Ediacaran Marine Redox Heterogeneity and Early Animal Ecosystems

Oxygenation has widely been viewed as a major factor driving the emergence and diversification of animals. However, links between early animal evolution and shifts in surface oxygen levels have largely been limited to extrapolation of paleoredox conditions reconstructed from unfossiliferous strata to settings in which contemporaneous fossils were preserved. Herein, we present a multi-proxy paleoredox study of late Ediacaran (ca. 560-551 Ma) shales hosting the Miaohe Konservat-Lagerstätte of South China and, for comparison, equivalent non-fossil-bearing shales at adjacent sections. For the fossiliferous strata at Miaohe there is geochemical evidence for anoxic conditions, but paleontological evidence for at least episodically oxic conditions. An oxygen-stressed environment is consistent with the low diversity and simple morphology of Miaohe Biota macrofossils. However, there is no evidence for euxinic (anoxic and sulphidic) conditions for the fossiliferous strata at Miaohe, in contrast to adjacent unfossiliferous sections. Our results indicate that Ediacaran marine redox chemistry was highly heterogeneous, even at the kilometre-scale. Therefore, our study provides direct-rather than inferred-evidence that anoxia played a role in shaping a landmark Ediacaran ecosystem. If the anoxic conditions characteristic of the studied sections were widespread in the late Neoproterozoic, environmental stress would have hindered the development of complex ecosystems.