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The Ediacaran biota were soft-bodied organisms, many with enigmatic phylogenetic placement and ecology, living in marine environments between 574 and 539 million years ago. Some studies hypothesize a metazoan affinity and aerobic metabolism for these taxa, whereas others propose a fundamentally separate taxonomic grouping and a reliance on chemoautotrophy. To distinguish between these hypotheses and test the redox-sensitivity of Ediacaran organisms, here we present a high-resolution local and global redox dataset from carbonates that contain in situ Ediacaran fossils from Siberia. Cerium anomalies are consistently >1, indicating that local environments, where a diverse Ediacaran assemblage is preserved in situ as nodules and carbonaceous compressions, were pervasively anoxic. Additionally, δ²³⁸U values match other terminal Ediacaran sections, indicating widespread marine euxinia. These data suggest that some Ediacaran biotas were tolerant of at least intermittent anoxia, and thus had the capacity for a facultatively anaerobic lifestyle. Alternatively, these soft-bodied Ediacara organisms may have colonized the seafloor during brief oxygenation events not recorded by redox proxy data. Broad temporal correlations between carbon, sulfur, and uranium isotopes further highlight the dynamic redox landscape of Ediacaran-Cambrian evolutionary events.

Photic zone euxinia (PZE) is a condition where anoxic, H₂S-rich waters occur in the photic zone (PZ). PZE has been invoked as an impediment to the evolution of complex life on early Earth and as a kill mechanism for Phanerozoic mass extinctions. Here, we investigate the potential application of mercury (Hg) stable isotopes in marine sedimentary rocks as a proxy for PZE by measuring Hg isotope compositions in late Mesoproterozoic (∼1.1 Ga) shales that have independent evidence of PZE during discrete intervals. Strikingly, a significantly negative shift of Hg mass-independent isotope fractionation (MIF) was observed during euxinic intervals, suggesting changes in Hg sources or transformations in oceans coincident with the development of PZE. We propose that the negative shift of Hg MIF was most likely caused by (i) photoreduction of Hg(II) complexed by reduced sulfur ligands in a sulfide-rich PZ, and (ii) enhanced sequestration of atmospheric Hg(0) to the sediments by thiols and sulfide that were enriched in the surface ocean as a result of PZE. This study thus demonstrates that Hg isotope compositions in ancient marine sedimentary rocks can be a promising proxy for PZE and therefore may provide valuable insights into changes in ocean chemistry and its impact on the evolution of life.