Milkov-2005-Gas hydrate systems at Hydrate Rid(3)

IsotopicpropertiesofcarboninhydrocarbongasesandCO2weremeasuredatWoodsHoleOceanographicInstitution.Sam-pleswereinjectedusingavalvedgas-tightsyringe?ttedwithasidehole/domeneedleintoaGerstelCIS-4injectoronanHP6890GCoperatedinsplitmode,withsplitratioselectedtooptimizethesignalintensity.Gascomponentswereseparatedona30m?0.32mmIDAlltechAT-Qcolumnwithheliumcarrier?owof3.0mL/min.Columntemperatureswereisother-malat40°Cforhighconcentrationmethanesamples,orpro-grammedfrom10°Cwith2-minholdto80°Cat50°C/minforCO2,orprogrammedfrom50°Cwith2-minholdto240°Cat50°C/minforC2?hydrocarbons.Anintegralfusedsilicacom-bustionsystemat950°CconvertedallorganiccomponentstoCO2.IsotopicpropertiesweremeasuredwithaFinniganMATDeltaPlusisotoperatiomonitoringmassspectrometerusingtheIsodatNTdatapackage.The?13Cvaluesarereportedaspermil(‰)relativetothePDBstandard.Theprecisionforcarbonisotopedataarebetterthan?0.5‰forC1and?1‰forC2?hydrocarbongasesandCO2.

IsotopiccompositionofhydrogenofC1wasmeasuredatIsotechLaboratoriesusinganof?inepreparationsystemanddualinletmassspectrometer.Inthisprocedure,customizedGowMacGCsareinterfacedwithavacuum/combustionsys-temtoseparateC1fromtheothercomponents,andoxidizetheC1intoCO2andwater.Theresultantwaterofcombustionispuri?edonthevacuumsystemandthensealedintoPyrextubesforisotopicanalysis.Thewaterisreactedwithzincturningsandconvertedtohydrogengas,whichisthenanalyzedoneitheraFinniganMATDeltaSorFinniganDelta?XLdual

GashydratesystemsatHydrateRidgeoffshoreOregon

1011

)599(1d))))

)2222e

an0000cl8v9))))))99999990000enHo999999(2(2(2(2re(1999999....llllfed.l(1(1(1(1(1(1aaaaRna......ttttatllllllaaaaaaeeeererrrrartttttteeeeceeeeeeellllikkkktnssssssitssssssccccsnnnnWhauuuiiiiKeeeeueueuSSSSSSWWWW)0000

1111(‰D

8rrrrrcn9rr1nnnnnn?????in–po1

1039oi9898tCti1111sfsoooIpmoC

80..5457117

46rrr.......3c166nnn8216979?––6676666–––––––?

3095626297380C1287rrrrrrr

/950nnnnnnn1C6161S

67437972..048rrrrrrr

H82...11312nnnnnnn2

74O1713123000rrrrrrr

)

C01000nnnnnn..000nse10...000sagl4

7arC04drrrrrrrrrr

u-0nnnnnnnnnnntn.a0nfo%64

loC10drrrrrrrrrr

-0nnnnnnnnnnnvi.(0niota2111r3

2t0d000r

nC0n000rrrrrr000nnnnnnne....c0000noC601112

35000rrrrrrr

C00000.....nnnnnnn00000451

44313rrr

C..951rrrr07...nn88687nnnnn999h)

tf7ps6eb.Dm3b

(766666666661??????????neivhgtpee.rd)

00na77rrrrrr6666

esrm66nnnnnn88887777ee(ivtgsayWelra.danaestee

cerl1egp2531tneimomAHHHHdryra2ABCDEFGGGGts9oosrr8eeeeeeeeeenlllllllllluooPppppppppp?wfotetiDmpmmmmmmmmmOaaaaaaaaaadmSSSSSSSSSSSnmoorr;fdfesseeteerttaaoululrpaade

rdevvygye

raHditeeeRHgdioggrnaaArrrne

rnRegetehdah?eevtirtrAvAroRdyunabNHSoinletmassspectrometer.The?Dvaluesarereportedaspermil(‰)relativetoSMOW(precisionbetterthan?2‰).

5.RESULTSANDDISCUSSION

5.1.MolecularPropertiesofHydrate-BoundGas5.1.1.GeneralCompositionalTrendsandRelationshipsThecompositionofhydrate-boundgasessampledonLeg204ispresentedinTable3.C1isthedominantcomponentofhydrate-boundgases(Fig.3a).CO2(Fig.3f)andC2(Fig.3b)arelesssigni?cantcomponentsbuttheyarepresentinallormostgashydrates(althoughnoC2wasdeterminedinsamplefrom53.4mbsfatSite1245C,thismayresultfromsigni?cantaircontamination).C3(Fig.3c)ispresentinminorconcentra-tionsinmostsamples,andinonesamplefrom7.4mbsfatSite1248BtheconcentrationofC3isrelativelyhigh(0.483%)andexceedstheconcentrationofC2.Butanes(i-C4andn-C4,Figs.3dand3e)andH2S(Fig.3g)arepresentonlyinalimitednumberofsamplesandthemedianconcentrationofthesecomponentsis0%.NoC5?hydrocarbongasesweredeter-mined.TheC1/C2?ratiovarieswidely(Fig.3h)anditsmedianvalueis1010.

HydrocarbongasesaregeneratedinmarinesedimentsmainlyviamicrobialreductionofCO2andthermaldecompo-sitionoforganicmatter(Whiticar,1999).Microbialhydrocar-bongasesforminshallowsedimentsbelowthesulfate-reduc-tionzoneandarecomposedmainlyofC1withtracesofC2andC3,andthushaveincreased(commonly?10,000)C1/C2?ratios(ClaypoolandKvenvolden,1983).Incontrast,thermo-genicgasesarerelativelyenrichedinC2?andhavelowC1/C2?ratios(?1000).

