Clay minerals and Sr-Nd isotopes of the sediments along the Western margin of India and their implication for sediment provenance

Abstract

Mineralogy and Sr-Nd isotopic composition of the <2-μm fraction of the Holocene and late Pleistocene sediments from 12 gravity cores collected along the western margin of India were used to delineate the provenance and understand the response of isotopes with respect to the source rock composition. Holocene sediments show distinct clay mineral associations representing three major sources: illite, chlorite-rich sediments from the Indus, smectite-dominated sediments from the Deccan Traps, and kaolinite and smectite-enriched sediments from gneissic rocks. The mixing of Indus-derived sediment with the Deccan Traps-derived sediment or the Deccan Traps-derived sediment with gneissic rock-derived sediment occurs in the deeper water cores. Sr isotopes are very sensitive to weathering and laterization. Weathering does not influence Nd isotopes. The ⁸⁷Sr/⁸⁶Sr ratios of the Holocene sediments in shallow water cores are higher and fluctuated widely than those in deeper water cores. ε_Nd correlates well with the clay mineral associations, indicating source rock composition or mixing of source sediments. Kaolinite-enriched sediments coupled with high Sr ratios and low ε_Nd on the SW margin of India suggest intense chemical weathering in the hinterland. The influence of the sediment from the Ganges-Brahmaputra, transported by the Bay of Bengal waters during the NE monsoon surface current or SW monsoon underwater current, on the composition of the sediments of the SW Indian margin is insignificant. The late Pleistocene sediments reflect a decrease of hydrolytic processes resulting in decrease of smectite and Sr isotopic ratio. Clay minerals and ε_Nd values suggest that the provenance and transport pathways of the late Pleistocene sediments are the same as those of the Holocene for each region.