Carbon and sulfur isotope chemostratigraphy of the Neoproterozoic Quanji Group of the Chaidam Basin, NW China: Basin stratification in the aftermath of an Ediacaran glaciation postdating the Shuram event?

The Neoproterozoic Quanji Group in the Chaidam Basin, northwest China, consists of, in ascending order, the Mahuanggou, Kubaimu, Shiyingliang, Hongzaoshan, Heitupo, Hongtiegou, and Zhoujieshan formations. It is a siliciclastic-dominated sequence with only two carbonate units, the ∼300-m-thick Hongzaoshan dolostone and the ∼4-m-thick Zhoujieshan cap dolostone, the latter of which lies above the Hongtiegou glacial diamictite. In this study, we analyzed the δ13C and δ18O of the Hongzaoshan dolostone and Zhoujieshan cap dolostone, and the δ34SCAS, δ34Spy, and trace element compositions (Fe, Mn, Sr, and S) of the Zhoujieshan cap dolostone. δ13C profile of the Hongzaoshan dolostone shows a shift from −5‰ to 0‰. This shift could be correlated with a similar shift in the Shuiquan Formation (<615 ± 6 Ma) in the Quruqtagh area of the Tarim Block (NW China), and may be equivalent to negative δ13C excursions in the middle or uppermost Doushantuo Formation, the latter of which is widely believed to be correlative with the Shuram negative δ13C excursion. This correlation, together with available paleontological evidence (fragments of Redkinia from the Heitupo Formation, a genus that also occurs in ∼555 Ma successions in the East European Platform), suggests that the Hongtiegou diamictite may represent an Ediacaran glaciation postdating the Gaskiers glaciation or occurring near the Precambrian–Cambrian boundary. The Zhoujieshan cap dolostone is characterized by positive δ13C values ranging from 0‰ to +1.7‰, in sharp contrast to cap carbonates overlying the Sturtian and Marinoan diamictites. δ34SCAS of the Zhoujieshan cap dolostone varies strongly between +13.9‰ and +24.1‰, probably due to relatively low-sulfate concentration in seawater. δ34Spy values, however, show a steady increase from +12.9‰ to +26.4‰. Thus, δ34SCAS and δ34Spy trends are decoupled in the Zhoujieshan cap dolostone, and the high δ34Spy values in the upper part of the cap dolostone indicate inverse sulfur isotope fractionations (δ34SCAS < δ34Spy). The decoupling of δ34SCAS and δ34Spy suggests that CAS and pyrite were derived from different sulfur pools (a possible scenario if the post-glacial Oulongbluq basin was restricted and stratified) and/or that aerobic oxidation of sulfide was intense in the post-glacial Oulongbluq basin.