Climate-ice sheet simulations of Neoproterozoic glaciation before and after collapse to Snowball Earth

Geologic evidence of tropical sea level glaciation in the Neoproterozoic is one of the cornerstones of the Snowball Earth hypothesis. However, it is not clear during what part of the Snowball Earth cycle that land-based glaciers or ice sheets could have grown: just before the collapse with tropical oceans still open, or after the collapse with oceans completely covered with sea ice. In the former state, the tropics may still have been too warm to allow flowing ice to reach sea level; in the latter, snowfall minus sublimation may have been too small to build significant ice. These possibilities are tested with a coupled global climate model and dynamic ice sheet model, with two continental configurations (similar to750 Ma, 540 Ma) and two CO2 levels bracketing the collapse to Snowball Earth (840, 420 ppmv). Prior to collapse large high-latitude ice sheets form at 750 Ma, but with flat continents, no low-latitude ice grows at 750 or 540 Ma. In the absence of reliable knowledge of Neoproterozoic topography, we apply a small-scale "test" profile in the ice sheet model, representing a coastal mountain range on which glaciers can be initiated and flow seaward. Prior to collapse, almost all low-latitude test glaciers fail to reach the coast at 750 Ma, but at 540 Ma many do reach the sea. After the collapse to full Snowball conditions, the hydrologic cycle is greatly reduced, but extensive kilometer-thick ice sheets form slowly on low-latitude continents within a few 100,000 years, both at 750 Ma and 540 Ma.