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 oxygen level.
• Stages 4 and 5 (0.85 Ga - present): Other O₂ reservoirs filled; gas accumulates in atmosphere.^[1]

The Great Oxidation Event (GOE), sometimes also called the Great Oxygenation Event, Oxygen Catastrophe, Oxygen Crisis, Oxygen Holocaust,^[2] or Oxygen Revolution, was a time period when the Earth's atmosphere and the shallow ocean first experienced a rise in oxygen, approximately 2.4 - 2.0 Ga (billion years ago) during the Paleoproterozoic era.^[3] Geological, isotopic, and chemical evidence suggest that biologically produced molecular oxygen (dioxygen, O₂) started to accumulate in Earth's atmosphere and changed it from a weakly reducing atmosphere to an oxidizing atmosphere,^[4] causing many existing species on Earth to die out.^[5] The cyanobacteria producing the oxygen caused the event which enabled the subsequent development of multicellular forms.^[6]

Oxygen accumulation

A chronology of oxygen accumulation suggests that free oxygen was first produced by prokaryotic and then later by eukaryotic organisms in the ocean. These organisms carried out photosynthesis, producing oxygen as a waste product.^[7][8] In one interpretation, the first oxygen-producing cyanobacteria could have arisen before the GOE,^[7][9] from 2.7-2.4 Ga and perhaps even earlier.^[3][10][11] However, oxygenic photosynthesis also produces organic carbon that must be segregated from oxygen to allow oxygen accumulation in the surface environment, otherwise the oxygen back-reacts with the organic carbon and does not accumulate. The burial of organic carbon, sulfide, and minerals containing ferrous iron (Fe²⁺) is a primary factor in oxygen accumulation.^[12] For example, when organic carbon is buried without being oxidized, the oxygen is left in the atmosphere. In total, the burial of organic carbon and pyrite today creates of O₂ per year. This creates a net O₂ flux from the global oxygen sources.

The rate of change of oxygen can be calculated from the difference between global sources and sinks.^[13] The oxygen sinks include reduced gases and minerals from volcanoes, metamorphism and weathering.^[13] The GOE started after these oxygen-sink fluxes and reduced-gas fluxes were exceeded by the flux of O₂ associated with the burial of reductants, such as organic carbon.^[14] For the weathering mechanisms, of O₂ per year today goes to the sinks composed of reduced minerals and gases from volcanoes, metamorphism, percolating seawater and heat vents from the seafloor.^[13] On the other hand, of O₂ per year today oxidizes reduced gases in the atmosphere through photochemical reaction.^[13] On the early Earth, there was visibly very little oxidative weathering of continents (e.g., a lack of redbeds) and so the weathering sink on oxygen would have been negligible compared to that from reduced gases and dissolved iron in oceans.

Dissolved iron in oceans exemplifies O₂ sinks. Free oxygen produced during this time was chemically captured by dissolved iron, converting iron ${\displaystyle {\ce {Fe}}}$ and ${\displaystyle {\ce {Fe^2+}}}$ to magnetite (${\displaystyle {\ce {Fe^2+Fe2^3+O4}}}$) that is insoluble in water, and sank to the bottom of the shallow seas to create banded iron formations such as the ones found in Minnesota and Pilbara, Western Australia.^[14] It took 50 million years or longer to deplete the oxygen sinks.^[15] The rate of photosynthesis and associated rate of organic burial also affect the rate of oxygen accumulation. When land plants spread over the continents in the Devonian, more organic carbon was buried and likely allowed higher O₂ levels to occur.^[16] Today, the average time that an O₂ molecule spends in the air before it is consumed by geological sinks is about 2 million years.^[17] That residence time is relatively short compared to geologic time - so in the Phanerozoic there must have been feedback processes that kept the atmospheric O₂ level within bounds suitable for animal life.

Eventually, oxygen started to accumulate in the atmosphere, with two major consequences.

• First, it has been proposed that oxygen oxidized atmospheric methane (a strong greenhouse gas) to carbon dioxide (a weaker one) and water. This weakened the greenhouse effect of the Earth's atmosphere, causing planetary cooling, which has been proposed to have triggered a series of ice ages known as the Huronian glaciation, bracketing an age range of 2.45-2.22 Ga.^[18][19][20] A fourth glaciation event found in South Africa is ~2.22 Ga in age. Because geological evidence suggests that the ice reached sea-level in some areas and that the South African event occurred at low latitudes, the latter is associated with a so-called Snowball Earth.^[21]

• Second, 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 morphology interacting in increasingly complex ecosystems, although these did not appear until the late Proterozoic and Cambrian.^[22]

Timeline of glaciations, shown in blue.

