# Great Oxygenation Event

O2 build-up in the Earth's atmosphere. Red and green lines represent the range of the estimates while time is measured in billions of years ago (Ga).
Stage 1 (3.85–2.45 Ga): Practically no O2 in the atmosphere. The oceans were also largely anoxic with the possible exception of O2 in the shallow oceans.
Stage 2 (2.45–1.85 Ga): O2 produced, rising to values of 0.02 and 0.04 atm, but absorbed in oceans and seabed rock.
Stage 3 (1.85–0.85 Ga): O2 starts to gas out of the oceans, but is absorbed by land surfaces. No significant change in terms of oxygen level.
Stages 4 and 5 (0.85 Ga–present): Other O2 reservoirs filled; gas accumulates in atmosphere.[1]

The Great Oxygenation Event, the beginning of which is commonly known in scientific media as the Great Oxidation Event (GOE, also called the Oxygen Catastrophe, Oxygen Crisis, Oxygen Holocaust,[2] Oxygen Revolution, or Great Oxidation) was the biologically induced appearance of molecular oxygen (dioxygen, O2) in Earth's atmosphere.[3] Geological, isotopic, and chemical evidence suggests a start of around 2.45 billion years ago (2.45 Ga),[4] during the Siderian period, at the beginning of the Proterozoic eon. The causes of the event remain unclear.[5] As of 2016, the geochemical and biomarker evidence for the development of oxygenic photosynthesis before the Great Oxidation Event is inconclusive.[6]

The first microbes to produce oxygen by photosynthesis were oceanic cyanobacteria.[7] They evolved into tufted microbial mats more than 2.3 billion years ago, approximately 200 million years before the GOE.[8] The free oxygen produced during this time was chemically captured by dissolved iron, converting iron ${\displaystyle {\ce {Fe}}}$ and ${\displaystyle {\ce {Fe^2+}}}$ to magnetite (${\displaystyle {\ce {Fe^2+Fe2^3+O4}}}$) which is insoluble in water, and sank to the bottom of the shallow seas to create massive, large scale, banded iron formations. Some of the oxygen was captured by organic matter. The GOE started after these oxygen sinks were filled to capacity.

Cyanobacteria: responsible for the build-up of oxygen in the Earth's atmosphere

The increased production of oxygen set Earth's original atmosphere off balance.[9] Free oxygen is toxic to obligate anaerobic organisms; the rising concentrations may have destroyed most such organisms.[10] Newer research indicates that accumulation of oxygen in the oceans could have started long before the gas started to change the atmosphere.[11]

A spike in chromium contained in ancient rock deposits formed underwater shows accumulated chromium washed off from the continental shelves. Chromium is not easily dissolved; its release from rocks requires the presence of a powerful acid. One such acid, sulfuric acid (H2SO4), may have formed through bacterial reactions with pyrite.[12] Mats of oxygen-producing cyanobacteria can produce a thin layer, one or two millimeters thick, of oxygenated water in an otherwise anoxic environment even under thick ice; thus, before oxygen started accumulating in the atmosphere, these organisms would already have adapted to oxygen.[13] Additionally, the free oxygen would have reacted with atmospheric methane, a greenhouse gas, greatly reducing its concentration and triggering the Huronian glaciation, called "snowball Earth", possibly the longest episode of glaciation in Earth's history.[14]

Eventually, the evolution of aerobic organisms that consumed oxygen established an equilibrium in the availability of oxygen. Free oxygen has been an important constituent of the atmosphere ever since.[14]

## Timing

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The most widely accepted chronology of the Great Oxygenation Event suggests that free oxygen was first produced by prokaryotic and then later eukaryotic organisms that carried out photosynthesis more efficiently, producing oxygen as a waste product. The first oxygen-producing organisms arose long before the GOE,[15] perhaps as early as 3,400 million years ago.[16][17]

Initially, the oxygen they produced would have quickly been removed from the atmosphere by the chemical weathering of reducing minerals, most notably iron. This dissolved iron easily oxidized ${\displaystyle {\ce {Fe}}}$ and ${\displaystyle {\ce {Fe^2+}}}$ to magnetite (${\displaystyle {\ce {Fe^2+Fe2^3+O4}}}$) which is insoluble in water, and sank to the bottom of the shallow seas to create massive, large scale, banded iron formations such as the sediments in Minnesota and in Pilbara, Western Australia. Only when all of the dissolved iron, and other reducing minerals, had been oxidized, was oxygen able to persist in the atmosphere. Depleting these reductive minerals took 50 million years.[18] Oxygen could have accumulated very rapidly: at today's rates of photosynthesis, much greater than those in the Precambrian without land plants, modern atmospheric O2 levels could be produced in just 2,000 years.[19]

Another hypothesis is that oxygen producers did not evolve until a few million years before the major rise in atmospheric oxygen concentration.[20] This is based on a particular interpretation of a supposed oxygen indicator used in previous studies, the mass-independent fractionation of sulfur isotopes. This hypothesis would eliminate the need to explain a lag in time between the evolution of oxyphotosynthetic microbes and the rise in free oxygen.

In either case, oxygen did eventually accumulate in the atmosphere, with two major consequences.

Firstly, it oxidized atmospheric methane (a strong greenhouse gas) to carbon dioxide (a weaker one) and water. This decreased the greenhouse effect of the Earth's atmosphere, causing planetary cooling, and triggered the Huronian glaciation. Starting around 2.4 billion years ago, this lasted 300-400 million years, and may have been the longest ever snowball Earth episode.[20][21]

Secondly, the increased oxygen concentrations provided a new opportunity for biological diversification, as well as tremendous changes in the nature of chemical interactions between rocks, sand, clay, and other geological substrates and the Earth's air, oceans, and other surface waters. Despite the natural recycling of organic matter, life had remained energetically limited until the widespread availability of oxygen. This breakthrough in metabolic evolution greatly increased the free energy available to living organisms, with global environmental impacts. For example, mitochondria evolved after the GOE, giving organisms the energy to exploit new, more complex morphologies interacting in increasingly complex ecosystems.[22]

Timeline of glaciations, shown in blue.