TwocontrastinggashydratesystemsappeartoexistatHydrateRidgebasedonthecompositionofhydrate-boundgases.Atshallowdepths(0–40mbsf)coredatSites1248to1250inthesummitareaofhighgas?ux,highlyconcentratedgashydrates(Milkovetal.,2003)commonlycontainrelativelyhighconcentrationofC2?gases,mostsamplescontainC3(meanis0.017%),andsomecontaini-C4,n-C4,andH2S(Figs.3c–3e;alsoseeFig.4fortypicalgasdistributions).TheC1/C2?ratioofhydrate-boundgasesis?1,500(Fig.5a).BecauseC3?hydrocarbongasesarenotincludedintosIgashydrate(Sloan,1998),themolecularpropertiesmaysuggestthatsomeshallowgashydratesrepresentcrystalintergrowthsofdominantsIgashydrate(composedofC1,C2,CO2andinsomecasesH2S)withsmallvolumeofstructureII(sII)gashydratethatincludesC3?gases(Lorensonetal.,1999;Yousufetal.,2003).Onegashydratefrom7.3mbsfatSite1248BcontainsmoreC3thanC2(Table3,Fig.4).Basedonthisobservation,thisgashydratemaycontainlargeportionofsIIcrystals(Sloan,1998).Toourknowledge,thisisoneofthe?rstevidenceofsIIgashydrateinthePaci?cOcean,onaconvergentcontinentalmarginwherenoactivepetroleumsystemisthoughttoexist.Pohlmanetal.(2003)alsoreportevidenceofsIIgashydrateonaconvergentmargininBarkleyCanyonoffVancouverIsland.

Incontrasttothesummit,nogashydratesarefoundatshallowdepthsatSites1244to1247locatedonthe?anksofHydrateRidgewheregas?uxisrelativelylow.AttheseSites,thegashydrateoccurrencezone(GHOZ)iswithinthedepthinterval?40to130mbsf(Milkovetal.,2004c),andall

Table2.Molecularandisotopicpropertiesofhydrate-boundgasescollectedbeforeLeg204.

GashydratesystemsatHydrateRidgeoffshoreOregon1013

Fig.3.Boxplotsillustratingmolecularproperties(concentrationsof

C1[a],C2[b],C3[c],i-C4[d],n-C4[e],CO2[f],H

Chydrate-boundgasescollectedonLeg2S[g],andratio

1/C2?[h])of204.The25thto

75thpercentiles(interquartilerange)ofthedatasetvaluesarerepre-

sentedbythebox.Thelineinsidetheboxcorrespondstothemedian.

Thehorizontallines(whiskers)thatextendtotherightandleftofthe

boxesstopatthelastdatapointwithinadistanceof1.5ofthe

interquartilerange.Circlescorrespondtotheoutliers(?1.5interquar-

tileranges).samplesexceptonecontainonlyC1,C2,andCO2(Table3).Onlythedeepestsamplefrom105mbsfatSite1246CcontainsaminoramountofC3(0.001%)whichcouldbecontaminationfromassociatedsediments.TheC1/C2?ratioofhydrate-boundgasesis?20,000(Fig.5a).Basedonmolecularpropertiesalone,itissuggestedthatdeepgashydratesatSites1244to1247arecomposedalmostexclusivelyofmicrobialgas.Fur-thermore,thesegashydratesarecharacterizedbystructureI(sI,Sloan,1998)becauseonlyC1,C2andCO2occurinthecages.Inadditiontotwogashydratesystemsdescribedabove,therearedeepintervals(50–100mbsf)atSites1248and1250,whichcontainhydrate-boundgaswithC1/C2?ratiobetween2000and35,000(Fig.5a).Thesegasesmaybeamixtureofmicrobialgas(similartotheoneatSites1244–1247)andgaswiththermogenicsignature(similartotheonefoundinshallowsedimentsatSites1248–1250).Themolecularpropertiesofhydrocarbongases(e.g.,C1/C2?ratio)ingashydratesfromSite892attheNorthernSummit(Whiticaretal.,1995;Kastneretal.,1998)aresimilartothoseattheSouthernSummitasfoundonLeg204(Fig.5a).How-ever,thecompositionofthreegashydratesamplesrecoveredfromthesea?oorofsummits(Suessetal.,1999)isdifferent.Thesesamplesappeartobesigni?cantlydepletedinC2?gasesrelativetothesamplesrecoveredinODPLegs146and204(Fig.5a).Thereasonsforthisdiscrepancyarenotclear.LowconcentrationofC2?gasesingashydrateofthehigh?uxsettingreportedbySuessetal.(1999)isinconsistentwiththeaboveinferencesaboutthecontributionofthermogenichydro-carbongasestothegashydratesystemnearthesea?oor.5.1.2.H2SinHydrate-BoundGasesSigni?cantenrichmentinH2S(upto18.5%ofnaturalgases)occursinrelativelydeep(3.7and17mbsf)sedimentsattheNorthernSummitofHydrateRidge(Table2,Whiticaret

al.,Fig.4.Gasconcentrationpro?lesdemonstratingtheabundanceofnaturalgasesindifferenthydrate-boundgasesfromHydrateRidge.CompositionofaBushHillgashydrateintheGulfofMexicoisalsoshownforcomparison(Sassenetal.,2001a).NotethatshallowgashydratesfromHydrateRidgeareenrichedinC