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• Carbonate weathering dominates the water chemistry of river water which accounts for about 80.5 % of solutes. • The sensitivity of chemical weathering to climate change and topographic factors were found. • Forward model is combined with the mass balance to calculate the weathering rates. • Weathering rate is 6.0 t km−2 a−1 for silicate and 70.1 t km−2 a−1 for carbonate. Chemical weathering acts as a linkage between the solid earth and atmospheric CO 2 and thus affects the global climate throughout the geologic time. However, these processes have not been well-documented in the Himalayan 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 CO 2 consumption rates, and their controlling factors. The total cation charge (1611 µEq) and average total dissolved solids (133 mg/L) of the UIRB waters were higher than those of the world's major rivers. HCO 3 − 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 carbonate 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 atmospheric CO 2 consumption rates of 91 × 103 and 503 × 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 CO 2 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. [ABSTRACT FROM AUTHOR]

Appropriate characterisation of isotope signatures (δ18O and δD) of groundwater, glaciers, and surface water in the Himalayan catchment are important for refining the sources of moisture and developing a modern isotope–elevation relationship for paleoelevation reconstructions. The Indus River located in Western Himalaya is previously unexplored in respect of the variation in water isotopic composition with elevation. In this study, water samples (n = 96) were collected from mainstream, tributaries, streams, glaciers, groundwater, lakes, hot springs, and pond water during the summer season. Results showed that δ18O signatures of the mainstream, tributaries and groundwater range from −15.1‰ to −12.3‰, −16.5‰ to −12‰, −13.8‰ to −12‰, with a mean value of −13.6‰, −14.7‰, −12.8‰, respectively. There was a northward rise in δ18O by 1.30‰ per degree of latitude in the tributary waters. The d -excess and δ18O values suggested that the key source of moisture in the studied river basin was mainly from westerlies and southwest monsoons. Thus, regional moisture recycling and subcloud evaporation contributed little in supplying moisture to the precipitation. The isotopic signature (δ18O) of the tributary stream and its surrounding regions conformed to a best-fit second-order polynomial relationship between δ18O and elevation over 3500 masl. Moreover, an δ18O elevation slope of −2‰/km is also established in this study area. This isotope–elevation relationship will be highly useful and can be applied to the Indus River and other Himalayan rivers for paleoelevation reconstruction. • Stable isotopes of surface water and groundwater in the UIRB are analyzed. • Surface waters showed variable isotopic spatial pattern in the upper Indus River. • Moisture sources in the UIRB mainly from southwest monsoons and westerlies. • Isotope-elevation link explored in the UIRB for paleo-elevation reconstruction. [ABSTRACT FROM AUTHOR]