Milkov-2005-Gas hydrate systems at Hydrate Rid(4)

containone-three2?hydrocarbongases

relativetodeepgashydrates,butordersofmagnitude

lessC2?gasesthantheBushHill

sample.

1014A.V.Milkovetal.

Fig.5.Variationofmolecular(ratiosC1/C2?[a]andC1/CO2[e])andisotopic(?13CofC1[b],C2[d],CO2[f],and?DofC1[c])propertiesofhydrate-boundgasesfromHydrateRidgewithdepth.Notethatpreviouslypublisheddata(Table2)arealsopresentedforcomparison.

1995;Kastneretal.,1998).ResultsfromLeg204suggestthatsuchasigni?cantH2SenrichmentisnottypicalfortheSouth-ernSummit.Onlysixsamplesofhydrate-boundgascontainH2S(Table3,Fig.3g).AllH2S-bearinggashydrateswererecoveredatrelativelyhighergas?uxSites1248and1250atdepths?2mbsf.TheH2Sconcentrationinthesesamplesdoesnotexceed1.2%,andthemeanconcentrationis0.27%.More-over,theH2Sconcentrationsmayinpartcomefromsedimentassociatedwithgashydrate.TheconcentrationofH2SinthesamplesofSuessetal.(1999)isslightlyhigherthanconcen-trationsfoundatSites1248and1250butalsoisroughlyoneorderofmagnitudelowerthanthosefoundatSite892.

OnlyalimitednumberofLeg204samplesrecoveredim-mediatelybelowthesea?oorcontainedlowconcentrationsofH2S.Itappearsthatdissolvedsulfateistotallyreducedjustbelow(0–15cm)thesea?oorattheSouthernSummit(Boetiusetal.,2000),whichistypicalofareaswithveryhighratesofadvectionofhydrocarbongases(Kastneretal.,1998).Weinferthatgas?uxatSites1248to1250oftheSouthernSummitisrelativelygreaterthanatSite892oftheNorthernSummitwheremoreH2Sindeepersedimentswasfound(Kastneretal.,1998).Paulletal.(1996)reportedlowconcentrationsofH2S(0.04–0.15%)fromdecomposedgashydratesintheshallowestcoresamplesfromSite996.Swartetal.(2000)suggestedthatgashydrateswithhigherH2Sconcentrationsmightbemorecommonthanpreviouslyrealized.Ourresultsdonotsupport

thishypothesis.TheC1-H2SgashydratefoundbyKastneretal.(1998)attheNorthernSummitofHydrateRidgeremainsuniqueandhasnotbeenfoundattheSouthernSummit.5.1.3.ComparisonofHydrate-BoundandVoidGasesin

DepthPro?lesVariationsofmolecularandisotopicpropertiesofhydrate-boundandvoidgaseswithdepthforSites1244to1250arepresentedinFigure6.Atlow-?uxSites1244to1247,gashydratesareoftenenrichedinC2relativetovoidgasesanddonotcontainC3(Figs.6a–6d).EnrichmentinC2anddepletioninC3areconsistentwiththeformationofsIgashydratethatcontainsC2butexcludesC3fromitscrystallattice(Sloan,1998).Milkovetal.(2004c)inferredthatgashydratecrystal-lization(insitu)anddecomposition(uponrecovery)mayresultintheobservedvariationofC2andC3concentrationsinvoidgases.Basedontheseinferences,discreteintervalsof?nelydisseminatedgashydratewereidenti?edincoredsediments(Milkovetal.,2004c).

Atthehigh-?uxSiteslocatedattheSouthernSummitofHydrateRidge,bothvoidgasesandhydrate-boundgasesareenrichedinC2?hydrocarbongasesrelativetolow-?uxSites(Figs.6e–6g).TheC1/C2?ratiosofadjacentgashydratesandvoidgasesaregenerallysimilar,althoughtherearetwoexcep-tions.Agashydratesamplefrom7.4mbsfatSite1248B

GashydratesystemsatHydrateRidgeoffshoreOregon1015

signi?cantlyenrichedinC2andC3relativetovoidgases.Asdiscussedabove,thissamplemaybecharacterized,atleastpartially,bysII.BecausevoidgasesdonothaveashighaconcentrationofC2?asthatgashydrate,itissuggestedthatfractionationofthemigratinggasesoccursduringsubsurfacegashydratecrystallization.Thisphenomenonisbestdocu-mentedintheGulfofMexico(Sassenetal.,2001a)buthasnotbeenpreviouslyrecognizedatHydrateRidge.

Incontrast,gashydratefrom100mbsfatSite1250isdepletedinC2?gasesrelativetotheadjacentgasvoids(Fig.6g).Moreover,noC3wasidenti?edinthatsample.Site1250ischaracterizedbyacomplex?uid?owpatternwherethermo-genicgasesappeartobepresentintheshallowgashydrateswhilemostlymicrobialgasesoccurindeepgashydrates(Fig.5a).Itispossiblethatthegashydrateat100mbsfcrystallizedfromgasthatisslightlydifferent(lessenrichedinC2?gases)fromthegasesthatcurrentlymigrateinsurroundingsediments.Thishypothesiswillbetestedwithisotopicdatainthefollow-ingsections.

SomegashydratesamplescontainrelativelylessCO2(Fig.5e)thanthesurroundinggasvoids(see,e.g.,thecomparisonforSite1248inFig.7).ThisobservationisinconsistentwiththeexpectedenrichmentofCO2inhydrate-boundgas(Sloan,1998).PaullandUssler(2001)haveshownthatgasvoidsincoreareenrichedinCO2comparedwithgassamplesatadja-centdepthsobtainedbythepressurecoresampler.ItislikelythatgasvoidsbecomeenrichedinCO2relativetogashydratesduringdecompositionofporewaterbicarbonateastemperatureofthecoreincreasesandpressuredecreasesduringrecovery:

2HCO?→COCO2?

