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The carbon pool stored in soil carbonate is comparable to the soil organic carbon. Therefore, secondary calcite precipitation in supersaturated catchment could be an important, yet poorly constrained, carbon sink within the modern global carbon cycle. The chemical analysis of some dissolved species transported by rivers, such as elevated Sr/Ca and Mg/Ca ratios but also heavy stable Ca isotopic compositions, witness the formation of secondary calcite in rivers draining arid regions. However, in areas affected by active tectonics and rapid physical erosion, co-variations in the fluvial Sr/Ca and Mg/Ca ratios could also be related to incongruent carbonate weathering processes. Here, we present a model to assess the roles played by incongruent carbonate dissolution and secondary calcite precipitation in modern weathering processes. We tested and applied the model to rivers draining the Himalayan-Tibetan region. The results suggest that regional aridity in the drainage basin promotes carbon sequestration as secondary carbonate but that for a given runoff, incongruent dissolution of carbonate possibly related to rapid physical erosion amplifies such sequestration. The isotopic compositions (C/C and O/O) of detrital carbonate transported by the main rivers in South and South-East Tibet imply that around 1% of the suspended material transported by those rivers corresponds to secondary carbonate and can represent between 5% and 15% of the alkalinity flux. Most of these alkalinity transported as particulate material is, nevertheless related to the weathering of carbonate lithologies and is also subjected to dissolution prior its final storage in sedimentary basins. However, on glacial-interglacial timescale this will amplify the significant role of mountain weathering on climatic variations. [ABSTRACT FROM AUTHOR]

The tectonically active NE Tibetan Plateau, which is located at the confluent area of the humid Asian monsoon region and dry Asian inlands, provides excellent archives of drainage pattern evolution in response to regional changes in topography and climate. However, the uplift of the NE Tibetan Plateau and formation of modern drainage patterns are still disputed. In this study, based on the Sr Nd isotopic distribution in modern surface water and sediments, we present integrated records of 87Sr/86Sr ratios in carbonate and 87Sr/86Sr-ε Nd (0) values in silicate of the Huaitoutala section in the northern Qaidam Basin to reveal the source-to-sink history of particulate and dissolved loads since ∼15 Ma. Our results show that changes in Sr Nd isotopes at ca. 10 Ma and ca. 5 Ma are indicative of two stages of source alteration and reorganization of the drainage system. Specifically, a remarkable increase in 87Sr/86Sr ratios in carbonate and silicate together with a decrease in ε Nd (0) values since ca. 10 Ma suggests a tectonic denudation event of Qilian Shan at ∼10 Ma. The subsequent declines in 87Sr/86Sr ratios in carbonate and silicate together with an increase in ε Nd (0) values since ca. 5 Ma, which approach the modern 87Sr/86Sr ratios near the study section site, suggest that a modern-like drainage pattern in the northern Qaidam Basin might have formed in response to tectonic uplift of Qilian Shan at ca. 5 Ma. Using coupled carbonate Sr-silicate Sr Nd isotopes, we provide reliable provenance tracers of drainage reorganization and the evolution of source-to-sink systems caused by tectonic deformation. • Carbonate and silicate Sr-Nd isotopic records since ∼15 Ma in the North Qaidam Basin. • Sr-Nd isotopic records show a three-phase evolution with shifts at ∼10 Ma and ∼ 5 Ma. • Particulate and dissolved loads with distinct Sr-Nd isotope changes synchronously. • Drainage reorganization caused by uplift and denudation of the Qilian Shan. • Modern-like drainage patterns in the North Qaidam Basin may have formed after ∼5 Ma. [ABSTRACT FROM AUTHOR]