Chemical weathering inferred from riverine water chemistry in the lower Xijiang basin, South China

Seasonal sampling was conducted on 13 sites involving the lower stem of the Xijiang river and its three tributaries to determine the spatial patterns of the riverine water chemistry and to quantify the chemical weathering rates of carbonate and silicate of the bedrock. Results indicate that the major ions in the Xijiang river system are dominated by Ca²⁺ and HCO₃⁻ with a higher concentration of total dissolved solids, characteristic of the drainages developed on typical carbonate regions. Obvious spatial variations of major ion concentrations are found at various spatial scales, which are dominantly controlled by the lithology particularly carbonate distribution in the region. The four selected rivers show similar seasonal variations in major ions, with lower concentrations during the rainy season. Runoff is the first important factor for controlling the weathering rate in the basin, although increasing temperature and duration of water-rock interaction could make positive contributions to the enhancement of chemical weathering. The chemical weathering rates range from 52.6 to 73.7 t/km²/yr within the lower Xijiang basin and carbonate weathering is over one order of magnitude higher than that of silicates. CO₂ consumption rate by rock weathering is 2.0 × 10¹¹ mol/yr, of which more than 60% is contributed by carbonate weathering. The flux of CO₂ released to the atmosphere-ocean system by sulfuric acid-induced carbonate weathering is 1.1 × 10⁵ mol/km²/yr, comparable with the CO₂ flux consumed by silicate weathering.