Hindawi Geofluids Volume 2017, Article ID 4240818, 13 pages https://doi.org/10.1155/2017/4240818 Research Article Gas Seepage along the Edge of the Aquitaine Shelf (France): Origin and Local Fluxes LivioRuffine,1Jean-PierreDonval,1ClaireCroguennec,1LaurentBignon,2 DominiqueBirot,1AnneBattani,3GermainBayon,1Jean-ClaudeCaprais,4 NadineLantéri,2DenisLevaché,5andStéphanieDupré1 1IFREMER,Unite´desGe´osciencesMarines,29280Plouzane´,France 2IFREMER,Unite´desRecherchesetDe´veloppementsTechnologiques,29280Plouzane´,France 3IFREMER,Unite´desEcosyste`mesProfonds,29280Plouzane´,France 4IFPEN,1-4avenuedeBois-Pre´au,92852Rueil-MalmaisonCedex,France 5TOTAL,avenueLarribau,64000Pau,France CorrespondenceshouldbeaddressedtoLivioRuffine;[email protected] Received 26 January 2017; Accepted 8 March 2017; Published 25 July 2017 AcademicEditor:TimothyS.Collett Copyright©2017LivioRuffineetal.ThisisanopenaccessarticledistributedundertheCreativeCommonsAttributionLicense, whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkisproperlycited. DuringthescientificexpeditionGAZCOGNE2attheBayofBiscayninegasseepsweresampledforthefirsttimeandtheirfluxwas measuredusinganinsitupressure-preservationsampler(PEGAZ,©IFREMER).Overall,threesiteswereinvestigatedtodetermine thenatureandtheoriginofthegasesbubblingattheseafloorandformingacousticplumesintothewatercolumn,asthiswasthe questionraisedfromthefirstgeologicstudyofthearea.Thishasguidedourstudyandaccordinglycorrespondstothemainpurpose ofthepresentarticle.Thus,themolecularandisotopic(𝛿Dand𝛿13C)analysesrevealedthatthegasseepswereprimarilycomposed ofmethane.Bothmethaneandethaneareofmicrobialorigin,andtheformerhasbeengeneratedbymicrobialreductionofcarbon dioxide.Heavierhydrocarbonsaccountedforlessthan0.06%molofthetotalamount.Despitethemicrobialoriginofmethane, thesamplesexhibitsubtledifferenceswithrespecttothe𝛿13C values,whichvariedbetween−72.7and−66.1‰.Ithasbeen CH suggestedthatsuchadiscrepancywaspredominantlygovernedbytheoccurrenceofanaerobicmethaneoxidation.ThePEGAZ sampleralsoenabledustoestimatethelocalgasfluxesfromthesam4 pledstreams.Theresultingvaluesareextremelyheterogeneous betweenseeps,rangingfrom35to368mLn⋅min−1.Assumingasteadydischarge,themeancalculatedmethaneemissionforthe nineseepsisof38kmol⋅yr−1.Consideringtheextentoftheseeparea,thisverylocalestimatesuggeststhattheAquitaineShelfisa veryappropriateplacetostudymethanedischargeanditsfateoncontinentalshelves. 1.Introduction attectonicallyactiveregionswithoutbeingassociatedwitha specificrelief[24–28]. Submarine natural gas seeps are the seafloor expression The study of submarine natural gas seeps is in many of gas migration from leaking deeply buried hydrocarbon aspectsatimelysubjectforthescientificcommunityandthe reservoirs or shallow depth methanogenesis. Any seep may offshoreindustry.First,theyrepresentasignificantsourceof be composed of a single gas stream or several ones very emitted hydrocarbons, mainly methane, into the water col- closetoeachothersuchthattheyclearlydefineitsperimeter. umnandperhapstheatmosphere[7].Inthecurrentcontext Submarine seeps, at cold seeps, occur worldwide along the ofon-goingclimatechange,itisthereforeessentialtolocate continental margins and are usually related to geological themoncontinentalmarginsandshelvesandthentoassess structureswitheitherpositivereliefssuchassubmarinepin- the fate of these hydrocarbons and quantify their potential goes[1–7],carbonateconcretionsandpavements[8–11],and contribution to