Chemical weathering and CO2 consumption rate of montane silicate in South China: a case study of the Xizhijiang River Basin

Consumed CO2 in silicate chemical weathering can be calculated by the chemical composition of the runoff, but trace carbonate weathering and pollutants have a significant impact on the hydrochemical signals. In this study, the chemical composition of runoff of four sub-basins with different trace carbonate contents in strata and land use types of the Xizhijiang River (XZR) basin was measured to investigate the impact of lithology and land use on the chemical runoff as well as estimating CO2 consumed by chemical weathering. Compared with the global rivers, the chemical runoff produced by weathering in the XZR basin with a hot and humid climate has a moderate concentration of total dissolved solids and a relatively higher concentration of dissolved silicon. Agricultural and industrial activities increased the riverine water Cl- concentration, and trace carbonate weathering released a large amount of Ca2+. More intensive chemical weathering in summer weakened the dilution effect of rainwater on runoff ions in the mid-upstream XZR basin. In the sub-basins affected by agricultural and industrial runoff, the ion concentration was significantly higher in the dry season than in the rainy season. The CO2 flux fixed by the chemical weathering is 3.13 x 10(5) mol km(-2) a(-1) for the mid-upstream XZR basin, which constitutes a significant geological carbon sink if extrapolated to the whole of South China.