The sulphur and oxygen isotopic composition of anhydrite from the Upper Pechora Basin (Russia): new data in the context of the evolution of the sulphur isotopic record of Permian evaporites

Abstract

This study describes a new determination of the S and O isotope composition of Lower Permian (Kungurian) anhydrites from the Upper Pechora Basin, Cis-Ural region, Russia. δ³⁴S values in sulphate facies vary from +13.7 to +15.1‰; and δ¹⁸O values range from +9.3 to +10.4‰. The values of d34S and d18O of anhydrite from halite facies varies from +12.6 to +15.0‰ and +7.5 to +10.9‰ respectively. The quantitative ratio of pyrite content from the water-insoluble residue (silty-sand fraction) is characterized by extremely low (<<1%) to high (4–5%) steep gradation values. The increased presence of pyrite indicates the influence of bacterial sulphate reduction. The sulphate reduction process was more intense, especially when evaporites were formed in mud. The narrow fluctuation range of sulphur and oxygen isotopes values of the measured anhydrite indicates low levels of fractionation. It was established that during the Permian, evolutionary changes in the content of sulphate ions in sea water correlate with the sulphur isotopic composition of marine evaporites