atmospheric methane contents [29, 30]. mudvolcanoes[9,12–18]ornegativereliefslikepockmarks Second,theyrepresentkeylocationsforthedevelopmentof [13, 19–23]. They also occur in seafloor-reaching fault areas microbialactivity,whichoxidizeshydrocarboncompounds, 2 Geofluids andsupply,inturn,energyforsymbioticcommunitiesliving neartheseafloor[31–35].Third,fortheoilandgasindustry, the detection of hydrocarbon seepages at the seafloor is 44∘30N indicative of the presence of either possible reserves that are economically exploitable or potential geohazards [36– 39]. Once a gas seep area is discovered, one of the major concernsisthedeterminationofitsorigin.Marinesediments hostmicrobeswhichdegradeorganicmatter,andthisleadsto 44∘20N insituproductionoflightgaseoushydrocarbonsatshallow depth [40–45]. Such gases are commonly called microbial gases and mainly consist of methane. When a submarine seepislinkedtoapetroleumsystem,thegasesareprimarily of thermogenic origin, resulting from the thermocatalysis of deeply buried organic matter [43, 46–49]. However, the 44∘10N two aforementioned gas types derive from organic matter; therefore they belong to biotic gases [43, 44, 48–51]. They represent by far the biggest hydrocarbon sources on earth. Besides,hydrocarbonswithanabioticoriginmaycontribute tothecompositionofseepgases[52].Theyareproducedfrom 44∘N either magmatic processes (mantle-derived compounds) or Victor 6000 ROV dive postmagmaticprocesses(gas-water-rockreactions). Bathymetry (m) −108 Furthermore,methanedischargeintheoceanisthought −2053 tohavebeenresponsibleforclimatechangeinthegeological (km) past [53–56]. However, the mechanisms governing the fate 0 5 10 43∘50N of methane and other hydrocarbons into the water column are still under debate, and their probability to reach the 2∘20W 2∘10W 2∘W 1∘50W atmosphereseemstobesite-dependent[57–64].Accordingly, Figure1:Locationmapofthestudyarea. although difficult, quantifying the contribution of oceanic methanetotheglobalatmosphericbudgetiscrucial,andthis necessarilyrequiresknowledgeoflocalmethanefluxesfrom frame of the GAZCOGNE2 expedition on board the R/V submarineseepsasinput. Pourquoipas?.Apartfromtheplethoraoffishespopulating Recently,anewmajorseepageareahasbeendiscovered thebottomwater,thesiteswerecharacterizedbythepresence at the Aquitaine Shelf, off SW France [65, 66], with a large ofmethane-derivedcarbonatepavements[68,69]capableof number of active gas discharges being observed at the sea- covering surface areas of few meters square on the seafloor floor. The present study combines geochemical data and in (Figure 2). Gases were bubbling through these carbonate situ flux measurements that aimed at determining both the pavementstothewatercolumnatmostoftheseeps.Gasseeps natureandoriginoftheemittedgasesanddiscussingthelocal werealsofoundwithinbacterialmats,withnooutcropping methanedischargedwithinthewatercolumn. carbonate pavements. However, push-core deployment for sedimentsamplingrevealedthepresenceofcarbonatescov- 2.DescriptionoftheStudyArea eredbysedimentsbeneaththebacterialmats,whichhindered thesamplingandledtounsuccessfulcorerecovery[66]. The study area is located in the Bay of Biscay, within the A total of nine gas streams have been sampled from offshore extension of the Parentis Basin [65]. The latter is differentseepsasshowninFigure2.SiteA,thenorthernmost the most prolific hydrocarbon basin in France [67]. Early one,wasthelessacousticallyactivesiteastheoveralldensity