Dissecting mechanisms of chemical weathering in a rapidly uplifted watershed

Abstract

Chemical weathering plays a vital role in controlling long-termed climatic fluctuation and landscape development. In particular, silicate minerals are attacked by protons produced from the transformation of atmospheric carbon dioxide into carbonic acid, releasing monovalent and divalent cations with bicarbonate into rivers and enabling the net removal of CO₂. The rates of silicate weathering have been correlated to various factors, such as lithology, temperature, runoff, and erosion rate for major river systems, providing a basis to assess the atmospheric CO₂ drawdown over a geological time scale. For comparison, small river systems in rapidly uplifting orogenic belts are characterized by short transport path and residence time, and often associated with torrential runoff and sediment discharge. River channels are deeply incised, producing large quantities of fresh materials and mineral surface readily for weathering processes. The patterns and reaction pathways of chemical weathering in such river systems and its impacts on the global solute budget and long-termed climatic fluctuation remain largely unknown. In this study, we carried out periodic field campaigns to sample various materials (river, creeks, seeps, hot springs, and suspended particulates) in a small river system in southeastern Taiwan where the uplift and exhumation is rapid. The analyses yielded that calcium, sulfate, and bicarbonate were the major ions in river, creek, and seep samples. Sulfate concentrations were highly correlated with divalent cations, but exhibited a weak and no correlation with the yields of suspended particulate, and the concentrations and isotopic compositions of dissolved inorganic carbon, respectively. Screening of 16S rRNA genes further suggests that pyrite oxidation is at least partly mediated by microbial processes. These data combined with isotopic compositions of sulfate suggest that pyrite oxidation, carbonate weathering, and mixing processes mostly account for the solute variation pattern. While carbon isotopic compositions indicate multiple sources of dissolved inorganic carbon, partial pressures of CO₂ often exceed the atmospheric equilibrium saturation state. These results suggest that instead of a sink such a river system is prone to CO₂ emission.