Ocean Drilling Program, United States

High-resolution analyses of the oxygen isotope ratio ( (super 18) O/ (super 16) O) of dissolved sulfate in pore waters have been made to depths of >400 meters below seafloor (mbsf) at open-ocean and upwelling sites in the eastern equatorial Pacific Ocean. delta (super 18) O values of dissolved sulfate (delta (super 18) O-SO (sub 4) ) at the organic-poor open-ocean Site 1231 gave compositions close to modern seawater (+9.5% vs. Vienna-standard mean ocean water, providing no chemical or isotopic evidence for microbial sulfate reduction (MSR). In contrast, the maximum delta (super 18) O values at Sites 1225 and 1226, which contain higher organic matter contents, are +20% and +28%, respectively. Depth-correlative trends of increasing delta (super 18) O-SO (sub 4) , alkalinity, and ammonium and the presence of sulfide indicate significant oxidation of sedimentary organic matter by sulfate-reducing microbial populations at these sites. Although sulfate concentration profiles at Sites 1225 and 1231 both show similarly flat trends without significant net MSR, delta (super 18) O-SO (sub 4) values at Site 1225 reveal the presence of significant microbial sulfur-cycling activity, which contrasts to Site 1231. This activity may include contributions from several processes, including enzyme catalyzed equilibration between oxygen in sulfate and water superimposed upon bacterial sulfate reduction, which would tend to shift delta (super 18) O-SO (sub 4) toward higher values than MSR alone, and sulfide oxidation, possibly coupled to reduction of Fe and Mn oxides and/or bacterial disproportionation of sulfur intermediates. Large isotope enrichment factors observed at Sites 1225 and 1226 (epsilon values between 42% and 79%) likely reflect concurrent processes of kinetic isotope fractionation, equilibrium fractionation between sulfate and water, and sulfide oxidation at low rates of sulfate reduction. The oxygen isotope ratio of dissolved pore water sulfate is a powerful tool for tracing microbial activity and sulfur cycling by the deep biosphere of deep-sea sediments.