acoustic surveys carried out during the expeditions Pegase and amplitude of the gas-related acoustic anomalies were 98/Pelgas2000–2011,originallydevotedtotheevaluationof less important. Two gas seeps were sampled there (GZ2- fish stock and recently examined for the detection of fluid pl536-PZ01-06andGZ2-pl536-PZ02-08).AtsiteD,threegas emission,revealedthattheseepareaislocatedatwaterdepth seeps have been sampled (GZ2-pl535-PZ01-01, GZ2-pl535- ranging from 140 to 185m [65] (Figure 1). New acoustic PZ02-03, and GZ2-pl535-PZ03-05). The southernmost site, surveysconductedin2013duringtheGAZCOGNE1expedi- called site G, was the most acoustically active one. It was tion aimed to better define the extent of the seep domain characterized by gas seeps close to each other and easier which appears to stretch over ∼80km length at the edge of to find with the ROV. Thus, four gas streams have been the Aquitaine Shelf, along a north-south oriented axis, and sampled at site G (GZ2-pl537-PZ01-09, GZ2-pl537-PZ02-11, upto8km(5kmanaverage)inWest-East[66]. GZ2-pl537-PZ03-13,andGZ2-pl537-PZ04-12). Three sites have been selected on the basis of both the occurrenceofgasemissionsdetectedacousticallyinthewater 3.SamplingandMethods columnandtheirlocationwithregardtotheParentisBasin and Landes High (Figure 1). They have been investigated 3.1.InSituGasSampling. DuringtheGAZCOGNE2cruise, by theremotelyoperatedvehicle (ROV) Victor-6000inthe gassampleswerecollectedusingagas-bubblesamplercalled Geofluids 3 GAZCOGNE2: SITE D (PL535-01) N ∘4410 Site A N ∘44930 N ∘449 Sampling BathPyEm−−G11e47Atr95Zy (m) ∘44830N 2∘130W 2∘1W 2∘030W 2∘W GAZCOGNE2: SITE A (PL536-02) N Site D ∘442410 N ∘4424 Sampling PEGAZ Bathymetry (m) −174 −189 2∘440W 2∘430W GAZCOGNE2: SITE G (PL537-03) Site G N ∘435726 Sampling PEGAZ 2∘410W Bat2hy∘m−−411e56Wtr51y (m) ∘435710N Figure2:DivesonsitesD,A,andGwiththecorrespondingsampledseeps. PEGAZ (PrEle`vement de GAZ), designed and built at the requireeithertwopressurechambersincludingonepressur- Unite´ de Recherches et De´veloppements Technologiques ized with fluid prior deployment or electrical control. Our (RDT) of IFREMER [70]. Several gas-tight samplers have gas-tight sampler was developed in order to correct these already been proposed in the literature for application to weaknesses. Thus, it allows an easy and fast deployment, coldseepandhydrothermalsystems[71–75].Althoughtheir depression-free fluid sampling with minimization of the reliabilityhasbeenprovenfromrepeateddeployments,they associatedseawatervolumeinthecaseofgassampling,and are characterized by a heavy weight (>5kg in air) and may can be operated at most sea bottom temperature range and 4 Geofluids witha25mlengthand0.32mmdiameterPorabond-Qcol- (1) umnfromAgilentandaninjectionvalve.Coldtrappingwas alsousedheretopreconcentratethesamplewhennecessary. The 𝛿D measurements were done on an Agilent 7890A gas chromatograph interfaced to a MAT 253 IRMS and using a GC-Isolink interface from Thermo.The gas chromatograph (3) was equipped with a 25m length and 0.32mm diameter (2) MolSieve column and an injection valve. Samples were run at least 3 times after which an average was calculated. They (4) were calibrated regularly against a calibration standard and the values are presented as part per thousand (‰) relative to the Vienna PeeDee Belemnite Standard (vPDB) and the Vienna Standard Mean Ocean Water (VSMOW) for 𝛿13C and 𝛿D, respectively. The uncertainties in the 𝛿13C and𝛿D