OASES ON SNOWBALL EARTH: A TALE OF THREE ICE-MASS TYPES

Evidence for ice-free coastal waters during Neoproterozoic glaciations in Death Valley, CA and elsewhere is said to falsify the snowball Earth hypothesis, as is d13C ≥0‰ PDB in carbonates precipitated from such waters (Kennedy et al., 2001). We interpret new data from NE Svalbard in light of sea ice dynamics models and reach a more congenial conclusion.

In Svalbard, two glacial units (Petrovbreen Mb and Wilsonbreen Fm) are separated by 200 m of glendonite-bearing suspension deposits (MacDonaldryggen Mb) and a regressive evaporitic carbonate £30 m thick (Slangen Mb). New isotopic and other data (Halverson, this meeting) imply that both glacial units are Marinoan, the 'type' Neoproterozoic snowball Earth episode. Is the sabkha-like Slangen Mb consistent with a snowball Earth?

According to Warren et al. (2002), and Goodman and Pierrehumbert (2002), marine ice on a snowball Earth is thick and flows equatorward over the open ocean like a glacier flows over land. Flowage causes the ‘sea glacier’ to be thicker at low latitudes and thinner at higher latitudes than thicknesses predicted by thermal diffusion alone, which pertain to landfast ice on shelves and inland seas. As snowball Earth slowly warms, tropical landfast ice eventually melts away, creating oases, while the tropical open ocean remains ice covered due to inflow of ice from higher latitudes.

We interpret the Svalbard succession as follows. Petrovbreen diamictites are ice-rafted debris from cold-based outlet glaciers. Abrupt cessation of IRD marks the invasion of sea glaciers into the tropical ocean (snowball phase begins). MacDonaldryggen Mb formed beneath landfast ice and Slangen Mb was precipitated in a snowball oasis, and records its rapid isotopic equilibration with a huge atmospheric reservoir of mantle-like carbon (higher d13C than non-snowball atmosphere). Wilsonbreen ice-contact deposits mark the advance of inland ice domes onto the platform. Eventual collapse of the sea glacier triggered cataclysmic deglaciation, with a typical Marinoan cap dolostone tracking the marine transgression.

The Svalbard succession reflects the different histories of three distinct ice-mass types: terrestrial ice domes, sea glaciers and landfast sea ice. Oases are a natural part of the snowball Earth cycle, provided shelves exist below sea level.