『Abstract
　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 133t/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.

Keywords: Carbon isotope; Dissolved inorganic carbon; Carbonate weathering; Karstic catchment』

1. Introduction
2. Materials and methods
　2.1. Site description
　2.2. Sampling and analytical methods
3. Results
　3.1. Major ions and DOC in the water
　3.2. PCO2 and calcite saturation index
　3.3. δ¹³C of dissolved inorganic carbon
4. Discussion
　4.1. Seasonal variation of carbon isotopic composition and DIC concentration
　4.2. Carbonate weathering versus the δ¹³C of DIC
　4.3. δ¹³C shift induced by soil CO2 infiltration
　4.4. CO2 evasion to the atmosphere
　4.5. Estimation of the carbonate weathering rate
5. Summary
Acknowledgements
References