Anaerobic carbon mineralisation through sulphate reduction in the inner shelf sediments of eastern Arabian sea

Abstract

Monsoon-induced coastal upwelling, land run-off, benthic and atmospheric inputs make the western Indian shelf waters biologically productive that is expected to lead to high rates of mineralisation of organic matter (OM) in the sediments. Dissimilatory sulphate reduction (SR) is a major pathway of OM mineralisation in near-shore marine sediments owing to depletion of other energetically more profitable electron acceptors (O2, NO3^-, Mn and Fe oxides) within few millimetres of the sediment-water interface. We carried out first ever study to quantify SR rates in the inner shelf sediments off Goa (central west coast of India) using the 35S radiotracer technique. The highest rates were recorded in the upper 10 cm of the sediment cores and decreased gradually thereafter below detection. Despite significant SR activity in the upper ~12 to 21 cm at most of the sites, pore water sulphate concentrations generally did not show much variation with depth. The depth integrated SR rate (0.066-0.46 mol m^-2 year^-1) decreased with increasing water depth. Free sulphide was present in low concentrations (0-3 Mu M) in pore waters at shallow stations (depth <30 m). However, high build-up of sulphide (100-600 Mu M) in pore waters was observed at two deeper stations (depths 39 and 48 m), 7-11 cm below the sediment-water interface. The total iron content of the sediment decreased from ~7 to 5 % from the shallowest to the deepest station. The high pyrite content indicates that the shelf sediments act as a sink for sulphide accounting for the low free sulphide levels in pore water. In the moderately organic rich (2-3.5 %) sediments off Goa, the measured SR rates are much lower than those reported from other upwelling areas, especially off Namibia and Peru. The amount of organic carbon remineralised via sulphate reduction was ~0.52 mol m^-2 year^-1. With an estimated average organic carbon accumulation rate of ~5.6 (±0.5) mol m^-2 year^-1, it appears that the bulk of organic matter gets preserved in sediments in the study region.