Distinguishing anthropogenic perturbations from natural carbon cycling in mega-rivers remains a critical challenge. Carbon isotopes (δ13CDIC) provide a powerful tracer to decode these complex interactions which are often masked in traditional hydrochemical assessments. This study investigated the hydrochemistry and multiple stable isotopes (δD, δ18O and δ13CDIC) of the Yangtze River surface water (YRSW) during the dry season to quantify these contributions. δD values ranged from -117.8‰ to -44.6‰ and δ18O from -16.3‰ to -7.0‰, aligning with the Global Meteoric Water Line, which confirms atmospheric precipitation as the primary water source shaped by continental and altitude effects. DIC concentrations ranged from 1560 to 5724.29 µmol L-1 (mean 2993.79 µmol L-1), acting as a net CO2 source with an average pCO2 of 522.08 µatm. Stoichiometric and isotopic analyses reveal that carbonate weathering dominated by soil CO2 is the primary DIC source. However, in the middle and lower reaches, anthropogenic sulfuric/nitric acid weathering and organic matter oxidation were identified as key drivers decoupling DIC from natural climatic controls. This study systematically reveals for the first time the spatial differentiation patterns of DIC and δ13CDIC on the scale of the entire Yangtze River Basin and the main controlling factors, providing a new perspective for the study of carbon cycle in large river basins under high-intensity human activity interference.
Pei et al. (Thu,) studied this question.