Gas Seepage along the Edge of the Aquitaine Shelf

15 Pages·2017·5.08 MB·English

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. Assuming steady discharge, the nine research collaborative project “Passive Margin Exploration
sampledstreamsreleaseannuallyaround38kmolofmethane Laboratories(PAMELA).”
10 Geofluids
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