SULFUR CYCLING IN GAS HYDRATE SYSTEMS OF THE GULF OF MEXICO, PART II: SULFUR AND OXYGEN ISOTOPE RELATIONSHIPS

A recent (June 2002) interdisciplinary research cruise in the Gulf of Mexico focused on the gas hydrate biogeochemistry of oil and gas seeps off the coast of Louisiana. As a complement to ongoing evaluations of the microbial ecology of anaerobic methane oxidation (AMO) and coupled sulfate reduction, we focused on isotopic variability for a wide array of solid-phase and aqueous sulfur species. Specifically, isotope profiles will be presented for dissolved sulfate (S and O) and sulfide (S) from the upper 20 to 30 cm of sediment across spatial gradients spanning from hydrate outcrops to control stations where sediments were not exposed to high fluxes of methane. Solid-phase species of interest are elemental S, Fe monosulfide and disulfide, extractable and kerogen organic S and sulfate trapped within the abundant authigenic carbonate. This "carbonate-associated sulfate" (CAS) should record the evolving S isotope environment coupled to the extreme alkalinity production of AMO-based sulfate reduction. Despite extensive recent work, comprehensive, multi-component S and O isotope studies of gas hydrate systems are lacking. Our results, particularly oxygen isotope ratios in dissolved sulfate, should also speak to the abundant sulfide oxidation that drives much of the associated micro- and macrofaunal ecology.

This S isotope mass balance is required for our quantification of AMO-related S cycling in the Gulf of Mexico. Furthermore, despite extreme lateral variability, the high rates of associated sulfate reduction and Fe sulfide formation should provide S isotope signatures that are diagnostic relative to analogous depositional settings in the geologic record that lacked gas hydrates and high methane fluxes. These isotope fingerprints with high preservation potential as pyrite, CAS and kerogen S, in combination with organic biomarkers, will provide a framework for recognition and quantification of ancient gas hydrate systems where unambiguous constraints are often lacking. Because our S isotope data are calibrated against measured rates of sulfate reduction and corresponding model estimates of methane oxidation, such rates can by inferred for ancient methane-rich deposits--particularly when sediment accumulation rates, Fe availability and deficiencies in particulate organic matter were similar