Sulfur isotope constraints on the formation of MgSO₄-deficient evaporites in the Qarhan salt Lake, western China

The Qarhan Salt Lake (QSL) in the Qaidam Basin in western China is the largest brine potash deposit in the world. The formation of MgSO4-deficient evaporites in the QSL, whether from the evaporation of river waters with high Na+ and Cl- levels, bacterial sulfate reduction (BSR) in brines resulting in SO42- depletion, or the mixing of the river and spring waters, remains controversial. The delta S-34 values of 33 intercrystalline brines and 10 gypsum samples from a sediment core (ISL1A) collected in the QSL were presented in this study. Combining these results with previously reported major ionic concentrations, total dissolved solids (TDS) and delta S-34 values of different waters, and Sr-87/Sr-86 ratios of sediments, it was concluded that (1) delta S-34 values of the intercrystalline brines (+ 6.7 parts per thousand to +12.1 parts per thousand) and gypsum (+ 8.1 parts per thousand to +10.1 parts per thousand) suggested that the BSR was insignificant in the QSL. These values were dominated by those of inflowing waters and the evaporation process; (2) river waters with high Na+ and Cl- levels were unable to form MgSO4-deficient evaporites in the QSL. A strong positive correlation of Na and Cl equivalents coupled with high (Na + Cl-)/TDS ratios suggested that the large amount of solutes in Golmud River waters results from the dissolution of halite rather than the chemical weathering of rocks; and (3) the flowing system of the river and spring in the past 50 kyr was similar to the modern supplying system, and mixing of river and spring waters was responsible for the precipitation of evaporites in the QSL.