The Great Oxygenation Event – when Earth took its first breath
Have you ever wondered where the oxygen we breathe came from? Why is our planet, unlike so many in the universe, capable of supporting such diverse forms of life?
All the clues we have lie in the rocks that were formed at the time – 2.3 billion years ago.
The Great Oxygenation Event
Despite how it sounds – like there was a sudden influx of oxygen into the oceans and atmosphere – this transition from an anaerobic earth (no oxygen) to an aerobic earth (lots of oxygen) happened extremely slowly. In fact, it took up to 2 billion years to reach today’s oxygen levels, and the geological record suggests it occurred in multiple steps, not one large burst.
The geochemical analysis of marine rocks deposited at this time suggested that some oxygen existed in shallow oceans around 2.6 billion years ago, and even 3 billion years ago in soils. Micrometeorites which fell from space 2.7 billion years ago even suggest oxygen existed in the upper atmosphere at this time. But what drove oxygen levels to increase from barely anything, to 21% of the earth’s atmosphere?
Cyanobacteria changed the world
Back in the Archean – a geological eon occurring 4 to 2.5 billion years ago – earth was anaerobic. Only very simple forms of life existed on earth, and they didn’t need oxygen to live.
But all this was about to change. A new type of organism – cyanobacteria – was starting to spread across the oceans. Unlike the organisms which came before them, they used a process known as photosynthesis to gain energy. Today, most plants use photosynthesis to live – they absorb sunlight for energy, and produce oxygen as a waste product.
At first, not much happened. The Archean oceans were full of dissolved iron, which chemically bonded with the oxygen the cyanobacteria released. This chemical reaction produced strange sedimentary rocks that can be found worldwide in Archean-age deposits.
BIFs – iron deposits suggest oxygenation events
These strange rocks are known as Banded Iron Formations, or BIFs. They were deposited when oxygen reacted with the iron-rich Archean ocean water to form iron oxides.
During the Archean, not very much oxygen was being produced, and most of it was reacting with iron in the oceans. The reaction produced iron oxide minerals known as goethite and hematite, which fell to the ocean floor to form vast deposits of BIFs.
So, if oxygen was initially soaked up during this reaction, how did the world eventually become oxygenated?
As cyanobacteria became more abundant, they released so much oxygen that there was not enough iron in the oceans to soak it all up. As oxygen levels increased and iron was removed from the ocean, these deposits stopped forming, and oxygen was free to be released into the oceans and atmosphere.
But, did cyanobacteria change the world all by itself, or did it have some help?
What do supercontinents have to do with it?
A recent study comparing the timing of oxygenation events with supercontinent formation events found that there was a distinct correlation between the two. Supercontinents form when landmasses, carried by tectonic plates, collide with each other to form one large land mass. These collisions created huge mountains.
This meant that more land was exposed to weathering and erosion, allowing lots of sediment full of nutrients to be washed into the ocean. These nutrient influxes could have supported huge plumes of algae and bacteria in the ocean, which released oxygen during photosynthesis.
Additionally, the abundant sediment being deposited into the ocean may have helped by burying organic carbon and a mineral called pyrite. Both of these consume oxygen in a process known as ‘oxidation’. Without this process, more oxygen was freely released into the oceans and atmosphere, contributing to rising oxygen levels.
In reality, both cyanobacteria and supercontinent formation likely contributed to the Great Oxygenation Event, and indeed many other factors were involved. It’s not easy to look back billions of years into the past, but the more geologists and earth scientists uncover, the closer we get to understanding how our world came to have the abundant oxygen we all need to survive.