Hydrochemistry of the middle and upper reaches of the Yarlung Tsangpo River system: weathering processes and CO₂ consumption

The study focuses on the chemical compositions of the river waters in the middle and upper reaches of the Yarlung Tsangpo river system. Samples were collected in two periods along the river system to analyze the spatio-temporal variation characteristics and to determine the weathering processes and CO₂ consumption. The results show that the chemical facies of river waters are dominated by Ca-HCO₃ type and the TDS values cover a wide range of 98.2–619.8 mg/l with an average value of 268.6 mg/l, which is higher than that of the global river waters. There are three major reservoirs (carbonates, evaporites, and silicates) contributing to the major ions, and the predominances of the reservoirs of different rivers show spatial heterogeneity. The chemical weathering rates of silicates and carbonates are 3.43 and 4.49 t/km²/year, respectively. Overall, the middle and upper reaches of Yarlung Tsangpo are responsible for 0.3 and 0.16 % of global atmospheric CO₂ consumption by silicates (1.7 × 10⁵ mol/km²/year) and carbonates (1.27 × 10⁵ mol/km²/year), respectively. In addition, the chemical weathering fluxes of the catchments consume atmospheric CO₂ of 0.5456 Tg C/year, accounting for 0.14 % of the total organic carbon flux to the oceans by rivers globally.