Carbonate weathering-related carbon sink fluxes in Chinese Loess Plateau with rich carbonate content: Insights from three little catchments

Continental rock weathering exerts negative feedback on global warming by removing atmospheric CO₂, with carbonate weathering being an important component of terrestrial carbon sinks. The fast kinetics of carbonate weathering leads the Chinese Loess Plateau (CLP), which is rich in carbonate minerals, to contribute greater HCO₃⁻ loads as the karst area. Therefore, carbonate weathering-related carbon sink fluxes (CCSFs) in the CLP are of great significance to the global carbon cycle. However, few studies have focused on the CLP, and the variations, mechanisms, and accurate estimates of CCSFs in loess areas with rich carbonate content remain unclear. To address this knowledge gap, we combined measurements of hydrochemistry (concentrations of HCO₃⁻, SO₄²⁻, Ca²⁺ and Mg²⁺), isotopic composition (δ¹³C_DIC and δ³⁴S-SO₄²⁻), and theoretical models to elucidate the temporal variations in riverine hydrochemistry and their controlling mechanisms. Our study focused on the Hong, Wuli, and Bainiu Rivers in Xi'an city, located on the southern margin of the CLP, during the hydrological years from November 2020 to May 2022. Our results show that seasonal variations in HCO₃⁻ concentrations in the three spring-river systems presented chemostatic behavior, which is also found in the karst areas of South China. Therefore, the variation in runoff depth (river discharge) may play a more important role in controlling CCSFs than variation in HCO₃⁻ for catchments in the CLP. The average concentrations of HCO₃⁻ in the spring sites of the Hong, Wuli, and Bainiu River catchments were 5.18 mmol/L, 6.2 mmol/L and 6.92 mmol/L, respectively, which are higher than the equilibrium concentrations of HCO₃⁻ at the local annual mean temperature of 13.1 °C under the given soil CO₂ concentration. The higher HCO₃⁻ concentrations indicate that the HCO₃⁻ is derived not only from the dissolution of carbonate minerals by carbonic acid but also by strong weathering acids (H₂SO₄ or HNO₃). After deducting the effect of strong acids, the net CCSFs of Hong, Wuli, and Bainiuhe Rivers were estimated to be 5.03 t C km⁻² a⁻¹, 4.61 t C km⁻² a⁻¹, and 5.39 t C km⁻² a⁻¹, respectively. Furthermore, we estimated that the CO₂ consumption of major rivers originating from or flowing through the loess region of the world could reach approximately 2.09 × 10⁷ t C a⁻¹, accounting for 9.86 % of the total carbonate weathering-related carbon flux from rivers to the ocean. Considering the extensive global distribution of loess (approximately 6 % of the total land area), further study on CCSFs in loess areas and the influence of climate and human activities is necessary.