CARBONATE VERSUS SILICATE WEATHERING IN THE RAIKHOT WATERSHED WITHIN THE HIGH HIMALAYAN CRYSTALLINE SERIES

The major element and Sr isotope geochemistry of surface waters, bedro ck, and river sands was investigated in the Raikhot watershed within t he High Himalayan Crystalline Series (HHCS) in northern Pakistan. Mass -balance calculations of mineral-weathering contributions to the disso lved flux of ions from the watershed indicate that 82% of the HCO3- fl ux is derived from the weathering of carbonate minerals and only 18% i s derived from silicate weathering, even though the bedrock is predomi nantly quartzofeldspathic gneiss and granite with only similar to 1% c arbonate in the watershed. This study demonstrates the importance of t race amounts of bedrock carbonate in controlling the water chemistry o f glacial watersheds. We suggest that the nux of Sr with a high Sr-87/ Sr-86 ratio in the major Himalayan rivers may be derived in large part from weathering of trace amounts of calcite,within the largely silica te HHCS. Models that use the flux of radiogenic Sr from the Himalayas as a proxy for silicate weathering rates may, therefore, overestimate the amount of CO2 consumption due to silicate weathering in the Himala ya.