The GREAT OXYGENATION EVENT: Oxygen Fills the Atmosphere

Great Oxygenation Event

Last Updated: Aug 22, 2018

What is the Great Oxygenation Event?

Over 4 billion years ago, Earth’s atmosphere was filled with methane and nitrogen.

Practically no free oxygen existed in the atmosphere. But as we know, it became oxygen-rich.

The Great Oxygenation Event marks a time when free oxygen filled the atmosphere.

What sparked this event? And how did it affect life on the planet?

Cyanobacteria released oxygen as waste

Long ago, oceanic cyanobacteria evolved to carry out photosynthesis to make energy for themselves.

The key to their existence at this time was that they didn’t need oxygen. They were completely anaerobic. In fact, oxygen was poison for cyanobacteria.

Over time, these cyanobacteria released oxygen as a waste product. So much oxygen, that it kept building and building up in the oceans.

Eventually, oxygen filled the atmosphere. This marked the start of the Great Oxygenation Event.

Free oxygen had profound effects on the planet. It triggered an oxygen crisis, it froze over the whole planet and it rusted iron.

Oxygen poisoned cyanobacteria

Without oxygen, anaerobic life flourished. But in an oxygen-rich environment, oxygen was poisonous for cyanobacteria.

The irony of cyanobacteria is that the oxygen they released was toxic to them.

As a whole, the number of anaerobic organisms dwindled to the brink of extinction. This wiped out over 90% of life on earth.

The reason why this event is called an “oxygen crisis” is because they threatened their own existence through their own waste oxygen.

Cyanobacteria didn’t completely vanish. By hiding in low oxygen environments, cyanobacteria avoided an extinction.

Eventually, life evolved to use this oxygen and now the ecosystem keeps itself in balance this way.

Snowball Earth from an oxygen-filled atmosphere

Oxygen played a key role in transforming the planet into a “Snowball Earth” or “Slushball Earth”.

Remember that the atmosphere before the great oxygenation event was mostly methane and nitrogen.

Methane is one of the most efficient greenhouse gases that exists. Long ago, it trapped heat in the atmosphere keeping temperatures warm.

So when oxygen combined with methane, it produced carbon dioxide. Because there was less methane in the atmosphere, the greenhouse effect wasn’t as strong.

Without heat trapped in the atmosphere, Earth froze over for about 300,000,000 years.

How come there wasn’t any free oxygen?

By nature, oxygen is a very volatile substance. This is because it steals electrons from other atoms.

Oxygen atoms existed at this time but were bonded with hydrogen in water molecules.

Instead of H2S, cyanobacteria used H20 as a source of electrons and hydrogen for fixing CO2.

During the CO2 fixation, this was the process that filled free oxygen in the skies.

Over time, life adapted to use oxygen for its own benefit. But that was not always the case for anaerobic organisms.

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