Hydrochemistry and carbon isotope characteristics of Nujiang River water: Implications for CO2 budgets of rock weathering in the Tibetan Plateau

Abstract

The Tibetan Plateau is one of the most complicated geographical units worldwide in terms of its tectonic and environmental background. Although a hotspot for continental weathering and carbon cycling studies, accurate determination of the weathering carbon budget is challenging in this area, especially sink and source flux quantification and the controlling mechanisms. Compared with other major rivers on the plateau, the Nujiang River is characterized by less human disturbance and maintains a relatively pristine state. This study investigates the high spatiotemporal resolution hydrochemistry and dual-carbon isotope composition (δ¹³C_DIC and Δ¹⁴C_DIC) of river water in the Nujiang River Basin. The results revealed that the solutes and dissolved inorganic carbon in the river water are predominantly derived from rock weathering by carbonic and sulfuric acids, mainly due to the carbonate weathering process, and significantly enhanced by deep carbon sourcing from hot springs in the fault zone. The average contributions of geological and modern carbon in the main stream of the Nujiang River are 35.2% and 64.8%, respectively, and sulfide oxidation contributes >90% of sulfate ions in the river water. After considering the involvement of sulfuric acid generated by sulfide oxidation during rock weathering, the calculated consumption fluxes of atmospheric CO₂ by silicate and carbonate weathering in the watershed were decreased by approximately 52.0% and 37.4%, respectively, compared with those calculated ignoring this process. Rock weathering of the Nujiang River Basin is a “CO₂ sink” on a short time scale, while the participation of sulfuric acid makes it a “CO₂ source” on a geological time scale. The high-frequency observations of ion concentrations, elemental ratios, and calculated contributions of different rock weathering materials indicate that carbonate rock weathering is more sensitive to temperature and runoff variations than silicate rock weathering, with the solute contribution from carbonate weathering increasing significantly during monsoon period. The material input from different rock types is dominated by the hydrological pathways and water-rock reaction times in the basin. This study reveals the river solute origins and weathering CO₂ sequestration effect in response to a monsoonal climate in one of the most representative pristine plateau watersheds in the world, which is of great importance for elucidating the weathering control mechanisms and CO₂ net source-sink effect in plateau watersheds.