Thermogenic gas hydrates and hydrocarbon gases in complex chemosynthetic communities, Gulf of Mexico continental slope

Organic Geochemistry 30 (1999) 485±497

Thermogenic gas hydrates and hydrocarbon gases in complex chemosynthetic communities, Gulf of Mexico continental slope Roger Sassen a,*, Samantha Joye b, Stephen T. Sweet a, Debra A. DeFreitas a, Alexei V. Milkov a, Ian R. MacDonald a a

Geochemical and Environmental Research Group, Texas A&M University, College Station, TX 77845, USA b Department of Marine Sciences, University of Georgia, Athens, GA 30506, USA Received 17 November 1998; accepted 16 March 1999 (Returned to author for revision 13 January 1999)

Abstract Where abundant at the sea ¯oor, thermogenic gas hydrates impact bacterially-mediated processes in chemosynthetic communities dependent on methane and H2S. Our main gas hydrate sites are at 0540 m water depth and relatively low temperature (078C). Gas hydrates outcrop as vein-®llings in hemipelagic muds near gas vents within chemosynthetic communities. Molecular and isotopic properties of hydrate-forming C1±C5 hydrocarbons and CO2 provide insight to bacterially-mediated processes. Hydrate-bound methane is altered by bacterial oxidation, as indicated by enrichment of 13C and deuterium (D), and by CO2 depleted in 13C. The degree of gas hydrate alteration appears related to duration of exposure at the sea ¯oor. In hydrate-associated sediments, bacterial oxidation of a mixed pool of hydrocarbons yields a net production of CO2 depleted in 13C. Bacterial oxidation of hydrate-bound methane and free hydrocarbon gases in adjacent sediments could contribute to gas hydrate decomposition. Some thermogenic carbon in sediments could be recycled via methanogenesis to yield a net production of bacterial methane depleted in 13C. Our results strengthen the hypothesis that gas hydrates could favor life in other extreme environments at low temperatures. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Gas hydrate; Hydrocarbons; d 13C and dD of methane; Chemosynthetic communities; Gulf of Mexico

1. Introduction Complex chemosynthetic communities on the Gulf of Mexico continental slope were ®rst discovered during trawling in areas characterized by sediments containing free hydrocarbon gases, gas hydrates, bacterially oxidized oil rich in toxic aromatic hydrocar-

* Corresponding author. Fax: +1-409-862-2361. E-mail address: [email protected] (R. Sassen)

bons, and authigenic carbonate rock at water depths in the 600±700 m range (Kennicutt et al., 1985). These low-temperature complex chemosynthetic communities (tube worms, methanotrophic mussels, clams, and various other fauna) derive energy from reduced carbon, mainly methane, and bacterial H2S. Later research con®rmed that a number of complex chemosynthetic communities were spatially associated with gas hydrate on the Gulf slope (Sassen et al., 1993; MacDonald et al., 1994; Sassen et al., 1994; Sassen and MacDonald, 1994, 1997; Fisher et al., 1998; Sassen et al., 1998).

0146-6380/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 6 - 6 3 8 0 ( 9 9 ) 0 0 0 5 0 - 9

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R. Sassen et al. / Organic Geochemistry 30 (1999) 485±497

Fig. 1. Location map showing study sites and Jolliet Field (Green Canyon protraction area) within a belt of shallow gas hydrates, seeps, chemosynthetic communities, and deep oil and gas accumulations from Garden Banks to Mississippi Canyon.

Because of the enormous global abundance of methane gas hydrates (Kvenvolden, 1993), evidence of a link between gas hydrates and chemosynthetic communities is signi®cant. Deep-sea gas hydrates are ice-like crystalline substances (minerals) in which hydrocarbon and non-hydrocarbon gases of speci®c molecular diameters are held by hydrogen bonding within rigid cages of water molecules. Structure I hydrate (body centered cubic lattice) generally contains methane and other non-hydrocarbon gases formed by bacterial activity at shallow depths in sediment (Sloan, 1990; Kvenvolden, 1993, 1995). In contrast, other gas hydrates contain thermogenic gases that have migrated to shallow sediments from deep hot subsurface hydrocarbon systems. Structure II gas hydrate (diamond lattice) encages C1± C4 hydrocarbons, and structure H hydrate (hexagonal lattice) encages C1±C5 hydrocarbons to include i-C5 (Sloan, 1990). Thermogenic hydrates also contain CO2

and other non-hydrocarbon gases formed in the deep subsurface (Brooks et al., 1986). Gas hydrates with all three crystal structures occur on the Gulf of Mexico continental slope (Kvenvolden, 1995; Booth et al., 1996; Sassen and MacDonald, 1994, 1997). Thermogenic gas hydrates outcrop on the Gulf of Mexico slope sea ¯oor (MacDonald et al., 1994). In contrast to gas hydrate buried at tens or hundreds of meters in sediments (Kvenvolden, 1995), outcropping gas hydrates of the Gulf slope exist in a relatively unstable environment. Shallow gas hydrates are subject to rapid temperature changes from periodic invasions of warm loop current water on the Gulf slope (MacDonald et al., 1994). Rates of gas venting on the Gulf slope ¯uctuate episodically over both long and short time scales because of changes in sea level, and because of dynamic geologic responses to rapid sediment loading (Roberts and Carney, 1997). Fluctuations in equilibrium conditions could a€ect gas

