TURBULENT TIMES FOR EARLY ANIMALS?

There is persistent debate about the role that surface oxygen levels played in controlling the diversification of complex life. To move forward our understanding on this topic we have undertaken palaeoredox work on organic-rich black shales of the Doushantuo Formation, South China. We have found evidence for pervasive oxic conditions, and likely even deep ocean oxygenation at three intervals in the lower (ca. 632 Ma), middle (ca. 580 Ma) and upper (ca. 551 Ma) Ediacaran. However, we have also found evidence for widespread anoxic conditions through much of the Ediacaran. The Doushantuo Formation hosts large enrichments of redox-sensitive trace element (e.g., molybdenum, vanadium and uranium) in anoxic shales, which are indicative of a globally oxic ocean-atmosphere system. However, much of the Doushantuo Formation in basinal sections is characterized by Fe speciation data and pyrite morphologies that indicate deposition under euxinic conditions with near-crustal enrichments of redox-sensitive element and positive pyrite-sulfur isotope values, which suggest low levels of marine sulfate and widespread euxinia. Our work reinforces an emerging view that the early Earth, including the Ediacaran, underwent numerous rises and falls in surface oxidation state, rather than a unidirectional rise as originally imagined. Further, this framework forces us to re-examine the relationship between Neoproterozoic oxygenation and metazoan diversification. Varying redox conditions through the Cryogenian and Ediacaran may help explain molecular clock and biomarker evidence for an early appearance and initial diversification of metazoans but with a delay in the appearance of most major metazoan crown groups until close to Ediacaran-Cambrian boundary.