Calcium isotope variations in the upper Yangtze River: evidence for the control of secondary carbonate formation.

Calcium (Ca) isotopes are increasingly recognized as sensitive tracers of chemical weathering and carbonate formation, yet the controls on riverine δ44/40Ca remain poorly constrained in large mountainous catchments. Here, we investigated dissolved Ca sources and isotopic behavior in the upper Yangtze River by comparing high-altitude and low-altitude sub-basins during the low-flow season. The δ44/40Ca values ranged from 0.74‰ to 0.98‰ in high-altitude regions (mean = 0.86‰) and from 0.77‰ to 0.87‰ in low-altitude regions (mean = 0.83‰), both within the global riverine range and showing only a small, statistically insignificant difference between altitude groups. Inversion modeling showed that carbonate weathering dominated dissolved Ca in low-altitude regions (20-89%, mean 60%), whereas high-altitude waters displayed more variable source contributions, with carbonate weathering contributing 15-80%, evaporite dissolution 6-53%, and silicate weathering 7-47%. Although source proportions were correlated with δ44/40Ca, conservative mixing remains an important control and cannot be fully excluded for all samples; secondary carbonate formation is therefore interpreted as a possible additional influence superimposed on source mixing. In high-altitude regions, δ44/40Ca was positively correlated with both Sr/Ca and Mg/Ca, consistent with possible preferential removal of light Ca isotopes during carbonate precipitation. In low-altitude regions, δ44/40Ca remained positively correlated with Sr/Ca, whereas the relationship with Mg/Ca was weak, suggesting stronger fractionation under longer water-rock interaction and slower flow conditions. Monte Carlo uncertainty propagation shows that the apparent α values for the high- and low-altitude groups are not statistically distinguishable within uncertainty. These results suggest that altitude-related hydrogeochemical settings modulate riverine Ca isotope behavior through combined effects of source mixing and possible secondary carbonate formation. This study demonstrates that altitude-driven hydrogeochemical variations govern riverine Ca isotope dynamics in the upper Yangtze, offering a robust tool for tracing secondary carbonate precipitation and quantifying carbonate weathering-related carbon fluxes in large river basins.