The Ediacaran palaeo-ocean environment in the northwest margin of South China Craton: Constraints of C–O–Mo isotopic and element geochemistry

Abstract

The coevolution of the redox environment, productivity and Ediacaran fauna is crucial to understanding the trigger of Early Cambrian explosion of the Chengjiang Biota and Qingjiang Biota in the southwestern and northern margins of the South China Craton, respectively. Multiple element and isotope geochemical proxies are effective indicators for palaeo-ocean environment and productivity. In this study, our new combination of geochemical proxies of FeT/Al, Th/U, V/(V + Ni), V/Sc, Ceanom, Uanom, CrEF, Al/Ti, MoEF, (Al, P, Ba, Cu, Zn, Ni)xs, and δ13C and δ98Mo isotopic data from the Ediacaran Dengying Formation in the Gaojiashan section suggest multiple rapid short-term pulsed oxygenation and reduction events. An approximate correlation between the redox environments, palaeoproductivity and life events can be concluded as well. Based on the stratigraphic position, lithology, fossil records and geochemical characteristics, the entire profile of the Ediacaran Dengying Formation can be divided into five intervals. The redox environment of Interval I experienced fluctuations from predominantly anoxic to slightly suboxic environments, and then increased to a flexible dysoxic–oxic–anoxic environment during Interval II. Meanwhile, the productivity also frequently fluctuated at a relatively low level. The relatively unstable seawater and pulsing fluctuations of the redox conditions and palaeoproductivity can be correlated to the hydrothermal activities. From Interval III, the redox environments have been improved by several phases of oxygenation, from the relatively oxic–suboxic conditions at the beginning of Interval III to highly oxic conditions during Interval IV. Meanwhile, the palaeoproductivity soared after the early stage of Interval III and continuously increased to a high level until the later stage of Interval III and Interval IV. The highly oxic environment and continuously increasing palaeoproductivity resulted in the flourishing of the Ediacaran Gaojiashan fauna. From Interval V, both the redox conditions and the palaeoproductivity were at a low level, although they improved during the later stage, resulting in the previous Cloudina fauna disappearing. After comparing other contemporaneous profiles in South China and worldwide, we found that most profiles experienced similar variation of Ediacaran seawater redox environments that were under a chronically turbulent, with rapid short-term pulsed oxygenation and reduction events. This type of pulsed oxygenation did not deplete the local life community significantly. The regional depositional settings, redox environments and palaeoproductivity are the most important controlling factors of the life events.