Chemical Geology 266 (2009) 390–402

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Chemical Geology

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Biogeochemical controls on authigenic carbonate formation at the Chapopote“asphalt volcano”, Bay of Campeche

Thomas H. Naehr a,⁎, Daniel Birgel b,1, Gerhard Bohrmann b,1, Ian R. MacDonald a,2, Sabine Kasten c,3

a Texas A&M University-Corpus Christi, 6300 Ocean Drive, Corpus Christi, TX 78412-5869, USAb MARUM-Center for Marine Environmental Sciences and Department of Geosciences, University Bremen, PO Box 33 04 40, D-28334 Bremen, Germanyc Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, D-27570 Bremerhaven, Germany

⁎ Corresponding author. Tel.: +1 361 825 2470; fax: +E-mail addresses: thomas.naehr@tamucc.edu (T.H. N

(D. Birgel), gbohrmann@uni-bremen.de (G. Bohrmann)(I.R. MacDonald), Sabine.Kasten@awi.de (S. Kasten).

1 Tel.: +49 421 218 65704/8639; fax: +49 421 218 652 Tel.: +1 361 825 2470; fax: +1 361 825 2025.3 Tel.: +49 471 4831 1936; fax: +49 471 4831 1149.

0009-2541/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.chemgeo.2009.07.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 January 2009Received in revised form 1 July 2009Accepted 6 July 2009

Editor: J. Fein

Keywords:AsphaltHydrocarbon seepsAuthigenic carbonateCarbon isotopesBiomarkersArchaea

The Campeche Knolls in the Bay of Campeche, southern Gulf of Mexico, were investigated through detailedseafloor mapping, ROV surveys, and sediment and pore water sampling. The knolls are elongated, submarinehills created by salt tectonics with a positive relief of up to 800 m above the surrounding seafloor. Several ofthe knolls are associated with sea-surface oil slicks identified from satellite data, indicating the presence ofhydrocarbon seeps on the seafloor. One of the knolls, named “Chapopote”, was studied in detail during twointernational research expeditions (SO174/2 and M67/2) and is characterized by extensive hydrocarbonseepage including large asphalt flows, oil and gas seeps, and seafloor gas hydrate deposits. Chemosymbioticbiological communities and authigenic carbonate deposits are associated with the seeps and are the result ofboth biogeochemical turnover and the interaction between downward-diffusing seawater and hydrocarbon-rich pore fluids at shallow sediment depth. Authigenic carbonates are characterized by aragonite, exhibit aporous texture, and are cemented by a matrix of microsparitic to sparitic aragonite. Macropores of thecarbonates were completely filled with liquid oil. Carbonate microfabrics include peloidal or clotted fabricsthat may indicate the existence of microenvironments resulting from microbial metabolism. Banded/botryoidal aragonite cements line the intra- and bioclasts and incompletely fill the pore spaces. The stablecarbon isotopic composition of authigenic aragonite varies between −28.6‰ and −17.9‰ (PDB), identifyingoil oxidation as the primary source of carbon to the DIC pool, while lipid biomarker data demonstrate theconcurrent existence of microbial communities responsible for anaerobic oxidation of methane (AOM). Theseobservations indicate the presence of additional, AOM-independent reactions responsible for carbonsequestration at hydrocarbon seeps and demonstrate the complexity of biogeochemical processes at seepsites in the Gulf of Mexico basin. Oxygen isotope data of authigenic aragonite vary from +2.5‰ to +3.8‰(PDB), indicating carbonate precipitation in slight disequilibrium with the surrounding pore fluids.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Large and numerous hydrocarbon seeps, locally called “chapopo-teras”, had been known to native populations of the southern GulfCoast region of Mexico for many centuries (cf. Wendt and Cyphers,2008). The occurrence of these onshore seeps was recorded in Olmec,Aztec and Mayan histories and reported by the earliest Spanishexplorers (Salvador, 1991). Onshore oil seeps also first attracted the

1 361 825 2025.aehr), dbirgel@uni-bremen.de, ian.macdonald@tamucc.edu

715/8664.

ll rights reserved.

attention of entrepreneurs in the late 19th century, and, after initialproduction activities west of Tampico, hydrocarbon exploration andproduction efforts in the southern Gulf of Mexico (GOM) basin movedoffshore and culminated in the 1976 discovery of the giant Cantarelloil field in the Bay of Campeche (Pemex, 1999). More recently, thediscovery of large asphalt extrusions on the Campeche continentalslope (Fig. 1) has added another component to the array of materialsassociated with deep sea hydrocarbon seepage (MacDonald et al.,2004). In addition to these unusual asphalt deposits, hydrocarbonseeps in the Bay of Campeche are characterized by chemosymbioticbiological communities and precipitates of authigenic minerals, whichare the result of both biogeochemical turnover and the interactionbetween downward-diffusing seawater and advection of hydrocar-bon-rich pore fluids (Suess et al.,1985; Hovland et al.,1985; Paull et al.,1995; Naehr et al., 2000a,b). Furthermore, chemosymbiotic benthicfauna is supported by the microbially driven anaerobic oxidation ofhydrocarbons, often methane (Thiel et al., 1999; Hinrichs et al., 1999;

Fig. 1. Map of the Gulf of Mexico Basin. Shaded box outlines study area on the Campeche continental slope. Solid line shows ship's track during R/V Sonne expedition SO174/2.

391T.H. Naehr et al. / Chemical Geology 266 (2009) 390–402

Elvert et al., 1999), which increases pore water alkalinity by theproduction of bicarbonate [HCO3¯],

CH4 þ SO2−4 b¼N HCO

−3 þ HS

− þ H2O ð1Þ

thus favoring precipitation of authigenic carbonate minerals in theshallow subsurface by the reaction (Baker and Burns, 1985).

2HCO−3 þ Ca

2þb¼N CaCO3 þ CO2 þ H2O ð2Þ

However, recent studies of organic biomarkers in sediments fromthe Gulf of Mexico (Pancost et al., 2005), observed disparities betweenmeasured rates of sulfate reduction andmethane oxidation (Joye et al.,2004), and carbon isotopic data from authigenic carbonates (Formoloet al., 2004) suggest that anaerobic oxidation of methane (AOM) maynot be the only process responsible for an increase in carbonatealkalinity at many seep sites. Rather, it seems likely that the oxidationof non-methane hydrocarbons provides a significant source ofmetabolic energy and bicarbonate production at these locations.

