Lithium isotopes as a chemical weathering proxy in lacustrine sediments: Implications from multiphase leaching analyses

Abstract

Reconstructing the history of continental silicate weathering is critical for understanding the mechanism that maintains Earth's habitability. Lithium (Li) isotopes have become a valuable indicator for tracing silicate weathering because of their negligible interference from carbonate and parent rock type. Although substantial effort has focused on the use of Li isotopes as a weathering proxy in marine settings, Li isotope distributions within terrestrial sediments remain largely unexplored. Here, we use a sequential leaching method to characterize Li isotopes across different phases of lacustrine sediments from the Qaidam Basin. The phases include the soluble salt, exchangeable, carbonate, silicate, and clay fractions. The results revealed that, compared with 0.05 M hydrochloric acid, 1 M acetic acid effectively extracted carbonates with minimal silicate contamination. The soluble salts and carbonates present similarly high and positively correlated δ⁷Li values, implying that these authigenic minerals reflect the dissolved Li isotopic composition of the palaeolake. The silicates in the whole rock and clay-size phases display similarly low δ⁷Li values and may capture trends in the Li isotopic compositions of weathering-related alteration products. However, both phases may overestimate the δ⁷Li value of the weathering-related alteration products by approximately 0.4–1.3 ‰ because of the presence of unweathered detrital minerals (e.g., quartz and feldspar). This study underscores the potential of multiphase Li isotope investigations in lacustrine sediments to yield integrated Li isotope signals from weathering solutions and alteration products, highlighting the importance of Li isotopes in terrestrial sediments for reconstructing catchment-scale silicate weathering histories.