IS THE SNOWBALL A "NO-BALL"?: THE CASE AGAINST THE SNOWBALL EARTH HYPOTHESIS

Whether the surface of the Earth achieved a totally frozen (snowball Earth) condition in Proterozoic times remains equivocal. The ages and even the number of Neoproterozoic glaciations are still debated so that temporal correlations - a sine qua non of the snowball Earth hypothesis (SEH) - remain conjectural. Evidence of low paleolatitude glaciers at sea level has been considered a pillar of the SEH. Some of these low latitude glaciogenic successions, however, contain puzzling evidence of strong seasonality in the form of a) finely laminated sediments (ancient varves?) that contain dropstones, and b) ice-wedge structures. Such evidence of strong seasonality at low latitudes demands an explanation other than those offered in the SEH (high obliquity of the ecliptic?). The great thickness (1000's of m in some cases) of some Neoproterozoic glacial successions, evidence of numerous advance-retreat cycles and their intimate association with waterlaid sediments (outwash deposits and, in some cases, tidally-influenced sediments) support the long-term existence of temperate glaciers and a vigorous hydrologic cycle. These data do not seem to be compatible with a completely frozen planet, on which the hydrologic cycle would presumably have been largely suppressed. Glacially-associated BIF in Neoproterozoic successions has been explained under the SEH as due to isolation of world oceans from the atmosphere, followed by oxidation at the end of the snowball condition. Field relationships (thickness/facies changes and stratigraphic position of the BIF) and geochemical studies support deposition of many of these Neoproterozoic iron-formations in rift-related, hydrothermally-influenced basins. The predicted iron-formations are also conspicuously absent from deposits of the Varanger glaciation. Paleoclimatic paradoxes such as those posed by both Palaeo- and Neoproterozoic successions are the stuff of which paradigm shifts are made but acceptance of conclusions such as those implicit in the SEH requires accommodation of all the geological observations and careful collection of additional facts so that dogma does not come to dominate data.