OCEAN HYPERSTRATIFICATION FOLLOWING THE MARINOAN (NEOPROTEROZOIC) GLACIATION

The development of a hyperstratified ocean was an inevitable consequence of the climatic rebound from a snowball glaciation. The evolution of such a stratified ocean is recorded globally in cap carbonate (transgressive-high stand) sequences to the 635 Ma Marinoan glaciation. For example, in northern Namibia, carbon isotope data from upper foreslope to inner shelf environments suggest the diachroneity of the basal, transgressive cap dolostone. In northwest Canada and central Australia, regionally extensive seafloor barite cements directly atop the cap dolostone indicate that the rise in sea level was accompanied by the mixing of chemically distinct water masses - presumably the surface and deep oceans. In Svalbard and East Greenland, the barite layer is absent, but the cap dolostone is everywhere succeeded by a transition from red siltstone to green then black shale at the level of maximum flooding, implying the migration of a redox front across the continental shelf during transgression. Based on these observations and other consistent stratigraphic, sedimentary, and geochemical patterns in Marinoan cap carbonates, we propose a simple model for the evolution of the post-glacial ocean. At the end of glaciation, catastrophic melting of sea and glacier ice delivered a large volume of fresh water to a surface ocean that capped a cold, saline, anoxic deep ocean residual from millions of years of hydrologic isolation. Radiative warming under an intense greenhouse atmosphere enhanced the buoyancy of the brackish surface mixed layer, rendered unusually thick by continuously tempestuous weather. Strong thermohaline, redox, and chemical gradients characterized the transition between the oxic mixed layer and the underlying anoxic deep water. Migration of this transition across continental shelves during the transgression generated strong lateral chemical gradients, which eventually decayed after mixing and diffusion diminished the salinity and temperature contrasts between the surface and deep oceans.