## Time lag theory

There may have been a gap of up to 900 million years between the start of photosynthetic oxygen production and the geologically rapid increase in atmospheric oxygen about 2.5–2.4 billion years ago. Several hypotheses propose to explain this time lag.

### Tectonic trigger

2.1 billion year old rock showing banded iron formation

The oxygen increase had to await tectonically driven changes in the Earth, including the appearance of shelf seas, where reduced organic carbon could reach the sediments and be buried.[23] The newly produced oxygen was first consumed in various chemical reactions in the oceans, primarily with iron. Evidence is found in older rocks that contain massive banded iron formations apparently laid down as this iron and oxygen first combined; most present-day iron ore lies in these deposits. It was assumed oxygen released from cyanobacteria resulted in the chemical reactions that created rust, but it appears the iron formations was caused by anoxygenic phototrophic iron-oxidizing bacteria, which does not require oxygen.[24] Evidence suggests oxygen levels spiked each time smaller land masses collided to form a super-continent. Tectonic pressure thrust up mountain chains, which eroded to release nutrients into the ocean to feed photosynthetic cyanobacteria.[25]

### Nickel famine

Early chemosynthetic organisms likely produced methane, an important trap for molecular oxygen, since methane readily oxidizes to carbon dioxide (CO2) and water in the presence of UV radiation. Modern methanogens require nickel as an enzyme cofactor. As the Earth's crust cooled and the supply of volcanic nickel dwindled, oxygen-producing algae began to out-perform methane producers, and the oxygen percentage of the atmosphere steadily increased.[26] From 2.7 to 2.4 billion years ago, the rate of deposition of nickel declined steadily from a level 400 times today's.[27]

### Bistability

Another hypothesis posits a model of the atmosphere that exhibits bistability: two steady states of oxygen concentration. The state of stable low oxygen concentration (0.02%) experiences a high rate of methane oxidation. If some event raises oxygen levels beyond a moderate threshold, the formation of an ozone layer shields UV rays and decreases methane oxidation, raising oxygen further to a stable state of 21% or more. The Great Oxygenation Event can then be understood as a transition from the lower to the upper steady states.[28]

### Hydrogen gas

Another theory credits the appearance of cyanobacteria with suppressing hydrogen gas and increasing oxygen.

Some bacteria in the early oceans could separate water into hydrogen and oxygen. Under the Sun's rays, hydrogen molecules were incorporated into organic compounds, with oxygen as a by-product. If the hydrogen-heavy compounds were buried, it would have allowed oxygen to accumulate in the atmosphere.

However, in 2001 scientists realized that the hydrogen would instead escape into space through a process called methane photolysis, in which methane releases its hydrogen in a reaction with oxygen. This could explain why the early Earth stayed warm enough to sustain oxygen-producing lifeforms.[29]

## Late evolution of oxy-photosynthesis theory

The oxygen indicator might have been misinterpreted. During the proposed lag era in the previous theory, there was a change in sediments from mass-independently fractionated (MIF) sulfur to mass-dependently fractionated (MDF) sulfur. This was assumed to show the appearance of oxygen in the atmosphere, since oxygen would have prevented the photolysis of sulfur dioxide, which causes MIF. However, the change from MIF to MDF of sulfur isotopes may instead have been caused by an increase in glacial weathering, or the homogenization of the marine sulfur pool as a result of an increased thermal gradient during the Huronian glaciation period (which in this interpretation was not caused by oxygenation).[20]

## Role in mineral diversification

The Great Oxygenation Event triggered an explosive growth in the diversity of minerals, with many elements occurring in one or more oxidized forms near the Earth's surface.[30] It is estimated that the GOE was directly responsible for more than 2,500 of the total of about 4,500 minerals found on Earth today. Most of these new minerals were formed as hydrated and oxidized forms due to dynamic mantle and crust processes.[31]

Great Oxygenation
End of Huronian glaciation
Palæoproterozoic
Mesoproterozoic
Neoproterozoic
Palæozoic
Mesozoic
Cenozoic
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Million years ago. Age of Earth = 4,560

## Origin of eukaryotes

It has been proposed that a local rise in oxygen levels due to cyanobacterial photosynthesis in ancient microenvironments was highly toxic to the surrounding biota, and that this selective pressure drove the evolutionary transformation of an archaeal lineage into the first eukaryotes.[32] Oxidative stress involving production of reactive oxygen species (ROS) might have acted in synergy with other environmental stresses (such as ultraviolet radiation and/or desiccation) to drive selection in an early archaeal lineage towards eukaryosis. This archaeal ancestor may already have had DNA repair mechanisms based on DNA pairing and recombination and possibly some kind of cell fusion mechanism.[33][34] The detrimental effects of internal ROS (produced by endosymbiont proto-mitochondria) on the archaeal genome could have promoted the evolution of meiotic sex from these humble beginnings.[33] Selective pressure for efficient DNA repair of oxidative DNA damages may have driven the evolution of eukaryotic sex involving such features as cell-cell fusions, cytoskeleton-mediated chromosome movements and emergence of the nuclear membrane.[32] Thus the evolution of eukaryotic sex and eukaryogenesis were likely inseparable processes that evolved in large part to facilitate DNA repair.[32][35] Constant pressure of endogenous ROS has been proposed to explain the ubiquitous maintenance of meiotic sex in eukaryotes.[33]

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