Elemental geochemistry and Sr–Nd isotopic fingerprinting of sediments in monsoon dominated river systems along the west coast of India.

Abstract:

Strontium-Neodymium isotopes have been widely utilized as provenance tracers of major global river basins. Using HR-ICPMS (elements) and MC-ICPMS (isotopes), we measured a suite of trace elements, ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotopic ratios from silicate (Si) and carbonate (CO₃^2-) fractions of 37 bottom sediment samples during 3 seasons along a transect from the estuary upstream in Narmada (ND) and Netravati (NT), the two large rivers along the west coast of India, discharge into the Arabian Sea.

Strontium isotopic values ranged from 0.70910 to 0.71778 (NR CO₃^2-), 0.71014 to 0.72427 (NT CO₃^2-) and 0.71085 to 0.72825 (NR Si), and 0.71025 to 0.73349 (NT Si). The εNd(0) values ranged from -22.1 to -0.8 (NR CO₃^2-), and -32.2 to -24.3 (NT, CO₃^2-), and -23.7 to -6.2 (NR Si) and -37.4 to -21.2 (NT Si). Variable extents of seasonal and spatial variations on the isotopic compositions of Sr and Nd (both in CO₃^2- and Si fractions), were observed in both rivers.

A comparison of the ‘model age’ of the sediments calculated from the time-depleted-mantle (T_DM) model using the concentrations of Nd and Sm and isotopic composition of Nd indicate the following: i) For NT, the T_DM–based ‘model age’, 2.8 to 3.3 Ga, agrees with the age of Peninsular Gneiss, an older gneissic complex (3.2-3.4 Ga) found in this watershed; ii) For NR, there is discordance between the T_DM–based ‘model age’ (1.3-2.4 Ga) and the age of the rocks in this watershed (Deccan basalt, ~65 Ma). We attribute this discordance to mixing of older material from the Precambrian rocks derived by weathering at the head waters of NR. From the observed differences in the Sr isotopes in the main stream and tributaries, we estimate that ~90% of the sediments are derived from main stream and ~10% is derived from tributaries using binary mixing equation in NT and additional sediment source(s) in NR. The Sr isotopic signatures of carbonate fractions in both rivers are close to the global average modern seawater (0.7092) and river water (0.7160) values, suggesting dominant authigenic origin representing the isotopic signatures of the dissolved phase of these rivers. Among the two rivers studied, the NR seems to be dominated by carbonate weathering and whereas silicate weathering dominates the NT. The contrasting differences between these two river systems serve as model for other large and small Indian rivers.