Rock weathering characteristics and process analysis in typical karst subterranean river basins, southeastern Chongqing

Rock weathering in river basins plays a vital role in the global carbon cycle by consuming atmospheric CO2. As a key water-carbon conduit in karst regions, the systematic analysis of the internal rock weathering processes in subterranean rivers provides a theoretical foundation for advancing both research and practical applications related to karst carbon sinks. This study focused on the Youyang Subterranean River(28°40′~29°03′N, 108°40′~108°53′E), the largest subterranean river system in southeastern Chongqing, where carbonate rocks are widely exposed, accounting for approximately 63 % of the total basin area. A total of 51 water samples were collected across high, normal, and low flow seasons. By integrating hydrochemical analysis, end-member diagrams, ionic ratios, and forward modeling, we deciphered the hydrochemical characteristics and ion sources, and quantitatively constrained the multiple pathways and their respective contributions to rock weathering. The results indicate that the watershed waters are moderately to weakly alkaline with low mineralization, predominantly of the HCO3-Ca·Mg type. The average aqueous pCO2 was approximately 7.3 times that of the atmospheric, with most samples reaching saturation with respect to carbonate minerals. Source analysis reveals that Ca2+, Mg2+, and HCO3- originate from carbonate rock weathering, while SO42- and NO3-, in addition to atmospheric deposition, have extra contributions from sulfide mineral oxidation and nitrogen fertilizer nitrification, respectively. Carbonate rocks were identified as the dominant source of weathered material, with silicate rocks making a subordinate contribution. In addition to soil CO2(carbonic acid), ionic charge balance and δ13C evidence collectively reveal the participation of sulfuric and nitric acids in the weathering process. Quantification of the weathering pathways shows that carbonic acid-mediated dissolution of carbonate rocks is predominant(64 % ~96 %; mean: 82 %), followed by sulfuric acid weathering(2 % ~24 %; mean: 10 %). The contributions of these distinct pathways, which are co-regulated by karst structure and hydrological conditions, exhibit significant spatiotemporal heterogeneity. The deeply buried lower reaches of the subterranean river, characterized by stable flow conditions, exhibit higher and more stable carbon sink potential compared to the upper reaches. The findings enhance the understanding of rock weathering processes in subterranean river basins and are expected to provide valuable insights for conducting reliable baseline assessments and informing regulation and management strategies for carbon sinks in similar areas.