32↑?3?H2O

ThederivedCO2wouldbearti?ciallyaddedtothemolecularcompositionofvoidgasesbutnotgashydrates.5.2.IsotopicPropertiesofHydrate-BoundGases5.2.1.IsotopicCompositionofMethane

C1isthemaincomponentofhydrate-boundgasesatHydrateRidgeandmuchinformationontheoriginofgashydratescanbeinferredfromitsisotopiccomposition.Inmarinesediments,microbialandthermogenicC1arecommonlydistinguishedbasedontheisotopicratiosofcarbonandhydrogen(Whiticar,1999).The?13CofC1ingashydratessampledonLeg204variesfrom?63‰to?70‰(Fig.8a),and?DofC1isintherangefrom?188‰to?208‰(Fig.8b).Althoughthedata-baseislimited(especiallyfor?Dvalues),thecomparisonofisotopiccompositionofC1fromgashydratesandfromgasvoidsshowsthatadjacentsampleshaveverysimilar?13Cand?Dvalues(Fig.6).Thereisnosigni?cantisotopicfractionationofC1duringgashydratecrystallizationanddecomposition.ThisconclusionisconsistentwithpreviousobservationsfromdeepsedimentsatBlakeRidge(Wallaceetal.,2000)andsea?oorseepsitesintheGulfofMexico(Sassenetal.,2001a,2001b).However,incaseswhere?13CofC1ingashydratesandinadjacentvoidsissigni?cantlydifferent,itispossiblethatgashydratesoccurinacomplexanddynamicsystemanddonotcontainthesamegasesthatarepresentinsurroundingsediments.Forexample,gashydratefrom100mbsfatSite1250isdepletedinC2andhasC1enrichedin12Crelativeto

adjacentgasvoids(Fig.6g).Itispossiblethatthegashydrate

crystallizedpredominantlyfromtheinsitumicrobialgasesbeforeC2?-enrichedgasesstartedtomigratethroughsurround-ingsediments.

IsotopicpropertiessuggestthatC1islargelymicrobialanditformedviaCO2reduction(Whiticar,1999),whichisconsistentwithconclusionsfrompreviousstudies(Whiticaretal.,1995;Suessetal.,1999;Winckleretal.,2002).However,thedistri-butionofisotopiccompositionthroughthestudiedareaanddepthsindicatesthattheoriginandpostgenerationhistoryofC1maybemorecomplexthanpreviouslyenvisioned.Similarlytotheresultsbasedonmolecularpropertiesofhydrate-boundgases,thevariationsintheisotopiccompositionofmethaneareconsistentwiththepresenceoftwoend-membergashydratesystems.Deep(50–105mbsf)gashydratescontainC?131withCvalues(meanis?68.2‰)consistentwithlargelymicro-bialandperhapsaninsituoriginofthegas(Fig.5b).ShallowgashydratesatSites1248to1250areenrichedinheavyisotopes13Cand2HrelativetodeepgashydratesatallstudiedSites(Figs.5band5c).Moreover,shallowgashydrateatSites1248to1250haveC)relativetogas1enrichedin13C(mean?13CofC?66‰voidsinshallowsedimentsatlow?ux1isSites1244to1247(?13CofC1isbetween?68‰and?78‰,Fig.6).TheseobservationssuggestthatsomeportionofC1fromshallowgashydratesinthehigh?uxenvironmentattheSouthernSummitwasnotgeneratedinsitubuthasmigratedfromgreaterdepthinthebasin.However,fromtheisotopicpropertiesofC1alone,itisunclearifthemigratedC1ispurelymicrobialorpartiallythermogenic.

5.2.2.CarbonIsotopicCompositionofC2?Gases

Mosthydrate-boundgasesfromLeg204containC2.Thedistributionandmainstatisticsof?13CofC2fromgashydratesarepresentedinFigure8c.Again,thecontentandisotopicpropertiesofC2suggestthattwodistinctivegashydratesys-temsarepresent.Shallow(0–40mbsf)gashydratesathigh?uxSites1248to1250haveC132enrichedinC(mean?13Cis?34.4‰,n?16)relativetodeep(50–105mbsf)gashydratesatSites1248and1250andatlow?uxSites1244to1247(mean?13Cis?45.5‰,n?12;Fig.5d).Thevaluesof?13CofC2inshallowgashydratesatSites1248to1250donotvarysigni?cantlyandareclearlyconsistentwithapredominantlythermogenicorigin(Clayton,1991).Incontrast,thevaluesof?13CofC2indeepgashydratesshowagreatrange.Themostnegativevaluesaround?53‰suggestC2ofmicrobialorigin(WasedaandDidyk,1995;Tayloretal.,2000).However,mostvaluesmeasuredatSite1248(Fig.6e)aremorepositive(??34‰)regardlessofdepth,andareconsistentwiththermo-genicoriginofC2.

Itappearsthatthereisatrendofdecreasing?13CvaluesofC2(i.e.,decreasingproportionofthermogenicC2)withincreas-ingdepth(Fig.5d).Thistrendisunusualandprobablyrelatestothemigrationpatternofthethermogenicgascomponent.GashydratesindeepsedimentscontainlittleornoC3(Table3,Fig.8d).Inshallowsediments,mostgashydratescontainC3withmeancarbonisotopiccomposition(measuredinninesamples)of?24.5‰(Table3),whichisconsistentwithathermogenicorigin(Clayton,1991).Thetrendofdecreasing?13CofC2withdepthandtheoccurrenceofthermogenicC3onlyinshallow