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487

Fig. 2. Sketch of typical gas hydrate mound (01±3 m across) modeled after the Bush Hill site including sampling stations BHHYD-1 (gas hydrate and mound sediment cover) and BHST-2 (nearby tube worm sediments). From photograph in Sassen et al. (1998).

hydrates and chemosynthetic communities (Carney, 1994). Recent research suggested that bacterial alteration of solid gas hydrate occurs via the oxidation of methane (Sassen et al., 1998). Here, we investigate the molecular and isotopic properties of C1±C5 gases and CO2 in the larger gas hydrate environment to provide insight to bacterially-mediated geochemical processes that impact complex chemosynthetic communities. Findings of this study expand the hypothesis that abundant sea-¯oor thermogenic gas hydrates could a€ect the stability of ecosystems in extreme environments at low-temperatures (Carney, 1994; Sassen et al., 1994, 1998). 2. Geologic and geochemical setting Our Green Canyon study area is part of a large belt spanning the Gulf of Mexico slope (Fig. 1) that contains sea-¯oor gas vents, oil seeps, gas hydrates, chemosynthetic communities, and subsurface oil and gas ®elds (Roberts and Aharon, 1994; Sassen et al., 1994; MacDonald et al., 1996). The most recent episode of hydrocarbon venting appears to have initiated across Green Canyon during the late Pleistocene (Aharon et al., 1997). Geochemical correlation of oil and gas from

our Green Canyon study areas o€shore Louisiana establishes a link to a deeply buried Mesozoic hydrocarbon system (6±10 km) that gave rise to shallower (2±3 km) oil and gas accumulations (Kennicutt et al., 1988). Vertical migration of ¯uids in Green Canyon is facilitated by actively moving salt bodies and faults, resulting in rapid ¯uid ¯ow and venting on the sea ¯oor. The Bush Hill study site is a fault-related seep feature in Green Canyon (GC) Block 185 (27847.5 ' N and 91830.5 ' W) at about 540 m water depth (Fig. 1). The faults are related to the subsurface hydrocarbon system that gave rise to nearby Jolliet Field on GC 184 (Reilly et al., 1996). Mean sea-bottom water temperature is 078C (MacDonald et al., 1994), and the pressure is 05400 kPa. Copious streams of thermogenic gas vent continuously to the water column at Bush Hill (MacDonald et al., 1994; Sassen and MacDonald, 1997). Sea-¯oor mounds, 01±3 m across, have gas hydrate outcropping on their ¯anks. Gas hydrate appears to rapidly crystallize during venting as vein ®llings within hemipelagic mud. Vein-®lling gas hydrate is typical in settings with rapid ¯uid ¯ow (Ginsburg and Soloviev, 1994; Ginsburg, 1998). Gas hydrate mounds at Bush Hill have persisted at the same locations for >5 years (MacDonald et al.,

488

R. Sassen et al. / Organic Geochemistry 30 (1999) 485±497

Fig. 3. Exposed structure II gas hydrate at ¯ank of gas hydrate mound at station BHHYD-1, Bush Hill site. Note scale bar.

1994), and are thus likely to be impacted by exposure at the sea ¯oor. Thin layers of hemipelagic mud (010± 30 cm) cap hydrate mounds at Bush Hill. Sediments are typically bioturbated by macro-fauna and contain aromatic-rich bacterially oxidized crude oil, free gas, dispersed gas hydrate nodules, authigenic carbonate rock, H2S, pyrite, and elemental sulfur (Sassen et al., 1993, 1994). Authigenic carbonate depleted in 13C occurs as linings of tubular polychaete burrows which extend down to the gas hydrate surface (Sassen et al., 1998). A complex chemosynthetic community inhabits the Bush Hill site (MacDonald et al., 1989). Speci®cally, white or orange Beggiatoa mats (Larkin et al., 1994) occupy the sediment±water interface on gas hydrate mounds; vestimentiferan tube worms (mainly Lamellibrachia n. sp.) are adjacent to the mounds (Fig. 2). The GC 234 study site (27844.8 ' N and 91813.3 ' W) is a fault-related seep area over shallow salt (Reilly et al., 1996) at 0543 m water depth (Fig. 1). Mean temperatures and pressures are similar to those observed at the Bush Hill site. Gas hydrate vein-®llings out-

cropped on an upthrown fault scarp. When sampled in 1997, the gas hydrate was decomposing, as evidenced by continuous formation and release of free gas bubbles from its surface. Gas hydrate instability resulted in undercutting of the scarp face, and by 1998 the feature was absent. We assume the GC 234 gas hydrate was more recently exposed than gas hydrate at the Bush Hill site, and was a transient feature. Hydrocarbon geochemistry (Brooks et al., 1986, Sassen et al., 1994), and chemosynthetic fauna (MacDonald et al., 1990a), are generally similar to those of Bush Hill. Fisher et al. (1998) observed unidenti®ed polychaete worms (``ice worms'') that clustered directly on outcropping gas hydrate at the GC 234 study site. The brine pool study site (GC 233: 27843.4 ' N and 91816.8 ' W) is an anoxic brine pool at a water depth of about 640 m (Fig. 1). The brine pool arises from ¯uid ¯ow along a salt-related fault (Reilly et al., 1996). The brine pool is rimmed by methanotrophic mussels, whereas tube worms and other chemosynthetic fauna are present in low abundance. Dissolved methane of bacterial origin saturates the brine itself, and free gas

