Secondary carbonate precipitation plays a crucial role in determining the CO2 outgassing from carbonate-saturated rivers and influences the short-term terrestrial carbon cycle. The fractionation of stable isotopes has been extensively employed to quantify post-weathering reactions in rivers and assess the quantities of metals removed by secondary carbonates. However, a major challenge is that water isotopic compositions usually reflect both lithological mixing and biogeochemical fractionation, which complicates distinguishing between these signals. In this study, we applied triple Sr isotopes (87Sr/86Sr and δ88/86Sr) to determine the "unfractionated" δ88/86Sr of water. This method allowed us to precisely quantify the precipitation-derived fractionation. Our results show that a median of 48 % of Sr was incorporated into carbonates in southwestern (SW) Taiwan, corresponding to the removal of 69 % of the originally weathered Ca. This process is closely related to a significant increase in HCO3- concentration in the water. In SW Taiwan, high river discharge during wet periods typically enhances carbonate dissolution, leading to increased carbonate precipitation along with CO2 outgassing. Thus, hydrology and river discharge constraints significantly influence CaCO3 precipitation and CO2 outgassing processes and their associated CO2 budget. Using a machine learning model, we estimated that the precipitation-derived CO2 outgassing from rivers is +7.9 × 108 mol per year. This flux is nearly twice the long-term (~10 kyr) emission flux of +4.2 ± 1.4 × 108 mol per year in this region. This indicates that precipitation-driven CO2 emissions from rivers constitute a significant carbon source within the terrestrial carbon cycle of orogenic regions.
Keywords: CO(2) outgassing; Carbonate precipitation; Carbonate weathering; Silicate incongruent weathering; Triple Sr isotopes.
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