Water Geochemistry of a Tropical River Draining Hainan Island: Implications for Catchment-Scale Chemical Weathering and Associated Carbon Budgets

Continental chemical weathering plays a crucial role in the regional and global carbon budgets. However, relevant research on tropical volcanic islands is still limited, particularly the influences of human activities on surficial chemical weathering, which remain scarce. This study analyzed the water geochemistry and dissolved inorganic carbon isotope (δ13CDIC) of the Changhua River draining Hainan Island, aiming to quantitatively elucidate the seasonal CO2 budgets at the catchment scale. We found that the major ions in the river were from atmospheric precipitation (6.79%), anthropogenic input (11.68%), carbonate weathering (41.96%), and silicate weathering (39.56%). The silicate weathering rate and the carbonate weathering rate were calculated as 16.91 t km–2 yr–1 and 30.68 t km–2 yr–1, respectively, resulting in CO2 consumption of 307.82 × 103 mol km–2 yr–1 and 400.95 × 103 mol km–2 yr–1. Considering sulfuric acid- and nitric acid-driven weathering reactions, the catchment became a net source of CO2 on a geological time scale. Furthermore, the drivers of chemical weathering and associated CO2 budget were deciphered. The significant positive correlations among chemical weathering fluxes, acids, and land uses quantitatively emphasized that the chemical weathering of CO2 budgets was largely disturbed by anthropogenic activities. This study showed that integrating river water geochemistry and geographical parameters could achieve a deep understanding of catchment weathering CO2 budgets and potential driving mechanisms.