Geochemical and isotopic signatures of surficial sediments from the western continental shelf of India: Inferring provenance, weathering, and the nature of organic matter

Abstract

Bottom sediments from thirteen transects sampled mainly at depth contours of 30, 100, and 200 m along the western continental margin of India, falling in two offshore sectors from Dwaraka to Goa and Cape Comorin to Goa, were analyzed for calcium carbonate, major elements in silicate fraction, trace and rare earth elements in the bulk fraction, and carbon and nitrogen isotopes in organic matter to understand the provenance of sediments, weathering patterns in the source areas, and nature of organic matter. Major elements such as Si, Ti, Mg, and discrimination plots involving alkalies and silica have shown a distinct north-south provinciality, with the source signatures pointing at Deccan basalt for sediments in the north (from Dwaraka to Goa) and gneisses and granulites of South India in the south. Shale-normalized REE patterns and REE content suggest additional local sources mainly in the southwest coast of India. Degree of alteration determined using Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA), and A-CN-K plots have suggested that the sediments are moderately weathered despite the intense orographic rainfall in the source area. Variability in degree of chemical weathering is seen with the change in provenance. In general, the sediments north of Goa with a provenance of Deccan basalts show a higher chemical weathering index. C sub(org)/N molar ratios are mostly between 10 and 12 (minimum and maximum being 6.9 and 14 respectively), which are closer to values for marine organic matter. This is corroborated by marine delta sup(13)C values (-18.2 to -21.6 ppt). While they broadly fall within the marine values, heavier values are seen in northern transects off Porbandar and Dwaraka, where the sediments have a provenance different from those in the south. This could be due to a higher contribution from C4-type vegetation in the north, consistent with heavier delta sup(13)C values in terrestrial plants in source regions. Heavier delta sup(15)N values are observed for the northern transects (north of Ratnagiri), which is supported by the intense denitrification in the northern Arabian Sea.