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 steady-state situation in a dominantly diffusion controlled system. Pore-water (ΔTA + ΔCa²⁺ + ΔMg²⁺)/ΔSO₄²⁻ ratios and δ¹³C_DIC depth profiles suggest the influence of dual sulfate reduction pathways (organo-clastic and anaerobic methane oxidation) on the sediment fluid chemistry. δ¹³C_CH4 values indicate microbial origin of methane. The lowest δ¹³C_CH4 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.