Chemical weathering of small catchments on the Southeastern Tibetan Plateau I: Water sources, solute sources and weathering rates

Hydro-geochemical study of small catchment provides important information to identify water and solute sources, understand chemical weathering processes and their controlling factors. In this work, 44 small catchments on the southeastern Tibetan Plateau were investigated. Stream, precipitation, glacier and spring waters in both high and low flow seasons and bed rocks samples were analyzed with a main purpose to understand the processes controlling the stream water chemistry and quantify the weathering rates. The stream waters are mainly recharged by precipitation and glacier meltwater. Glacier meltwater and precipitation account for 25.8% and 73.9% of the total discharge in high flow season, and 44.4% and 54.1% in low flow season on average. Hydrograph separation and chemical mass balance are jointly used to estimate the contributions of major reservoirs (precipitation, glacier, spring, carbonates and silicates) to the total dissolved loads of the streams. Rock weathering accounts for similar to 90% of the total dissolved cations for most streams. Silicate and carbonate weathering account for 15.9% and 75.2% of total dissolved cations in high flow season, and 9.5% and 77.2% in low flow season on average. Lack of basic hydrological data in the ungauged remote area is a problem for quantified weathering study. The Noah LSM model is applied to obtain the annual runoff of these un-gauged catchments in this study. Based on these approaches, the chemical weathering rates and total denudation rates (TDR) are calculated for each of the small catchments. The silicate cation weathering rates (SCWR) range between 0.6 and 5.2 t/km(2)/yr, with the area-weighted mean value about 1.8 t/km(2)/yr. The TDR range between 8.9 and 1907.9 t/km(2)/yr. The comparisons between the small catchments and with other river basins in different tectonic and climatic environments indicate that lithology, climatic factors (temperature and runoff) and physical erosion rate are the key parameters controlling chemical weathering rate. The average SCWR of the small catchments is about 6 times higher in high flow season than in low flow season, which could be attributed to the higher temperature and runoff in high flow season. Meanwhile, the positive relationship between SCWR and TDR supports the view that physical erosion has an important effect on chemical weathering in the Tibetan Plateau.