On discrimination between carbonate and silicate inputs to Himalayan rivers

We review new and published analyses of river waters, bedloads and their constituent minerals from the Dhauli Ganga and Alaknanda, headwaters of the Ganges in Garhwal, and the Marsyandi in Nepal and their tributaries. These data are used to discriminate between the inputs of major cations and Sr from silicate and carbonate sources. Methods of estimating the proportion of the carbonate and silicate inputs to river waters using mixing arrays in Sr-Ca-Mg-Na-K 87Sr/86Sr space are shown to suffer from systematic correlations between the magnitude of the precipitation of secondary calcite and the fraction of the silicate component. This results in factor-of two overestimates of the fractions of silicate-derived Ca, Mg and Sr. To correct for this the magnitude of secondary calcite precipitated and relative fractions of silicate and carbonate-derived cations are instead calculated by modeling the displacement of water compositions from the compositions of the carbonate and silicate components of the bedload in subsets of Sr-Ca-Mg-Na-K 87Sr/86Sr space. The compositions of the carbonate and silicate end-members in the bedload are determined by sequential leaching. The results of this modeling are compared with modeling of the modal mineral inputs to waters where mineral compositions are derived from electronmicroprobe analyses of the minerals in the bedload. In the upper Marsyandi catchment, which drains low-grade Tethyan Sedimentary Series formations, a set of mainstem samples collected over a two-year period define tight correlations in Sr-Ca-Mg-Na-K- 87Sr/86Sr space. Modeling of the magnitude of secondary carbonate precipitation and fractions of silicate-derived Ca, Mg and Sr in Sr-Ca-Mg 87Sr/86Sr space gives selfconsistent results that are compatible with both the calculations of mineral modes and published Mg-isotopic compositions, if the ratio of chlorite to biotite weathering is hig