A BASIN REDOX TRANSECT AT THE DAWN OF ANIMAL LIFE

Multiple eukaryotic clades make their first appearance in the fossil record between ~850-715 Ma. Molecular clock studies suggest that the origin of animal multicellularity may have been part of this broader eukaryotic radiation. Animals require oxygen to fuel their metabolism, and low oxygen levels have been hypothesized to account for the temporal lag between animal origins and the Cambrian radiation of large, ecologically diverse animals. Here, paleoredox conditions were investigated in the Fifteenmile Group, Ogilvie Mountains, Yukon, Canada, which hosts an 811 Ma ash horizon and spans the origin and early evolution of animals. Iron-based redox proxies, redox-sensitive trace elements, and carbon and sulfur isotopes were analyzed in six stratigraphic sections along two parallel basin transects. These data suggest that for this basin, oxygenated waters on the shelf overlay generally anoxic deeper waters. The anoxic water column likely oscillated between euxinic and ferruginous conditions, with the lower portion of the Reefal Assemblage characterized by euxinia and the upper portion by ferruginous conditions. Theoretical considerations and the ecology of modern oxygen-deficient settings suggests that the inferred oxygen levels would not be prohibitive to the presence of sponges, eumetazoans or bilaterians. Thus the appearance of the earliest animals was probably not limited by the low oxygen levels that characterized the Neoproterozoic, although these inferred levels would limit animals to very small sizes and low metabolic rates.