Sr and Nd isotopes in river sediments from the Ganga Basin: sediment provenance and spatial variability in physical erosion

Abstract

Sr and Nd concentrations and isotope compositions in sediment of the Ganga River, from Gangotri to Rajmahal, and its tributaries have been measured to determine provenance and the spatial variability in physical erosion among the Ganga subbasins. Sr and Nd in silicates range from 37 to 138 and from 10 to 36 μg/g, with ⁸⁷Sr/⁸⁶Sr and ε_Nd of 0.7474-0.8428 and -25.5 to -15.5, respectively. The results suggest that >65% of Ganga mainstream sediments are derived from the Higher Himalayan Crystallines highlighting intense physical erosion in this region. The ⁸⁷Sr/⁸⁶Sr values of sediments in the Gangetic plain show nearly identical trends during two seasons, with a sharp and significant decrease at Barauni downstream of Gandak confluence. This brings out the major impact of the sediment contribution from the Gandak to the Ganga mainstream. Model calculation suggests that about half of the Ganga sediment at Rajmahal is sourced from the Gandak. The erosion rates in the Himalayan subbasins of the Ganga range between 0.5 and 6 mm/a (where a is years), with the Gandak having the highest erosion rate. High relief and intense precipitation over the headwater basins of the Gandak appear to drive the rapid and focused erosion of this basin. The results of this study and those in literature suggest that the eastern syntaxis (Brahmaputra), the western syntaxis (Indus), and the Gandak have much higher physical erosion rates than the other Himalayan basins. Focused erosion in the hot spots of these river basins contributes significantly to the global riverine sediment budget and influence regional tectonics.