Spatial variations in chemical weathering and CO2 consumption in Nepalese High Himalayan catchments during the monsoon season

The major ion chemistry of the Marsyandi basin and six of its tributaries in the Nepalese Himalaya have been investigated during the monsoon months of 2002. Weekly water samples taken at 10 river monitoring stations in the Annapurna watershed over the course of 4 months provide chemical weathering data for the region at an unprecedented temporal and spatial resolution. The river chemistry of all but one basin is heavily dominated by carbonate weathering which, compared to silicate weathering, contributes 80 to 97% of the total solute load. This prevalence is due to a combination of (a) intrinsically faster dissolution kinetics of carbonates, (b) relatively high runoff and (c) glacial meltwater and low temperatures at high altitudes resulting in enhanced carbonate solubilities. Monitoring stations with headwaters in the Tethyan Sedimentary Series (TSS) are particularly carbonate-rich and slightly supersaturated with respect to calcite through half of the monsoon season. Silicate weathering in the TSS is driven largely by sulfuric acid and therefore does not contribute significantly to the drawdown of atmospheric CO₂. With respect to the tributaries in the Greater Himalayan Sequence (GHS), carbonate weathering is practically as predominant as for the TSS, in spite of the largely felsic lithology of the GHS. Relative to the TSS, the primary proton source in the GHS has shifted, with at least 80% of the protons derived from carbonic acid. Averaged over the whole field area, the CO₂ fluxes, based on silicate-derived Ca and Mg, are considerably lower than the global average. Assuming that this study area is representative of the entire range, we conclude that in situ weathering of the High Himalayas does not represent a significant sink of atmospheric carbon dioxide, despite the presence of a watershed south of the GHS that is characterized by a four times higher CO₂ consumption rate than the global average. Silicate weathering rates of all basins appear to be climate controlled, displaying a tight correlation with runoff and temperature. Given the extremely low chemical weathering under transport-limited conditions in high-altitude crystalline terrains outside of the monsoon season, this would result in virtually no chemical exhumation for 2/3 of the year in such a cold and arid climate, north of the rain shadow cast by the High Himalayas.