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489

Fig. 4. Exposed and undercut vein-®lling gas hydrate layers at station GCHYD-1, GC 234 site. Note scale bar.

vents from the brine pool to the water column. The hydrocarbon geochemistry of the brine pool is dissimilar to that of the Bush Hill and GC 234 sites in that bacterial methane dominates (MacDonald et al., 1990b). Gas hydrate has not been documented at this site. 3. Samples and methods During the 1995 and 1997 dives of the Johnson SeaLink (JSL) research submersible, the main hydrocarbon pools related to gas venting and abundant gas hydrates within chemosynthetic communities (vent gases, gas hydrates, sediment gases) were sampled. The sampling strategy at Bush Hill and GC 234 was intended to encompass zones of maximum potential impact from abundant gas hydrates. Therefore, the gas-hydrate rich zones of our study may not be representative of the entire expanse of the chemosynthetic communities we studied, nor of other low-temperature communities elsewhere. Each gas hydrate site includes multiple exposures of

gas hydrate, and for this reason, speci®c localities are de®ned for future reference. At the Bush Hill site, samples of vent gases, gas hydrates, and associated sediments were collected at stations identi®ed as BHHYD-1 (Figs. 2 and 3) and BHHYD-2; the GC 234 site samples were collected at a single station identi®ed as GCHYD-1 (Fig. 4). Samples of tube worm sediments were acquired in the vicinity of gas hydrate localities at stations de®ned as BHST-2 (see Fig. 2) and GCAT-2. For purposes of comparison with a setting lacking gas hydrate, samples of venting gas were collected at the GC 233 brine pool at a station identi®ed as BPRN-1 (Fig. 5). We sampled free gas naturally venting into the water column at Bush Hill and GC 234, within 00.5 m of underlying massive gas hydrates at mounds. This intimate association suggests that the vent gas is representative of the hydrocarbon pool from which the gas hydrate crystallized. The mechanical arm of the submersible was used to hold gas-tight Lexan tubes (30 cm in length and 7.5 cm inside diameter) over gas bubble trains until the ambient sea water in the tube was displaced. Gas venting from the GC 233 brine pool was

490

R. Sassen et al. / Organic Geochemistry 30 (1999) 485±497

Fig. 5. Methanotrophic mussel beds at edge of dark methane-saturated brines at station BPRN-1, brine pool site at GC 233. Note scale bar.

collected the same way. Intact vent gas samples were recovered at the sea surface, transferred to vacutainers, and frozen at ÿ208C (Sassen and MacDonald, 1997). Samples of solid gas hydrate were acquired at mound ¯anks at Bush Hill and GC 234 where they outcrop, or where best situated beneath a thin veneer of underconsolidated hemipelagic sediment. The mechanical arm of the research submersible allowed us to collect solid gas hydrate (Sassen and MacDonald, 1997). An inverted steel cup with sharpened edges was used to chip fragments from larger hydrate masses, which ¯oated up into the cup. The cup and gas hydrate were transferred to a pressure vessel containing ambient sea water. After sealing, the pressure vessel was used to transport the gas hydrate samples to the sea surface at approximate in situ pressure and temperature. Discrete gas hydrate was decomposed within the pressure vessel on the surface, and the resultant gas was transferred to vacutainers and frozen at ÿ208C.

Samples of sediments (generally representing 15± 25 cm sections) were collected from crests of gas hydrate mounds (Fig. 2) using Lexan push cores 30 cm in length and 7.5 cm inside diameter. Depth of core penetration was generally controlled by direct contact with the hard upper surface of massive gas hydrate. Sediment samples from the mounds were thus intimately associated with gas hydrate. Other sediment samples were collected near tube worms adjacent to gas hydrate mounds (Fig. 2). Upon recovery, sediment samples were canned with sodium azide bactericide and frozen at ÿ208C. Analyses of gas samples focused on C1±C5 gas chromatography, measurement of d 13C and dD of methane, and measurements of d 13C on C2±C5 hydrocarbons and CO2. Some degassing of sediment core samples occurred during the ascent of the research submersible; C1±C5 gas concentrations reported in Table 2 should be regarded as minimums. The d 13C values are reported as parts per thousand (-) relative to the

Peedee belemnite (PDB) standard (20.2-); the dD values are reported as - relative to the standard mean ocean water (SMOW) standard (2ÿ 5-). C1±C5 hydrocarbon gases were separated using a Hewlett±Packard 5890 gas chromatograph using an activated alumina column (1 m F-1 80/100) temperature programmed from 1008 to 1608C at 328C/min. Known volumes of authentic hydrocarbon standards were used to identify and quantitate peak responses. Concentrations of each hydrocarbon is expressed in ppm by sediment volume, and normalized as percent of total C1±C5 hydrocarbons. Analyses of d 13C of C1± C5 hydrocarbon gases and CO2 were performed at the Houston Area Research Council (HARC; The Woodlands, Texas) using a Varian 3400 gas chromatograph with a Finnigan MAT 252 IRMS. Gas chromatographic separation was performed on a Poraplot Q capillary column (25 m), temperature programmed from 508 to 2108C at 508C/min. Analysis of methane JSL Dive 2874 2875 2892 2637 2637 2637 2891 2881 2881 2876 2860

Sample Vent gasa Vent gasa Gas hydratea Gas hydratea Gas hydratea Gas hydratea Gas hydrate Vent gas Gas hydrate Vent gas Vent gas

Data from Sassen et al. (1998).