Geological evidence

Continental indicators

Paleosols, detrital grains, and redbeds are evidence of low-level oxygen.^[13] The paleosols older than 2.4 Ga have low iron concentrations that suggests anoxic weathering.^[23]Detrital grains older than 2.4 Ga also have material that only exists under low oxygen conditions.^[24]Redbeds are red-colored sandstones that are coated with hematite, which indicates that there was enough oxygen to oxidize iron.^[25]

Banded iron formation (BIF)

Iron speciation

The concentration of ferruginous and euxinic states in iron mass can also provide clues of the oxygen level in the atmosphere.^[26] When the environment is anoxic, the ratio of ferruginous and euxinic out of the total iron mass is lower than the ratio in an anoxic environment such as the deep ocean.^[27] One of the hypotheses suggests that microbes in the ocean already oxygenated the shallow waters before the GOE event around 2.6-2.5 Ga.^[13][27] The high concentration of ferruginous and euxinic states of sediments in the deep ocean showed consistency with the evidence from banded iron formations.^[13]

Isotopes

There are two types of isotope fractionation considered: mass-dependent fractionation (MDF) and mass-independent fractionation (MIF). Isotopes in marine sediments of the accumulation of oxygen such as carbon, sulfur, nitrogen, transitional metals (chromium, molybdenum and iron) and other non-metal elements (selenium) are considered as MDF evidence.^[13]

For example, a spike in chromium contained in ancient rock deposits formed underwater shows accumulated chromium washed off from the continental shelves.^[28] Since chromium is not easily dissolved, its release from rocks requires the presence of a powerful acid such as sulfuric acid (H₂SO₄) which may have formed through bacterial reactions with pyrite.^[29]

The critical evidence of GOE was the MIF of sulfur isotopes that only existed in anoxic atmosphere and disappeared from sediment rocks after 2.4-2.3 Ga.^[30] MIF only existed in an anoxic atmosphere since oxygen (and its photochemical product, an ozone layer) would have prevented the photolysis of sulfur dioxide. The process of MIF sedimentation is currently uncertain.^[13]

Fossils and biomarkers

Stromatolites provide some of the fossil evidence of oxygen, and suggest that the oxygen came from photosynthesis. Biomarkers such as 2?-methylhopanes from cyanobacteria were also found in Pilbara, Western Australia. However, the biomarker data has since been shown to have been contaminated and so results are no longer accepted.^[31]

Other indicators

Some elements in marine sediments are sensitive to different levels of oxygen in the environment such as transition metals molybdenum and rhenium.^[32] Non-metal elements such as selenium and iodine are also indicators of oxygen levels.^[33]

Hypotheses

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.

Increasing flux

Some people suggest that GOE is caused by the increase of the source of oxygen. One hypothesis argues that GOE was the immediate result of photosynthesis, although the majority of scientists suggest that a long-term increase of oxygen is more likely the case.^[34] Several model results show possibilities of long-term increase of carbon burial,^[35] but the conclusions are indecisive.^[36]

Decreasing sink

In contrast to the increasing flux hypothesis, there are also several hypotheses that attempt to use decrease of sinks to explain GOE. One theory suggests that composition of the volatiles from volcanic gases was more oxidized.^[12] Another theory suggests that the decrease of metamorphic gases and serpentinization is the main key of GOE. Hydrogen and methane released from metamorphic processes are also lost from Earth's atmosphere over time and leave the crust oxidized.^[37] Scientists realized that hydrogen would escape into space through a process called methane photolysis, in which methane decomposes under the action of ultraviolet light in the upper atmosphere and releases its hydrogen. The escape of hydrogen from the Earth into space must have oxidized the Earth because the process of hydrogen loss is chemical oxidation.^[37]

Tectonic trigger

2.1-billion-year-old rock showing banded iron formation

One hypothesis suggests that 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.^[38][39] 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 were caused by anoxygenic phototrophic iron-oxidizing bacteria, which does not require oxygen.^[40] 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.^[41]

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.^[42] From 2.7 to 2.4 billion years ago, the rate of deposition of nickel declined steadily from a level 400 times today's.^[43]

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.^[44][45]

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.^[46] 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.^[47]

Role in cyanobacteria evolution

In a field research done in Lake Fryxell, Antarctica, researchers found out that mats of oxygen-producing cyanobacteria can produce a thin layer, one to 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 could have possibly adapted to oxygen.^[48][49] 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.

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.^[50]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.^[51][52] 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.^[51] 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.^[50] Thus the evolution of eukaryotic sex and eukaryogenesis were likely inseparable processes that evolved in large part to facilitate DNA repair.^[50][53]

See also

• Geological history of oxygen - Timeline of the development of free oxygen in the Earth's oceans and atmosphere
• Medea hypothesis
• Pasteur point
• Rare Earth hypothesis - Hypothesis that complex extraterrestrial life is an extremely rare phenomenon
• Stromatolite – Layered sedimentary structures formed by the growth of bacteria or algae

References

External links

• Lane, Nick (5 February 2010). "First breath: Earth's billion-year struggle for oxygen". New Scientist. No. 2746. Archived 2011-01-06 at the Wayback Machine