The Sulfur Isotopic Characteristics of Evaporites in the Yarkand Basin of Xinjiang Province in the Paleocene and Its Paleoenvironmental Evolution

Abstract

The Yarkand Basin, located in the southwest of the Tarim Basin, is a northeastern part of the eastern Paratethys ocean in the Paleocene, and a significant amount of evaporites, with gypsum, anhydrite, and halite as the main types, were developed in this area. These evaporites record the sedimentary environment at that time. A study was conducted on the sulfur isotopic composition of gypsum in the Paleocene of the Yarkand Basin to explore the origin of the evaporites and interpret the sedimentary environment. The experimentally measured sulfur isotope δ34SCDT values of 187 gypsum samples ranged from 6.69‰ to 25.92‰ with an average value of 18.64‰. The overall trend of the Paleocene gypsum sulfur isotopic curve is consistent with the global seawater sulfur isotopic curve, which shows a decreasing trend. In the early and middle Paleocene, the curve shows four stages of sulfur isotope increase, indicating that the sedimentary environment during that time was mainly influenced by bacterial reduction and a relatively open sedimentary environment, while the late period shows a decreasing trend, suggesting that the late period may be primarily influenced by terrigenous freshwater. In addition, the sulfur isotope value has the characteristics of decreasing from northwest to southeast of the basin, which may indicate that the sedimentary environment of Paleocene evaporites in the Yarkand Basin may also be related to paleotopography and distance from the estuary, resulting in differences in sedimentary environments. The mainly original sulfur isotope values of the Paleocene evaporites in the Yarkand Basin should be in the range of 18‰–20‰, which is a supplement to the Paleocene global paleoseawater and is of great significance for the reconstruction of the marine transgressive–regressive cycle and sulfur isotopic composition of the eastern Paratethys ocean during this period.