Bush Hill Bush Hill Bush Hill Bush Hill Bush Hill Bush Hill Bush Hill GC 234 GC 234 Brine Pool Brine Pool

BHHYD-1 BHHYD-1 BHHYD-1 BHHYD-1 BHHYD-1 BHHYD-1 BHHYD-2 GCHYD-1 GCHYD-1 BPRN-1 BPRN-1

a

Location

Station ÿ 46.0 ÿ 44.1 ÿ 42.9 ÿ 42.2 ÿ 43.5 ÿ 42.5 ÿ 42.9 ÿ 48.7 ÿ 48.4 ÿ 64.3 ÿ 65.5

d 13C C1 ÿ 198 ÿ 200 ÿ 163 ÿ 190 ÿ 177 ÿ 193 ÿ 115 ÿ 203 ÿ 203 ÿ 165 ÿ 200

dD C1 95.9 90.4 83.1 71.7 80.2 72.1 85.7 93.7 73.8 97.8 97.4

% C1 ÿ 29.7 ÿ 30.2 ÿ 28.6 ÿ 29.0 ÿ 29.7 ÿ 29.2 ÿ 28.6 ÿ 29.2 ÿ 29.4

d 13C C2

Fig. 6. Diagrams summarizing molecular (bottom) and isotopic (top) properties of vent gas and gas hydrate samples at the BHHYD-1 and -2 sample stations, Bush Hill.

Table 1 Molecular and isotopic compositions of vent gas and gas hydrate samples

2.4 4.5 7.6 10.6 9.4 12.4 6.3 2.4 12.6 0.1 0.1

% C2

ÿ 25.0 ÿ 26.3 ÿ 24.9 ÿ 25.5 ÿ 25.5 ÿ 25.7 ÿ 25.6 ÿ 26.4 ÿ 26.4

d 13C C3

1.2 3.7 8.1 12.6 7.3 11.4 6.1 2.4 9.9 2.1 2.5

% C3

ÿ 27.9 ÿ 27.2 ÿ 27.6 ÿ 27.9 ÿ 27.8 ÿ 26.8 ÿ 27.9 ÿ 28.4

d 13C i-C4

< 0.1 0.6 0.9 2.6 1.6 2.3 1.1 0.5 1.6 0.0 0.0

% i-C4

ÿ 25.2 ÿ 24.9

ÿ 22.6 ÿ 23.3 ÿ 22.1 ÿ 22.8 ÿ 23.0 ÿ 22.7

d 13C n-C4

0.3 0.6 0.2 1.7 1.2 1.6 0.8 0.6 1.8 0.0 0.0

% n-C4

ÿ 26.5

ÿ 26.2 ÿ 24.8 ÿ 24.7

ÿ 26.1 ÿ 26.1

d 13C i-C5

0.2 0.2 0.0 0.8 0.3 0.3 0.0 0.2 0.2 0.0 0.0

% i-C5

ÿ 25.0

d 13C n-C5

< 0.1 < 0.1 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.0 0.0

% n-C5

ÿ 25.3

+2.9

ÿ 4.9 ÿ 5.4 ÿ 27.8 ÿ 20.0 ÿ 23.7 ÿ 21.6

d 13C CO2

R. Sassen et al. / Organic Geochemistry 30 (1999) 485±497 491

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R. Sassen et al. / Organic Geochemistry 30 (1999) 485±497

dD was performed by Coastal Science Laboratories (Austin, Texas) using an upgraded Micromass 602 mass spectrometer. Gas chromatographic separation was performed using a Porpak Q column (1.5 m).

