The anaerobic oxidation of methane and sulfate reduction in sediments from Gulf of Mexico cold seeps

Abstract

We determined the geochemical characteristics of sediments and measured rates of the anaerobic oxidation of methane (AOM) and sulfate reduction (SR) in samples collected near thermogenic (structure II) gas hydrate mounds and in areas lacking hydrates along the continental slope in the Gulf of Mexico. We used radiotracer (C-14 and S-35) techniques to determine rates of AOM and SR over depth in sediment cores. Abundant mats of white and orange Beggiatoa spp. were common in areas of active seepage and these sediments were enriched in hydrogen sulfide and methane. In cores collected from areas without Beggiatoa or hydrate, concentrations of redox metabolites showed little variation over depth and these sites were inferred to be areas of low seepage. Integrated AOM rates were low in Beggiatoa-free cores (<0.05 mmol m⁻² day⁻¹) and averaged 2.8±4.6 mmol m⁻² day⁻¹ in seep cores that contained Beggiatoa or gas hydrate. Integrated SR rates were also low in Beggiatoa-free cores (<1 mmol m⁻² day⁻¹) and averaged 54±94 mmol m⁻² day⁻¹ in cores with Beggiatoa or hydrate. Rates of SR generally exceeded rates of AOM and the two processes were loosely coupled, suggesting that the majority of SR at Gulf of Mexico hydrocarbon seep sites is likely fueled by the oxidation of other organic matter, possibly other hydrocarbons and oil, rather than by AOM.

Introduction

Gas hydrates are solid ice-like structures, made up of gas (predominantly methane) encaged inside ice crystals. They are stable at low temperatures (<10 °C) and high pressures (≥5 MPa). Gas hydrates represent an important and dynamic organic carbon reservoir on the Earth and the amount of methane in gas hydrates probably exceeds reserves of conventional oil and gas (Collett and Kuuskraa, 1998). Most of the gas hydrates on the Earth occur in the oceans, and most of the hydrates in the ocean occur along continental margins (Kvenvolden, 1988). Methane seeps and associated gas hydrates occur along both active (e.g., the Cascadia margin, Suess et al., 1999; the Nankai trough Ashi et al., 2002, Tsunogai et al., 2002; the Costa Rican margin, MacDonald et al., 1996; Kahn et al., 1996; the Peru-Chile triple junction, Brown et al., 1996) and passive (e.g., the Blake Plateau, Dickens et al., 1997; the Gulf of Mexico, Kennicutt et al., 1985) continental margins.

The sediments in the northern Gulf of Mexico overlie enormous reservoirs of liquid and gaseous hydrocarbons that rest upon Jurassic-age salt deposits Kennicutt et al., 1988a, Kennicutt et al., 1988b, Roberts et al., 1999. Salt-driven tectonics creates fault networks that serve as conduits for the rapid transfer of oil, gas and brines from deep reservoirs through the overlying sediments and ultimately into the water column Kennicutt et al., 1988a, Kennicutt et al., 1988b, Aharon et al., 1992, Roberts and Carney, 1997. On the seafloor, such conduits give rise to gas vents and seeps, subsurface and sediment surface-breaching gas hydrates, brine pools, and mud volcanoes Aharon, 1994, Sassen et al., 1994. Gulf of Mexico gas hydrates are structure II hydrates, containing methane (44%), ethane (11%), propane (32%), iso-butane (9.5%), butane (3%) and pentane (0.5%) Sassen et al., 1998, Orcutt et al., 2004. Sediments in and around areas of active seepage are characterized by elevated concentrations of simple (C₁–C₅) and complex (oils) hydrocarbons and hydrogen sulfide (H₂S).

