Iron and phosphorus biochemical systems and the Cryogenian-Ediacaran transition, Jacadigo basin, Brazil: Implications for the Neoproterozoic Oxygenation Event

Abstract Termination of the Marinoan Snowball Earth ice age (635 Ma) marked the transition to a greenhouse world. This climate change forever modified the biogeochemical cycling of iron and phosphorus, ended large-scale iron formation deposition, began accumulation of phosphorus on marine shelves, and led to the Ediacaran radiation of eukaryotes. The Jacadigo Basin, Brazil, contains a nearly complete record of this critical transition. Glaciomarine diamictites and iron formation accumulated during two Marinoan ice advances with hydrothermal input of iron delivered via ice-margin upwelling. Biochemically precipitated rhythmites of siderite, sedimentary apatite, and hematite represent microbially mediated, sub-sea ice precipitation. Siderite laminae preserve microbial textures and have a mean ∂13C = −8.80‰, PDB (+/−0.86‰) reflecting degradation of organic matter at the seafloor. These millimeter-scale rhythmites are a sensitive record of sub-ice dynamics because they formed in response to short-term fluctuations of O2 due to seasonal sub-ice photosynthesis. They demonstrate the connection between ice cover, O2, and cycling of iron and nutrient elements such as phosphorus. These biochemical rhythmites suggest that Cryogenian sea ice limited oxygen production prior to the onset of the Neoproterozoic Oxygenation Event. O2 increased enough to concentrate bioavailable phosphorus at the seafloor, which was essential for later diversification of metazoans in the Ediacaran. Such sub-ice Cryogenian biochemical systems may provide Earth-based analogs for life on ice-covered worlds, such as Europa and Enceladus.