Geochemistry of dissolved inorganic carbon and carbonate weathering in a small typical karstic catchment of Southwest China: Isotopic and chemical constraints

The sources of dissolved inorganic carbon (DIC) and rock weathering processes were studied in the Houzhai catchment, a typical karstic catchment in the Changjiang River Basin, Southwest China. The carbon isotopic compositions ( δ 13C DIC) of DIC in the samples collected from the catchment vary from − 13.5‰ to − 6.9‰, with a mean value of − 9.8‰. The DIC in the catchment is thereby considered to be mainly derived from soil CO 2 and weathering of carbonate rocks. The DIC concentrations and the δ 13C DIC values show pronounced seasonal variations, with the lowest values being observed during the high flow season (from May to October). The logPCO 2 values in the waters were positively correlated with the DIC contents and negatively correlated to the saturation index of calcite (SIc) and δ 13C DIC values. These observations indicate that CO 2 derived from organic matter oxidation plays an important role in the dissolution of carbonate for this typical karstic environment. Based on a chemical mass balance, the weathering rate of carbonate rocks in the Houzhai catchment was estimated to be approximately 133 t/km 2/year or 584 × 10 3 mol/km 2/year in terms of CO 2 consumption rate, which is higher than the reported values for other rivers in Southwest China. ► Soil CO 2 controls composition of dissolved inorganic carbon in karstic river. ► Isotopic composition of DIC varies in response to partial pressure of CO 2. ► Typical karstic catchment has a high weathering rate of carbonate rocks.