Reconciling the elemental and Sr isotope composition of Himalayan weathering fluxes: Insights from the carbonate geochemistry of stream waters

Determining the relative proportions of silicate vs. carbonate weathering in the Himalaya is important for understanding atmospheric CO2 consumption rates and the temporal evolution of seawater Sr. However, recent studies have shown that major element mass-balance equations attribute less CO2 consumption to silicate weathering than methods utilizing Ca/Sr and Sr-87/Sr-86 mixing equations. To investigate this problem. we compiled literature data providing elemental and Sr-87/Sr-86 analyses for stream waters and bedrock from tributary watersheds throughout the Himalaya Mountains. In addition, carbonate system parameters (P-CO2, mineral saturation states) were evaluated for a selected suite of stream waters. The apparent discrepancy between the dominant weathering source of dissolved major elements vs. Sr can be reconciled in terms of carbonate mineral equilibria. Himalayan streams are predominantly Ca2+-Mg2+-HCO3- waters derived from calcite and dolomite dissolution, and mass-balance calculations demonstrate that carbonate weathering contributes similar to87% and similar to76% of the dissolved Ca2+ and Sr2+, respectively. However, calculated Ca/Sr ratios for the carbonate weathering flux are much lower than values observed in carbonate bedrock, suggesting that these divalent cations do not behave conservatively during stream mixing over large temperature and P-CO2, gradients in the Himalaya. The state of calcite and dolomite saturation was evaluated across these gradients, and the data show that upon descending through the Himalaya, similar to50% of the streams evaluated become highly supersaturated with respect to calcite as waters warm and degas CO2. Stream water Ca/Mg and Ca/Sr ratios decrease as the degree of supersaturation with respect to calcite increases, and Mg2+ Ca2+, and HCO3- mass balances support interpretations of preferential Ca2+ removal by calcite precipitation. On the basis of patterns of saturation state and P,,, changes, calcite precipitation was estimated to remove up to similar to70% of the Ca2+ originally derived from carbonate weathering. Accounting for the nonconservative behavior of Ca2+ during riverine transport brings the Ca/Sr and Sr-87/Sr-86 composition of the carbonate weathering flux into agreement with the composition of carbonate bedrock. thereby permitting consistency between elemental and Sr isotope approaches to partitioning stream water solute sources. These results resolve the dissolved Sr2+ budget and suggest that the conventional application of two-component Ca/Sr and Sr-87/Sr-86 mixing equations has overestimated silicate-derived Sr2+ and HCO3- fluxes from the Himalaya. In addition, these findings demonstrate that integrating stream water carbonate mineral equilibria, divalent cation compositional trends, and Sr isotope inventories provides a powerful approach for examining weathering fluxes. Copyright (C) 2002 Elsevier Science Ltd.