THE BIOGEOCHEMICAL CYCLING OF SULFUR, CARBON, AND OXYGEN AT GULF OF MEXICO COLD SEEP ENVIRONMENTS

Methane hydrate deposits are a major sink for reduced carbon. In fact, their total mass exceeds all other organic carbon reservoirs with the exception of carbon dispersed in the lithosphere. A refined understanding of the biogeochemical cycling in these systems is fundamental to understanding present and past environments. Cold seeps in the Gulf of Mexico provide a natural laboratory to study the micro- and macrofaunal cycling of sulfur, carbon, and oxygen in these systems. Of particular interest are the coupled metabolic pathways between sulfate reduction and anaerobic oxidation of methane (AOM) and other liquid and gaseous hydrocarbons. Our in-depth study of sulfur emphasizes a comprehensive speciation of solid and dissolved phases, including concentration and isotopic determinations for acid volatile sulfur, pyrite, elemental sulfur, dissolved sulfate and sulfide and plans for carbonate-associated sulfate. We have also measured sulfate reduction rates using S-35 and are continuing to address the carbon and oxygen isotope relationships expressed in the authigenic calcium carbonate that pervades these systems. Specifically, a mass balance of the carbon isotope data has allowed us to delineate the relative contributions from the various carbon substrates, including methane and other hydrocarbons. Many of the preliminary data show the dominant substrates to be hydrocarbons other than methane.

When combined, these individual geochemical proxies elucidate the essential geochemical cycles and their relationships to complex micro- and macrofaunal communities. Our results indicate extreme heterogeneity over narrow spatial and temporal scales. In addition, we have measured diagnostic enrichments in the concentrations of Fe sulfide and authigenic carbonate. These enrichments and their carbon and sulfur isotope expressions provide fingerprints for recognition of similar systems in the past. Furthermore, our concentration and isotopic data, when calibrated against the rate measurements, provide a framework for quantifying levels of AOM at ancient cold seeps.