A 4 MILLION YEAR LONG MARINOAN SNOWBALL EARTH: ASYMMETRY BETWEEN THE NEOPROTEROZOIC PANGLACIALS

Twice during the Neoproterozoic Era, Earth experienced runaway ice-albedo catastrophes that resulted in multi-million year, low-latitude glaciations: the Cryogenian Snowball Earths. In the snowball climate state, CO2 consumption through silicate weathering collapses, and atmospheric CO2 accumulates via volcanic outgassing until a sufficiently strong greenhouse causes deglaciation. Long-lasting (>10 Myr) Snowball glaciation requires air-sea gas exchange through small regions of open water, whereby CO2 is consumed by pH-dependent seafloor weathering. In contrast, shorter durations can be explained either by a more extreme, hard snowball in which CO2 accumulates without a significant seafloor sink, or a lower albedo that decreases the threshold CO2 level for deglaciation. Radiometric ages have defined the duration of the first Snowball glaciation, the Sturtian, to 56 Myr, but the duration of the second, Marinoan, glaciation currently has 11 Myr of uncertainty. We present four CA-ID-TIMS U-Pb zircon dates from the Marinoan Ghaub Formation at Fransfontein Ridge, Namibia, which constrain on the onset of the Marinoan snowball glaciation to ca 639 Ma, yielding a 4 Myr duration. The Marinoan was an order of magnitude shorter than the Sturtian snowball, evidencing major asymmetry in the Snowball climate states. This asymmetry suggests a lack of seafloor weathering sink during the Marinoan and/or lower albedo that triggered early deglaciation.