Coupled C-S-Fe geochemistry in a rapidly accumulating marine sedimentary system: Diagenetic and depositional implications

Abstract

In the present study, we have investigated the C-S-Fe systematics in a sediment core (MD161-13) from the Krishna-Godavari (K-G) basin, Bay of Bengal. The core covers the late Holocene period with high overall sedimentation rate of 573 cm kyr-1. Pore fluid chemical analyses indicate that the depth of the present sulfate methane transition zone (SMTZ) is at 6 mbsf. The (ΔTA + ΔCa+ΔMg)/ ΔSO42- ratios suggest that both organoclastic degradation and anaerobic oxidation of methane (AOM) drive sulfate reduction at the study site. The positive correlation between total organic carbon content (TOC) and chromium reducible sulfur (CRS) content indicates marked influence of organoclastic sulfate reduction on sulfidization. Coupled occurrence of 34S-enriched iron sulfide (pyrite) with 12C-enriched authigenic carbonate zones is the possible records of paleo-sulfate methane transition zones where AOM-driven-focused sulfate reduction was likely fueled by sustained high methane flux from underlying gas-rich zone. Aluminum normalized poorly reactive iron (FePR/Al) and La/Yb ratios suggest increasing contribution from Deccan basalts relative to that of Archean-Proterozoic granitic complex in sediment flux of Krishna-Godavari basin during the last 4 kyr.