Sea Level Variations During Snowball Earth Formation: A Preliminary Analysis

The sea level changes that would have accompanied the "Snowball Earth" events of the Late Neoproterozoic (1000 Ma - 540 Ma) have remained enigmatic. Emplacement of the voluminous ice sheets on the continents during these events should have drawn down eustatic sea level by ~1000 m but geological evidence of this having actually occurred is scant. The only reliable estimate is apparently that based upon observations from the Otavi Group in Northern Namibia (Hoffman et al. (2007), Earth Planet Sc Lett, 258(1-2), 114-131, doi:10.1016/j.epsl.2007.03.032) which indicates a sea level change of approximately 500 m to have occurred. In this work, we provide a preliminary theoretical estimate of the extent to which the ocean surface could have fallen with respect to the continents during such snowball Earth events. Our analyses are performed by solving the Sea Level Equation for a spherically symmetric Maxwell Earth subject to extensive continental glaciation for a realistic land-sea distribution. We find, as expected, that the fall of relative sea level must have been highly non-uniform, with the collapse of the geoid least near the coastal regions and greatest in the ocean basin interiors. For a 720 Ma (Sturtian) continental configuration, the ice sheet volume in a snowball state is ~824 m sea level equivalent, but ocean surface lowering (relative to the original surface) can be as little as ~300 m near the coast of a continental fragment that was host of a major ice dome. Although the mean water depth (relative sea level) is reduced by ~824 m, because the mean elevation of the ocean floor is increased by ~193 m, the mean ocean surface fall is only ~631 m. The change of continental freeboard (which may be recorded in the sedimentary record) at the edge of the continents is usually larger than the lowering of ocean surface due to the forebulge effect, ranging from 400 - 600 m. For the 570 Ma (Marinoan) continental configuration, ice sheet volumes is ~1180 m in eustatic sea level equivalent in a "soft snowball" event and ~1230 m in a "hard snowball" event. For this most recent of the two major Neoproterozoic glaciations the inferred freeboard generally ranges from 500 - 850 m, but can be as small as 360 m. We therefore find that the expected continental freeboard during a snowball Earth event is broadly consistent with expectations based upon the inferences from Otavi Group sediments.