The aim of this paper is to develop a more comprehensive un-derstanding of biogeochemical processes responsible for authigeniccarbonate precipitation and to elucidate the relative contributions ofmethane andnon-methane hydrocarbons to the total carbonpool that isbeing sequestered bymeans of carbonate authigenesis.We present datathat illustrate the complexity of biogeochemical processes at ahydrocarbon seep site in the southern Gulf of Mexico. We will discussthese data with respect to the relative importance of various microbialreactions that lead to authigenic carbonate precipitation at a deep-seaseep site in the Bay of Campeche. Neither detailed visual imaging norsystematic sampling had occurred prior to our research cruises and ROV

dives to this relatively unexplored part of the deep Gulf of Mexico basin(MacDonald et al., 2004).

1.1. Gulf of Mexico hydrocarbon seepage

In the Gulf of Mexico, a thick sequence of continental marginsediments overlies enormous reservoirs of liquid and gaseous hydro-carbons that rest upon Jurassic-age salt deposits. Salt-driven tectonicsgenerate fault networks that act as conduits for the rapid transfer of oil,gas and brines from deep reservoirs through the overlying sedimentsand into the water column (Roberts, 2001). On the sea floor, theseconduits give rise to gas vents and seeps, subsurface and surficialmethane hydrates, brine pools and mud volcanoes (Aharon, 1994;Roberts, 2001; MacDonald et al., 2003). Sediments around areas ofactive seepage are characterized by elevated concentrations of simple(C1–C5) and high-molecular weight hydrocarbons (oils) and hydrogensulfide. Complex chemosymbiotic communities proliferate in this cold,high-pressure environment by exploiting the abundance of energy-rich,reduced substrates including CH4 and H2S, the latter produced byanaerobic oxidation of hydrocarbons in gas, liquid, and gas hydratephases (Aharon and Fu, 2000; MacDonald et al., 2003).

The Campeche Knolls in the Bay of Campeche, southern Gulf ofMexico, are distinct, elongated submarine hills averaging roughly5×10 km in size (Fig. 2a), which have been generated by salttectonics (Garrison and Martin, 1973). The knolls extend from waterdepths of about 1000 m to the deepest part of the continental slope(∼3500 m water depth) and occur over a total area of about20,000 km2. Of the more than 50 knolls mapped during R/V Sonnecruise SO174/2 in 2003 and R/V Meteor cruise M67/2 in 2006, mosthave a distinct southwest–northeast alignment (Ding et al.,

Fig. 2. (a) Bathymetric map of the study area as outlined in Fig. 1. The Campeche Knolls area is characterized bymostly elongate, SW–NE trending submarine hills that are the surfaceexpression of underlying salt diapirism. Water depths range from ∼800 m to ∼3500 m. (b) Detailed bathymetric mapwith 25 m contours of one of the knolls, named Chapopote. Theknoll is characterized by a central depression and several crescent-shaped features perpendicular to its slopes, which probably represent mass-wasting features. Shaded arearepresents approximate outline of asphalt flow present at Chapopote. TVG-6 (red) is the sample location for the authigenic carbonates discussed in this study, TVG-5 and GeoB 10602(grey) are sample locations for background sediment. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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submitted for publication; Bohrmann et al., 2004, 2009). They rangein relief above the surrounding seafloor from ∼450 to 800 m andhave maximum slopes of 20 to 30%. Many of the knolls mapped havecrests and flanks dissected by either crescent or linear scarps, whichmay correspond to slump scars and faults, respectively. Several ofthese dissected knolls are associated with sea-surface oil slicksidentified from satellite data (MacDonald et al., 2004). Visualsurveys of one cratered knoll, named “Chapopote” (Fig. 2b), locatedat approximately 21°54′N and 93°26′W (MacDonald et al., 2004),revealed extensive surface deposits of solidified asphalt, apparentlyemanating from points along the southern rim of a broad depressionnear the crest of the structure (Figs. 2b, 3a). The complex sequenceof partially overlapping flows that comprises the Chapopote asphaltdeposits suggests separate deposition events and/or developmenthistories and most likely represents asphalt flows of differing ages(Fig. 3b). Recently, Ding et al. (2008) proposed that a combination oflocal salt tectonics and a large, shallow reservoir with deeplysourced, relatively heavy petroleum is responsible for the transportof asphalt and, presumably, other hydrocarbons from the subsurfaceto the seafloor. In addition to its manifestation as asphalt flows,hydrocarbon seepage at Chapopote is also evident by the occurrenceof liquid oil, dissolved and free gas, gas hydrate deposits both in theshallow subsurface and on the seafloor (Klapp et al., in press), and

the occurrence of dense chemoautotrophic biological communities(Fig. 3c) and authigenic carbonate deposits. Formation of massiveauthigenic carbonates at seeps is the consequence of extensivemicrobial utilization of hydrocarbons as an energy source (Robertsand Aharon, 1994; Canet et al., 2006) and provides an importantgeological archive, as these carbonates represent one of the fewpermanent records of an otherwise ephemeral phenomenon(Bohrmann et al., 1998; Naehr et al., 2000b; Teichert et al., 2003).

2. Methods

2.1. Carbonate mineralogy and petrography

Bulk mineralogy and the relative abundance of carbonate mineralsin each sample were determined by X-ray diffraction using a RigakuUltima III X-ray diffractometer at Texas A&M University-CorpusChristi. Samples for XRD analyses were prepared following standardprocedures using an internal corundum standard (cf. Naehr et al.,2000a). Scans were run from 2° to 60° 2θ at a scanning speed of 0.01°2θ/s. The relative proportions of different carbonate minerals werequantified on the basis of the (104) peak areas of calcite, Mg-calciteand dolomite, and the (111) peak area of aragonite using calibrationcurves after Greinert et al. (2001). The position of the (104) peak was

Fig. 3. Hydrocarbon-related features at Chapopote: (a) relatively fresh asphalt flow with ropey surface morphology and thin sediment veneer. White areas are bacterial mats,probably Beggiatoa. Small clusters of vestimentiferan tubeworms are visible near the center of the image. Field of view is approximately 3.5 m. (b) Two generations of asphalt flows(separated by dashed line): younger flow with ropey surface morphology and thin sediment cover overlies older, more strongly degraded flow with blocky morphology. Note thetubeworms extending into the gap between the two flows. Field of view is approximately 5m. (c) Outcrop of altered, blocky asphalt deposits. The area is colonized by typical seep-endemic organisms including mussels (m; Bathymodiolus sp.), vestimentiferan tubeworms (t; Lamellibrachia sp.), galatheid crabs (c; Munidopsis sp.), unidentified sponges, andbacterial mats. Field of view is approximately 2 m. (d) Authigenic carbonate sample recovered from TVG-6 (see Fig. 2). The sample was recovered from shallow (b1 m) sedimentdepth. Liquid oil began seeping out of the porous sample immediately after recovery. Scale bar is 25 cm long.

