Lithium isotope compositions of the Yangtze River headwaters: Weathering in high-relief catchments

Tectonically active high relief areas are proposed to have a substantial impact on climate regulation throughout the Cenozoic. Here, we present lithium (Li) isotope data of the dissolved load and suspended particulate material from the basins of the Yalong River and the Jinsha River, which drain the high-relief borders of the eastern Tibetan Plateau and constitute the headwaters of the Yangtze River. Dissolved delta Li-7 values are relatively low and constant in the Jinsha River mainstream (+6.8 parts per thousand to +9.7 parts per thousand) compared to the higher and downstream increasing values in the Yalong River mainstream (+10.8 parts per thousand to +17.5 parts per thousand). Furthermore, dissolved delta Li-7 values in the major tributaries of the Jinsha and Yalong River are highly variable and increase downstream from +11.4 parts per thousand to +29.4 parts per thousand and from +9.0 parts per thousand to +23.6 parts per thousand, respectively. Robust correlations between Li, Na, and Cl concentrations identify that evaporite dissolution in the uppermost headwaters of the Jinsha River basin controls the Li budget and riverine delta Li-7 values of the mainstream and has a significant impact on dissolved Li in the Yangtze River. In contrast to the Jinsha River mainstream, dissolved Li in the Yalong River mainstream and the Jinsha and Yalong River tributaries are dominantly derived from the dissolution of silicates. delta Li-7 variations in the Yalong River basin and the Jinsha tributaries reflect little Li uptake by clays in the upper reaches and more Li uptake by clays in the lower reaches of the river basins. SWR/D ratios (where SWR is the chemical silicate weathering rate, and D is the total denudation rate) highlight an intermediate silicate weathering intensity under a kinetically limited weathering regime for the study area. However, positive correlations between dissolved delta Li-7 value and silicate weathering intensity proxies (such as Si/(Na-sil + K) and K/(Na-sil + K)) indicate an increasing weathering intensity downstream. A positive correlation between dissolved delta Li-7 values and the annual precipitation and a negative one between delta Li-7 values and elevation indicates that larger annual precipitation under a flattening relief results in a stronger silicate weathering intensity, thus, more Li uptake in clays and higher riverine delta Li-7 values in the lower reaches. Compared to the lower reaches of Yangtze River basin (similar to+20 parts per thousand), lower dissolved delta Li-7 values in its headwater of this study suggest that the increase in delta Li-7 of seawater over the Cenozoic is influenced by weathering in floodplains that developed due to erosion of high mountains, rather than weathering of mountain belts itself. Combined with silicate weathering rates of the eastern border of the Tibetan Plateau, the highest riverine delta Li-7 values in its catchments come along with the highest silicate weathering rates and high CO2 consumption, suggesting that a more elevated topography may have an important role in the drawdown of CO2 during the Cenozoic. (C) 2020 Elsevier Ltd. All rights reserved.