Chemical weathering processes impacted by pyrite oxidation in the upper Indus River basin, Western Himalaya

Chemical weathering acts as a linkage between the solid earth and atmospheric CO2 and thus affects the global climate throughout the geologic time. However, these processes have not been well-documented in the Hima-layan Rivers due to the lack of information on the sources of solutes and pyrite oxidation. Here we present chemical compositions of surface water in the upper Indus River basin (UIRB), Western Himalayas, aiming to explore the sources of major ions, chemical weathering rates, associated CO2 consumption rates, and their controlling factors. The total cation charge (1611 mu Eq) and average total dissolved solids (133 mg/L) of the UIRB waters were higher than those of the world's major rivers. HCO3- and Ca2+ dominated the ionic compositions of river water. The total major ions of analyzed rivers were mainly sourced from reservoirs including carbonates, evaporites, silicates, and atmospheric inputs. Quantitative investigation of solute sources indicated that car-bonate weathering dominates the dissolved loads in river water, which accounts for approximately 80.5 %. The chemical weathering rates of silicate and carbonate were estimated to be 6.0 and 70.1 t km -2 a-1, with atmo-spheric CO2 consumption rates of 91 x 103 and 503 x 103 mol km-2 a-1, respectively. The highest carbonate weathering rate (128.2 t km -2 a-1) in the UIRB was observed in the Suru River basin, which has the steepest catchment average slope. After considering pyrite oxidation, the actual CO2 consumption rates by the silicates and carbonate in the basin were only approximately 80 % of the amount estimated under the presumption that carbonic acid donates all protons included in the chemical weathering. This study highlights the importance of pyrite oxidation for riverine carbon budget in the mountain rivers, Western Himalayas.