Sr and <SUP>87</SUP>Sr/<SUP>86</SUP>Sr in the Yamuna River System in the Himalaya: sources, fluxes, and controls on Sr isotope composition

Sr and 87Sr/86Sr have been measured in the Yamuna river headwaters and many of its tributaries (YRS) in the Himalaya. These results, with those available for major ions in YRS rivers and in various lithologies of their basin, have been used to determine their contributions to riverine Sr and its isotopic budget. Sr in the YRS ranges from 120 to 13,400 nM, and 87Sr/86Sr from 0.7142 to 0.7932. Streams in the upper reaches, draining predominantly silicates, have low Sr and high 87Sr/86Sr whereas those draining the lower reaches exhibit the opposite resulting from differences in drainage lithology. 87Sr/86Sr shows significant co-variation with SiO2/TDS and (Na* + K)/TZ+ (indices of silicate weathering) in YRS waters, suggesting the dominant role of silicate weathering in contributing to high radiogenic Sr. This is also consistent with the observation that streams draining largely silicate terrains have the highest 87Sr/86Sr, analogous to that reported for the Ganga headwaters. Evaluation of the significance of other sources such as calc-silicates and trace calcites in regulating Sr budget of these rivers and their high 87Sr/86Sr needs detailed work on their Sr and 87Sr/86Sr. Preliminary calculations, however, indicate that they can be a significant source to some of the rivers. It is estimated that on an average, &#732;25% of Sr in the YRS is derived from silicate weathering. In the lower reaches, the streams receive &#732;15% of their Sr from carbonate weathering whereas in the upper reaches, calc-silicates can contribute significantly (&#732;50%) to the Sr budget of rivers. These calculations reveal the need for additional sources for rivers in the lower reaches to balance their Sr budget. Evaporites and phosphorites are potential candidates as judged from their occurrence in the drainage basin. In general, Precambrian carbonates, evaporites, and phosphorites ''dilute" the high 87Sr/86Sr supplied by silicates, thus making Sr isotope distribution in YRS an overall two end member mixing. Major constraints in quantifying contributions of Sr and 87Sr/86Sr from different sources to YRS rivers are the wide range in Sr and 87Sr/86Sr of major lithologies, limited data on Sr and 87Sr/86Sr in minor phases and on the behavior of Sr, Na, and Ca during weathering and transport. The Ganga and the Yamuna together transport &#732;0.1% of the global Sr flux at the foothills of the Himalaya which is in the same proportion as their contribution to global water discharge. Dissolved Sr flux from the Yamuna and its mobilization rate in the YRS basin is higher than those in the Ganga basin in the Himalaya, a result consistent with higher physical and chemical erosion rates in the YRS