Geochemistry of sediments from the Mangalore-Cochin shelf and upper slope off southwest India: Geological and environmental factors controlling dispersal of elements

Abstract

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 SiO 2, CaCO 3 and Sr amongst which SiO 2 is present as terrigenous quartz sand while CaCO 3 and Sr, being well correlated with skeletal components, are biogenous. Extremely good positive correlation between Sr and CaCO 3 ( 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 SiO 2 content and very high in Al 2O 3, TiO 2, Fe 2O 3, MgO, Na 2O, K 2O, P 2O 5, Mn, Cu, Ni, Zn and Li contents as compared to the sandy sediments of the outer shelf. Except for organic carbon, P 2O 5 and TiO 2, strong positive correlations of these components with Al 2O 3 ( 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, P 2O 5 and TiO 2 also show positive correlation with Al 2O 3 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.