The molecular and isotopic compositions of two vent gas samples from Bush Hill (Fig. 6) are similar to unaltered hydrocarbon gases in subsurface reservoirs of Jolliet Field, as reported by Kennicutt et al. (1988). Methane is the major component of two vent gas samples from Bush Hill (90.4% and 95.9%); individual C2+ hydrocarbons decrease in relative abundance with increasing carbon number, and n-C5 occurs in trace amounts (<0.1%) (Table 1). The d 13C of methane (ÿ44.1- and ÿ46.0- PDB) varies by 1.9-, and dD shows little variation (ÿ198- and ÿ200- SMOW). The isotopic properties of methane and associated C2± C4 hydrocarbons indicate a thermogenic origin (Sassen and MacDonald, 1997). The d 13C of CO2 from the vent gas samples shows little variation (ÿ4.9- and ÿ5.4- PDB; Table 1). Thermogenic vent gas from GC 234 is similar in composition to the vent gas from Bush Hill (compare Figs. 6 and 7). One sample of GC 234 vent gas has methane (93.7% of the C1±C5 distribution) with a d 13C of ÿ48.7- PDB and a dD of ÿ203- SMOW (Table 1). The molecular distributions and d 13C values of the C2±C5 hydrocarbons are also similar to vent gas from Bush Hill (Table 1). The d 13C of vent gas CO2 is +2.9- PDB (Table 1). Compositional similarities suggest vent gases at both sites are derived from the same Mesozoic subsurface hydrocarbon system. In contrast, vent gas from the brine pool in GC 233 is largely bacterial methane (MacDonald et al., 1990b). Two vent gas samples show high percentages of methane (97.8% and 97.4%); thermogenic C2 and C3 are minor components, and the C4 and C5 hydrocarbons are absent or below detection limits (Table 1). The d 13C values of methane are ÿ64.3- and ÿ65.5PDB, and the dD values are ÿ200- and ÿ165SMOW; the latter gas contains CO2 with a d 13C of ÿ25.3- PDB (Table 1). The isotopic properties of the vent methane are consistent with an origin from reduction of CO2 via bacterial methanogenesis (Jenden and Kaplan, 1986; Coleman et al., 1996).

4. Results and discussion

4.2. Gas hydrate crystallization

4.1. Vent gases

Structure II gas hydrate near the ori®ces of gas vents at the GC 234 site is assumed to have crystallized from the associated vent gas. The change in phase from vent gas to solid structure II gas hydrate involves molecular fractionation favoring gases of appropriate molecular diameter (Sloan, 1990). Methane decreases in abundance relative to vent gas, and C2±C4 hydrateforming gases increase in abundance in structure II gas hydrate (Sloan, 1990). The molecular fractionation during gas hydrate crystallization from vent gas at GC 234 is illustrated in Fig. 7. Relative to vent gas, methane of gas hydrate occurs in lower percentage

Fig. 7. Diagrams summarizing molecular (bottom) and isotopic (top) properties of vent gas and gas hydrate samples at the GCHYD-1 sampling station, GC 234 site.

The bulk of thermogenic hydrocarbon gases and CO2 from the subsurface hydrocarbon system bypass shallow sediments, and exit by venting to the water column. However, a fraction of the vent gas undergoes a rapid phase change (Sassen and MacDonald, 1997) under ambient temperature (078C) and pressure (05400 kPa), and crystallizes as solid gas hydrate. It is important to characterize vent gases because they are the primary starting materials from which other hydrocarbon pools are derived.

ÿ 22.4 ÿ 23.3

0.0 0.0 0.0 0.0 0.0 0.0 0.5 1.0 1.4 ÿ 23.7 ÿ 22.4 ÿ 25.2 ÿ 25.1

1.9 5.7 1.1 0.3 0.0 3.5 7.3 13.4 13.8 ÿ 24.9 ÿ 25.3 ÿ 21.3 ÿ 18.1

ÿ 19.8 ÿ 21.9 ÿ 22.4 ÿ 22.8

3.7 3.4 0.6 1.6 0.0 3.1 2.9 1.0 3.5 ÿ 21.5 ÿ 19.6 ÿ 20.4 ÿ 20.6

ÿ 27.8 ÿ 28.3 ÿ 28.2 ÿ 26.5

3.3 0.7 5.8 1.1 0.0 2.7 1.8 1.8 2.0 ÿ 28.5 ÿ 31.0 ÿ 28.5 ÿ 28.4

ÿ 22.1 ÿ 19.3 ÿ 20.5

13.7 2.4 5.6 4.6 0.0 5.7 0.9 0.7 0.6 ÿ 24.7 ÿ 20.3 ÿ 20.5 ÿ 22.9

16.4 30.7 25.4 17.4 9.2 24.5 1.4 1.3 1.4 ÿ 29.5 ÿ 28.8 ÿ 29.6 ÿ 29.3 ÿ 24.7 ÿ 26.7 ÿ 27.7 ÿ 27.2 61.0 57.1 61.5 71.5 90.8 60.5 85.2 80.8 73.3 ÿ 140 ÿ 134 ÿ 158 ÿ 136 ÿ 125 ÿ 121 ÿ 125 ÿ 119 ÿ 104 ÿ 45.4 ÿ 42.4 ÿ 42.3 ÿ 51.0 ÿ 41.6 ÿ 49.3 ÿ 53.1 ÿ 53.9 ÿ 54.2 Hydrate mound. Tube worm. a

2637 2641 2641 2651 2875 2881 2886 2886 2886 Sedimenta Sedimenta Sedimenta Sedimenta Sedimentb Sedimenta Sedimentb Sedimentb Sedimentb Bush Hill Bush Hill Bush Hill Bush Hill Bush Hill GC 234 GC 234 GC 234 GC 234 BHHYD-1 BHHYD-1 BHHYD-1 BHHYD-1 BHST-2 GCHYD-1 GCAT-2 GCAT-2 GCAT-2

b

d 13C i-C4 % C3 d 13C C3 % C2 d 13C C2 % C1 dD C1 d 13C C1 C1±C5 ppm JSL Dive Sample Location Station