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used to determine the Mg content of carbonate minerals (Goldsmithet al., 1961; Lumsden, 1979). For the purpose of this study, calcite withless than 5 mol% MgCO3 is considered low-Mg-calcite (LMC) andcalcite compositions of 5–20 mol% MgCO3 are referred to as high-Mg-calcite (HMC) after Burton and Walter (1991). Carbonate content[expressed as weight percent (wt.%) CaCO3] of the samples wasdetermined by comparing sample weight before and after removingthe soluble CaCO3 fraction using 10% hydrochloric acid. Textural,compositional, and morphological features of carbonate precipitateswere studied on hand specimens, slabs, and polished thin sections(50×75 mm), partially stained with alizarin red, using standardoptical microscopy.

2.2. Carbonate and pore water chemical and stable isotope analysis

Samples for carbonate oxygen and carbon isotope analysis wereobtained from the surfaces of polished slabs using a hand-heldmicrodrill. Stable isotopes of carbon and oxygen were measured at theUniversity of Bremen stable isotope laboratory using a FinniganMAT251mass spectrometer. The CO2 for analysis was obtained by reactingsamples with 100% orthophosphoric acid in vacuo at 75°. The purifiedCO2 gas was analyzed using standard mass spectrometric techniques.Results are reported in standard ∂ notation relative to the V-PDBstandard. Precision is on the order of 0.02‰ for carbon and 0.05‰ foroxygen. Oxygen isotope data were corrected according to Kim et al.(2007a) using an aragonite-phosphoric acid fractionation factor ofα=1.00898 (at 75 °C) to account for the aragonitic nature of thesamples. Porewater samples were sealed in serumvials, refrigerated forisotopic measurement of ΣCO2 and are reported in standard ∂ notation

relative to the V-SMOW standard. Our calculations of equilibriumformation temperatures for authigenic aragonite or pore water isotopiccompositions are based on the equation by Kim et al. (2007b):

1000 lnαaragonite�water = 17:88� 0:13ð103=TÞ–31:14� 0:46:

Details regarding sediment and pore water sampling and chemicalanalysis can be found in Bohrmann et al. (2004) for sites TVG-5 andTVG-6 and Bohrmann et al. (2009) for site GeoB 10602.

2.3. Lipid extraction and analysis

Onemassive authigenic carbonate sample from site TVG-6 (Fig. 2b)was chosen for lipid biomarker extraction. After physical removal ofthe outer rim, the sample was crushed into small pieces. Prior todecalcification of the authigenic carbonate, the pore-spacefilling crudeoil was removed by extracting the sample 8 times with DCM(dichloromethane)/MeOH (3:1). Themass of the authigenic carbonateafter the extensive oil extraction was 178 g. After removing the crudeoil, the authigenic carbonates were decalcified as described in Birgelet al. (2006a). After the decalcification procedure, the samplewas firstsaponified by using 6% KOH in methanol, and afterwards extractedthree times with DCM/MeOH (3:1) using a microwave extractionsystem (CEM, MARS X) at 80 °C and 600 W. Subsequently, the organicextracts were separated into four fractions of increasing polarity usingSupelco LC-NH2® glass cartridges (500 mg sorbent) with a sequenceof solvents of increasing polarity: (1) hydrocarbons, 4 ml n-hexane;(2) ketones/esters, 6 ml n-hexane/DCM 3:1; (3) alcohols, 7 ml DCM/

394 T.H. Naehr et al. / Chemical Geology 266 (2009) 390–402

acetone; (4) carboxylic acids, 8 ml 2% formic acid in DCM. Alcoholswere analyzed as their trimethylsilylesters (TMS-derivatives). TMS-derivatives were produced after reaction of alcohols with N,O-bis(trimethylsilyl)trifluoracetamide in dichloromethane and pyridine.Only hydrocarbon and alcohol fractions contained abundant lipidbiomarkers andwere examined by coupled gas chromatography–massspectrometry using a Thermo Electron Trace GC–MS equipped with a30 m RTX-5MS fused silica capillary column (0.32 mm i.d., 0.25 µmfilm thickness). Helium was used as the carrier gas. The GCtemperature program used routinely for all fractions was as follows:60 °C,1 min isothermal; from 60 to 150 °C at 10 °Cmin−1; from 150 to320 °C at 4°min−1; 22 min isothermal. Identification of the individualcompounds was based on GC retention times and mass spectra incomparison with published data. Compound-specific carbon isotopeanalyses were carried out with a Hewlett Packard 5890 GC via aThermo Electron GC-combustion-interface to a Finnigan MAT 252 massspectrometer. GC conditions were identical to those described above.Carbon isotopes are given as ∂ values (∂13C in ‰) relative to the V-PDBstandard and have been corrected for the addition of carbon duringpreparation of TMS-derivatives. Several pulses of CO2 gas of known ∂13Cvalue at the beginning and end of the runs were used for calibration.Instrument precision was checked using a mixture of n-alkanes withknown isotopic composition. Standard deviations are b0.8‰. Eachcompound-specific isotopic value reported here is an average valueobtained from duplicate measurements.

3. Results

Avideo-guidedgrab sample (TVG-6; Fig. 2b), targetinganarea coveredwith white bacterial mats (possibly Beggiatoa) recovered materialcontaining liquid oil, gas-rich sediment with dispersed layers of gashydrate, and several slabs of authigenic, oil saturated carbonate (Fig. 3d).

Table 1Mineralogy and stable isotopic composition of authigenic carbonate samples from Chapopo

Sample ID Arag. Mg-Cc Mg in Mg-Cc Sample

(%) (%) (mol%) ∂18O (‰

TVG-6-I a 98b 2 1.0 3.82TVG-6-I b 4.00TVG-6-I c 3.84TVG-6-I d 3.98TVG-6-I e 4.47TVG-6-II a 99 1 0.5 4.10TVG-6-II b 3.95TVG-6-II c 3.93TVG-6-II d 3.99TVG-6-II e 4.04TVG-6-III a 98 2 1.0 4.21TVG-6-III b 3.69TVG-6-III c 4.21TVG-6-III d 3.90TVG-6-III e 3.72TVG-6-IV a 98 2 0.4 3.89TVG-6-IV b 3.24TVG-6-IV c 4.02TVG-6-IV d 3.88TVG-6-IV e 4.06

TVG-6-V a 97 3 1.0 3.62TVG-6-VI a 100 – – 3.89TVG-6-VI b 3.81TVG-6-VII a 96 4 2.0 3.75TVG-6-VIII a 97 3 1.5 3.88TVG-6-VIII b 3.59TVG-6-IX a 98 2 0.4 3.82TVG-6-X a 99 1 1.0 3.15TVG-6-X b 3.21TVG-6-XI a 98 2 2.0 3.45

a Corrected after Kim et al. (2007a).b Based on one bulk XRD analysis for each sample TVG-6-I through XI.

