Clay minerals and organic matter in shelf sediments off Coromandel Coast of India: Implications for provenance, transportation and depositional processes

Abstract

Surface sediments collected from 25 stations in the Bay of Bengal were analyzed for grain size, organic matter and clay minerals to understand the spatial distribution, transport pathways, depositional process and provenance of the marine sediments. Granulometric analysis reveals the decreasing trend of grain size with increasing depth from coast to offshore region. The predominance of sandy sediments at shallower region represents their terrigenous origin and prevalence of high energy condition. The sediments from 50-100 m water depth are found to contain higher concentration of organic matter (TOC, TN and Sulfur) suggesting the high productivity zone. The C/N value ranges from 4 to 8 in 52% of sediments which indicates the source of both marine and terrestrial inputs. And 36% of sediment having C/N ratio <4 denotes that they are mainly derived from a marine source of phytoplankton and zooplankton remain and faeces while 12% of sediments C/N ratio ranges from 8 to 14 which suggests terrestrial organic matter input through local riverine systems. The C/S ratio of samples varies from 0.57 to 4.69 with mean value of 2.47 indicates the prevalence of semi-anoxic to anoxic marine conditions. The positive correlation of mud with TOC and sulfur denotes that their distribution is mainly controlled by finer fractions of sediments. Illite, Kaolinite and Smectite are the dominant clay minerals in the order of abundance occurring in the study area, which indicates the terrestrial sources. High amount of illite and kaolinte in the study area is influenced by Pennar River which drains through Archean granites, charnockites, khondalites suites of rock formations and Nellore pegmatites. The abundance of smectite in the study area is mainly from Deccan basalts by Krishna River and also from Cuddapah basin by Pennar River. In the study area, the clay minerals, organic carbon, nitrogen and sulfur distribution in surface sediments are chiefly controlled by the provenance and currents.