Evidence of deep subsurface carbon–sulfur geochemistry in a sediment core from the eastern Arabian Sea

Deep biospheric anaerobic microbial sulfate reduction and oxidative sulfur cycling have been studied in long sediment cores mainly acquired as part of IODP explorations. The most remarkable observation in many of these studies is the existence of an active sulfur cycle in the deep subsurface sediments that have very low organic carbon content and are presumably refractory. Here, we investigate the interstitial sulfate concentrations and sulfur isotope ratios in a 290 m-long sediment core collected from the eastern Arabian Sea at a water depth of 2663 m. Continuous decrease in porewater-sulfate concentrations with depth (up to 75 mbsf) coupled with enrichment in δ³⁴S_SO4 values suggests organoclastic sulfate reduction (OSR) processes attributed to the activity of sulfate-reducing bacteria (SRB) and retention of labile organic substrates amenable to the SRBs. Below a depth of 75 mbsf, the absence of further reduction in sulfate concentration indicates insufficient labile substrate to drive sulfate-reduction activity. An increase in sulfate concentrations at the deeper subsurface (below 128.5 mbsf) coupled with decreasing δ³⁴S_SO4 values may be attributed to the oxidation of Fe-sulfide to sulfate. The increase in porewater alkalinity in the lower part of the core has been linked to the silicate degradation process by CO₂ produced via the dissolution of CaCO₃. Compilation of previous studies from this core, along with our investigation, intrigues future research on organic matter reactivity and microbiological activity in deeper subsurface under oligotrophic depositional regimes.