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 (δ13CDIC) 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 CO2 and weathering of carbonate rocks. The DIC concentrations and the δ13CDIC values show pronounced seasonal variations, with the lowest values being observed during the high flow season (from May to October). The logPCO2 values in the waters were positively correlated with the DIC contents and negatively correlated to the saturation index of calcite (SIc) and δ13CDIC values. These observations indicate that CO2 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/km2/year or 584 × 103 mol/km2/year in terms of CO2 consumption rate, which is higher than the reported values for other rivers in Southwest China.

Table Highlights
► Soil CO2 controls composition of dissolved inorganic carbon in karstic river.
► Isotopic composition of DIC varies in response to partial pressure of CO2.
► Typical karstic catchment has a high weathering rate of carbonate rocks.