Dissecting chemical weathering pathways and CO2 fluxes in a mountainous catchment with rapid uplifting rate

Chemical weathering plays a critical role in the global carbon cycle, landscape development, and ecosystem stability. The central themes focus on the pathways of acid production, rock availability for dissolution, and correlations between rates and climatic or tectonic factors. Most previous efforts are diverted to measuring exports of solutes and sediments from large rivers systems. As small rivers in active orogens across Asia and Oceania deliver solutes and sediments at a rate surpassing large rivers on the per-area basis, the exact flux and reaction pathway of chemical weathering in these catchments remain poorly quantified.

This study aims to investigate the spatial and temporal variations in patterns of chemical weathering along the Beinan river system in southeastern Taiwan where the flux of sediment export is among the largest in Taiwan. Analyses of river water collected monthly from different tributaries yielded Ca²⁺, SO₄^2- and dissolved inorganic carbon (DIC) as the major constitutes. In particular, SO₄^2- concentrations are two to sixteen times higher than the average world value. Isotopic compositions of sulfate combined with the solute pattern and geological occurrence indicated that the measured sulfate is primarily attributed to being produced from pyrite oxidation. Screening of 16S rRNA genes further suggests that pyrite oxidation is at least partly mediated by microbial processes. While carbon isotopic compositions indicate multiple sources of DIC, partial pressures of CO₂ often exceed the atmospheric equilibrium saturation state. Using a new computational approach, we demonstrate that carbonate weathering was mostly driven by sulfuric acid and accounted for >70% solute export at all sites. The catchment-wide flux derived from carbonate weathering was at a level four-five orders of magnitude higher than the average of major large rivers. The computation also indicated that the short-termed CO₂ efflux was correlated with the flux of pyrite-derived carbonate weathering with the net emission occurring in upstream regions in all seasons and the net consumption in the downstream plain in dry seasons. Overall, these results demonstrate a strong modulation of the oxidation of pyrite inherited with metamorphic rocks in chemical weathering and CO₂ emission in a rapidly uplifting catchment.