Chemical weathering, atmospheric CO2 consumption, and the controlling factors in a subtropical metamorphic-hosted watershed

Abstract

The hydrochemistry of the small subtropical Xishui River watershed, which drains mainly gneisses and amphibolites, is here systematically investigated. By collecting samples from the source area to the river mouth in both summer and winter, we evaluated temporal and spatial variations of the major ion concentrations. We also evaluated the contributions from atmospheric input, anthropogenic activities and rock weathering (silicate, carbonate, and evaporite), and we calculated the chemical weathering and atmospheric CO2 consumption rates in the catchment. The major ion concentrations found in the summer are higher than those found in the winter, except for Si, which may indicate intensive weathering, agricultural activities, acid rain, and that the aquifer in the summer exceeds the dilution effect. Contributions from atmospheric input are 13.2% and 7.8% in the winter and summer, respectively. Anthropogenic activities provide only 2.4% and 4.2% of the major ions in the winter and summer, respectively, to the Xishui River. Contributions from silicate weathering are 52.1% and 48.7%, and carbonate weathering contributes 28.4% and 24.2% in the winter and summer, respectively. Although carbonate rocks account for only <5% of the drainage area, they significantly contribute to the major cations in the Xishui River. Contributions from evaporite are 3.8% and 15.1% in the winter and summer, respectively, indicating that the influence of temperature on evaporite dissolution is more noticeable. Silicate weathering rates are 0.64–4.44t/km2y and 4.1–21.2t/km2y in the winter and summer, respectively. The rates are lower for the Xishui River compared with those calculated for small watersheds that have a higher temperature and runoff in the tropical zone. The rates are also remarkably higher than those found for Siberia and Canada watersheds that have low temperature and low runoff, indicating that climate may be the most important factor influencing the chemical weathering rates in silicate watersheds.