Tracing the Paleo sulfate-methane transition zones and H2S seepage events in marine sediments: An application of C-S-Mo systematics

Microbially mediated anaebic oxidation of methane (AOM) coupled with sulfate consumption within the sulfate methane transition zone (SMTZ) in marine sediments is a widely recorded biogeochemical reaction and has profound influence on the atmospheric CH4 budget, marine carbon cycle and composition of sediment pore fluids. Recognizing the paleo-SMTZs in the marine sediments/rock records can throw light on the variation of paleo-methane fluxes and occurrences of cold seep (H2S + CH4) events through geologic time. Here, we present results from carbonate carbon, pyrite sulfur and molybdenum analyses for two sediment cores overlying the methane hydrate deposits in the Bay of Bengal. The results show intimate association of isotopically depleted carbonate carbon and enriched pyrite sulfur, constraining the paleo SMTZ within the sediment column. In addition, anomalous enrichments of Mo concentrations indicate hydrogen sulfide seepage events. Here, we propose a geochemical tool using C-S-Mo sytematics to decipher the paleo-SMTZs in marine sediments and rocks.