measurementsaregivenas±0.1–0.3‰and±1‰,respectively. Figure3:PictureofthePEGAZgas-bubblesampler:(1)triggering 4.ResultsandDiscussion system,(2)chassis,(3)high-pressurecylinder,and(4)funnel. The molecular and isotopic compositions of the sampled gases are listed in Table 1. All samples are characterized by very high methane content. This compound accounts for for water depth varying from 30 to 3000 meters. Briefly, morethan99.948%molofthegasmixtures.Carbondioxide includingtheframe,PEGAZis522mmhighand∼210mmin isthesecondmostabundantcomponent,whereasonlytraces diameterandweighsonly2.7kginair(Figure3).The50mL ofethanetobutaneweremeasured.Thestablecarbonisotopic sampling cell is made of Teflon(cid:2)-coated titanium TA-6V. It ratiosareveryheterogeneous(𝛿13C)andrangebetween−72.7 consistsofacentralshaftendedwithapistonandacylindrical and −66.1‰ for methane (CH4), −49.1 and −40.9‰ for wallwithagraduatedPMMAfunnelattachedatitsbottom, ethane (C2H6), −30.1 and −27.9‰ for propane (C3H8), and allowinggasfluxmeasurementduringsampling. −34and−14‰forcarbondioxide(CO2);and𝛿Dvaluesfor methane vary between −182 and −178‰. Methane with the 3.2.AnalyticalMethods. Afterrecovery,thePEGAZsampler lightest carbon isotope comes from the two sampled seeps wasconnectedtoagastransfersystemonboardforsubsam- located at site A, the northernmost site. The measured gas pling[19].Thecollectedgaseswerestoredat∼3barsin12mL fluxes are highly contrasted from one seep to another and preevacuated vials from Labco(cid:2). The latter was analyzed evenbetweengasstreamsfromthesameseep,rangingfrom at the Laboratoire des Cycles Ge´ochimiques et Ressources 35to368mLn⋅min−1 (mLnbeingthevolumebroughtatthe (LCG)ofIFREMERformolecularcomposition.Agaschro- ambientpressure)(Table2). matograph𝜇GCR3000fromSRAequippedwitha𝜇TCDand 4.1. Hydrocarbon Origin. Molecular and isotopic composi- aPoraPlotUcapillarycolumnwasusedforthedetermination tions of natural gas are commonly used to determine their ofbothmethaneandcarbondioxideconcentrations.Forthe origin.Figure4(a)representsthemethaneversusethaneand analysisofheavierhydrocarbons,apreconcentrationmethod using activated silica at −80∘C was applied to separate the propaneconcentrationratiosasafunctionof𝛿13CCH (after 4 methanefromtheotherhydrocarbons.Theliquefiedfraction Bernardetal.(1978))foroursamples.Thegasesemittedalong of the sample was then analyzed on a gas chromatograph the western edge of the Aquitaine Shelf clearly fall into the Agilent 7890A equipped with a 32m, 0.32mm Porapak Q microbialdomain.ThisisstrengthenedbyFigure4(b)which column. The uncertainty in the measurements was of±2% further confirms that the methane has been generated by for methane and carbon dioxide concentrations and ±4% the microbial reduction of carbon dioxide. Such a result is for the heavier hydrocarbons. Subsamples were sent to the in agreement with calculated values from the study of the laboratory Isolab (Netherlands) for carbon and hydrogen associatedmethane-derivedauthigeniccarbonatescollected stableisotopeanalyses.Theanalyseswerecarriedoutongas bothontheseafloorandfrombox-cores[69].Inaddition,the chromatograph-isotope ratio-mass spectrometers (GC-IR- plot of 𝛿13CCH versus 𝛿13CCH shown in Figure 4(c) indi- 2 6 4 MS).Stablecarbonisotopesofmethaneandcarbondioxide cates that ethane is also generated from microbial process. were analyzed with an Agilent 6890N gas chromatograph Theverylowvaluesof𝛿13Cforcarbondioxide(Table1)are