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Great Oxidation Event
Paleoproterozoic surge in atmospheric oxygen / From Wikipedia, the free encyclopedia
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The Great Oxidation Event (GOE) or Great Oxygenation Event, also called the Oxygen Catastrophe, Oxygen Revolution, Oxygen Crisis or Oxygen Holocaust,[2] was a time interval during the Earth's Paleoproterozoic era when the Earth's atmosphere and shallow seas first experienced a rise in the concentration of free oxygen.[3] This began approximately 2.460–2.426 Ga (billion years) ago during the Siderian period and ended approximately 2.060 Ga ago during the Rhyacian.[4] Geological, isotopic and chemical evidence suggests that biologically produced molecular oxygen (dioxygen or O2) started to accumulate in the Archean prebiotic atmosphere due to microbial photosynthesis, and eventually changed it from a weakly reducing atmosphere practically devoid of oxygen into an oxidizing one containing abundant free oxygen,[5] with oxygen levels being as high as 10% of modern atmospheric level by the end of the GOE.[6]
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- 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 appearance of highly reactive free oxygen, which can oxidize organic compounds (especially genetic materials) and thus is toxic to the then-mostly anaerobic biosphere, may have caused the extinction/extirpation of many early organisms on Earth – mostly archaeal colonies that used retinal to utilize green-spectrum light energy and power a form of anoxygenic photosynthesis (see Purple Earth hypothesis). Although the event is inferred to have constituted a mass extinction,[7] due in part to the great difficulty in surveying microscopic organisms' abundances, and in part to the extreme age of fossil remains from that time, the Great Oxidation Event is typically not counted among conventional lists of "great extinctions", which are implicitly limited to the Phanerozoic eon. In any case, isotope geochemistry data from sulfate minerals have been interpreted to indicate a decrease in the size of the biosphere of >80% associated with changes in nutrient supplies at the end of the GOE.[8]
The GOE is inferred to have been caused by cyanobacteria, which evolved chlorophyll-based photosynthesis that releases dioxygen as a byproduct of water photolysis. The continually produced oxygen eventually depleted all the surface reducing capacity from ferrous iron, sulfur, hydrogen sulfide and atmospheric methane over nearly a billion years. The oxidative environmental change, compounded by a global glaciation, devastated the microbial mats around the Earth's surface. The subsequent adaptation of surviving archaea via symbiogenesis with aerobic proteobacteria (which went endosymbiont and became mitochondria) may have led to the rise of eukaryotic organisms and the subsequent evolution of multicellular life-forms.[9][10][11]