Subcores taken from TVG-6 showed H2S concentrations of 8 to 13 mM inthe immediate subsurface. Alkalinitywas also greatly elevatedwith valuesranging from 29 to 35 mM (seawater: 2.3 mM). Molecular and isotopiccompositions of gas hydrate and sediment headspace indicate thepresence of moderately mature, thermogenic gas (MacDonald et al.,2004). Oily sediment extracts and asphalt pieces were composed of adegradedmixtureof hydrocarbonswith apeak atn-C30 anda fewresolvedC29 to C32 hopanes (MacDonald et al., 2004). Aliphatic and aromaticbiological markers indicate an Upper Jurassic-sourced, carbonate-rich oilof at leastmoderatematurity, which is typical of deep-water hydrocarbonseeps in the Gulf of Mexico (Cole et al., 2001).

Additional sediment cores collected about 50 m from the asphaltflow (TVG-5 and GeoB 10602; Fig. 2b) were typical of deep-seasediments, apart from containing some small asphalt fragments. Insubcores from TVG-5, a thin surficial layer of brown, clay-rich sedimentindicative of oxic bottom-water conditions, was overlying clayey,nannofossil ooze. No H2S was observed and the presence of nitrate inall samples, which showed a gradient from 14 μM to about 4 μM in10 cmbsf, indicates that the sediment was oxic to at least 10 cm depthbeneath the sediment surface (Bohrmann et al., 2004). No alkalinityincrease was detected in the upper 30 cmbsf. Carbonate contents ofbackground sediment ranged from 29.6 to 40.2 wt.% CaCO3 with anaverage of 33.6 wt.% CaCO3 for the upper 100 cmbsf in core GeoB 10602.Based on macroscopic observations and pore water alkalinity data(Bohrmann et al., 2009), no authigenic carbonate had formed in thesesediments and the carbonate content data from GeoB 10602 areconsidered to represent background CaCO3 concentrations.

3.1. Mineralogy and petrography

The carbonate mineralogy of authigenic carbonates from Chapo-pote is summarized in Table 1. Carbonate contents of the massive

te TVG-6.

vs. PDB Correcteda vs. PDB Comments

) ∂13C (‰) ∂18O (‰) ∂13C (‰)

−28.18 3.14 −28.18 Bulk−9.94 3.32 −9.94 Shell

−21.70 3.16 −21.70 Bulk−26.75 3.31 −26.75 Clast−28.62 3.80 −28.62 Bulk−27.68 3.43 −27.68 Bulk−27.92 3.28 −27.92 Bulk−18.79 3.25 −18.79 Cement−21.67 3.31 −21.67 Bulk−17.94 3.36 −17.94 Bulk−23.96 3.53 −23.96 Clast−3.69 3.01 −3.69 Shell

−24.59 3.53 −24.59 Bulk−20.09 3.22 −20.09 Clast−26.16 3.04 −26.16 Cement−23.63 3.21 −23.63 Clast−22.91 2.56 −22.91 Clast−23.96 3.34 −23.96 Cement−20.70 3.20 −20.70 Bulk−23.67 3.39 −23.67 Bulk−22.97−22.14 2.94 −22.14 Bulk−26.62 3.22 −26.62 Clast−22.50 3.13 −22.50 Bulk−26.46 3.08 −26.46 Clast−20.77 3.21 −20.77 Bulk−27.00 2.92 −27.00 Bulk−27.50 3.14 −27.50 Bulk−23.42 2.47 −23.42 Clast−25.66 2.53 −25.66 Bulk−26.57 2.78 −26.57 Clast

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carbonates vary between 82.6 wt.% and 89.0 wt.% CaCO3 with a meanof 84.8 wt.%. These values are significantly higher than the CaCO3

values of the unconsolidated near-surface (0–100 cmbsf) sediments inthe area (core GeoB 10602). Carbonate mineralogy at Chapopote isdominated by aragonite, which represents the only authigeniccarbonate mineral at TVG-6 (Table 1).

Macroscopic textures of the authigenic carbonates are character-ized by large irregular shaped intra- and bioclasts (up to several cm indiameter; Fig. 4), held together by a matrix of microsparitic to spariticaragonite cement. Bioclasts predominantly consist of broken bivalveshells (probably Vesicomyidae), up to several cm in length. Commonlyspaces between intraclasts are incompletely filled with aragonite,creating large irregular pore spaces (Fig. 4b). In fresh samples, thesepore spaces were completely occupied by liquid oil (Figs. 3d, 4a–d).Tubes of vestimentiferan tubeworms (cf. Lamellibrachia sp.) occurbetween some of the intraclasts (Fig. 4b).

The intraclasts themselves consist of microcrystalline aragonite-cemented mud- and siltstones. Clay minerals and silt-sized quartz andfeldspar constitute the clastic and non-carbonate components. For-aminifers and small bivalve fragments are the dominant biogeniccomponents. Intraclasts with clotted or peloidal fabrics, that are typicalfeatures of microbial carbonates (Burne andMoore,1987) and probablyrepresent an early stage of cementation, are present in all samples(Fig. 5a). Individual clots are irregularly shaped, range in sizefrom ∼50 µm to several 100 µm, and are surrounded by acicular toradiaxial-fibrous aragonite cement. More pervasively cemented intra-clasts show “ghosts” of earlier clots in a microcrystalline aragonite-cemented sedimentmatrix (Fig. 5b). Some of the intraclasts show signsof brittle deformation of the semi-indurated sediments such ascarbonate-cemented fractures (Fig. 5c). Aggregations of framboidalpyrite are common, frequently filling foraminifer tests (Fig. 5d). Fibrousor banded/botryoidal aragonite cements line the intra- and bioclasts andincompletelyfill the pore spaces (Fig. 5e). These aragonite cements form

Fig. 4. Authigenic carbonates from site TVG-6: (a) sample TVG-6-I; irregular, relatively denseliquid oil; cross section of large Vesicomyidae shell visible in lower right half of sample. (bmacroporosity; pores were originally filled with liquid oil; fragments of bivalve shellsvestimentiferan tubeworm is visible along the upper edge of the sample. (c and d) Sammacroporosity, indicating a more pervasive cementation by sparitic aragonite; close-up (d) sharagonite cement; available pore space was originally filled with liquid oil. See text for add

a continuous spectrum of crystal aggregates, from isopachous rimcements to perfect botryoids (Fig. 5e, f). Fibrous to botryoidal aragonitealso grows on exterior and interior surfaces of the biogenic sedimentcomponents (Fig. 5f). Carbonate dissolution events, presumably relatedto the breakdown of liquid hydrocarbons under aerobic conditions oroxidation by reduced sulfur components, are recorded by corrosionsurfaces on the isopachous and banded/botryoidal aragonite cements.

