ORIGIN AND DIAGENESIS OF BIOCHEMICAL RHYTHMITES DEPOSITED AT THE END OF THE MARINOAN ICE-COVERED SNOWBALL EARTH WORLD, SOUTHWESTERN BRAZIL

The Neoproterozoic glaciations were among the most extreme in Earth history and the climate change that followed the last of these Snowball Earth glaciations forever modified the biogeochemical cycling of Fe and P, ended large-scale iron formation deposition, initiated widespread accumulation of P on the seafloor, and led to the Ediacaran radiation of eukaryotic life. Little is known, however, regarding marine sediments during the transition out of the ice ages.

The Cryogenian of southwestern Brazil, records Marinoan Snowball Earth ice advances, hydrothermal input of Fe, ice-margin upwelling, and ice-rafted debris. Above the uppermost diamictite, fluvial outwash deposits are overlain by tidally influenced sandstones and mudstones that are punctuated by glacial dropstones and intervals of biochemically precipitated rhythmites that are devoid of clastic sediment. The siderite, francolite, and hematite rhythmites represent bacterially-mediated, sub-sea ice precipitation. Millimeter laminae of siderite with peloidal and filamentous structures reflect anoxic microbial degradation of organic matterdriven and increased alkalinity. Siderite ranges from small clusters of crystallites that characterize low-temperature formation to euhedral, Mg-rich crystals reflecting burial diagenesis. The Mg-rich overgrowths suggest continued crystal growth and recrystallization typical of all carbonate sediments. ∂¹³C of siderite laminae reflect this organic process and ranges from -14 to -2, with a mean of -9.5‰ and ∂¹⁸O ranges from -8 to +2, with a mean of -2.4‰ reflecting authigenic values and diagenetic overprinting. Francolite-rich microbial laminae precipitated on siderite layers, which are in turn, overlain by hematite mudstone, resulting from suspension settling of iron-(oxyhydr)oxides precipitated in the water column.

These millimeter-thick cycles record short-term changes in seawater oxygen, possibly due to seasonal fluctuations in sub-ice photosynthesis. Theses biochemical rhythmites suggest oxygen periodically increased enough to increase bioavailable phosphorus at the seafloor. This remarkable Cryogenian sub-ice biochemical system reflects microbial, biochemical processes and could be an analog for life on other worlds with ice-covered oceans, such as Europa and Enceladus.