Calcium isotopic evidence for secondary carbonate precipitation in a large river catchment, Southeastern Tibetan Plateau

Carbonate weathering consumes atmospheric CO₂, thereby regulating global climate by sequestering carbon transported by rivers in marine carbonate minerals in short timescales (<10 kyr). Calcium serves as one of the major elements that participate in carbonate weathering and global C cycle, and the riverine biogeochemical Ca cycle has attracted great concern in these decades due to its significant effect on the regulation of atmospheric CO₂ concentration and seawater chemistry. Therefore, investigating dissolved Ca and inorganic C fluxes could help understand carbonate weathering and its impact on C-Ca cycles under global warming circumstances. However, riverine solute fluxes based on the conservative-mixing models may not be conservative due to secondary processes, such as secondary carbonate precipitation. Ca isotopes provide valuable insights into the complex chemical weathering processes. Significant progress among chemical weathering studies has been constructed using Ca isotopes, but studies focusing on carbonate-dominated river catchments are limited. Carbonate weathering contributes ~80% of riverine dissolved Ca flux to global oceans, therefore, it is necessary to clarify the Ca isotope geochemical behaviors, especially in the catchment dominated by carbonate. Lancang River (LR) is located at the upper Mekong and originates from the southeastern Tibetan Plateau (TP), which is widely covered by carbonates and evaporites, making it an ideal natural setting for investigating carbonate weathering processes.

To address the above scientific issue, Prof. Han Guilin and Dr. Li Xiaoqiang of the China University of Geosciences (Beijing) conducted a detailed investigation of the Ca isotope compositions in river water and suspended sediments from the Lancang River. By integrating multi-isotope datasets (Li-K-Fe-Zn), they systematically elucidated the geochemical behavior of Ca isotopes during carbonate weathering.

Results shows that dissolved Ca showed wide variations of δ^44/40Ca values (0.64‰–1.05‰) with a declining trend along the river flow, with most mainstream values being close to 0.88‰. In contrast, suspended sediments were enriched in light Ca isotopes (0.46‰–0.75‰), which is controlled by mineralogy (clay vs. carbonate) based on Ca/Al ratios and Ca-Zn isotopic evidence.

The conservative mixing of different rock weathering inputs made it hard to demonstrate the fluctuation of dissolved Ca isotopes in the Lancang River and most of the world’s rivers. The riverine dissolved Ca isotopes in Ca-rich bedrock catchments were heavier than those of the catchment bedrock, and dissolved δ^44/40Ca were positively related to Sr/Ca, Mg/Ca, and the saturation index of calcite. This finding indicated that secondary carbonate precipitation may serve as a possible mechanism enriching heavier Ca isotopes in river water.

However, such an effect was not observed in lowland tributaries, where high dissolved δ^44/40Ca and δ⁷Li values reflected the influence of secondary clay mineral formation. When considering global rivers, dissolved Ca isotopic ratios were dominated by carbonate weathering and the precipitation of secondary carbonate. These processes controlled riverine Ca flux to the ocean, thereby highlighting the significance of secondary carbonate precipitation in river catchments.

 

See the article:

Han G, Li X. 2025. Calcium isotopic evidence for secondary carbonate precipitation in a large river catchment, Southeastern Tibetan Plateau. Science China Earth Sciences, 68(12): 4163–4174, https://doi.org/10.1007/s11430-025-1691-6

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