Organic Carbon in Sediments of the Southwestern Margin of India: Influence of Productivity and Monsoon Variability During the Late Quaternary

The texture, organic carbon (OC), CaCO₃ and Rock-Eval parameters of the sediments from two gravity cores collected at depths below the oxygen minimum zone (OMZ) of the southwestern margin of India are presented and compared the results with those within the OMZ. Clayey silt/silty clays are the characteristic sediments. The OC in the core top sediments between Cape Comorin and Mangalore is higher below the OMZ than those from the OMZ. However, it is higher within the OMZ than those below the OMZ in the sediments between Mangalore and Goa. The down-Core variations of OC are identical in these cores. In both the cores, relatively high OC content and low sedimentation rates correspond to the intervals of late Holocene and Last Glacial Maximum (LGM) and, low OC and high sedimentation rates to the early Holocene sediments. The CaCO₃ follows sand content in a core off Cape Comorin, with low values at ,the core top, increase marginally in the early Holocene and LGM and then decrease in the late Pleistocene sediments. The CaCO₃ values in a core off Mangalore are higher in the intervals of the late Holocene and early deglaciation than in early Holocene and LGM intervals. Rock-Eval parameters distinguish the sources of organic matter only at high OC concentrations.

The high OC during the LGM may be related to the productivity, associated with convective mixing occurring during the NE monsoon. The low OC/CaCO₃ and high clay content during the early Holocene may be the consequences of the intensified SW monsoon that results in stronger near-Surface stratification leading to low productivity. High OC and low CaCO₃ during the late Holocene suggest increased productivity and early diagenesis in the near surface sediments. We suggest that the variations in productivity and downslope transport of sediment controlled the OC enrichment.