CO2 Release Driven by the Combination of Sulfide Oxidation and Carbonate Dissolution in the Upper Changjiang River: Effect of Erosion and Lithology on Chemical Weathering

AbstractThe positive link between weathering and orogeny suggests that the drawdown of atmospheric carbon dioxide (CO2) over the Cenozoic should be closely related to the enhanced silicate weathering caused by Cenozoic orogeny, especially the uplift of the Tibetan plateau. However, the uplift of orogens could also exhume carbonate and sulfide minerals, the coupling weathering of which can act as a source for atmospheric CO2 at the long‐term timescale and counteract the CO2 consumption by silicate weathering. In this study, the solute concentrations and isotopic ratios in the upper Changjiang River are determined to investigate the co‐weathering of silicate, carbonate, and sulfide minerals and its impact on the long‐term carbon cycle. An inverse mixing model incorporating elemental and isotopic ratios is established to quantitatively identify the sources of cations and weathering acids. The inversion of our samples reveals that carbonate weathering dominates the chemical composition of river water, even with the presence of secondary calcite precipitation. The sulfide oxidation widely occurs in the study area and the bulk of generated sulfuric acid is buffered by carbonate minerals. The erosion is decoupled from sulfide oxidation and chemical weathering, likely due to lithology and secondary calcite precipitation. Overall, we find that the CO2 release from sulfide oxidation coupled with carbonate dissolution exceeds the CO2 drawdown from silicate weathering in the Changjiang River basin, indicating the weathering will act as a source for atmospheric CO2 at the timescale between marine carbonate compensation (105 years) and sulfate reduction (<10 Ma).