Oxygenation variations in the atmosphere and shallow seawaters of the Yangtze Platform during the Ediacaran Period: Clues from Cr-isotope and Ce-anomaly in carbonates

Abstract

The late Neoproterozoic Era witnessed a significant oxygenation in Earth’s surface in line with the naissance and diversity of early animals; however, detailed variations of the oxygenation during this period remain controversial. Chromium-isotope in ancient carbonates has shown its ability to trace the oxygenation history of the atmosphere-hydrosphere system. In this study, we analyzed Cr-isotope and Ce-anomaly data of carbonates from the Ediacaran Doushantuo and Dengying formations in the Yangtze Gorges area, South China, in order to reconstruct oxygenation variations in the atmosphere and shallow seawaters of the Yangtze Platform during the Ediacaran Period.After evaluating and excluding detrital and diagenetic effects, well-preserved δ53Crauth signatures were obtained. Post-Marinoan carbonates record slightly positive δ53Crauth values (0.18–0.43‰) and slightly negative Ce-anomaly, suggesting generally oxidizing conditions in the atmosphere (with pO2 over 1% PAL) and in shallow seawaters of the Yangtze Platform during the post-Marinoan phase, which may have been related to the diversity of early Ediacaran organisms. Shuram/Wonoka correlative carbonates are characterized by the fluctuations of δ53Crauth values (−0.25 to 0.97‰) and a significant negative δ13C excursion, which may have resulted from partial oxidation of the large deep-water DOC reservoir under enhanced oxidative weathering conditions. This process released 13C-depleted carbon into DIC pool and isotopically unfractionated Cr (III) into water columns, the latter of which immediately precipitated by adsorption onto the detrital materials. Meanwhile, an increase in local anoxia (and even euxinia) in the intrashelf basin of the Yangtze Platform driven by enhanced microbial sulfate reduction and the resultant H2S, possibly limited regional biological evolution during this phase. Late Ediacaran carbonates record elevated δ53Crauth values (0.40–0.96‰) together with the decreasing trend in Ce/Ce∗ values, indicating a more oxygenated environment in the atmosphere and shallow seawaters of the Yangtze Platform during the late Ediacaran phase, which may have been beneficial to the diversification of late Ediacaran animals.