Table 2 Molecular and isotopic compositions of sediment samples

The C1±C5 molecular distributions of hydrocarbons from ®ve samples of structure II gas hydrate from Bush Hill are illustrated in Fig. 6. Methane occurs in lower percentages (71.7±85.7%) than in associated vent gas; the C2, C3, i-C4, and n-C4 hydrocarbons are present in higher percentages (Table 1). Although low percentages of i-C5 are present, n-C5 is absent or below detection limits. Relative to vent gas, hydrate-bound methane (ÿ42.2- to ÿ43.5- PDB) is enriched in 13C by as much as 3.8- (Table 1). The dD of hydrate-bound methane (ÿ115- to ÿ193- SMOW) shows much variation (Fig. 6), and is enriched in D relative to vent gas by as much as (an atypical) 85- (Table 1). Enrichment in 13C and D of hydrate-bound methane are best explained by bacterial oxidation e€ects (Sassen et al., 1998). During bacterial oxidation, 12C and 1H are preferentially used from the methane reactant pool, resulting in enrichment of 13C and D in the residual methane (Coleman et al., 1981). Free-living methanotrophs attach to surfaces; they are able to utilize methane, but are unable to utilize reduced compounds with C-C bonds (Nelson and Fisher, 1995). The d 13C values of C2±C4 hydrocarbons from gas hydrates and from vent gases sampled at the Bush Hill site are nearly identical (Table 1). The lack of oxidation e€ects of hydrate-bound C2±C4 hydrocarbons o€ers strong evidence that methanotrophic activity occurs on the surfaces of the sampled gas hydrate. We also considered an alternate hypothesis. Conceivably, molecular replacement of hydrate-bound hydrocarbons by free hydrocarbon gases of sediments enclosing the gas hydrates could occur. However, in contrast to the unaltered C2±C4 hydrocarbons of gas hydrate samples, speci®c C2±C4 hydrocarbon gases in sediment samples display strong bacterial oxidation e€ects (see below). This lack of correlation excludes

% i-C4

4.3. Gas hydrate alteration

241,808 88,687 107,902 225,079 137,631 138,864 75,249 46,530 12,960

d 13C n-C4

% n-C4

d 13C i-C5

% i-C5

d 13C n-C5

% n-C5

d 13C CO2

(73.8%), and the C2, C3, i-C4, and n-C4 hydrocarbons are present in higher percentages (Table 1). Meaningful carbon isotopic fractionation is not thought to occur during gas hydrate crystallization (Claypool et al., 1985; Brooks et al., 1986; Kennicutt et al., 1988; Sassen and MacDonald, 1997). The d 13C (ÿ48.4- PDB) and dD (ÿ203- SMOW) of hydratebound methane are nearly identical to vent gas, as are the d 13C values of the hydrate-bound C2±C4 hydrocarbons (Table 1; Fig. 7). Measurable isotopic fractionation of carbon or hydrogen is not observed at this site, perhaps because the gas hydrate was only exposed at the sea ¯oor recently. The GC 234 gas hydrate serves as a benchmark in terms of isotopic properties, allowing comparison to other gas hydrates with a longer duration of exposure at the sea ¯oor.

493

ÿ 17.5 ÿ 21.8 ÿ 18.2 ÿ 19.7 ÿ 27.3 ÿ 28.3 ÿ 28.0 ÿ 29.4 ÿ 34.4

R. Sassen et al. / Organic Geochemistry 30 (1999) 485±497

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22.9- (Tables 1 and 2). The depletion in 13C of CO2 is consistent with bacterial oxidation of hydrate-bound methane (Sassen et al., 1998). In contrast, previously analyzed natural structure II and structure H hydrates from the Gulf of Mexico contain CO2 (d 13C=+18.5- to ÿ4.4- PDB) usually assumed to be largely of deep thermogenic origin direct from vent gas (Brooks et al., 1984, 1986; Sassen and MacDonald, 1994). Gas hydrate mounds at Bush Hill have persisted at the same localities for years (MacDonald et al., 1994). Long duration of exposure at the sea ¯oor could explain increased alteration e€ects on gas hydrates at the Bush Hill site relative to the GC 234 site. 4.4. Hydrocarbon alteration in hydrate-mound sediments

Fig. 8. Diagrams summarizing molecular (bottom) and isotopic (top) properties of sediments overlying gas hydrate mounds and from near tube worm colonies at Bush Hill (BHST-2) and GC 234 (GCAT-2) sampling stations.

molecular replacement by sediment gases as a viable hypothesis. Little is known about the surfaces of natural gas hydrates. Gas hydrate crystallized rapidly during sea¯oor experiments at the Bush Hill study site; nucleation and crystallization of dendrites and botryoidal masses of gas hydrate occurred within minutes (Sassen and MacDonald, 1997). Rapid crystal growth is commonly known to increase ¯uid and solid inclusions on growth surfaces, and to increase the frequency of interconnecting ¯aws and ®ssures (Van Hook, 1961). The exterior surface area and part of the internal surface area of rapidly grown gas hydrate crystals thus could be accessible by methanotrophic bacteria. Detailed optical petrologic study of natural gas hydrate thin sections represents a critical area of future research. Relative to vent gas, hydrate-bound CO2 (ÿ20.0to ÿ27.8- PDB) is depleted in 13C by as much as

