Recurrent photic zone euxinia limited ocean oxygenation and animal evolution during the Ediacaran

The Ediacaran Period (similar to 635-539 Ma) is marked by the emergence and diversification of complex metazoans linked to ocean redox changes, but the processes and mechanism of the redox evolution in the Ediacaran ocean are intensely debated. Here we use mercury isotope compositions from multiple black shale sections of the Doushantuo Formation in South China to reconstruct Ediacaran oceanic redox conditions. Mercury isotopes show compelling evidence for recurrent and spatially dynamic photic zone euxinia (PZE) on the continental margin of South China during time intervals coincident with previously identified ocean oxygenation events. We suggest that PZE was driven by increased availability of sulfate and nutrients from a transiently oxygenated ocean, but PZE may have also initiated negative feedbacks that inhibited oxygen production by promoting anoxygenic photosynthesis and limiting the habitable space for eukaryotes, hence abating the long-term rise of oxygen and restricting the Ediacaran expansion of macroscopic oxygen-demanding animals.

Automated summary

We used mercury isotope compositions from black shale sections in South China to reconstruct the redox conditions of the Ediacaran ocean. Our results provide compelling evidence for recurrent photic zone euxinia occurring on the continental margin of South China during times of ocean oxygenation events. We suggest that this euxinia was driven by increased availability of sulfate and nutrients from a transiently oxygenated ocean, but also may have inhibited oxygen production and restricted the expansion of macroscopic oxygen-demanding animals in the Ediacaran.