Complex chemosynthetic communities comprised of a variety of microorganisms and bacteria-metazoan symbioses thrive around hydrocarbon seeps in the Gulf of Mexico Kennicutt et al., 1985, MacDonald et al., 1989, MacDonald et al., 1990, MacDonald et al., 1996, Fisher, 1990, Ferrell and Aharon, 1994, Larkin et al., 1994. These communities proliferate in a cold, high-pressure environment by exploiting the abundance of energy-rich reduced substrates, such as methane and H₂S. While the diversity and distribution of seep macrofauna has been the focus of intense study, the activity of free-living bacteria in seep sediments and around gas hydrates has received little attention. This lack of information is surprising given that microbial activity may impact the flux and composition of both liquid and gaseous hydrocarbons and oils as they transit the seep ecosystem Kennicutt et al., 1988a, Kennicutt et al., 1988b, Sassen et al., 1998 and may even be responsible for the formation of seep deposits, such as carbonate reefs, chimneys, and mounds Ferrell and Aharon, 1994, Suess et al., 1999, Michaelis et al., 2002.

Because of their occurrence along continental margins, the stability of gas hydrate reservoirs is sensitive to changes in global climate that could increase water temperatures or decrease sea level, thereby altering the hydrate stability field Paull et al., 1991, Kvenvolden, 1993. As such, gas hydrates represent one of the most dynamic organic carbon reservoirs on Earth. Gas hydrate dissociation has been correlated with significant variations in global climate Dickens et al., 1995, Katz et al., 1999, Norris and Röhl, 1999, Kennett et al., 2000, Hesselbo et al., 2000, suggesting that periodic pulses of hydrate-derived methane to the atmosphere contributed to past increases in global temperatures and changes in global carbon fluxes Hesselbo et al., 2000, Kennett et al., 2000. Hence, methane oxidation in sediments near gas hydrate deposits represents a globally important biological sink for hydrate-derived methane. Understanding linkages and feedbacks between gas hydrates and global climate on a large scale requires a firm understanding of microbial methane cycling in hydrate ecosystems on a small scale.

The anaerobic oxidation of methane (AOM) and sulfate reduction (SR) are dominant microbial processes in methane-rich sediments (Hinrichs and Boetius, 2002). In marine sediments, rate measurements of SR and AOM and modeling of pore water geochemical parameters suggest that most (if not all) of the upward CH₄ flux is oxidized anaerobically near the sulfate–methane interface Reeburgh, 1976, Devol et al., 1984, Iversen and Jørgensen, 1985. Syntrophic coupling between methane oxidizing and sulfate reducing microorganisms supposedly mediates AOM Hoehler et al., 1994, Hoehler et al., 2001, Hoehler and Alperin, 1996. Organic geochemical biomarker and molecular biological data from marine sediments have provided further evidence that a syntrophic consortium of sulfate reducing bacteria and methanotrophic archaea mediate AOM Hinrichs et al., 1999, Boetius et al., 2000, Orphan et al., 2001a, Orphan et al., 2001b, Michaelis et al., 2002. Syntrophic metabolism as the mechanism of AOM infers that the processes of AOM and SR are closely coupled. Though multiple putative methanotrophic archaea and SO₄²⁻-reducing bacterial (SRB) partner organisms have been identified in several environments Hinrichs et al., 1999, Orphan et al., 2001a, Orphan et al., 2001b, Michaelis et al., 2002, the biochemical mechanism of the process remains unknown.

Documenting patterns of sediment biogeochemistry and microbial CH₄ processing is important because rising global temperatures could destabilize gas hydrates in shallow basins like the Gulf of Mexico where surface breaching structure II gas hydrates are abundant. Methane oxidation in the sediments and in the water column represents a sink for methane derived from hydrate dissociation and provides a mechanism to reduce the flux of hydrate-derived methane to the atmosphere (Hinrichs and Boetius, 2002). Investigating rates and controls on methane oxidation in sediments associated with gas hydrates represents a unique opportunity to elucidate how biological oxidation influences the cycling and fate of methane in these unique habitats. Here, we present data describing the biogeochemical signature of seep sediments and the first contemporaneous measurements of the anaerobic oxidation of methane (AOM) and sulfate reduction (SR) in sediments influenced by both complex hydrocarbon and oil seepage and the presence of gas hydrates.