Hygroscopic Properties of Plateau Surface Salts: Insights from Chemical Composition and Isotope Signatures

Abstract

Evaporite salts from saline lakes and playas play active roles in the atmospheric cycles and the climate system, especially in the context of changing climate. This study investigates the chemical, isotopic, and hygroscopic characteristics of surface salt samples from two saline lakes, i.e., Mang’ai and Dalangtan (MA and DLT), in the Qaidam Basin. Samples from both lakes shared similar ionic compositions, with brines rich in Cl–, Mg2+, and Na+, and lakebed salts being primarily NaCl-based. Disparities in the composition between MA and DLT crust salts were observed. Isotopic analyses revealed consistent δ34S values within samples from a single site, hinting at a common origin. The sulfur source for the MA saline lake likely arises from nearby freshwater inflows and atmospheric deposits. The δ37Cl values varied by sample type, with solid samples typically exhibiting higher values than brines, attributed to 37Cl depletion during precipitation. Ionic composition largely determines hygroscopic properties. While brines started moisture absorption around 40% relative humidity (RH), lakebed salts commenced at 70% RH. The DLT playa salt, enriched in Na2SO4, demonstrated a unique behavior, responding significantly only above 80% RH. The layered DLT samples showcased variable hygroscopic behaviors, particularly the early moisture uptake of the topmost layer, despite its ionic similarity to another layer, hinting at molecular or hydration disparities. In conclusion, this investigation unravels the multifaceted relationship between salt evaporites composition and their implications for atmospheric chemistry.