Geochemistry of shelf sediments off Mulki on the southwestern coast of India and their Palaeoenvironmental significance

A sediment core collected from the shelf (50 m water depth) off the Mulki-Pavanje river mouth has been studied for down-core variations of base metals, magnetic susceptibility (chi (m)) and partitioning of metals in various accumulative phases of sediment, to understand the impact of late Quaternary sea level fluctuations on shelf sedimentation processes. The 192 cm-long core may be demarcated into two zones: the lower (140 to 192 cm) dark green to greenish black clayey silt with abundant plant debris; and the upper (0 to 140 cm) light green to dark green silty sand. Sediments of the lower zone are characterised by higher Cu, Zn, Pb, Co, Mn, Fe, Al, chi (m) and organic matter contents and higher metal/Al ratios, but lower Ca, and Pb/Al and Mn/Al ratios when compared to the upper zone indicating textural control on the geochemistry of sediments. Although Mn is high in the lower zone, the low Mn/Al ratios suggest that a part of Mn has been remobilised under anoxic-sulphidic conditions. Higher Pb/Al ratios in the upper zone may not imply mobilisation of Pb, but may be suggestive of anthropogenic input of this element from the use of leaded petrol in modern times. Geochemical and textural variations of these sediments suggest changes in the depositional environments: Sediments of the lower zone were deposited in a marshy environment when the core site was close to the shore during the late Pleistocene low stand of sea level. The late Quaternary sea level rise led to a shift in the depositional environment - from marsh to a still stand beach environment- when silty sands were deposited. Although the core site lias been in the shelf since the beginning of Holocene, there has not been much sedimentation not only because of the negligible quantity of sediment supplied by the small west-flowing rivers but also due to the rapid rise in sea level during early Holocene. The reconstructed depositional history of the core is in consonance with the recently proposed sea level curve for the western continental margin of India.