Sr Fluxes from the Himalayas: Calculation of Carbonate and Silicate Inputs

The marked increase in seawater ⁸⁷Sr/⁸⁶Sr ratio since 40 Ma suggests that enhanced physical erosion associated with the Himalayan-Tibetan orogeny impacted on global chemical weathering fluxes. Whether this reflects weathering of high ⁸⁷Sr/⁸⁶Sr ratio silicates, high ⁸⁷Sr/⁸⁶Sr ratio carbonates or increased silicate chemical weathering is controversial. To resolve this we need to determine the proportions of Sr and ⁸⁷Sr which are derived from carbonate and silicate minerals. Here we present a new method for doing this which uses arrays of tributary compositions to define the cation ratios of silicate and carbonate inputs. This reveals the importance of incongruent precipitation and dissolution reactions in controlling water chemistry. The use of end-members defined by the water compositions to calculate Sr sources (but not the partition of major cations) is robust against many of the processes which perturb water compositions. The method is applied to the headwaters and flood plain of the Ganges. In the headwaters we estimate that 50% of the Sr flux and 57 ± 14% of the "excess" ⁸⁷Sr flux, which forces changes in seawater Sr-isotopic composition, is derived from silicate. In the flood plain 33 ± 5% of the Sr flux is silicate derived. Overall the 55% of the "excess" ⁸⁷Sr flux from the Ganges is silicate-derived. About 60% of the impact of S.E. Asian rivers on seawater ⁸⁷Sr/⁸⁶Sr is due to high ⁸⁷Sr/⁸⁶Sr ratio silicate or carbonate sources, the remaining ~ 40% being due to the higher than average silicate Sr fluxes.