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

Abstract

Spatiotemporal variations in the redox conditions during the Ediacaran-Cambrian (E-C) transition are closely related to the extinction of Ediacaran biota and the Cambrian explosion. However, the detailed variations in the ocean redox gradients remain a controversial issue. In this study, a large dataset of major and trace elements was obtained from the leachates of carbonate rocks sampled from three sections of the E-C transition in the Tarim Basin, northwest China. The Ce anomaly in the late Ediacaran Qigebulage Formation exhibited a regular gradient from anoxic conditions at its base to oxic conditions at its top, suggesting a gradually oxidizing environment. This could have been a beneficial factor that led to the diversification of late Ediacaran fauna. In addition, significantly negative carbon isotope excursions within the E-C boundary of the three sections studied were identified at the base of the Yuertusi Formation. This is coincident with the extinction of Ediacaran biota 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 this 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 evidence of the Redlichiid-Olenellid Extinction Carbon isotopic Excursion event recorded at the boundary of the Cambrian Series 2 and Cambrian Miaolingian. From the Cambrian Series 2 Xiaoerbulak Formation to the Cambrian Miaolingian Awatage Formation, carbonate rocks were deposited in deep water conditions and an anoxic environment. Redox fluctuations could be ascribed to widespread early Cambrian transgressions and tectonic activity, 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.