Rb-Cs systematics in salt lakes and their abnormal enrichment in lake sediments on the Tibetan Plateau

Highlights:Salt lakes on the Tibetan Plateau are generally enriched in rare alkali metals, including lithium (Li), rubidium (Rb) and cesium (Cs), which are mainly sourced from extensive geothermal fluids. However, geochemical behaviors of Rb and Cs during the source-sink process in salt lake basins are different from Li, resulting in their limited enrichment in brines and deviation from the geothermal source signatures. Using Damxung Co Salt Lake as a representative case, this study investigated the source-sink dynamics through hydrochemical analysis, H-O isotopes, and sediment sequential extraction. Results indicate that Rb and Cs are mainly derived from geothermal fluids, with deep high-temperature water-rock interactions and input of minor magmatic fluid. Sinters (Rb: 24.65 ppm; Cs: 13.65 ppm), lake brines (Rb: 13.64 mg/L; Cs: 0.54 mg/L), and sediments (Rb: 77.0 ppm; Cs: 32.0 ppm) act as the three major gathering carriers for Rb and Cs, in which lake sediments represent the dominant occurrence reservoir. Mass balance calculations indicate that approximately 69% of Rb and 97% of Cs have been removed from the water column and sequestered in the sediments. Rb and Cs are specifically associated with the acid-insoluble fraction of illite. This enrichment is controlled by salinity: in low-salinity environments (fresh-brackish water), the low hydration energies of Rb⁺ and Cs⁺ facilitate their preferential adsorption onto the illite. In contrast, during the high-salinity (salt lake) stage, part of Rb and Cs were desorbed from the sediments due to the competitive absorption of other high concentrations of cations (Na⁺, K⁺). This study elucidates Rb-Cs behavior in salt lakes, highlighting their preferential enrichment in sediments. It further suggests that ongoing lake desalination favors the preservation of Rb and Cs in the sedimentary sink.