S and O Isotope Studies of Microbial S Cycling in the Deep Biosphere of Marine Sediments: Eastern Equatorial Pacific Ocean

We have determined the oxygen (¹⁸O/¹⁶O) and sulfur (³⁴S/³²S) isotope ratios of porewater sulfate to depths of over 400 mbsf in sediments from open-ocean and upwelling sites in the Eastern Equatorial Pacific ocean. Sulfate δ ¹⁸O ranges from near-normal seawater values (9.5 permil) at organic-poor open-ocean sites, to approximately 30 permil at sites with higher organic matter content and higher associated microbial activity. Depth-correlative trends of δ ¹⁸O, δ ³⁴S, alkalinity, methane, ammonium and the presence of sulfide, indicate significant oxidation of sedimentary organic matter by sulfate-reducing microbial populations as well as anaerobic oxidation of methane. δ ¹⁸O-SO₄ values at low-activity sites reveal the presence of significant microbial sulfur-cycling activity despite relatively flat sulfate concentration and δ ³⁴S profiles. This activity may include contributions from several processes including: enzyme-catalyzed equilibration between oxygen in sulfate and water superimposed upon microbial sulfate reduction, sulfide oxidation, and bacterial disproportionation of sulfur intermediates. Large isotope enrichment factors observed at low-activity sites (40-80 permil) likely reflect concurrent processes of: kinetic isotope fractionation, equilibrium fractionation between sulfate and water, and sulfide oxidation at low rates of sulfate reduction. Results of this study indicate that coupled measurements of S and O isotope ratios of porewater sulfate are a powerful tool for tracing microbial activity and sulfur cycling in marine sediments.