabundancediameterhydrategasesrelative(Sloan,favoringgasesofappropriateincreasetoinvent1990).Methanedecreasesabundancegas,andinCstructure2±C4hydrate-IIgasduring234gas(Sloan,hydrate1990).crystallizationThemolecularfromventfractionationgasatGCmethaneisillustratedofgashydrateinFig.occurs7.Relativeinlower

topercentage

ventgas,

R.Sassenetal./OrganicGeochemistry30(1999)485±497493

(73.8%),areandtheC2,C3,i-C4,andnthoughtMeaningfulpresentinhighercarbonpercentages-C4hydrocarbonsisotopic(Tablefractionation1).

isnot(Claypooltoetetoccural.,1985;duringBrooksgasethydrateal.,1986;crystallization(boundà48.4al.,1988;-SassenandMacDonald,1997).KennicuttThed13Cthebonsd13CmethanePDB)andvaluesareofthenearlydD(àhydrate-boundidentical203-SMOW)toofhydrate-Cventgas,asare2±Cation(Tablesite,ofcarbon1;Fig.orhydrogen7).Measurable4hydrocar-isisotopicfraction-atservestheperhapsseabecausethegashydratenotwasobservedonlyexposedatthisallowingasabenchmarkˉoorrecently.intermsTheofGCisotopic234gasproperties,hydratelongerdurationcomparisonofexposuretootheratthegasseahydratesˉoor.

witha4.3.Gashydratealteration

fromTheBush?veC1±Csamples5molecularofstructuredistributionsIIofhydrocarbonslowerHillgas;percentagesareillustrated(71.7±85.7%)inFig.6.MethanegashydrateoccursfrominsentthecentagesinhigherC,Cthaninassociatedvent2percentages3,i-C4,and(Tablen-C4hydrocarbons1).Althoughlowarepre-per-detectionofi-C5arepresent,n-C5isabsentorbelow(muchàRelativelimits.

42.2-totoà43.5vent-PDB)gas,hydrate-boundmethanemethaneas3.8-(Table1).TheisenricheddDin13Cbyasvariationgas(Fig.(à115-toà193-SMOW)ofhydrate-boundshowsmuchEnrichmentbyasmuch6),andasis(anenrichedinDrelativetoventare(Sassenbestin13CandDofatypical)hydrate-bound85-(Tablemethane1).etexplainedal.,1998).byDuringbacterialbacterialoxidationoxidation,e??ects12and1Ctantresidualpool,Hareresultingpreferentiallyinenrichmentusedfromofthe13Cmethanereac-areFree-livingmethanemethanotrophs(Colemanetal.,andDintheattach1981).

tosurfaces;reducedabletoFisher,compoundsutilizemethane,withC-Cbutaretheybondsunabletoutilizefrom1995).Thed13CvaluesofC(Nelsonand2±C4hydrocarbonsBushgasofHillhydratessiteareandnearlyfromidenticalventgases(Tablesampledatthebonsoxidationtivityo??erse??ectsofhydrate-boundC1).Thelack2±C4hydrocar-hydrate.occursstrongConceivably,Wealsoontheevidencesurfacesthatofmethanotrophicthesampledgasac-hydrocarbonsmolecularconsideredenclosingbyfreehydrocarbonreplacementanalternatehypothesis.gasesofhydrate-boundofcontrasthydratetothethegasunalteredhydratesCcouldoccur.However,sedimentsin2±Csedimentsamples,speci?cC4hydrocarbonsofgas2±C4hydrocarbongasesine??ects(seesamplesbelow).displayThislackstrongofcorrelationbacterialoxidationexcludes

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494R.Sassenetal./OrganicGeochemistry30(1999)485±497

Fig.pic8.mounds(top)Diagrams(BHST-2)andpropertiessummarizingmolecular(bottom)andisoto-andfromGC234nearof(GCAT-2)tubesedimentswormoverlyinggashydratesamplingcoloniesstations.

atBushHillmolecularhypothesis.

