Oxygen Isotopes in porewater sulfate: evidence for unrecognized sulfur cycling

Changes in the major element and related isotope profiles in porewaters of organic-rich sediments suggest that various microbial processes using a succession of electron acceptors are in play during the remineralization of organic matter. Of the electron acceptors, sulfate is by far the most abundant and bacterial sulfate reduction (BSR) is responsible for most organic matter remineralization in sediments. In addition, nearly all the methane produced during methanogenesis below the sulfate minimum zone is oxidized anaerobically through sulfate reduction (anaerobic methane oxidation (AMO)). In places where AMO occurs, recent studies have demonstrated that the majority of the sulfate is reduced by methane. This results in linear diffusive profiles of sulfate concentrations over tens and even hundreds of meters. Oxygen isotopes in marine sulfate (δ18OSO4) from porewater profiles from ODP leg 175 were measured to better understand microbial sulfur cycling and the coupling between sulfate reduction and methane oxidation. In these sites, sulfate concentrations are depleted with depth, mainly through AMO. The δ18OSO4profiles show a rapid increase near the top of all sites from seawater values of 9% to maximums between 22 and 25%. The δ18OSO4 remains enriched and constant (between 22 and 25%) through the rest of the core as sulfate is continually depleted, then decreases at the bottom of the core as sulfate is consumed in the zone of AMO. The δ18OSO4 increase at the top of the cores is difficult to explain without significant rates of sulfate reduction, yet reoxidation rates must approach 100 percent because of the lack of depletion in sulfate concentrations and lack of change in sulfur isotopes. This suggests that sulfate is recycled in the system. The isotopic decrease in δ18OSO4 into the zone of AMO in all cores indicates that isotopically heavier sulfate is preferentially reduced during sulfate reduction associated with methane oxidation.