Sedimentary environment of Middle Jurassic anhydrite in the Qiangtang Basin, eastern Tethys: constraints from sulfur isotope

Middle Jurassic marine anhydrite deposits are extensively distributed in the Qiangtang Basin, Northern Tibet, which is significant for a better understanding of the marine environment conditions of eastern Tethys Ocean during the Middle Jurassic. In this study, we present a continuous and high-resolution record of sulfur isotopes (δ34S) composition of seawater sulfate from Middle Jurassic Lower Xiali Formation anhydrite (CaSO4) obtained from a deep core (~ 300 m) in the Qiangtang Basin, eastern Tethys. XRD results show that samples mainly consist of anhydrite. Combined with the microscopy by SEM, we suggest the anhydrite transformed from the Middle Jurassic gypsum in the Qiangtang Basin during burial, which formed under the arid climatic condition in eastern Tethys Ocean. The δ34S values of anhydrite samples range between + 9.1 and + 15.4 ‰ with an average value of + 13.2‰, which is lower than the sulfur isotopic composition of Jurassic seawater previously reported. Combined with the remarkable changes in 87Sr/86Sr, δ13C and δ18O, we suggest that the Qiangtang basin in the eastern Tethys Ocean was partly influenced by the chemical weathering, and enhancing of evaporite dissolution leading to a large flux of sulfate to the ocean, permitting high rates of pyrite burial, increasing atmospheric and ocean O2. This study contributes to improve understanding of anhydrite formation in modern and ancient playas, and of the environment conditions of the eastern Tethys Ocean.