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

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 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 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. On the hygroscopic properties, the ionic composition is identified as a key determinant. While brines started moisture absorption around 40% RH, lakebed salts commenced at 70% RH. The DLT playa salt, enriched in Na₂SO₄, demonstrated 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.