Li isotopic seasonality in a small catchment at the northeastern Tibetan Plateau: Roles of hydrology and temperature dependency.

Silicate weathering is critical to sustain our habitable planet. Lithium (Li) isotopes enable us to investigate the nature of silicate weathering. A number of riverine Li isotope (δLi) investigations have been made from polar to equatorial terrains, but there remains no consensus about the controlling mechanisms of both weathering and δLi. Here we investigated δLi response to climate by collecting weekly river water samples in a small catchment (the Buha River within the Lake Qinghai basin) on the northeastern Tibetan Plateau, with stable tectonic, lithology, and topography. In the hydrology year of 2007 of the Buha River, we find that during the dry seasons, δLi ratios show temperature dependency typically, when the groundwater fed the river. During the monsoon seasons, δLi were obviously lower than the temperature dependency predicted values, when abundant rock dissolved and thereby fresh Li release into rivers. We propose that the hydrology and temperature dependency together play important roles in regulating δLi ratios in such an alpine small catchment. The mechanism is that long residence time facilitates the equilibrium chemical and Li isotopic fractionation during the dry seasons, so a temperature dependency of δLi is achieved. In contrast, rapid erosion and weathering contribution of fresh rock-like δLi to river water would significantly decrease δLi ratios during the monsoon seasons. This hypothesis can better interpret previously reported data of seasonal δLi variation, as a superposition between temperature dependency and hydrology regulation on silicates weathering in the small catchments besides tectonics.