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

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 partial derivative C-13 = -8.80 parts per thousand, PDB (+/-0.86 parts per thousand) 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 O-2 due to seasonal sub-ice photosynthesis. They demonstrate the connection between ice cover, O-2, 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. O-2 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.