Enhanced carbonate weathering and CO2 release in a typical karst watershed (Southwest China): Evidence from hydrochemical and multi-isotopic data.

Carbon sinks formed by carbonate weathering are a major component of the carbon sink deficit and play a significant role in the carbon cycle. However, exogenous acids (HNO₃ and H₂SO₄) are also involved in carbonate weathering, complicating the overestimation of the formation of carbon sinks and fluxes. In this study, we analyzed the Hongjiadu karst underground river basin in Guizhou using isotopes (δ¹³C-DIC, δ³⁴S-SO₄, δ¹⁸O-SO₄, δ¹⁸O-H₂O, and δ²H-H₂O) and hydrochemical methods to investigate carbon cycling characteristics and its influencing factors, and quantitatively evaluate the effects of HNO₃ + H₂SO₄ on CO₂ sink fluxes in the basin. The following results were obtained: (1) The nitrate in the watershed mainly comes from fertilizers, and the sulfate mainly comes from local coal seams. (2) The relationship between isotopes (δ¹³C-DIC, δ³⁴S-SO₄, δ¹⁸O-SO₄, δ¹⁸O-H₂O, and δ²H-H₂O) and hydrochemicals (Ca²⁺, Mg²⁺, NO₃^-, HCO₃^-, and SO₄^2-) indicates that H₂CO₃, HNO₃, and H₂SO₄ are involved in carbonate rock dissolution. Most of the samples in the watershed, particularly the acid mine drainage water, were affected by acid rain and sulfide oxidation, resulting in increased δ¹³C-DIC values. Another portion of the springs was affected by soil CO₂, resulting in decreased δ¹³C-DIC values. (3) Quantitative calculations showed that exogenous acids enhanced the weathering rate of carbonate rocks in groundwater by 67.24 % (dry season) and 76.31 % (wet season) while decreasing the carbon sink flux in groundwater by 55.31 % (dry season) and 66.64 % (wet season). Rainfall enhances the weathering of carbonate rocks. In the process, CO₂ is released, increasing the carbon source. Our study highlights the influence of anthropogenic, natural, and hydrological changes on karst carbon sinks in mixed-pollution karst basins. This information is important for future studies on the carbonate weathering processes in karst regions.