The Great Oxidation Event: How our atmosphere went from deadly gas to fresh air

In the beginning, Earth’s atmosphere had no oxygen. Then photosynthesis made life as we know it possible, produced much of our mineable iron, and caused an ice age.

Image by Tim Bertelink. CC BY-SA 4.0. No changes made.
Artist’s rendering of the Archean Eon. Note the shallow sea full of stromatolites, or mats of cyanobacteria, on the right.
The Great Oxidation Event

Today’s atmosphere is 21% oxygen, but during the early Archean Eon (3.8 billion to 2.5 billion years ago) the atmosphere was anoxic – that is, until the Great Oxidation Event, the world’s first appearance of oxygen in the atmosphere. This event changed Earth’s environment forever, paving the way for life to flourish.

While there is not yet a complete consensus on what started the Great Oxidation Event, one prominent theory points to the appearance of photosynthetic cyanobacteria. Cyanobacteria, or blue-green algae, are organisms that produce oxygen as a byproduct of photosynthesis. Though single-celled, they can live together as a mat of sediments and cells on the ocean floor. These structures, referred to as stromatolites, are common fossils in Archean rock worldwide. Some of these fossils contain microbial filaments with evidence of phototaxis, meaning they followed the sun and carried out photosynthesis. Although this fossil evidence strongly supports the role of cyanobacteria, their presence has not been definitively proven.

Image by James St. John. CC BY 2.0. No changes made.
A fossilized stromatolite from Wyoming. Notice the horizontal stripes formed by layers of sediment and cyanobacteria.
Boom and Bust

If cyanobacteria are responsible for oxygen in the atmosphere during the Archean Eon, their population growth was not without interruption. The presence of oxygen proved deadly for cyanobacteria, meaning they were poisoning themselves over time. Oxygen would react with iron in the water, which the cyanobacteria likely used as a catalyst for nitrogen fixation. Saturation of the environment with oxygen led to the near-extinction of cyanobacteria when all the iron near the cyanobacteria had been “soaked up” by the oxygen they produced. Over time, iron would reenter the water column via weathering and erosion from land as well as from volcanoes and mid-ocean ridges underwater. When iron was present, oxygen production was no longer deadly, allowing the few remaining cyanobacteria to reproduce exponentially again. Thus the “Great Oxidation Event” may be a slight misnomer as Archean oxygenation was a drawn-out, cyclical process involving hundreds of recolonizations and near-extinctions rather than a single event.

Banded Iron Formations

The effects of this cycle of life and death can be observed in banded iron formations, thick geologic deposits that formed starting 2.7 billion years ago during the Late Archean and Early Proterozoic. Banded iron formations consist of gray hematite or magnetite containing a high abundance of Fe2O3 layered with red, iron-poor chert. They formed in shallow marine environments with high dissolved iron that reacted with oxygen and precipitated into iron-rich layers. Between the gray, iron-rich layers lie red iron-poor layers of chert, which reflect cyanobacteria population decline caused by either a lack of iron or an excess of oxygen. These formations are widespread and can be found globally. Besides their usefulness in investigating ancient history, banded iron formations are the world’s primary source of iron today, making up 60% of iron reserves worldwide.

Image by Abigail Donahue.
This banded iron formation from Michigan’s Upper Peninsula is composed of gray, iron-rich layers and red, iron-poor layers.
Snowball Earth

In addition to leaving behind minable iron, the Great Oxidation Event caused atmospheric change which is at least partially responsible for the Huronian glaciation, around 2.29-2.25 billion years ago. This event covered much of Earth with snow and ice and was caused by oxygen from the ocean entering the atmosphere and reacting with methane to produce carbon dioxide. Methane is a more potent greenhouse gas than carbon dioxide, so this reaction decreased the greenhouse effect of the atmosphere, partnering with a faint young Sun (70-80% as bright as it is today) to cause global cooling and a global glaciation event.

Image by Acaro. CC BY-SA 3.0. No changes made.
The introduction of oxygen to the atmosphere caused an ice age.

The Great Oxidation Event most likely marks the advent of photosynthesis and biological oxygen production in Earth’s history, a landmark for understanding evolution and the environment. While O2 levels in the atmosphere probably didn’t rise all the way to today’s levels by the end of the Great Oxidation Event, the appearance of detectable O2 was a dramatic change and served as a stepping stone to our richly oxygenated atmosphere.


Berman-Frank, I., Lundgren, P., & Falkowski, P. (2003) ‘Nitrogen fixation and photosynthetic oxygen evolution in cyanobacteria’, Research in Microbiology, vol. 154, no. 3, pp. 157-164.

Coogan, L., & Cullen, J. (2009) ‘Did natural reactors form as a consequence of the emergence of oxygenic photosynthesis during the Archean?’, GSA Today, vol. 19, no. 10, pp. 4-10.

Hofmann, H. (2000) ‘Archean Stromatolites as Microbial Archives’, in Riding, R., Awramik, S. (eds) Microbial Sediments, Springer, Berlin, Heidelberg, Germany, pp. 315-316.

Knoll, A. (2008) Cyanobacteria and earth history. The cyanobacteria: molecular biology, genomics, and evolution, 484.

Konhauser, K., Hamade, T., Raiswell, R., Morris, R., Ferris, F., Southam, G., & Canfield, D. (2002) ‘Could bacteria have formed the Precambrian banded iron formations?’, Geology, vol. 30, no. 12, pp. 1079-1082.

Lyons, T., Reinhard, C., & Planavsky, N. (2014) ‘The rise of oxygen in Earth’s early ocean and atmosphere’, Nature, vol. 506, pp. 307-315.

Robb, L., Knoll, A., Plumb, K., Shields, G., Strauss, H., & Veizer, J. (2009) The Precambrian: the Archean and Proterozoic Eons Cambridge University Press, Cambridge, England.

Sessions, A., Doughty, D., Welander, P., Summons, R., & Newman, D. (2009) ‘The Continuing Puzzle of the Great Oxidation Event’, Current Biology, vol. 19, no. 14, pp. R567-R574.

Tang, H. & Chen, Y. (2013) ‘Global glaciations and atmospheric change t ca. 2.3 Ga’, Geoscience Frontiers, vol. 4, no. 5, pp. 583-596.

Zhu, X., Tang, H., & Sun, X. (2014) ‘Genesis of banded iron formations: A series of experimental simulations’, Ore Geology Reviews, vol. 63, pp. 465-469.

Reviewed by

Share this:
Abigail Donahue

Abigail Donahue

I graduated in 2021 from the University of Notre Dame with a B.S. in environmental sciences and a minor in theology. My research there was focused on Notre Dame's Museum of Biodiversity and the ecology of various aquatic macroinvertebrates. After graduating, I moved to Ireland to pursue an M.Sc. in environmental science from Trinity College Dublin. So far, my research has considered the impact of parasites in freshwater fish invasions and the impact of multiple anthropogenic stressors on stream macroinvertebrates. Besides the environment, I am passionate about museums, theology, hiking, and travel!

Leave a Reply