Chemical Weathering in Small Catchments with Contrasting Lithologies and the Influence of Exogenous Acids on CO2 Consumption in Headwater Catchments of South China

Geological carbon sequestration is an important component of the global terrestrial carbon sink and plays a crucial role in the global carbon cycle and contributes to regional carbon-neutrality goals. However, acid deposition and exogenous acid pollution can substantially modify the hydrochemical characteristics of surface runoff, thereby affecting the assessment of regional geological carbon sequestration. In this study, we investigated stream-water chemistry in two small headwater catchments in the Pearl River Basin, South China, with contrasting lithologies: the Dinghushan (DH) catchment, which is dominated by silicate rocks, and the Niudu stream (ND) catchment, which contains both silicate and carbonate rocks. We examined the effects of lithology and exogenous acid inputs on the solute composition of stream water and estimated CO2 consumption associated with chemical weathering. The results indicate that lithological composition, atmospheric precipitation, and human activities jointly influence mineral weathering and stream-water chemistry in both catchments. Under the humid and warm climate, the two catchments exhibited different chemical weathering rates, and the intensity of carbonate mineral weathering was substantially greater than that of silicate mineral weathering. After excluding the contribution of exogenous acids to rock and mineral weathering, the estimated carbon dioxide consumption for silicate weathering in the DH basin was 1.34 × 105 mol km−2 yr−1. In the ND basin, the corresponding estimated values for silicate and carbonate weathering were 1.30 × 105 and 8.93 × 105 mol km−2 yr−1, respectively. However, ignoring the influence of exogenous acids would lead to an overestimation of the silicate weathering estimate in the DH basin by approximately 167%, while for the ND basin, the silicate and carbonate weathering would be overestimated by 15.38% and 60.13%, respectively. Exogenous acid inputs have different effects on the enhancement of silicate and carbonate mineral weathering, but neglecting their effects leads to an overestimation of regional carbon sinks.