Role of anthropogenic sulfuric and nitric acids in carbonate weathering and associated carbon sink budget in a karst catchment (Guohua), southwestern China

Abstract

Accurate estimate of carbon sink flux resulted from carbonate weathering in the karst area is of great significance for advancing the current understanding of the global carbon cycle and climate change. However, the carbon sink flux may be overestimated when the natural reaction with carbonic acid is influenced by protons of anthropogenic sulfuric and nitric acids, which can reduce the carbon sink flux during carbonate weathering. Here, we quantitatively evaluated the impact of anthropogenic sulfuric and nitric acids on carbonate weathering and carbon sink flux under base flow condition based on the stoichiometry of chemical compositions of groundwater from a typical karst catchment (Guohua), Guangxi, southwestern China. Seventy groundwater samples were analyzed for the characteristics of hydrochemistry and carbon isotopes of dissolved inorganic carbon (δ13CDIC) during the dry season in 2015. The results show that: (1) Ca2+ and Mg2+ dominate 92.4%–99.7% of the total cations, while HCO3− accounts for 83.0%–97.4% of total anions, indicating that the compositions of groundwater are primarily controlled by carbonate weathering; (2) The [Ca2++ Mg2+]/[HCO3−] equivalent ratios (1.00 to 1.41) and the corresponding δ13CDIC values (−16.8‰ to −8.1‰) of groundwater are all distributed between carbonate weathering by carbonic acid, and carbonate weathering by sulfuric and nitric acids. Meanwhile, the [Ca2++ Mg2+]/[HCO3−+ SO42−+ NO3−] ratios of groundwater are about 1, and the [Ca2++ Mg2+] equivalent concentrations have a good positive correlation with [HCO3−+ SO42−+ NO3−] and [SO42−+ NO3−], suggesting that sulfuric and nitric acids, in addition to carbonic acid, have been involved in carbonate weathering; (3) The contribution of sulfuric and nitric acids involved in carbonate weathering to (Ca2++ Mg2+) and HCO3− in groundwater varies from 0.6% to 58.0% (mean value of 20.9%) and from 0.3% to 40.9% (mean value of 12.2%), respectively. The carbonate weathering rate has increased by 20.9% while the karst carbon sink flux has decreased by 12.2%. Therefore, the carbon sink flux produced by carbonate weathering should be carefully evaluated when anthropogenic sulfuric acid and/or nitric acid are involved and the role of sulfuric and nitric acids in carbonate weathering could not be ignored in the global carbon cycle.