A frozen planet

After the Archaean-Proterozoic transition, where we left off, the Earth went through severe glaciations that would put the last ice ages to shame. These glaciations lasted from 100 to 200 million years at the beginning and the end of the Proterozoic (Hoffman, 2002). The extent of these glaciations is still debated, with some arguing for Snowball Earth events (Hoffman, 2002), while others suggest the evidence is inconclusive for such widespread events (Young, 2012).

The Huronian glaciations occurred at the beginning of the Proterozoic (2.45-2.22 billion years ago), with at least 3 being identified (Young, 2012). The Sturtian (715 million years ago) and Marinoan (635 million years ago) glaciations occurred at the end of the Proterozoic. 

Fig. A: Current location of Neoproterozoic glacial deposits, and
their estimated original latitudes. Hoffman, 2002

The evidence for these events is found spread around the modern day continents, mainly in the form of glacial deposits, like ice rafted debris. By estimating their original latitudes (figures A and B), it was found that many of these glacial features were formed near the equator (Hoffman, 2002), leading to the idea of frozen world, or Snowball Earth.

Fig. B: Location of Sturtian glacial deposits on the
supercontinent Rodinia. Young, 2012

However, as mentioned before, the extent of these glaciations is not yet clear. A ‘Slushball Earth’ scenario, with open water, is an alternative. Among the arguments against a Snowball Earth (Young, 2012) is that without open water, the hydrological cycle would basically “shut down” and additionally there is evidence for glacial advance and retreat cycles. Another important consideration is how or where microorganisms could have survived in a completely frozen world - photosynthetic organisms had already evolved as discussed previously. Hoffman (2002) argues a weak hydrological cycle would be present and enough for snow to accumulate slowly, eventually allowing for glaciers to flow. Regarding the microorganisms, he says that there were many places where they could survive, like transient meltwater ponds or hot springs around volcanic islands. He argues that microorganisms did not only survive, but benefited from these harsh conditions, since it was only after the Neoproterozoic glaciations that there was an increase in their diversity, and the appearance of the first macrofossils came soon after.

Fig. C: Macrofossil from the Edicaran period (635-542 million years ago). Source: Wikipedia 

In the next post we will explore the possible causes that led to these extensive glaciations and why they occurred so long ago!