Impacts of hydrologic variations on chemical weathering and solute sources in the Min River basin, Himalayan-Tibetan region.

Feedback between hydrologic variations and chemical weathering is thought to play a crucial role in modulating global carbon cycling. The mechanisms associated with the impacts of hydrologic variations on solute sources and chemical weathering were evaluated by examining the relationships between river discharge and hydrochemistry based on high-frequency sampling of the Min River, which originates in the Himalayan-Tibetan region. Fluid transit times and flow pathways vary with changes in discharge, thereby affecting various biogeochemical processes. Although shorter transit times occur during the high-flow season than during the low-flow season, concentrations of chemical weathering products exhibit chemostatic behaviour (less variation than changes in discharge) in response to increasing discharge due to hydrologic flushing of minerals, which increases the amount of reactive mineral surface area. The contributions of various sources to dissolved loads in the Min River were estimated using a forward model. The calculated annual carbonate and silicate weathering fluxes are 24.1 and 9.6 t/km2 year, respectively. Atmospheric contributions increase with increasing discharge, whereas the contributions of silicate weathering decrease with increasing discharge. Both the carbonate weathering flux (FCarb) and silicate weathering flux (FSil) are positively correlated with the discharge, indicating that temporal variations in chemical weathering fluxes in the Min River are highly affected by hydrologic variations. The slope of the relationship between FCarb and discharge is much greater than that between FSil and discharge due to the rapid dissolution of carbonate minerals, suggesting that carbonate weathering is more sensitive than silicate weathering to hydrologic variations. This study demonstrates that high-frequency sampling is necessary when investigating solute sources and chemical weathering processes in river basins influenced by a monsoon climate.