TellUs: "The Earth Story" Series, Snowball Earth & Thriving Life

The Earth’s Story Series: The Proterozoic Eon

This blog will be going into more detail about the Proterozoic eon, which is a very exciting time in our Earth’s history! This is leading on from the previous Hadean and Archaean eons discussed in my last blog, so check that out first if you haven’t already and would like to!

Proterozoic Eon

Here’s where things get interesting - this eon is nicknamed “time of early life”! But before we jump into that, let us place this eon. This eon occurred 2500-545 million years ago, spanning a huge chunk of the Earth’s history. 

So, as discussed in the latest blog cyanobacteria are beginning to change the environment. Also life is diversifying and similar organisms are flourishing. Due to the effect of photosynthesis there is an increase of oxygen in the atmosphere. At first this process of creating an oxygen-rich atmosphere occurs slowly. However when we approach the middle of the eon (2200-2000 million years ago ago) this process reaches a tipping point. It’s likely that most oxygen, that was first released, was initially absorbed by the abundant iron and sulphur-bearing rocks. Oxygen reaches significant levels in the atmosphere only when these rocks are transformed into oxygen-rich minerals. This event is marked in the geological record by the sudden ending of banded iron formations. This is still the source of most iron ore today and their replacement red, oxygen-rich iron-bearing rocks. 

The banded iron layers form only in oceans - banding or layering is a sign of deposition in water. The presence of these deposits early in geological record provide evidence of the lack of oxygen in ancient oceans. In later deposits, iron compounds combined with oxygen to form un-branded, red minerals. Therefore the disappearance of branded iron layers is a sign that now the oceans have taken up oxygen from the atmosphere. 

Due to the event above, two changes transpired. First is that carbon dioxide and methane in the atmosphere were being reduced. This resulted in the climate cooling. In fact it even caused a period of global glaciation, with glaciers and ice covering the continents and oceans. This is the first time an event like this had occurred in the Earth’s history. 

An early tillites deposit, a mix of
broken rocks & other debris

Secondly this new oxygen-rich environment favours more complex life-forms than bacteria and wipes out many previous forms of life. The specific timing for this however is still debated today. Four kingdoms of life start to become present: protista, fungi, plants and animals. This new life is characterised by cells where genetic material contains a nucleus. This leads to the opportunity of sexual reproduction and the acceleration of evolution. The potential of differential cells leads to multicellular life. 

Finishing off another glaciation occurs, some deeming this a “snowball Earth”. Signs of past glaciations are known due to deposits of ancient tillites. 

Nevertheless, conditions overall are now ripe for life!