Dissolved inorganic carbon in the Xijiang River: concentration and stable isotopic composition

Dissolved inorganic carbon (DIC) is one the major products of chemical weathering of the land, and it is also one of the most active components in the riverine carbon species. Concentration and stable isotopic composition of DIC in the mainstream of the Xijiang River (XJR) and its two tributaries (the Guijiang and Hejiang rivers) were investigated during a hydrological year and a massive flooding. DIC concentration of most selected sections of the XJR system varied from 750 to 2,400 μM with an average of (1,630 ± 390) μM. DIC was significantly diluted in the massive flooding period. The stable isotopic composition of DIC (δ13CDIC) varied from −14.75 to −8.25 ‰ [with an average of (−11.19 ± 1.18) ‰], from  17.22 to −10.65 ‰ [with an average of (−13.00 ± 1.86) ‰], and from −15.94 to −9.71 ‰ [with an average of (−12.68 ± 1.28) ‰] in the mainstream of the XJR, the Guijiang River and the Hejiang River, respectively. This is jointly determined by the outcrop area of carbonate rocks and the properties of soil organic carbon within the catchments. δ13CDIC is less negative in the dry season because of the increasing contribution from karst groundwater and more negative in the wet season because of the increasing contribution from surface soil water. Calcite was saturated sometimes at the mainstream sections of the XJR. Carbonate-sourced DIC contributes 10.32–43.95 % with an average of about (27.69 ± 6.03) % of the riverine DIC for the XJR. The XJR exports about 4.27 × 1012 g of DIC in the surveyed year, of which about 83 % occurred in the wet season.