Hydrogeochemical Controls on δ13C-DIC in a Dolomite Cave: Insights from 7-Year Monitoring at Mahuang Cave, Southwest China

Differences in the dissolution kinetics between dolomite and limestone constrain both the understanding of regional carbon cycling and the precision of speleothem-based paleoclimate reconstructions. To elucidate the mechanisms by which hydrogeochemical processes in a dolomite cave regulate dissolved inorganic carbon isotopes (δ13C-DIC), this study analyzed hydrochemical parameters and δ13C-DIC of five hydrological flow types, coupled with soil and cave CO2 and their isotopic compositions, based on monitoring data from 2019 to 2025 at Mahuang Cave (dolomite) in southwestern China, and compared them with six limestone caves. The results show that the hydrochemistry of Mahuang Cave is of the Ca·Mg-HCO3 type, with major ions exhibiting higher concentrations in the warm season and lower in the cold season, while δ13C-DIC is lighter in the rainy season and heavier in the dry season. On the interannual scale, δ13C-DIC shows only a weak correlation with ENSO phase variability, suggesting that the direct modulation of ENSO signals on the cave system is relatively limited. Water–rock–gas interactions and hydrological flow paths regulate the spatial gradient of δ13C-DIC (approximately 2.7‰): rapid fracture flow transmits light carbon signals of soil CO2 in an open system, slow matrix flow enriches heavy isotopes through enhanced prior calcite precipitation (PCP) in a near-closed system, and the underground river exhibits decoupling between Sr/Ca and δ13C-DIC due to multi-source mixing. Compared with limestone caves, Mahuang Cave is characterized by high soil CO2, low cave CO2, and heavier δ13C-DIC, which is attributed to the slow dissolution of dolomite, inhibited CO2 transport through thick overlying strata, and limited degassing due to low cave temperature. This study provides empirical evidence for understanding carbon cycling mechanisms in dolomite regions and emphasizes the need to account for lithological constraints.