Geomorphic and climatic controls on chemical weathering in the High Himalayas of Nepal

The hypothesis that CO2 consumption by chemical weathering is enhanced by the exposure of fresh minerals provides a mechanism linking tectonic, geomorphic, and atmospheric processes. We present data from the High Himalayas of Nepal indicating a positive linear relationship between cation weathering rate and suspended sediment yield. Because of a steep precipitation gradient across the field area, the weathering rates are normalized according to runoff to isolate erosion as the controlling factor. Adjusted for runoff, increases in weathering rates do not keep pace with increasing erosion rates where suspended sediment yields > 2000 t/km2/y. This result supports studies that predict a nonlinear relationship between denudation and weathering in rapidly eroding landscapes. In addition, we document a decoupling of the weathering of Si and silicate cations from a glacierized watershed. The proportions of silicate-derived Na and K are high relative to Si during the warm summer months and then drop when subfreezing temperatures arrive in the fall. We propose that, during the summer, glacial abrasion creates fresh mineral surfaces that favor the release of cations, and abundant meltwater flushes these weathering products from the subglacial bed. As temperatures fall below freezing and the production of meltwater slows dramatically, river discharge becomes dominated by groundwater bearing the chemical signature of more congruent weathering reactions.