3.2. Stable carbon and oxygen isotopes

A total of 30 aragonite samples from TVG-6were analyzed for stablecarbon and oxygen isotopes (Table 1, Fig. 6). The ∂13C values ofauthigenic aragonite vary between −28.6‰ and −17.9‰ with ∂18Ovalues ranging from+2.5‰ to+3.8‰. Two samples taken frombivalveshells that had been incorporated in the carbonate crusts as bioclastshave carbon isotopic compositions of −9.9‰ and −3.7‰ and oxygenisotopic compositions of +3.0‰ and +3.3‰, respectively. No correla-tion is recognizable between ∂13C and ∂18O values and variationswithinindividual carbonate samples are almost as great as the overall variationbetween all data. Carbon isotopic values formethane and carbondioxidegas extracted from headspace and gas hydrate samples varyfrom−55.1‰ (headspacemethane) to−50.1‰ (gashydratemethane),and from −19.9‰ (headspace CO2) to −7.5‰ (gas hydrate CO2). Fordetails see MacDonald et al. (2004). Pore water samples extracted fromour background core (GeoB 10602) show oxygen isotopic compositionsbetween−0.47‰ (676 cmbsf) and+0.09‰ (70 cmbsf) ∂18O relative tothe V-SMOW standard. The bottomwater at this site has a ∂18O value of0.06‰ at a temperature of 4.02 °C.

3.3. Lipid biomarker inventory after decalcification

Prior to the removal of the pore-filling crude oil, only a massiveunresolved complex mixture (UCM) was identified, comprising

, aragonite-cemented carbonate crust; available pore space is completely saturated with) Sample TVG-6-III; sample with aragonite-cemented intraclasts and well developed(presumably Vesicomya sp.) are dominant bioclasts. Small posterior fragment of aple TVG-6-IV; Macroscopic texture of this sample is similar to (b), but with smallerows numerous, partially brecciated intraclasts (medium grey), held together by spariticitional discussion. Length of scale bar is 5 cm for all samples.

Fig. 5. Authigenic mineral phases of the Chapopote carbonate crusts. (a) Detail of intraclast; clotted or peloidal fabric represents an early stage of cementation. Individual clots areirregular shaped, range from ∼50 µm to several 100 µm in size, and are surrounded by acicular to radiaxial-fibrous aragonite cement. (b) More pervasively cemented intraclastsshowing “ghosts” of earlier clots (arrows) in a microcrystalline aragonite-cemented sediment matrix. (c) Intraclast with large fracture (arrow) that was subsequently filled witharagonite cement indicating episodes of dynamic conditions during authigenic carbonate formation. (d) Aggregation of framboidal pyrite (py), filling foraminifer tests. (e) Banded/botryoidal aragonite cements (bbc) line the intra- and bioclasts and incompletely fill the pore spaces. These aragonite cements form a continuous spectrum of crystal aggregates,from isopachous rim cements to perfect botryoids. Thin dark layer visible in upper half of image is residual oil that used to fill the entire pore space. (f) Fibrous aragonite cementgrowing on interior surfaces of a gastropod shell.

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degraded petroleum-derived hydrocarbons (Fig. 7a). After theremoval of the oil, the decalcified and newly extracted samplecontained traces of the tail-to-tail C25 isoprenoid pentamethylicosane(PMI) and the more abundant tail-to-tail C20 isoprenoid crocetane(Fig. 7c). Despite pre-cleaning the sample (see “Methods” section), anUCM was still present in the hydrocarbon fraction. In the alcoholfraction (Fig. 7b), the major peaks detected contained archaeal lipidsincluding archaeol and sn2-hydroxyarchaeol (hydroxyarchaeol).Furthermore, the bacterial-derived C34:2-dialkylglyceroldiether(C34:2-DAGE; structure IId in Pancost et al., 2001), as well as minoramounts of not further identified C30:0 and C31:0 DAGEs were found.The carboxylic acid fractions did not contain any detectable lipidbiomarkers.

The carbon isotopic composition of themeasured compounds in thedecalcified limestone sample varied from−105‰ (crocetane) to−71‰(C34:2-DAGE; see also Fig. 7b,c). The strongest 13C-depletion was foundfor crocetane (∂13C:-105‰). Crocetane and its unsaturated derivativesare known frommodern seeps (e.g., Elvert et al.,1999; Blumenberg et al.,2004) as reliable biomarkers for euryarchaea performing anaerobicoxidation of methane (AOM), but were also identified at ancient coldseeps as old as Pennsylvanian age (e.g., Elvert et al., 1999; Thiel et al.,1999;Blumenberget al., 2004; Birgel et al., 2008). Furthermore, archaealdiethers archaeol and hydroxyarchaeol were found with ∂13C valuesof −98‰ and −97‰, respectively. The C34:2-DAGE had a more 13C-enrichedcomposition of−71‰. Concentrationsof PMI andotherDAGEswere too low for compound-specific carbon isotope measurements.

Fig. 6. Carbon and oxygen stable isotope composition for authigenic carbonates fromChapopote. The ∂18O data were corrected according to Kim et al. (2007a) using anaragonite-phosphoric acid fractionation factor of α=1.00898 (at 75 °C) to account forthe aragonitic nature of the samples. Shaded area represents the range of ∂18O valuesexpected for carbonate precipitation under equilibrium conditions with T=4.02 °C and∂18Opore water=+0.09‰ according to Kim et al. (2007b). See text for further discussion.