Concentrations of C1±C5 hydrocarbons of four hydrate-mound sediment samples from Bush Hill are in the 88,687 to 241,808 ppm range (Table 2). The C1± C5 molecular characteristics resemble gas hydrates; speci®cally, hydrate-forming hydrocarbons such as C2, C3, and i-C4 occur in higher concentrations than in vent gas (Fig. 8). The sediment gas could therefore originate, at least in part, by slow or episodic decomposition of the underlying gas hydrate. Methane comprises 57.1±90.8% of C1±C5 distributions. The d 13C of methane from these sediments (ÿ42.3- to ÿ51.0- PDB) shows a wider range than the vent gas (ÿ44.1- and ÿ46.0- PDB) (Tables 1 and 2). Some methane is depleted in 13C relative to vent gas by as much as 6.9- PDB, indicating an in situ component of methane other than from thermogenic venting. Some sediment samples contain both thermogenic vent methane and bacterial methane. Our limited isotopic data do not allow distinction between pathways involving reduction of CO2 or acetate fermentation (Coleman et al., 1996). However, the GC 233 brine pool emphasizes the volumetric importance of bacterial methane production in our Green Canyon study area (Table 2). In addition, bacterial methane gas hydrates are well documented in the study area (Brooks et al., 1986). The mixed methane pool in Bush Hill sediments has been subject to extensive bacterial oxidation. Methane from hydrate-mound sediments is strongly enriched in D (ÿ134- to ÿ158- SMOW) relative to associated vent gas (ÿ198- to ÿ200- SMOW), by as much as 66- (Tables 1 and 2). We could expect to observe enrichment of 13C in residual methane of sediments based on strong enrichment of D in the methane samples. However, this is not observed because of mixing with bacterial gas strongly depleted in 13C. Measurements of d 13C suggest preferential bacterial oxidation of speci®c C2+ hydrate-forming molecules in sediments. This alteration is not observed in associated

R. Sassen et al. / Organic Geochemistry 30 (1999) 485±497

gas hydrates. Although C2 is not greatly a€ected, C3 in hydrate-associated sediments (ÿ24.7- to ÿ20.3PDB) is enriched in 13C relative to vent gases (ÿ25.0to ÿ26.3- PDB) by as much as 6.0- (Tables 1 and 2). Although i-C4 is not greatly a€ected, n-C4 in hydrate associated sediments (ÿ19.6- to ÿ21.5PDB) is enriched in 13C relative to vent gases (ÿ22.6to ÿ23.3- PDB) by as much as 3.7- (Tables 1 and 2). These kinetic isotope e€ects are consistent with preferential bacterial oxidation of straight-chain hydrocarbons (such as C3 and n-C4) in the geologic environment (Winters and Williams, 1969; James and Burns, 1984), and in laboratory simulations (Stahl, 1980). CO2 of hydrate-mound sediments is depleted in 13C relative to thermogenic vent gas by as much as 16.4(Tables 1 and 2). Closed-system oxidation of hydratebound methane could result in greater 13C depletion of hydrate-bound CO2 than observed here (Barker and Fritz, 1981; Alperin et al., 1988). However, the CO2 is likely a mixture of thermogenic vent-gas CO2 and bacterially-derived CO2 (Sassen et al., 1998). Our data are consistent with a net production of CO2 from bacterial hydrocarbon oxidation in a complex natural system. A sample of hydrate-associated sediment from GC 234 (C1±C5=138,864 ppm) shows a similar pattern of bacterial hydrocarbon oxidation and net CO2 production (Table 2). The d 13C of methane is similar to vent gas. However, relative to vent gas from the same site (ÿ203- SMOW), sediment methane is enriched in D (ÿ121- SMOW) by 82- (Tables 1 and 2). The C3 and n-C4 hydrocarbons are enriched in 13C, being consistent with the kinetic isotope e€ects of bacterial oxidation (see Table 2). Relative to vent gas, CO2 is depleted in 13C by 31.2- (Tables 1 and 2). 4.5. Hydrocarbon alteration in tube worm sediments Concentrations of C1±C5 hydrocarbons from three core samples of tube worm sediment from GC 234 are in the 12,960 to 75,249 ppm range, and methane comprises 73.3±85.2% of the total (Table 2 and Fig. 8). Relative to methane of vent gas from the same site (ÿ48.7- PDB), tube worm sediments (ÿ53.1- to ÿ54.2- PDB) are depleted in 13C by as much as 5.5(Tables 1 and 2). The depletion in 13C of methane is consistent with production of bacterial methane in sediments. Relative to vent gas methane (ÿ203SMOW), the sediments are enriched in D (ÿ104- to ÿ125- SMOW) by as much as 99- (Tables 1 and 2). The enrichment in D is best explained by extensive bacterial oxidation of methane. Straight-chain hydrocarbons (particularly C3 and n-C4; see Tables 1 and 2) are enriched in 13C, also consistent with bacterial oxidation e€ects. Relative to vent CO2 (+2.9- PDB), the CO2 from sediment samples (ÿ28- to ÿ34.4-

