Vanadium isotope evidence for expansive ocean euxinia during the appearance of early Ediacara biota

For reasons that remain unclear, the initial appearance of large, morphologically complex life on Earth seems to have taken place in deep-marine environments. We provide new perspective on this topic by applying for the first time the vanadium (V) isotope paleoredox-proxy. We use shales in two different sections that preserve Doushantuo Member IV (South China) to reconstruct a global seawater V isotope composition (δ51V=∼−0.23±0.06‰) during the late-Ediacaran (∼567 to ≥ 560 million years ago) that is much lighter than today. A mass-balance model informed by this composition is only reconciled by a global ocean in which hydrogen sulfide-rich (‘euxinic’) conditions were commonly present on continental shelves. Higher surface temperatures are a known driver of widespread euxinia in Earth's past, and if this was also the case during the late-Ediacaran, then relegation of large complex life to deep-marine settings at this time was probably driven to some extent by the persistently cooler and sulfide-poor conditions offered by this refuge. •New data for Ediacaran shales confirm the utility of the V isotope paleoredox proxy.•Lower seawater δ51V (∼−0.23‰) indicates expanded euxinia in marginal setting.•Widespread euxinia in shelf relegated Avalon assemblage to deep-marine settings.