Chemical denudation and silicate weathering in Himalayan glacier basins: Batura Glacier, Pakistan
The major ion, SiO2 and minor element contents of Batura Glacier meltwaters in the headwaters of the Indus Basin, Pakistan are examined. The data are used to examine the controls upon solute acquisition by runoff and to estimate solute yields. Solute yield calculations include estimation of the total and silicate-derived cationic denudation rates following the application of simple solute provenance models. Examination of meltwater composition shows that, like other glacier basins reported in the literature, much of the solute content of runoff is acquired from carbonate weathering. Hence, HCO3− accounts for 80% of the equivalent anionic content of runoff, whilst [Ca2++Mg2+] accounts for 89% of the equivalent cation content. The small excess of [Ca2++Mg2+] over HCO3− is largely balanced by non-marine SO42−. Otherwise, silicate weathering accounts for ca. 15% of the cation content and appears to produce runoff with high Sr2+/Ca2+ ratios. Chemical denudation in the Batura Glacier Basin produces a yield of ca. 88 ton km−2 a−1, or 75% of the entire solute yield. The corresponding dissolved SiO2 yield is 2.4 ton km−2 a−1 and the cationic denudation rate is ca. 1600 meq+ m−2 a−1. Data from three other studies suggest that a range of 660–4200 meq+ m−2 a−1 is likely for Himalayan glacier basins with between 1.6 and 6.5 m a−1 specific annual runoff. These rates are high relative to Arctic glaciers and similar to upper values observed in temperate and Icelandic glacier basins. The silicate-derived cationic denudation rate for the Batura Glacier Basin is approximately 240 meq+ m−2 a−1, whilst the corresponding range for all of the Himalayan glacier basins is 240–970 meq+ m−2 a−1. These yields are again high, which is most likely to reflect high flushing rates caused by ablation and monsoon precipitation delivery. However, the very incongruent nature of silicate weathering by the dilute glacial meltwaters means that these cation yields are accompanied by only modest dissolved SiO2 yields relative to global means.