Chemical weathering in Manas River Basin: Driven by sulfuric acid or carbonic acid?

Abstract

Carbonic acid produced by the dissolution of atmospheric and soil CO₂ in water is usually the most dominant catalyst for chemical weathering, but a sulfuric acid-driven phenomenon, different from usual, was found in the orogenic belt watersheds dominated by silicate bedrock. This study, rooted in comprehensive field investigations in the Manas River Basin (MRB) north of the Tianshan Mountains, delves into the mechanisms and impacts of sulfuric and carbonic acid as catalysts driving different types of chemical weathering in the Central Asian Orogenic Belt. Quantitative analyses elucidate that carbonate weathering constitutes 52.4% of the total chemical weathering, while silicate and evaporite account for 18.6% and 25.3%, respectively, with anthropogenic activities and atmospheric precipitation having little effect. The estimated total chemical weathering rate in MRB is approximately 0.075 × 10⁶ mol/km²/year. Quantitative findings further suggest that, preceding carbonate precipitation (< 10⁴ year), chemical weathering can absorb CO₂. Subsequently, and following carbonate precipitation (10⁴–10⁷ year), it will release CO₂. The release significantly surpasses the global average CO₂ consumption, contributing to a noteworthy climate impact. This study underscores the distinctive weathering mechanisms, wherein sulfuric acid emerges as the predominant catalyst. The quantity of sulfuric acid as a catalyst is approximately three times that of carbonic acid. Sulfuric acid-driven carbonate rock weathering (SCW) is identified as the sole chemical weathering type with a net CO₂ release effect. SCW CO₂ release flux (5176 mol/km²/year) is roughly 2.5 times the CO₂ absorption by Ca–Mg silicate weathering, highlighting the pivotal role of chemical weathering in sourcing atmospheric CO₂ over the timescales of carbonate precipitation and sulfate reduction. Lastly, this study posits that catalyst and transport limitations are the most plausible critical factors in MRB. The interplay between sulfuric acid and dissolved CO₂ competitively shapes the types and rates of chemical weathering reactions.