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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]

Hydrothermal systems, including hot springs and hydrothermal calcite, are common features of orogenic belts. The hot springs and hydrothermal calcite in the Himalayas are characterized by radiogenic Sr related to metamorphic processes. Such metamorphism-driven processes have contributed significant amounts of radiogenic Sr to rivers and groundwater, which has affected the evolution of global seawater 87Sr/86Sr during the Cenozoic. However, whether such hydrothermal systems with highly radiogenic Sr occur in other continental collisional belts with intense metamorphism remains unknown, which hinders a better understanding of regional and global Sr cycles. Compiled 87Sr/86Sr ratios in surface water around the Tibetan Plateau show that the Qilian-North Qaidam region displays a higher 87Sr/86Sr ratio than other regions, except for the Himalayas. Here, we report 87Sr/86Sr ratios of a hydrothermal system on the northern margin of the early Paleozoic North Qaidam ultrahigh-pressure metamorphic belt, NE Tibetan Plateau. The high 87Sr/86Sr ratios (0.757–0.734) of the hot spring water and the acetic acid leachates of the surrounding rocks (gneiss and schist) with other isotopic tracers (e.g., oxygen, hydrogen, lithium, and boron isotopes) indicate that 1) Sr was exchanged during metamorphism between carbonate and silicate phases in crustal rocks and the high 87Sr/86Sr ratio of the hot spring water was acquired from a deep water-rock reaction process; and 2) hydrothermal systems around the Qilian-North Qaidam region operate like those in the Himalayas. Mass balance calculations indicate that the metamorphism-forced hydrothermal system plays a key role in delivering radiogenic Sr to rivers and lakes and yielding higher 87Sr/86Sr ratios in surface water around the Qilian Shan region than in the surrounding areas. The reactivated early Paleozoic Qilian orogenic belts with ultrahigh-pressure metamorphism caused by the India-Asia collision have significantly impacted the regional water 87Sr/86Sr ratio, suggesting that the reactivated old orogenic belts with intense metamorphic processes formed by continental subduction/collision can play a pivotal role in regulating past seawater 87Sr/86Sr evolution. [Display omitted] • Hot spring with a high 87Sr/86Sr of 0.757 was found in North Qaidam metamorphic belt. • Spring radiogenic Sr was formed during intense metamorphism and fluid-rock interaction. • Metamorphism-forced processes caused radiogenic Sr in surface water in Qilian Shan. • Hydrothermal systems in Qilian-North Qaidam region operate like those in Himalayas. • Reactivated orogen with intense metamorphism could affect regional or global Sr cycle. [ABSTRACT FROM AUTHOR]

The Paleogene environment of today's semi-arid and arid Central Asia is poorly quantified and knowledge of the paleoelevation of northern Tibet remains elusive, yet both are crucially important for understanding inter-relationships between growth of the Tibetan Plateau and Asian monsoon development. Here, we apply the Climate-Leaf Analysis Multivariate Program (CLAMP) and moist enthalpy method to the newly discovered early Oligocene (30.8 Ma) fossil leaves from the Qaidam Basin, to reconstruct quantitatively the paleoclimate and paleoelevation of this critical part of northern Tibet. We find the Qaidam Basin floor vegetation was likely at ∼3.3 ± 1.4 km in the early Oligocene, similar to that of the present basin floor but higher than the rising Himalaya at that time, and experienced a temperate, moderately wet climate. Near-freezing (1.4 ± 3.5 °C) winters accompanied cool summers (∼23 ± 2.9 °C). Annual precipitation likely exceeded 1000 mm (growing season precipitation of 1229 ± 643 mm) with subdued (non-monsoonal) seasonality in which summers were drier than winters. This finding challenges geodynamic models that envisage a Miocene or later uplift of northern Tibet and progressive uplift from the south. • Early Oligocene climate of Qaidam estimated based on physiognomy of plant fossils. • Qaidam Basin was temperate and wet with low precipitation seasonality at ∼30.8 Ma. • Qaidam Basin floor was likely at its present height of ∼3.3 km in early Oligocene. • High elevations present in northern Tibet before the rise of the Himalaya. [ABSTRACT FROM AUTHOR]