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4. Discussion

Seafloor seepage of hydrocarbons from leaky subsurface reservoirsand its impact on the geology and biology of the Gulf of Mexico basinis a well-known phenomenon and the co-migration of oil and gascreates a unique biogeochemical regime at Gulf of Mexico cold seeps(e.g., Sassen et al., 1993; Roberts and Aharon, 1994; MacDonald et al.,2003). To date, however, most studies have focused on the Louisiana–Texas continental slope in the northern part of the basin. Ourdiscovery of a complex hydrocarbon seep assemblage consisting ofasphalt, liquid oil, hydrocarbon gas, and near-seafloor gas hydratedeposits on the deep Campeche continental slope in the southern Gulfof Mexico has significantly expanded the inventory of seeps and seep-related features in the basin (MacDonald et al., 2004). Authigeniccarbonates and accompanying sediment and pore water samplescollected during R/V Sonne cruise SO174/2 and R/V Meteor cruiseM67/4 have allowed us to elucidate some of the biogeochemicalprocesses driven by the flux of a complex mixture of hydrocarbonsfrom the subsurface to the seafloor.

4.1. Carbonate authigenesis

Elevated total alkalinity concentrations of up to 35 mM in pore watersamples obtained from TVG-6 sediments indicate that conditionsappropriate for the formation of hydrocarbon-derived authigenic carbo-

nates exist in the shallow subsurface at Chapopote. High H2S values of upto 13 mM in the upper sediment column also suggest that sulfatereduction via hydrocarbon oxidation is active and responsible for theformation of authigenic carbonates at shallowsediment depths. Aragoniteis the only authigenic carbonate mineral observed in samples fromChapopote (Table 1), which is in contrast to conditions on the continentalslope in the northern GOM, where seep-related seafloor deposits show agreater mineralogical complexity, including high-Mg-calcite (HMC),dolomite, and barite (Roberts and Aharon, 1994; Fu et al., 1994). Ofcourse, this perceived lack of complexity may simply be a function of ourlimited spatial coverage and future expeditions may recover authigenicseafloor deposits with a greater range of mineralogical compositions. Ingeneral, a high degree of carbonate supersaturation in combination withhigh Mg/Ca ratios and the presence of sulfate would promote aragoniteprecipitationby inhibitingHMCcrystallization (Burton,1993; Savard et al.,1996; Morse et al., 1997). Such conditions are typical for the shallowsubsurface, slightly above the base of the sulfate reduction zone. Thebrecciated and porous texture of the carbonates fromChapopote togetherwith the shallow depth fromwhich the sampleswere recovered (a few to∼80 cmbsf) support formation close to the sediment-water interface.Brecciation of semi-consolidated sediments may be triggered by tectonicor seepage-induced disturbances, such as rapidly increasing flow rates orthe sudden release of trapped fluids and/or gases (Matsumoto, 1990;Greinert et al., 2001). Carbonate microfabrics in samples from Chapopoteare similar to fabrics reported from other modern and ancient seepdeposits (Roberts and Aharon, 1994; Peckmann and Thiel, 2004; Naehret al., 2007). The occurrence of clotted microfabrics may be related to theexistence of microenvironments caused by microbial metabolism, whichwould provide evidence for microbial mediation of carbonate precipita-tion (Peckmann et al., 2002). Indeed, some of the clots in the mostincomplete cemented samples (e.g., TVG-6-VIII b; Fig. 5a) appear to haveoriginated fromnucleationcentersofpossiblemicrobial origin. Isopachousand banded/botryoidal aragonite cements incompletely fill the pore spacebetween individual clots and intraclasts and are responsible for themacroscopic texture of the samples (Fig. 5e,f). Sparitic aragonite cementssuch as the ones illustrated above are typical for many seep-relateddeposits and have been described by various authors from both modernand ancient seep environments (e.g., Naehr et al., 2000a; Peckmann et al.,2001b; Himmler et al., 2008).

Oxygen isotopes of seep-related authigenic carbonates are known toshow considerable variation, documenting the effects of temperatureand/or fluid composition on carbonate precipitation (cf. Naehr et al.,2007). Assuming equilibrium conditions during crystallization ofauthigenic aragonite from Chapopote, ∂18O values between +2.5‰and+3.8‰ (Fig. 6) and a porefluid∂18O composition of+0.09‰ (vs. V-SMOW)would indicate temperatures of carbonate precipitation rangingfrom 6.6 °C to 0.9 °C with an average of ∼3.6 °C (mean ∂18O: 3.16±0.3‰). The measured bottom water temperature at Chapopote was4.02 °C, suggesting carbonate precipitation in isotopic disequilibrium atleast for samples at the extreme ends of the calculated temperaturespectrum. Alternatively, pore fluid ∂18O compositions varying over arange from about−0.5‰ to +0.8‰with an average of +0.18±0.03‰(vs. V-SMOW)at4.02 °Cwouldbe required forequilibriumprecipitationof authigenic aragonite from Chapopote (Fig. 8). Advection of 18O-depleted pore fluids from greater depth or partial dissociation of 18O-enriched gas hydrate (αhydrate–seawater=1.0028 to 1.0032;Maekawa andImai, 2000) could explain the observed ∂18O variations in our samples.Oxygen isotope values of the two bivalve samples (+3.3‰ and+3.0‰)indicate precipitation at 2.9 to 4.2 °C, close to equilibrium conditions.

4.2. Carbon sources at Chapopote—AOM versus oil oxidation

The carbon isotopic composition and biomarker inventory ofauthigenic carbonates from Chapopote allow us a glimpse at thecomplexity of competing biogeochemical processes at Gulf of Mexicohydrocarbon seeps. Authigenic carbonates from areas of hydrocarbon

Fig. 7. (a) Pore-space filling crude oil, total lipid extract; unresolved complex mixture (UCM) comprising degraded petroleum-derived hydrocarbons. (b) Alcohol fraction of sample TVG-6-IV; major peaks detected contained archaeal lipidsincluding archaeol (Ar) and sn-2-hydroxyarchaeol (sn2-OH-Ar), but also bacterial C30:0, C31:0, and C34:2-dialkylglyceroldiether (C34:2-DAGE). The carboxylic acid fractions (not shown) did not contain any detectable lipid biomarkers.(c) Hydrocarbon fraction of sample TVG-6-IV after removal of pore-space filling crude oil; sample shows traces of tail-to-tail C25 isoprenoid pentamethylicosane (PMI) and the more abundant tail-to-tail C20 isoprenoid crocetane (UCM:unresolved complex mixture; is: internal standard).

