Sulfate-oxygen isotope insight into anaerobic methane oxidation in estuarine sediments

Methane is an important greenhouse gas whose production is driven mainly by microbially-mediated methanogenesis. This methane is almost entirely consumed by anaerobic methane oxidation (AOM) coupled bacterial sulfate reduction (BSR). The details of this coupling between AOM and BSR remain enigmatic. This study seeks to use the sulfur and oxygen isotope composition of sulfate (δ18OSO4 and δ34SSO4, respectively) consumed through AOM to further our understanding of the mechanism of this critical microbially-mediated process. We focus on highly stratified estuaries in the coastal area of Israel (the Yarkon and the Qishon). At these sites, sulfate is rapidly consumed and methane concentrations subsequently increase, suggest intensive production of methane deeper within the sediment. The pore fluid sulfate and DIC concentration profiles are similar at all sites. However, the δ34SSO4, δ18OSO4, and δ13CDIC in the pore fluid and the δ34S of the sedimentary pyrite are fundamentally different among the different sites. Because the sulfur and oxygen isotopes in pore fluid sulfate are indicative of the relative intracellular fluxes of sulfur intermediates during BSR, we conclude that the isotope geochemistry require that the mechanism of BSR differs among the studied sites and in different sulfate-methane transition zones. We use a model for the various intracellular steps during BSR to explore what may cause these differences. We conclude that the geochemical interpretation of these sites may under represent the processes occurring in the subsurface as suggested by the isotope data. This suggests that recycling of sulfur intermediates may be fundamentally different in BSR coupled to AOM among estuary sites.