Assessment of dissolved inorganic carbon sources and dynamics in a large catchment based on major ions and multiple stable isotopes

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.