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Fig. 8. Temperature–∂18O plot illustrating the range of pore water compositionsrequired for equilibrium crystallization of aragonite with ∂18O values of +2.5‰ to+3.8‰ at 4.02 °C (solid horizontal line). Diagonal lines: hypothetical oxygen isotopiccomposition of pore fluids in isotopic equilibrium with aragonite. See text for furtherdiscussion.

Fig. 9. Calculated contributions from available carbon sources to the total carbon poolpresent during the formation of authigenic carbonates from Chapopote after Formoloet al. (2004). See text for discussion.

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seepage are known to show a wide spread of carbon isotopic composi-tions, ranging from 13C-depleted samples with ∂13C values as low as−60‰ produced by the anaerobic oxidation of biogenic methane, to 13C-enriched samples with ∂13C values of up to +30‰ derived frommethanogenesis deeper in the sediment column (e.g., Claypool andKaplan,1974;Naehr et al., 2000a;Greinert et al., 2001;Orphan et al., 2004;Gieskes et al., 2005). Even in theGulf ofMexico,∂13Cvaluesof seep-relatedcarbonates vary from roughly −60‰ to −15‰ (Roberts and Aharon,1994; Ussler and Paull, 2008), althoughmany samples cluster around∂13Cvalues of about −25‰, indicative of a crude oil carbon source (e.g.,Formolo et al., 2004; Canet et al., 2006). Similarly, authigenic carbonatesfrom TVG-6 show ∂13C values of −28.6‰ to −17.9‰, with an averageof−24.2±3.0‰ (Fig. 6). These values are in stark contrast to the carbonisotopic composition of methane at Chapopote, which varies between−55.1‰ and−50.1‰, eliminatingmethane-derived carbon (viaAOM) asthe primary carbon source for carbonate authigenesis. Crude oil from theGOMbasin, on theotherhand, has anaverage carbon isotopic compositionof−27±1.0‰ (Anderson et al.,1983; Cerqueira et al., 2004) andmaybe alikely source for the carbon incorporated in the TVG-6 carbonates. To testthis hypothesis, we used the carbon isotope mass balance introduced byFormolo et al. (2004) that allowedus to evaluate the inputs of carbon fromavarietyof sources (seawater, organicmatter,methane, andnon-methanehydrocarbons; Table 2) during the formation of authigenic carbonates.According to their model, which we applied to four carbonate samplesfrom Chapopote, non-methane hydrocarbons contributed around 90% tothe total carbon incorporated into the carbonate structure, whilemethane-derived carbon only accounted for 0% to 5% of the total carbonpool (Fig. 9).

Table 2Boundary conditions used for the calculation of relative contributions from available carbon

CaCO3 ∂13Ccarbonate⁎ CaCO3background ∂13CNMH

(wt.%) (‰) vs. PDB (wt.%) (‰) vs.

TVG-6-I 89.0 −26.31 (n=4) 33.6 −27.0TVG-6-II 82.6 −22.80 (n=5)TVG-6-III 84.5 −23.70 (n=4) (Ø 0–100 cmbsf for core GeoB 10602✝) AndersoTVG-6-IV 83.0 −22.97 (n=5)

⁎Average for each sample; ✝Background core; wNMHC = non-methane hydrocarbons.

These data are seemingly in contrast to the lipid biomarkerinventory of the authigenic carbonates and the stable carbon isotopiccomposition of the archaeal compounds (crocetane: −105‰ to−97‰; see also Fig. 7), which demonstrate carbon uptake eitherdirectly or indirectly by anaerobic methane oxidation in sedimentsfrom Chapopote. The observed predominance of crocetane over PMI isunusual, but was described, for example, from cold seeps in theAleutian trench (Elvert et al., 2000). At most seep locations PMIpredominates over crocetane, or crocetanemay not be present at all asshown for some modern and ancient methane seeps by variousauthors (e.g., Hinrichs et al., 2000; Birgel et al., 2006b). Hydro-xyarchaeol is most likely sourced from Methanosarcinales of theANME-2 group (cf., Orphan et al., 2001). ANME-2 as tentative sourceorganisms are also suggested by an hydroxyarchaeol/archaeol ratio ashigh as 2.8 (cf., Blumenberg et al., 2004). Crocetane was alsosuggested by these authors as a derivative of ANME-2, but it is notclear if ANME-2 or other, unknown methanotrophic archaea areproducing this compound (Orphan et al., 2001). The presence ofabundant C34:2-DAGE with a ∂13C of −71‰ (Fig. 7b) can beinterpreted as a signature of sulfate-reducing bacteria (SRB), mostlikely living in a consortium with methane-oxidizing archaea (cf.,Pancost et al., 2001). However, SRBs of the genus Desulfosarcina, thesyntrophic partners of anaerobic methane oxidizers, have not yet beenfound to contain DAGEs. Nevertheless, other, unidentified seep-thriving SRBs still appear to be the most probable source organismsof DAGEs. In sediments from Hydrate Ridge, C34:2-DAGE andcyclopropyl C17-carboxylic acid, both with very low ∂13C values,were interpreted as SRB-derived biomarkers (Elvert et al., 2003; Elvertet al., 2005). Interestingly, terminally-branched carboxylic acids werenot identified at Chapopote, although they were abundant, togetherwith isotopically-depleted DAGEs, at other seep locations (see Elvertet al., 2005). One explanation for the lack of SRB-derived carboxylicacids at Chapopote may be a preferential degradation of carboxylicacids compared with DAGEs.

sources to the total carbon pool at Chapopote.

Cw ∂13Cmethane ∂13Corg. matter ∂13Cseawater

PDB (‰) vs. PDB (‰) vs. PDB (‰) vs. PDB

−55.1 −20.0 1.0

n et al. (1983) MacDonald et al. (2004) Goni et al. (1998) Aharon et al. (1991)

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Despite these uncertainties, it is unambiguous that AOM does occurin sediments fromChapopote and that authigenic carbonates fromTVG-6 contain a lipid biomarker inventory that reflects this importantmicrobial process. Even the difference of compound-specific carbonisotope ratios between archaeal and bacterial biomarkers (up to−34‰,see Fig. 7) lies within the range that was observed at other, non-oilcontaining seep sites (e.g., Pancost et al., 2001; Elvert et al., 2005).However, the carbon isotopic composition of the authigenic carbonatesclearly indicates that AOM does only play a minor role in supplyingmethane-derived carbon to the DIC pool from which carbonateprecipitation is taking place. These observations are supported by datafrom Formolo et al. (2004)whodescribe the considerable role that non-methanehydrocarbonsplay in the formation of authigenic carbonates insediments from the Green Canyon area in the northern Gulf of Mexico.Similarly, Joye et al. (2004)measured sulfate reduction rates thatwere inexcess of the corresponding rates of anaerobic 14CH4 oxidation at GOMseep sites, which is consistent with our assertion that microbialprocesses in the Gulf are driven by the consumption of methane andnon-methane hydrocarbons. Finally Pancost et al. (2005) observed thepresence of both archaeal biomarkers and biodegraded petroleum inauthigenic carbonate rocks, again from the Green Canyon area,consistent with our data for the lipid biomarker inventory of theChapopote carbonates.