495

PDB) is depleted in 13C by as much as 37.3- (Tables 1 and 2). A sample of Bush Hill tube worm sediment shows hydrocarbon characteristics consistent with extreme bacterial alteration (Table 2). The concentration of hydrocarbons is 137,631 ppm, but only methane (90.8%) and C2 (9.2%) are signi®cant components; other hydrocarbon gases are absent or below detection limits (Table 2). Relative to vent methane (ÿ44.1and ÿ46.0- PDB), sediment methane (ÿ41.6- PDB) is enriched in 13C by as much as 4.4- (Tables 1 and 2). Similarly, sediment methane (ÿ125- SMOW) is enriched in D by as much as 75- (Tables 1 and 2). Relative to vent C2 (ÿ29.7- and ÿ30.2- PDB), sediment C2 (ÿ24.7- PDB) is enriched in 13C by as much as 5.5- (Tables 1 and 2). Relative to vent CO2 (ÿ4.9- to ÿ5.4- PDB), the CO2 from sediment (ÿ27.3- PDB) is depleted in 13C by as much as 22.4- (Tables 1 and 2). 5. Synthesis and conclusions Our conclusions cannot be extended to all chemosynthetic communities but, where present in abundance, gas hydrates can have a profound localized e€ect on diverse bacterially-mediated geochemical processes in chemosynthetic communities. Alteration of gas hydrates appears related to duration of exposure on the sea ¯oor. Our ®ndings in this regard are novel. Carbon isotopic properties of hydrate-bound methane and CO2 from Bush Hill are consistent with limited bacterial oxidation (Tables 1 and 2). The d 13C of methane from thermogenic vent gas (n=3; mean= ÿ 46.3- PDB) and altered gas hydrate (n=5; mean= ÿ 42.8- PDB) is consistent with bacterial oxidation of hydrate-bound methane. The mean d 13C of methane in hydrate mound sediments (Bush Hill and GC 234: n=5) is = ÿ 46.1- PDB and in tube worm sediments (Bush Hill and GC 234: n=4) is = ÿ 50.7- PDB. It should be emphasized that variability within and between sediment sample sites is large (see Table 2). However, methane from one sample of Bush Hill hydrate mound sediment has a d 13C of ÿ51.0- PDB, and all sediment samples from GC 234 have methane d 13C values that are depleted in 13C relative to thermogenic methane from vents. This suggests that thermogenic vent gas is not the only source of methane in these sediments, and that in situ bacterial methane is also present. The dD of methane provides an indicator of bacterial oxidation of thermogenic gas hydrate and associated gases in sediments (Tables 1 and 2). For example, the trend of dD from thermogenic vent gas (Bush Hill and GC 234: n=3; mean= ÿ 200- SMOW), to altered gas hydrate (Bush Hill: n=5; mean= ÿ 168-

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SMOW), to hydrate mound sediments (n=5; mean= ÿ 138- SMOW), and to tube worm sediments (n=4; mean= ÿ 118- SMOW) shows kinetic isotope e€ects consistent with increasing oxidation of methane. Previous studies have only shown that bacteriallymediated processes occur in sediments associated with buried methane gas hydrates (Cragg et al., 1996; Borowski et al., 1997). The d 13C of CO2 in samples provides another indicator of bacterial oxidation (Tables 1 and 2). The trend of d 13C of CO2 from thermogenic vent gas (Bush Hill and GC 234: n=3; mean= ÿ 2.5- PDB), to altered gas hydrate (Bush Hill: n=4; mean= ÿ 21.1- PDB), to hydrate mound sediments (n=5; mean= ÿ 22.7- PDB), and to tube worm sediments (n=4; mean= ÿ 29.8- PDB) shows increasing depletion of 13C, and is broadly consistent with increasing oxidation e€ects. Factors such as ¯uctuating temperature, pressure, and rates of venting have been noted to potentially impact gas hydrate stability (Roberts and Carney, 1997). Unaltered gas hydrate at GC 234 appears to be recently exposed, but altered gas hydrates at the Bush Hill site experienced longer exposure at the sea ¯oor. Gas hydrates at both sites appear to be decomposing when sampled. We suggest, based on data of the Bush Hill site, that gas hydrate stability could be impacted by (1) bacterial depletion of hydrate-bound methane, and (2) by bacterial depletion of hydrate-forming gases in sediments associated with gas hydrates. For example, one sample of tube worm sediment from the Bush Hill site contains only methane and C2 (Table 2); the gas is thus unlikely to be in equilibrium with structure II gas hydrate because C3 and i-C4 are absent or in low abundance. The occurrence of bacterial methane in this extreme environment dominated by gas hydrates and thermogenic hydrocarbons is signi®cant. We hypothesize that thermogenic carbon from vent methane and other hydrocarbons is partially recycled by methanogenic bacteria, which could potentially favor methanotrophic fauna in other extreme environments at low temperatures and high pressures. Acknowledgements We acknowledge the support provided to Sassen, MacDonald, and Joye through the Minerals Management Service Gulf of Mexico contract 1435-0196-CT-30318. We appreciate the support provided by the JSL team and the NOAA National Undersea Research Center, University of North Carolina at Wilmington during sample collection by research submersible. Fig. 3 is credited to Ian MacDonald, Fig. 4 to Chuck Fisher, and Fig. 5 to Ian MacDonald.

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