interfacedtoaFinniganDeltaSIRMSusingaFinniganGC-C indicativeofanorganicsource,which,again,furtherprovesa IIinterfacefromThermo.TheGCwasequippedwitha12m microbialoriginforthegases.Infact,CO2 originatingfrom lengthand0.32mmdiameterMolSievecolumnfromAgilent inorganic sources is characterized by higher values of 𝛿13C andaninjectionvalve.Theminimumconcentrationrequired [82,83].Thus,thetwomajorhydrocarboncomponentsofthe was25–50ppm.Stablecarbonisotopesofethaneandpropane emitted gases are of microbial origin. Components heavier wereanalyzedonanAgilent7890Agaschromatographinter- thanethane(C3+)arepresentintraceamountsinallsamples. facedtoaMAT253IRMSusingaGC-IsolinkoraFinnigan Their presence in our samples either reflects a very small GC-C III interface. The gas chromatograph was equipped thermogeniccontributionofdeep-seatedreservoirsfromthe Geofluids 5 𝜀c‰ 42.8 34.1 55.3 48.7 42.1 D‰ 81 81 79 80 82 81 78 82 82 𝛿 −1 −1 −1 −1 −1 −1 −1 −1 −1 O2 CC‰ −24 −34 −14 −24 −24 13𝛿 CH38‰ 9.4 8.3 0.1 8.3 7.9 eeps. 13𝛿C −2 −2 −3 −2 −2 s elf h uitaineS 13𝛿CCH26‰ −40.9 −41.9 −49.1 −42.7 −45.6 q A e h asesoft 13𝛿CCH4‰ −66.8 −68.1 −69.3 −71.2 −72.7 −68.6 −66.1 −68.8 −69.1 g d e pl ol m m sa % 032 051 020 012 015 014 015 016 010 he O2 0. 0. 0. 0. 0. 0. 0. 0. 0. t C of s n mpositio nCH%410mol 0.000094 0.000110 0.000136 0.000100 0.000083 0.000075 0.000089 0.000105 0.000123 o c c isotopi H%10mol 00015 00005 00023 00008 00005 00004 00002 00083 00003 and iC4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 r a ul Molec H%8mol 00067 00056 00102 00051 00054 00043 00042 00051 00042 1: C3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 e l b a T H%6mol 00313 00464 00600 00357 00368 00429 00423 00435 00359 C2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ol m 7 8 9 8 5 5 5 3 0 6 4 7 8 8 8 8 8 9 % 9 9 9 9 9 9 9 9 9 H4 99. 99. 99. 99. 99. 99. 99. 99. 99. C 01-01 02-03 03-05 01-06 02-08 01-09 02-11 03-13 04-12 Z Z Z Z Z Z Z Z Z P P P P P P P P P 5- 5- 5- 6- 6- 7- 7- 7- 7- 3 3 3 3 3 3 3 3 3 5 5 5 5 5 5 5 5 5 ple -pl -pl -pl -pl -pl -pl -pl -pl -pl m 2 2 2 2 2 2 2 2 2 Z Z Z Z Z Z Z Z Z Sa G G G G G G G G G 6 Geofluids 106 105 ) 104 （8 ＃3 + （6 103 ＃2 /( 4 （ ＃ 102 101 100 −100 −80 −60 −40 −20 13C CH4 (a) −100 H4on) −70 ₀%C()CH4 −−6800 (meMthiyclr foebrimale CntHat4ion) Microbial C(COreducti2 ₀%PDB) −50 Miacnrodb Cia2lH C6H4 13 Mixed gas (H6 C2 Thermogenic CH4 C −40 Thermogenic 13 −30 Microbial CH4 and C2H6 CH4 and thermogenic C2H6 −350 −300 −250 −200 −150 −100 −80 −70 −60 −50 −40 −30 −20 D (%₀) 13C (%₀PDB) CH4 CH4 (b) (c) Figure4:DiagramsforthestudyventinggasesoftheAquitaineShelfof(a)𝛿13CCH4 versusmethaneto(ethaneandpropane)molecular compositionratio(modifiedafterBernardetal.,1978);(b)𝛿13DCH4versus𝛿13CCH4(modifiedafterSchoell,1983,andWhiticar,1999);and(c) 𝛿13CCH4 versus𝛿13CCH4 (modifiedafterBernardetal.,1978). ParentisBasinorareproductsgeneratedatshallowdepthand carbondioxidewhicharethemainsubstratefordegradation lowtemperatureviamicrobialpathway[84–86].Ourdataset and methanogenesis, respectively. This is not supported by cannotallowustoproceedtoacleardiscriminationbetween neither Figure 5(a) nor Figure 5(b), where one can clearly thesetwosources[87–89].However,the𝛿13Cmeasuredfor see that 𝛿13CCH remains nearly constant with increas- 3 8 bothethaneandpropaneareclosetovaluesfoundforgases ing C2H6/C3H8 ratio and 𝛿13CCH does not decrease with 4 claimedasbeingmicrobialintheliteratureandlighterthan increasing𝛿13CCH ,respectively.Moreover,Figure5(c)com- valuesobtainedfromthermogenic-claimedgases[16,82,87, 3 8 parestheisotopicpatternsoftwogasseepscharacterizedby 90]. biodegradation followed by methanogenesis (SoM Western Because the study area is located within the offshore High and SoM Central High which were collected on the ParentisBasin,onemaythinkthatthemicrobialmethanewas WesternandCentralHighsintheSeaofMarmara,resp.)with generatedfromthermogenichydrocarbonsatreservoirlevel ourstudiedsamplesfromtheAquitaineShelf(BayofBiscay) orduringsecondarymigrationbyhydrocarbondegradation andamicrobialgasseep(SoM-CinarcikBasin)collectedin followed by methanogenesis [91–95]. Such biogeochemical theeasternmostbasinoftheSeaofMarmara[87].Onecan 13 processesleadtoanenrichmentin Cforbothpropaneand clearly see that the Bay of Biscay gas seeps have chemical Geofluids 7 15 −20 −22 12 −24 9 −26 CH38 CH38 /CH26 6 13C −28 −30 3 −32 −34 0 −40 −30 −20 −10 0 −100 −80 −60 −40 −20 0 13C 13C C3H8 CH4 (a) (b) 1/n 0 0.2 0.4 0.6 0.8 1 0 −20 n −40 o b drocar Hy C 13 −60 −80 −100 GZ2-pl535-PZ01-01 GZ2-pl537-PZ02-11 GZ2-pl535-PZ02-03 SoM_Cinarcik Basin GZ2-pl535-PZ03-05 SoM_Western High GZ2-pl536-PZ02-08 SoM_Central Basin (c) Figure5:DiagramsforthestudyventinggasesattheAquitaineShelfof(a)𝛿13CC3 versusethanetopropanemolecularcompositionratio (modifiedafterPrinzhoferandDeville,2013)and(b)𝛿13CC1 versus𝛿13CC3 (modifiedafterPrinzhoferandDeville,2013);(c)diagramofthe reciprocalnumberofcarbonatominmethane,ethane,andpropaneversustheir𝛿13CforthestudyventinggasesattheAquitaineShelfand othergassamplesfromtheSeaofMarmara(afterChungetal.,1988). pattern which is similar to the microbial gases collected in 4.2. Factors Affecting the Component-Isotopic Signatures. As the Sea of Marmara. They are also characterized by close mentioned earlier, the isotopic signatures of the different 𝛿13Cvalues.Thus,ourresultsclearlyindicatethatthesampled components are unevenly distributed from one seep to seepsdischargemicrobialmethaneandethaneintothewater another. Several factors can contribute to this scattering column,thoughtheoriginoftheC3+fractionremainselusive. [44, 96, 97] such as the heterogeneity and the quality of 8 Geofluids Table2:Gasfluxesmeasuredfromtheninegasstreamssampled 20 c=90 80 70 55 c=40 fromtheAquitaineShelf(normalizedtotheatmosphericpressure). 10 Carbonate 30 reduction atm GSGaZZm22p--pplell553355--PPZZ0021--0013 Flux/m364L51n/min ₀%() −100 Production (marinef,e sramMlienenteht)aytlionCH4, CO2 20 O2 (freshwater) GZ2-pl535-PZ03-05 306 C −20 C  =5 GZ2-pl536-PZ01-06 35 13 Oxidation c −30 GZ2-pl536-PZ02-08 136 Methane GZ2-pl537-PZ01-09 368 −40 oxidation GZ2-pl537-PZ02-11 199 −50 GZ2-pl537-PZ03-13 244 −100 −90 −80 −70 −60 −50 −40 −30 −20 GZ2-pl537-PZ04-12 51 13C (%₀) CH4 Figure6:Diagramofcarbonstableisotopecompositionofcarbon dioxideversusmethaneforthestudysites(modifiedafterWhiticar, thesubstrates,thehydrocarbon-generationtemperature,the 1999). occurrence of hydrocarbon oxidation coupled or not with carbonate precipitation, or multiple hydrocarbon-transport processes. Mixing of several gas sources can also lead to differences in the isotopic signatures as observed from our in Figure 6. Pore-water geochemistry from cores would samples.However,mixingofseveralsourcescanbediscarded certainlyallowustogetmoreinsightintotheseprocesses;but here at the Aquitaine Shelf as the isotopic signatures of reliablecoringforpore-watergeochemistrywasunsuccessful methane and ethane clearly show that they were generated