But what are the microbial communities that can anaerobicallyoxidize complex hydrocarbons while reducing seawater sulfate?Aeckersberg et al. (1991, 1998) describe two strains of sulfate-reducing bacteria that can grow under anoxic conditions on n-alkanesin the range of C12–C20 and C14–C17, reducing sulfate to sulfide incontrolled laboratory experiments. Although, to our knowledge, nosuch strains have been observed in natural seep environments, similarmicrobial communities may be responsible for anaerobic sulfatereduction at seep sites and may contribute to authigenic carbonateprecipitations in these extreme environments. For an overview ofrecent findings on the metabolism of n-alkanes and n-alkenes byanaerobic bacteria see Grossi et al. (2008) and references therein.

Our findings may also have implications for the study of ancientseep deposits, which are commonly characterized by carbon isotopiccompositions that do not unequivocally identify methane as theprimary carbon source, such as the Devonian Hollard Mound and theSidi Amar location in Morocco (Buggisch and Krumm, 2005;Peckmann et al., 2007), the Early Carboniferous (Mississippian)Iberg in the Harz Mountains, Germany (Peckmann et al., 2001a), orthe Late Jurassic to Late Cretaceous seeps in western California(Campbell et al., 2002). At least for the latter locations, a scenariosimilar to the one proposed in this study can be imagined. The∂13Ccarbonate composition of the three sites described by Campbellet al. (2002) has been found to vary between−44‰ and−20‰ andwas interpreted as being derived from anaerobic oxidation ofmethane and/or oil degradation (Campbell et al., 2002). In afollow-up lipid biomarker study it was shown that althoughprominent AOM signatures were present in all three locations,degradation of oil might have played amajor role aswell (Cold Fork ofCottonwood Creek, Wilbur Springs, Paskenta; see Birgel et al., 2006a;Campbell et al., 2002 for details). Typical SRB-derived carboxylicacids, such as iso- and anteiso-carboxylic acids were identified in twoof the studied sites with ∂13C-values ranging from −59‰ to −38‰(Birgel et al., 2006a, their Table 2), whichmay represent signatures ofsulfate-reducing bacteria that were not primarily involved in AOM,but were probably oil degraders.

5. Summary and conclusions

(1) Hydrocarbon seepage in the Bay of Campeche is extensive and ischaracterized by the occurrence of at least four types ofhydrocarbon species: liquid oil, gas, gas hydrate, and asphalt.The distribution of seeps appears to be controlled by salt

tectonics, which produces local topographic highs (the Cam-peche Knolls), as well as associated faults and anticlinalstructures that may act as pathways for ascending fluids andgases.

(2) Anaerobic oxidation of hydrocarbons in the shallow subsurfaceleads to an increase in pore water alkalinity that causesprecipitation of authigenic aragonite at shallow sediment depths.

(3) Authigenic carbonate samples from Chapopote are character-ized by a porous texture, produced by large irregular shapedintra- and bioclasts, held together by a matrix of microspariticto sparitic aragonite cement. Macropores of the carbonatesamples were completely filled with liquid oil. Carbonatemicrofabrics are typical for seep carbonates and includepeloidal or clotted fabrics that may indicate the existence ofmicroenvironments resulting from microbial metabolism.Banded/botryoidal aragonite cements line the intra- andbioclasts and incompletely fill the pore spaces.

(4) Oxygen isotope data indicate carbonate precipitation inslight disequilibrium with the surrounding pore fluids. Gashydrate destabilization or advection of pore fluids from greaterdepth may be responsible for the observed variations in ∂18Ovalues.

(5) The stable carbon isotopic composition of the authigenicaragonite identifies oil oxidation as the primary source ofcarbon to the DIC pool at Chapopote, while lipid biomarker datademonstrate the concurrent existence of microbial commu-nities responsible for anaerobic methane oxidation. Theseobservations are supported by studies of seep sites in otherparts of the Gulf of Mexico basin, where sulfate reduction ratesexceed the corresponding rates of anaerobic 14CH4 oxidation,indicating the presence of additional, AOM-independent reac-tions responsible for sulfate depletion at hydrocarbon seeps.These findings may also have implications for the study ofancient seep deposits, which are commonly characterized bycarbon isotopic compositions that do not unequivocallyidentify methane as the primary carbon source.

(6) The study of hydrocarbon seepage in the Campeche Knolls areais in its early stages. Future expeditions and improved access todeep-diving submersibles and ROVs will allow us to answermany of the still open questions regarding the formation andbiogeochemical consequences of hydrocarbon seepage in thislittle explored region of the Gulf of Mexico basin.

Acknowledgements

We would like to thank the captain and crew of R/V Sonne, R/VMeteor with ROV Quest, and R/V Justo Sierra for their support duringresearch cruises SO174/2, M67/2, and Chapopote III. We thankMonika Segl (MARUM Bremen) for the carbonate stable isotopeanalyses, M. Zabel (MARUM Bremen) for pore water oxygen isotopedata, Torsten Wilhelm for determining carbonate contents of back-ground core GeoB 10602, and M.J. Formolo (MPI for MarineMicrobiology, Bremen) for his support and comments on quantifyingcarbon sources at hydrocarbon seeps. Naehr would like to thank theHanse Institute for Advanced Study and its staff in Delmenhorst/Germany for their generous support during his 2007/2008 HanseFellowship. Support for the joint U.S.–Mexican–German participationwas provided by the NOAA Office of Ocean Exploration, NSF grant no.OCE-0085548, Texas Research Development Fund, Universidad Nacio-nal Autonoma de Mexico, and the Harte Research Institute. Alsosupported by the U.S. Department of Energy, National EnergyTechnology Laboratory (I.R.M.) and through the DFG-ResearchCenter/Excellence Cluster “The Ocean in the Earth System”, Bremen,Germany (G.B.). Rosalie Shapiro, Michael Formolo, and one anon-ymous reviewer provided comments that greatly improved earlierversions of this manuscript.

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