How lithium isotope cycles in Qinghai Lake?

Recently, Prof. XIAO Yilin's team from the University of Science and Technology of China (USTC), in collaboration with the Qinghai Institute of Salt Lakes, University of Pennsylvania and Nanning Normal University, revealed the lithium (Li) cycling process and lithium isotope (δ⁷Li) fractionation mechanism in Qinhai Lake through their analysis of the water and sediments of the lake. Their research was published in Applied Geochemistry.

δ⁷Li has been a powerful tool to trace the silicate cycling at Earth's surface and paleoclimate evolution due to their large fractionation and immunity to biological effects. With minimal human activities, the pristine environment of Qinghai Lake makes it an ideal recorder of the paleoclimate evolution of the Qinghai-Tibetan Plateau. However, to understand the region's past climate evolution, we need to first unravel the mechanisms of δ⁷Li fractionation and element cycling in the lake’s water and sediment systems.

To tackle this problem, the team collected and analyzed the elemental compositions of the samples from Qinghai Lake’s water, sediments and recharge waters. The results indicated that the Li concentration shows slight fluctuations, mainly influenced by interactions with iron oxides or suspended particles. However, the δ⁷Li value demonstrates remarkable uniformity across the lake at 32.1‰ (±0.4‰). The δ⁷Li value of lake sediments varies minimally across sampling sites, approximately 30‰ lower than that in lake water, primarily due to the formation of authigenic clays.

Further, this team constructed a Li mass balance model for Qinghai Lake to ascertain the net Li isotopic fractionation between bulk sediment and lake water. This model revealed that Qinghai Lake is currently out of steady state, with Li inputs (46.8 t/a) slightly higher than Li outputs (44.4 t/a). Based on the assumption that the climate conditions remain constant, the lake's Li reserves are expected to gradually increase, reaching a steady state within 1.2 thousand years. The δ⁷Li value of the lake water will continue to rise until it reaches approximately 45‰ after 6 thousand years.

Given the similarities between Qinghai Lake and oceanic systems, this study provide potential insights into the evolution of Li isotopes in marine environment. Moreover, this study deepens our understanding of the geochemical processes within closed-basin water systems and holds great significance for reconstructing the paleoclimate history of the Tibetan Plateau.