Chemical erosion rates of river basins of the Ganga system in the Himalaya: reanalysis based on inversion of dissolved major ions, Sr, and ⁸⁷Sr/⁸⁶Sr

Abstract

Available data on chemical and Sr isotopic composition of the headwaters of the Ganga and its tributaries in the Himalaya are analyzed using inverse and the forward models to quantify the silicate and carbonate erosion rates in their basins and to parameterize the controlling factors of contemporary chemical erosion. The estimates of silicate-derived cations based on the inverse model are lower on average by ∼20% than that derived from the forward model, a result attributable to differences in the release ratios of major ions between the two models. The inverse model yields the best fit values for the release ratio of various major ions based on measured river chemistry and therefore is more likely to be representative of the basins, unlike the forward model, in which the release ratios are assumed. Among the various tributaries, the Kosi and the Bhagirathi have relatively larger contributions from silicate weathering to the cation budget with high ⁸⁷Sr/⁸⁶Sr, consistent with the lithologies of their basins, which are dominated by silicates. The Ganga river system in the Himalaya is known to be supersaturated with respect to calcite during lean flow periods. The inverse model calculates the extent of Ca loss to be between ∼10%–40% with the highest for the Gandak. The silicate and carbonate erosion rates for these Himalayan rivers are relatively higher, in the ranges of 9.9–17.6 and 12.4–92.3 tons/km²/yr, respectively, compared to other global rivers. The higher chemical erosion rates in these river basins could be due to intense physical erosion caused by their higher relief and runoff.