Trace and rare earth elements geochemistry of sedimentary rocks in the Ediacaran-Cambrian transition from Tarim Basin, Northwest China: Constraints for redox environments

Abstract Temporal and spatial variations in the redox conditions during the Ediacaran-Cambrian (E-C) transition have a close relationship with the extinction of Ediacaran biota and the Cambrian explosion. However, detailed variations of ocean redox gradients remain controversial. In this study, a large dataset of major and trace elements is obtained from the leachates of carbonate rocks sampled from three sections of the E-C transition in the Tarim Basin. The cerium anomaly of the late Ediacaran Qigebulage Formation, exhibited a regular gradient from anoxic conditions at the base to oxic conditions at the top, suggesting a gradually oxidizing environment. This could have been a beneficial factor leading to the diversification of the late Ediacaran fauna. Significantly negative carbon isotope excursions within the E-C boundary of the three studied sections were identified at the base of the Yuertusi Formation. This is coincident with the Ediacaran biota extinction due to an anoxic sedimentary environment. The base of the Yuertusi Formation was deposited under anoxic or/and euxinic conditions and intense hydrothermal activity during the early Cambrian. The upper layers of the Yuertusi Formation exhibited significant Ce anomalies following a general increase in the oxidation level, which reflected an environment transitioning from anoxic/euxinic to oxic following the early Cambrian transgression. A significantly negative δ13Ccarb excursion was observed at the boundary of the Cambrian Series 2 and Cambrian Miaolingian (the “ROECE” event). From the Cambrian Series 2 Xiaoerbulak Formation to the Cambrian Miaolingian Awatage Formation, carbonate rocks were deposited in deeper water conditions and a more anoxic environment. Redox fluctuations could be ascribed to the widespread early Cambrian transgression and tectonic movements, such as the formation of the Tianshan Ocean during the early Cambrian, and the accompanying volcanic activity in the Tarim Basin. Marine redox fluctuations Coupled with the paleontological records indicate that the increased concentration of oxygen in the water promoted evolutionary transitions and accelerated the diversification of species during the E-C transition.