Life › The microbial world
Great Oxidation Event
c. 2.43–2.1 billion years ago· Event· Earth, climate & extinctions
Well establishedStrong fossil, genetic or geological evidence.
The period, starting about 2.43 billion years ago, when free oxygen first built up in Earth's air and shallow seas.
See it on the Life map →What happened
Before it, the air held almost no oxygen, under a hundred-thousandth of today's level. Over roughly 200 million years, oxygen rose in fits and starts to perhaps a few percent of today's level; estimates vary widely. It rusted minerals on land, destroyed much of the air's methane and allowed an ozone layer to form. For many oxygen-free microbes it was toxic, pushing them into mud and deep water.
Why it matters
It permanently changed Earth's chemistry, opened the way for oxygen-breathing life and complex cells, and may have triggered climate upheavals such as the Huronian ice ages.
The evidence (fossils, genes, rocks)
An unusual sulfur-isotope signal, possible only in oxygen-free air, vanishes about 2.43–2.33 billion years ago; easily rusted minerals disappear from river gravels; red, rusted soils appear.
Still debated
Why oxygen took perhaps hundreds of millions of years to accumulate after photosynthesis began, and whether the rise was one step or several swings.
Inside this (4)
Connections (7)
▸Oxygen-making photosynthesis caused Great Oxidation EventWell established
The connection
For perhaps hundreds of millions of years, the oxygen that cyanobacteria made was mopped up by iron in seawater and by volcanic gases. When oxygen production outran these 'sinks', the gas finally accumulated in the air, permanently changing Earth's chemistry.
When and how
Oxygen built up worldwide from about 2.43 billion years ago; the key sulfur-isotope evidence came from James Farquhar and colleagues in 2000.
The evidence
Oxygen-making photosynthesis is the only known source big enough; geological signs of free oxygen follow other evidence for cyanobacteria.
Debates and open questions
Why the delay was so long if oxygen photosynthesis began by 3 billion years ago; some argue it evolved only shortly before the rise.
▸Banded iron formations record the build-up to Great Oxidation EventStrong evidence
The connection
Banded iron formations peaked as oxygen first appeared and mostly stopped around 1.85 billion years ago. They record the build-up: dissolved iron absorbed oxygen until the supply was used up, after which oxygen could accumulate in air and water.
When and how
The largest deposits formed about 2.6–2.4 billion years ago, such as Western Australia's Hamersley Range; they returned briefly during Snowball Earth about 700 million years ago.
How sure are we?
Dating shows the biggest deposits cluster around the oxidation event, and their iron chemistry tracks changing ocean oxygen.
Debates and open questions
Why they stopped around 1.85 billion years ago: an oxygenated deep ocean, sulfur-rich seas trapping iron, or less iron from vents.
▸Great Oxidation Event may have triggered Huronian glaciationDebated
The connection
Early Earth was kept warm partly by methane, a strong greenhouse gas. When oxygen built up, it destroyed much of that methane, so the planet may have cooled sharply, and glaciers spread, perhaps to the tropics. Weathering of new continents, drawing down carbon dioxide, may also have helped.
When and how
About 2.45–2.2 billion years ago, in rocks around Lake Huron, Ontario, and in South Africa; Robert Kopp, Joseph Kirschvink and colleagues argued the methane link in 2005.
How sure are we?
Glacial deposits sit close in time to sulfur-isotope signs of the oxygen rise; the sequence fits, but the precise order is still being refined.
Debates and open questions
Some glaciations may predate the oxygen rise, which would weaken the link; whether ice truly reached the equator is debated.
▸Great Oxidation Event allowed the formation of The ozone layerWell established
The connection
Ozone is made from oxygen. Before the Great Oxidation Event the air held almost no free oxygen, so no shield could form. Once oxygen from cyanobacteria built up, ultraviolet light split some of it high in the atmosphere, forming ozone that absorbed the harshest radiation.
When and how
Globally, from roughly 2.4 billion years ago, as oxygen reached a small fraction of today's level.
The evidence
The chemistry is basic: ozone requires oxygen. The disappearance of UV-driven sulfur isotope signals in rocks about 2.4 billion years ago marks when oxygen rose.
Debates and open questions
How fast the shield strengthened, and how much it weakened when oxygen dipped during the Boring Billion, are uncertain.
▸Great Oxidation Event fueled the spread of Aerobic respirationStrong evidence
The connection
Once oxygen was common, microbes that could use it to burn food gained a huge energy advantage: about 30 or more units of ATP per sugar, versus 2 by fermentation. Oxygen-using microbes spread widely, and one of them later became the mitochondria inside complex cells.
When and how
Spread across many bacterial groups after about 2.4 billion years ago; some key enzyme families may be older.
How sure are we?
Gene studies date many oxygen-using enzyme families to around or after the oxidation event; the energy arithmetic is textbook biochemistry.
Debates and open questions
Whether some oxygen-using enzymes evolved earlier, near local 'oxygen oases' made by cyanobacteria before the air changed.
▸Plate tectonics begins may have helped trigger Great Oxidation EventDebated
The connection
Moving plates may have helped oxygen win. As continents grew and rose, more volcanoes erupted on land, and their gases use up less oxygen than undersea ones. Weathering of new land fed phosphorus to the seas, feeding cyanobacteria, and organic carbon buried in sediments left its oxygen behind in the air.
When and how
Continents grew and rose above the sea roughly 3.0–2.5 billion years ago; the ideas were developed by Lee Kump and others in the 2000s and 2010s.
How sure are we?
Indirect: rock chemistry suggests crust became more like modern continents around then, and the timing lines up with the oxygen rise.
Debates and open questions
Because the start of plate tectonics is itself debated, with estimates from 4.2 to 1 billion years ago, any causal link remains speculative.
▸Francevillian biota (Gabon) followed the oxygen rise of Great Oxidation EventDebated
The connection
The Francevillian fossils formed about 2.1 billion years ago, near the end of the Great Oxidation Event, when a spike in oxygen left shallow seas unusually well oxygenated. Their discoverers argue the extra oxygen let many-celled organisms try bigger bodies, which vanished when oxygen later declined.
When and how
Franceville Basin, Gabon, about 2.1 billion years ago; described by Abderrazak El Albani and colleagues in 2010.
How sure are we?
Rock chemistry in the same shales indicates oxygenated bottom water, and the timing matches a global carbon isotope spike (the Lomagundi event) linked to high oxygen.
Debates and open questions
Whether the fossils are organisms at all, and whether the oxygen spike caused them or merely coincided with them, remain disputed.