SULFUR CYCLING IN GAS HYDRATE SYSTEMS OF THE GULF OF MEXICO, PART I: RATES OF SULFATE REDUCTION AND CORRESPONDING PORE-WATER AND SOLID-PHASE SPECIATION

During a recent (June 2002) cruise of the R/V Seward Johnson II, gas hydrate systems on the continental slope off Louisiana were sampled extensively by submersible for detailed analysis of the biogeochemical sulfur cycle. In contrast to control stations away from the ubiquitous oil and gas seeps, the hydrate localities showed abundant hydrogen sulfide, Fe sulfide formation and authigenic carbonate--all reflective of the very high rates of bacterial sulfate reduction. In this study--emphasizing pathways of anaerobic methane oxidation (AMO) coupled to sulfate reduction--we employed S-35 radiotracer and S speciation techniques to assess spatial and down-core variability in rates of bacterial sulfate reduction. Our analytical protocol is designed to be complementary to extensive ongoing microbiological community analysis and S stable isotope characterization.

Preliminary results reveal tremendous lateral variability over very small (sub-meter to meter) scales. Such variation also manifests in the macrofaunal ecosystem driven largely by chemosynthetic pathways and in corresponding distributions of surface mats of sulfide oxidizing bacteria. Extensive sulfide oxidation may be expressed in differences between sulfate reduction rates as measured through the radio-labeled incubations versus those modeled using sulfate concentrations. Down-core profiles for rates of sulfate reduction will delineate key intervals (i.e., subsurface maxima) wherein AMO was most active--in contrast to a near-surface maximum driven perhaps largely by particulate organic matter unrelated to seep activity. Carefully measured rates will also provide at least semi-quantitative estimates of methane fluxes and rates of oxidation, which can be compared to sulfate reduction coupled to non-methane electron donors. In this presentation we are emphasizing the distributions of bacterial rates of sulfate reduction and the transient and preservable solid and dissolved species related to S cycling. Iron sulfides will also provide a rate-calibrated, long-term (geologic) constraint on spatial and temporal distributions of gas hydrates and related AMO.