The evolution of lithium resources and isotopic composition in salt lakes on the Qinghai-Tibet Plateau: A source–transport–sink perspective

The source–transport–sink dynamics of salt lakes are fundamentally tied to resource source and mineralization, which are crucial for sustainable resource development and environmental protection. By integrating published and experimental datasets on lithium (Li) concentrations, Li isotopes, and Li/TDS–Li/Na ratios, this study systematically investigates the characteristics, evolutionary patterns, and driving mechanisms of Li and its isotopes throughout source–transport–sink processes in salt lakes across the Qinghai–Tibet Plateau. The results demonstrate that: (1) Li in salt lakes primarily originates from geothermal fluids, with significant contributions from Li-rich rocks and paleosediments. (2) Li transport mechanisms can be classified into source- and process-control. In source-control systems, Li is largely derived from Li-rich endmembers; although secondary inputs and attenuation occur during transport, the persistently high dissolved Li load governed by the original source retains a diagnostically traceable isotopic composition. This system is marked by high dissolved Li fluxes (>300 μg/L), elevated Li × 10³/TDS ratios (>0.7), and relatively depleted δ⁷Li values (1 ‰ to 6 ‰, occasionally as low as −4.8 ‰). In process-control systems, Li mainly comes from silicate weathering within catchments, resulting in lower riverine Li fluxes (20–80 μg/L) that are highly sensitive to environmental conditions, where source signals are frequently overprinted by secondary inputs and adsorption. These systems exhibit lower Li × 10³/TDS ratios (0.05–0.22) and enriched δ⁷Li values ranging from 6 ‰ to 18 ‰. (3) The sink evolution of Li and its isotopes is controlled by clay adsorption and evaporite precipitation, closely correlating with developmental phases of salt lake. Clay adsorption causes Li depletion and isotopic fractionation, leading to elevated δ⁷Li signatures in the early evolutionary phase. In later phases, evaporate becomes the dominant control on brine Li isotope evolution due to evaporite formed aquicludes, reduced adsorption capacity of ancient clays, and suppression of adsorption under high salinity. (4) This study offers valuable references for understanding Cenozoic marine Li isotope evolution by establishing a source–transport–sink framework within a small sink basin. Tectonic uplift has enhanced continental weathering and physical erosion, increasing supplies of dissolved Li and fresh clay minerals in runoff, while climate change has reduced continental discharge and extended water–rock interaction time. These processes collectively enhance water–rock interactions through increased reactant supply and prolonged reaction duration, elevating riverine δ⁷Li fluxes into the ocean and influencing marine Li isotope evolution.