Calcium isotope evidence for secondary carbonate formation in the Upper Yellow River, Northeastern Tibetan Plateau

Abstract

Chemical weathering and secondary mineral formation, both of which involve Ca cycling, are intricately linked to the carbon cycle. However, the impact of these geochemical processes on Ca cycling, particularly through secondary carbonate formation in terrestrial environments, remains poorly understood. In this study, we analysed Ca isotopic compositions in the Upper Yellow River, northeastern Tibetan Plateau, to investigate how chemical weathering and secondary processes affect Ca cycling in this region. Inversion modeling reveals the relative contributions of different sources to dissolved Ca in river water. Although Ca fractions from different sources show linear correlations with riverine δ^44/40Ca values, detailed analysis reveals that these correlations do not imply a source control. Strong positive correlations between Sr/Ca, Mg/Ca, and δ^44/40Ca suggest that the formation of secondary carbonate predominantly regulates δ^44/40Ca in both river and lake water. Using Monte Carlo simulations, the Rayleigh fractionation model best fits a Ca isotope fractionation factor of 0.9997. The highest δ^44/40Ca values (1.49 ‰ to 1.76 ‰) are attributed to secondary carbonate formation in lakes. Meanwhile, secondary carbonate, such as carbonate coatings, formed within the soil matrix and primarily influenced by rainfall, evaporation, and evaporite dissolution, explains ~0.3 ‰ of the variation in riverine δ^44/40Ca. Based on literature data, we propose that secondary carbonate formation could be the primary cause of the Ca isotope imbalance in river water. This study highlights the influence of secondary carbonate formation on the carbon cycle, particularly through its impact on CO₂ outgassing from lakes and the soil inorganic carbon pool.