Sulfuric acid as an agent of carbonate weathering constrained by δ13C DIC: Examples from Southwest China

Rock weathering by carbonic acid is thought to play an important role in the global carbon cycle because it can geologically sequestrate atmospheric CO ₂. Current model of carbon cycle evolution usually assumes that carbonic acid is the major weathering agent and that other acids are not important. Here, we use carbon isotopic evidence and water chemistry of springs and rivers from the Beipanjiang River basin (Guizhou Province, Southwest China) to demonstrate that sulfuric acid is also an important agent of rock weathering. The δ¹³C of dissolved inorganic carbon (DIC) in the water samples ranges from - 13.1‰ to - 2.4‰, and correlates negatively to [HCO ₃^-]/([Ca ²⁺] + [Mg ²⁺]) ratios and positively to [SO ₄^2-]/([Ca ²⁺] + [Mg ²⁺]) ratios. These relationships are interpreted as mixing diagrams between two reactions of carbonate weathering, using carbonic acid and sulfuric acid as a proton donor, respectively. Mixing proportions show that around 42% of the divalent cations in the spring water from Guizhou are originated from the interaction between carbonate minerals and sulfuric acid. It is shown that 40% of this sulfuric acid is derived from the atmosphere and has an anthropogenic origin. The remaining 60% are derived from the oxidative weathering of sulfide minerals in sedimentary rocks. Our results show the positive action of sulfuric acid on the chemical weathering of carbonate. Particularly, we show that sulfuric acid generated by coal combustion has increased by almost 20% the weathering rates of carbonate in Southwest China. This is a clear evidence that human activities are changing the weathering rates of rocks and demonstrates a negative feedback on the acidification of the ocean by greenhouse gases. Because of the involvement of sulfuric acid in weathering reactions, 63% of the alkalinity exported by rivers is derived from carbonate, instead of 50% when atmospheric CO ₂ is the only acid involved in chemical weathering of carbonate. In the Guizhou Province, the weathering of carbonate is thus, at least transiently, a net source of CO ₂ to the atmosphere. When extrapolated at global scale, sulfuric acid-induced carbonate weathering could counterbalance a significant part of the CO ₂ consumed by silicate weathering. This paper highlights the competition between silicate weathering by carbonic acid and carbonate weathering by sulfuric acid for the regulation of the atmospheric CO ₂ level.