Response of weathering carbon sink effect to anthropogenic sulfuric acid in different lithological catchments: A case study from Southwest China

Accurately quantifying the carbon sink effect resulting from chemical weathering caused by anthropogenic H₂SO₄ is imperative to improve the assessment of the global carbon budget. Nevertheless, there is still a lack of precise understanding regarding the impact of anthropogenic H₂SO₄ on CO₂ consumption during chemical weathering. Here, spring water samples were collected monthly from three catchments with distinct bedrock lithologies affected by severe acid precipitation in Southwest China for analyses of hydrogeochemistry and δ¹³C_DIC to quantitatively estimate the effect of anthropogenic H₂SO₄ on the weathering carbon sink budget. The results show that carbonates contribute 97.4 %, 95.0 % and 88.8 % of the total cationic load using a straightforward method in the Beidiping carbonate catchment, as well as in the Shegengyan and Bianyan silicate catchments, respectively. The [Ca²⁺+Mg²⁺]/[HCO₃⁻] (0.98–1.19) and [Ca²⁺+Mg²⁺]/[HCO₃⁻+SO₄²⁻] (approximately 1) equivalent ratios, and δ¹³C_DIC values (−16.8 to −8.0 ‰) of the samples suggest that besides H₂CO₃, H₂SO₄ is involved in carbonate weathering. The stoichiometry of the chemical compositions of spring water indicates that the presence of H₂SO₄ enhances carbonate weathering rates by 14.8 %, 8.1 % and 7.5 % while decreasing the CO₂ consumption by 8.2 %, 4.3 % and 4.0 % in Beidiping, Shegengyan and Bianyan, respectively. Thus the reduced proportion of karst carbon sink in the carbonate catchment is approximately 2 times that in the silicate catchment, suggesting that carbonate weathering in the karst catchment is more sensitive to acid precipitation. The impact of acid precipitation on rock weathering in the silicate catchment is constrained by the soil buffering effect. Our study highlights the important role of anthropogenic H₂SO₄ in carbonate weathering, which should be critically evaluated in regional and global carbon cycles in future studies.