duetothewideoccurrenceofcarbonatepavements[69]. by microbial processes. Moreover, even though there is a small thermogenic contribution, its amount would be so 4.3. Local Methane Flux from the Nine Gas Streams. The smallthatitwouldnotexplainsuchanimportantscattering occurrence of a large number of seeps with multiple gas for𝛿13Cofmethane.Duringmicrobialmethanogenesis,the streams in the shallow water of the Aquitaine Shelf for at methanogens preferentially use the lighter carbon dioxide least 18 years [65] raises questions about the amount of [44].IftheCO2poolisseverelydepleted,itwillmechanically methanedischargedintothewatercolumn.Theunfavorable beenrichedintheheaviercarbonandashiftinthe𝛿13CCH seafloorconditionsforhydrateformationthatprevailonthe 4 towardshighervalueswillbeobservedovertime.However, shelf(shallowwaterdepthwithbottomwatertemperatureof the carbon fractionation factor 𝜀c, which is the difference ∼12∘C)discardthepossibilityofmitigatingmethanerelease between𝛿13CCO and𝛿13CCH ,shouldremainconstantupon bybothstorageashydratedepositandtrappingofthemigrat- 2 4 depletion[44].Here,thisfactorvarieswidelywithin34.1and inggasbeneaththesemipermeablehydrate-bearinglayer. 55.3‰.Thus,itisunlikelythatthequalityortheheterogene- As mentioned previously, each investigated seep was ity of the substrate is the major factor responsible for the composed of several streams and only one stream was contrastinthe𝛿13CCH .Furthermore,inmarinesediments, selected for gas sampling and flux measurement. The seeps 4 methanogenesisfrommicrobialCO2-reductionusuallyleads wereseparatedfromeachothertoacoupleofmeterstotens to𝜀crangingbetween49and100‰[44].Thevaluesobtained ofmeters.Inaddition,disturbanceofthesedimentduringthe here fall below this range, except for the gas sample GZ2- samplingbyROVcreatednewandlessvigorousgasstreams, pl535-PZ03-05. Such low 𝜀c values may be partly explained which may persist for the duration of the sampling. This bythefactthatmethaneistransportedbybothdiffusionas indicates that the seep is frequently subject to both spatial dissolvedphaseandadvectionofgasbubbles.Suchtwo-phase andtemporalvariations.AccordingtoTable2,themeasured transport is not surprising in gas seeps. However, methane gasfluxesvaryfrom35to368mLn⋅min−1.Thesevaluesare advection in the gas phase is by far the dominant process in the range of volumetric fluxes measured in the Central due to the elevated gas fluxes measured (discussed in the Nile Deep-Sea Fan in the Eastern Mediterranean Sea [76], following section); and the plot of 𝛿13CCH as a functionof theKerchseepareainthenortheasternBlackSea[79],and 4 thegasfluxdidnothighlightanycorrelation.Thus,webelieve the convergent Makran continental margin [78]. However, that the two-phase transport cannot solely explain such an these settings are located at much deeper depth than the importantscatteringforboth𝛿13CCH and𝜀c.Themostcon- AquitaineShelf. Ifoneconsidersthegascompositiongiven 4 vincingexplanationremainstheoccurrenceofbiogeochem- in Table 1 and assuming steady gas fluxes over time for the icalprocesses,morespecificallytheAnaerobicOxidationof measured streams, about 38.2kmol of methane bubbles is Methane(AOM)coupledwithcarbonateprecipitationwithin annually discharged into the water column from only nine thesedimentarycolumn[31,40,44,98].Thisisinagreement streamsoverthousandswhichcreatethenumerousdetected withtheisotopicsignaturesoftheinvestigatedMDAC(Pierre acousticplumes[65].Atfirstsight,thisvalueappearssmall