Dynamics in riverine inorganic and organic carbon based on carbonate weathering coupled with aquatic photosynthesis in a karst catchment, Southwest China.

Carbonate mineral weathering coupled with aquatic photosynthesis, herein termed 'coupled carbonate weathering' (CCW), represents a significant carbon sink which is determined by riverine hydrochemical variations. The magnitudes, variations and mechanisms responsible for the carbon sink produced by CCW are still unclear. In this study, major ions, TOC and discharge data at the Darongjiang, Lingqu, Guilin and Yangshuo hydrologic stations in Li River basin, a karst catchment typical of this geographic region, were analysed from January 2012 to December 2015 to elucidate the temporal variations in riverine inorganic and organic carbon and their controlling mechanisms. The results show that (1) HCO was sourced from carbonate weathering and silicate weathering, carbonate weathering by carbonic acid being predominant; (2) TOC was created chiefly by the transformation of bicarbonate to organic carbon by aquatic phototrophs during the non-flood period; (3) The carbon sink produced by coupled carbonate weathering in the Li River basin was calculated to be 14.41 tC·km·yr, comprised of the sink attributable to carbonate weathering (12.17 tC·km·yr) and sink due to the "biological carbon pump" (S) (2.24 tC·km·yr). The S thus accounted for approximately 15.54% of the total carbon sink, indicating that the proportion of riverine TOC sourced by the transformation from bicarbonate to organic carbon by aquatic phototrophs may be high and must be considered in the estimation of carbonate weathering-related carbon sinks elsewhere.