Influence of dual sulfate reduction pathways on pore-fluid chemistry and occurrences of methane hydrate in sediment cores (IODP-353) off Mahanadi basin, Bay of Bengal

Abstract

Pore fluid chemistry of sediment cores from two coring sites collected from Mahanadi basin (Bay of Bengal) during IODP-353 was studied to understand the intra-basinal variation in microbially mediated sediment biogeochemistry as well as methane hydrate occurrences. The ionic concentrations of SO42- shows quasi-linear profiles reflecting the steadystate situation in a dominantly diffusion controlled system. Pore-water (?TA + ?Ca2+ + ?Mg2+)/?SO42- ratios and d13CDIC depth profiles suggest the influence of dual sulfate reduction pathways (organo-clastic and anaerobic methane oxidation) on the sediment fluid chemistry. d13CCH4 values indicate microbial origin of methane. The lowest d13CCH4 values in the present study are reported from the sulfate methane transition zone (SMTZ) and attributed to secondary methane generation via microbial recycling of 13C depleted DIC produced through AOM. Based on the drop in Cl- concentrations relative to the ambient sea-water, we predict the occurrence of disperse/pore-filling methane hydrates in the site U1445 proximal to the basin in which the SMTZ is slightly deeper from the surface compared to the distal (northern) U1446 site.