etal.2017).Thelatterprocesspreferentiallytakesupmethane compared to the estimates of gas fluxes from other settings with the lighter carbon atom [99], therefore producing a like the aforementioned ones, the Hydrate Ridge [80] and 13C-depleted CO2 which leads to a decrease of 𝜀c as shown The UK Block 15/25 [81] (Table 3). However it is important Geofluids 9 Table3:Comparisonofgas-bubblefluxesfromdifferentsettings. Waterdepth Gasflux Fluxmeasurement Setting (m) (×103mol⋅yr−1) technique Gasorigin Comment Reference Videorecordingofgas Measurementfor9 Aquitaineshelf 140–180 38.2 bubblingthrougha Biogenic Thiswork individualstreams graduatedvolume Combinationofgas Average Eastern analysis,ship-based measurementfor MediterraneanSea 1500–1800 230–2300 hydroacousticsurveys, Biogenic [76] 20individual (Hydratesite) bubblesizemodeling, streams andROVobservations Measurementof HakonMosbyMV gasdischargefrom 1250 6307 ROVobservations Biogenic [77] (Hydratesite) openfaults andmudvolcano Measurementfrom Combinationofgas 3gasflares Makrancontinental analysis,ship-based composedof margin 575–2870 10330 hydroacousticsurveys, Biogenic severalstreamsand [78] (Hydratesite) bubblesizemodeling, extrapolationtoall andROVobservations identifiedflares (>20) Combinationofgas Average Don-Kubanpaleo-fan analysis,ship-based measurementfrom (BlackSea) 890 2000–87000 hydroacousticsurveys, Biogenic ∼600acoustic [79] 6 2 (Hydratesite) bubblesizemodeling, flaresover10 m andROVobservations surfacearea Measurementfor Northernsummitof Videorecordingofgas 10individual HydrateRidge 1250 21900 displacingafluid Biogenic 2 [80] streamsover80m (Hydratesite) throughaPVCfunnel surfacearea Surveyedareaof UKBlock15/25 167 1064 Gasratemeasurements Thermogenic 6.5×105m2 [81] surfacearea toemphasizethatourcalculatedvaluedoesnotrepresentan into the water column. Knowing that thousands of plumes estimateforthetotalamountofmethanedischargedintothe were detected from the water column acoustics, this puts watercolumnoverthewidespread-seepareaoftheAquitaine forwardthe concern regardingthe fate ofthe methaneinto Shelf, an area with a surface > 60 ∗ 103 times bigger than theshallowwatercolumnoftheAquitaineShelf.Suchastudy theUKBlock15/25,forinstance.Itonlycorrespondstothe shoulddeservemoreconsiderationsinfutureprospects. cumulatedvaluesofninehighlylocalizeddischarges. ConflictsofInterest 5.ConclusionandPerspectives Theauthorsdeclarethattheyhavenoconflictsofinterest. This paper reports on the first results of gas geochem- istry from samples collected from submarine seeps at the Acknowledgments Aquitaine Shelf (southwestern French margin). The origin andfluxofthegasesemittedintothewatercolumnweredis- TheauthorsaregratefultotheROVVictor-6000teamforthe cussed.Ourdatasetindicatedthatthegasesmainlycontain in situ sampling. The Captain of the Pourquoi pas? and his methane,whichwasgeneratedbymicrobe-mediatedcarbon crewarealsoacknowledgedfortheirtechnicalsupport.The dioxidereduction.Heavierhydrocarbonsandcarbondioxide oceanographicexpeditionsGAZCOGNE1(http://dx.doi.org/ were present in trace and minor amounts, respectively. The 10.17600/13020070) and GAZCOGNE2 (http://dx.doi.org/ gas fluxes of the sampled streams were very heterogeneous 10.17600/13030090) as well as the study were financially between each other, with values ranging in between 35 supportedbybothIFREMERandTOTALthroughthejoint and 368mLn⋅min−1. 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