Uranium isotope evidence for an expansion of anoxia in terminal Ediacaran oceans

Anoxic and iron-rich oceanic conditions prevailed throughout most of the Archean and Proterozoic (4000 to c.540 million years ago, Ma), but the oceans are hypothesised to have become progressively oxygen-rich during the Ediacaran–Cambrian transition interval, coincident with the rise of animal life. We utilise the uranium isotope ratio of seawater (²³⁸U/²³⁵U; reformulated as δ²³⁸U), an effective tracer of oceanic redox conditions, as a proxy for changes in the global proportion of anoxic seafloor. We present a new δ²³⁸U dataset for carbonate rocks from the Lower Nama Group, Namibia, deposited in a shelf ramp succession during the terminal Neoproterozoic (∼550 to ∼547 Ma). These data capture a transition from δ²³⁸U similar to the modern ocean towards persistently low δ²³⁸U (average = −0.81 ± 0.06‰). Such low δ²³⁸U are consistent with enhanced U drawdown from the water column under anoxic conditions, and the preferential export of ‘heavy’ ²³⁸U to sediments following U(VI)–U(IV) reduction. Placing our results into a steady state ocean box model suggests at least a third of the global seafloor was covered by anoxic bottom waters compared with only 0.3% in today's oxygenated oceans. Comparison with δ²³⁸U from older sediments deposited in other basins further supports an expansion of anoxic bottom waters towards the end of the Ediacaran. Our data are consistent with an emerging picture of a dominantly anoxic Ediacaran ocean punctuated by brief ocean oxygenation events. In the Nama Group, the transition towards globally widespread anoxic conditions post-dates the first appearance of both skeletal metazoans and soft-bodied fauna of the Nama Assemblage. This suggests that the global expansion of anoxia did not coincide with the decline of the Ediacaran biota, or drive the biotic turnover between the White Sea and Nama Assemblages. The impact of this global redox change on metazoan ecosystems is unclear, since the expansion of anoxia, if contained mainly within deeper waters, may not have impinged significantly upon continental shelves that host the majority of biodiversity.