GEOCHEMISTRY OF SEDIMENTS FROM THE MANGALORE-COCHIN SHELF AND UPPER SLOPE OFF SOUTHWEST INDIA - GEOLOGICAL AND ENVIRONMENTAL-FACTORS CONTROLLING DISPERSAL OF ELEMENTS

The geochemical investigations on 136 surficial sediment samples of the Mangalore-Cochin shelf and upper slope were undertaken to understand the distribution, sources and processes by which various major and trace elements are incorporated into the sediments. Texturally the inner shelf (<50-m water depth) and upper slope (>120-m water depth) are dominantly composed of clayey sediments whereas the outer shelf (50-120-m water depth) is carpeted by relict (∼10,000 yr. in age) sands. The results show that sediment texture largely controls the distribution of elements. The sandy relict sediments of the outer shelf (59 samples) and of the upper slope (restricted coverage) have high contents of SiO2, CaCO3 and Sr amongst which SiO2 is present as terrigenous quartz sand while CaCO3 and Sr, being well correlated with skeletal components, are biogenous. Extremely good positive correlation between Sr and CaCO3 (r=0.77) suggests presence of Sr in the lattices of aragonite which is the main carbonate mineral. The clay-rich sediments of the inner shelf (52 samples) and upper slope (25 samples) are both very rich in organic carbon content and low in SiO2 content and very high in Al2O3, TiO2, Fe2O3, MgO, Na2O, K2O, P2O5, Mn, Cu, Ni, Zn and Li contents as compared to the sandy sediments of the outer shelf. Except for organic carbon, P2O5 and TiO2, strong positive correlations of these components with Al2O3 (r=0.41-0.90) suggest their association mainly with clay minerals. Within the clay minerals, these are present in the crystal lattice either as essential constituent or incorporated by cation exchange or adsorbed onto clay platelets. Organic carbon, P2O5 and TiO2 also show positive correlation with Al2O3 mainly due to their physical association with fine-grained clayey sediments. Phosphate in these sediments is mainly present in association with organic matter and to some extent it also occurs either as ferric phosphate or as authigenic apatite. In general, the geochemistry of these sediments clearly reflects their mineralogy. Formation of Fe- and Mn-hydroxides in the nearshore regions is responsible for scavenging of Ni, Cu and Zn and effectively incorporating them into the inner shelf sediments. These trace elements also show significant correlation with organic carbon (r=0.51-0.56), suggesting that a part of their content is biogenous. © 1990.