[1] Chemical and strontium isotope analyses of solutes in a series of streams representing the bulk of the water reaching the similar to8500 km(2) Owens Lake watershed provide a unique opportunity to evaluate factors that control stream chemistry on a regional scale. The streams drain terrain that is dominated by Mesozoic granitoids of the Sierra Nevada batholith but that also contains irregularly distributed roof pendants of Paleozoic metasedimentary sequences. Mass balance modeling indicates that plagioclase feldspar weathering is the dominant contributor to stream chemistry and that carbonate dissolution is also a significant source of solutes for most streams. The calculated ratio of plagioclase to carbonate is highly sensitive to the assumed weathering product of plagioclase (kaolinite versus smectite), and we suggest that the importance of carbonate could be exaggerated in cases where only the plagioclase-to-kaolinite reaction is considered. Strontium isotope (Sr-87/Sr-86) ratios range from 0.70685 to 0.71958, with a flow-weighted mean of 0.70911. To the extent that this value represents the water that would be flowing into Owens Lake (were it not diverted by the Los Angeles Aqueduct system), the flow-weighted mean Sr-87/ Sr-86 ratio is consistent with ratios measured in Owens Lake core carbonates ranging in age up to 25 kyr. The overall Sr budget of the watershed is determined by the relative contributions of carbonate dissolution and plagioclase feldspar weathering and is dominated by reactions related to pyrite oxidation and hydrothermal fluid input from just two subbasins. The impact of these processes needs to be considered when modeling regional and global riverine Sr budgets. Climate-related shifts in the relative importance of silicate mineral weathering reactions appear to be only second-order effects on a regional scale. [References: 73]
展开▼