replacementbysedimentgasesasaviablehydrates.LittleisknownˉoortionexperimentsGashydrateaboutatthecrystallizedthesurfacesBushHillrapidlyofnaturalgasstudysite;duringnuclea-sea-massesandandmonlyMacDonald,ofcrystallizationgashydrateofdendritesandbotryoidal1997).occurredRapidwithincrystalminutesgrowthis(Sassencom-growthknownconnectingsurfaces,toincreaseˉuidandsolidinclusionsonexteriorˉawsandandto?ssuresincreasethefrequencyofinter-areasurfaceareaandpart(VanoftheHook,1961).Thebeofrapidlygrowngashydratecrystalsinternalthussurfacecouldopticalaccessiblesectionspetrologicbymethanotrophicstudyofnaturalbacteria.gasDetailedtoRelativerepresentsà27.8-toacriticalareaoffuturehydrateresearch.thinPDB)ventisgas,depletedhydrate-boundin13CbyCOas2(muchà20.0-as

22.9is-(Tables1and2).Thedepletionin13CofCO2methaneconsistentwithbacterialoxidationofhydrate-boundanalyzed(Sassenetal.,1998).Incontrast,previouslyfrom(thenaturalGulfstructureofIIMexicoandstructurecontainHhydratesCO2bed13

gaslargelyC=+18.5-toà4.4-PDB)usuallyassumedto1994).

(Brooksofetdeepal.,thermogenic1984,1986;SassenoriginanddirectMacDonald,fromventtheGasLongsamehydratelocalitiesmoundsforatBushHillhavepersistedatexplaindurationofexposureyears(MacDonaldattheseaetˉooral.,1994).couldtheBushincreasedHillsiterelativealterationtoe??ectstheGCon234gassite.

hydratesat4.4.Hydrocarbonalterationinhydrate-moundsedimentshydrate-moundConcentrationsofC1±C5hydrocarbonsofinCthe88,687tosediment241,808ppmsamplesrangefrom(TableBush2).TheHillfourCare1±speci?cally,5molecularChydrate-formingcharacteristicshydrocarbonsresemblegassuchhydrates;asC2,vent3,andi-C4occurinoriginate,gas(Fig.higherconcentrationsthanincompositionatleast8).Theinpart,sedimentbyslowgasorcouldepisodicthereforebutions.Methaneoftheunderlyinggashydrate.

de-(theà42.3-Thecomprisestoàd1351.0C-ofPDB)methane57.1±90.8%ofC1±C5distri-showsfromawidertheserangesedimentsandvent2).Somegas(àmethane44.1-andisdepletedà46.0-inPDB)13(Tablesthan1ventsitugasbyasCrelativetogeniccomponentmuchofmethaneas6.9-otherPDB,thanindicatingfromaninthermogenicventing.limitedventSomemethanesedimentandbacterialsamplesmethane.containthermo-bothOurpathwaysisotopicmentationinvolvingdatareductiondonotallowofCOdistinctionbetween2or233ofbrinepool(Colemanemphasizesetal.,the1996).However,acetatethefer-GCstudybacterialgasarea(Tablemethane2).productionvolumetricIninourGreenimportanceCanyon(Brookshydratesetal.,are1986).

welldocumentedaddition,inbacterialthestudymethaneareabeenThefromsubjectmixedtomethaneextensivepoolbacterialinBushoxidation.HillsedimentshasDvent(à134hydrate-mound-toàsedimentsisstronglyenrichedMethanein66enrichment-gas(Tables(à1981-158andto-à2).200SMOW)relativetoassociatedWe-SMOW),couldexpectbyastomuchasbasedsamples.onstrongof13CenrichmentinresidualobserveofmethaneofsedimentsingwithHowever,thisisnotobservedDinbecausethemethaneofmix-oxidationMeasurementsbacterialofgasdstrongly13Csuggestdepletedpreferentialin13C.

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R.Sassenetal./OrganicGeochemistry30(1999)485±497495

gashydrate-associatedhydrates.AlthoughPDB)toisenrichedinsedimentsC2isnot13Crelative(àgreatlyto24.7a??ected,C3invent-gasesto(àà20.325.0-2).àhydrateAlthough26.3-PDB)byasmuchas6.0-(Tables1and-PDB)associatedi-C4issedimentsnotgreatly(à19.6a??ected,-n-C4intoisenrichedin13Crelativetoventgasesto(àà21.522.6-2).àferentialThese23.3-kineticPDB)byasmuchas3.7-(Tables1and-bonsenvironment(suchbacterialisotopee??ectsareconsistentwithpre-asoxidationCandofnstraight-chain-Chydrocar-34)inthegeologicBurns,1980).

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