page_title: Great Oxidation Event
section_title: Oxygen accumulation
hierarchical_section_title: Great Oxidation Event / Oxygen accumulation
caption_attribution_description: English: File:GlaciationsinEarthExistancelicenced.jpg, annotated with ice age names
context_page_description: The Great Oxidation Event, sometimes also called the Great Oxygenation Event, Oxygen Catastrophe, Oxygen Crisis, Oxygen Holocaust, or Oxygen Revolution, was a time period when the Earth's atmosphere and the shallow ocean experienced a rise in oxygen, approximately 2.4 billion years ago to 2.1–2.0 Ga during the Paleoproterozoic era. Geological, isotopic, and chemical evidence suggests that biologically produced molecular oxygen started to accumulate in Earth's atmosphere and changed Earth's atmosphere from a weakly reducing atmosphere to an oxidizing atmosphere, causing many existing species on Earth to die out. The cyanobacteria producing the oxygen caused the event which enabled the subsequent development of multicellular forms.
context_section_description: 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 more efficiently, producing oxygen as a waste product. In one interpretation, the first oxygen-producing cyanobacteria could have arisen before the GOE, from 2.7–2.4 Ga and perhaps even earlier. 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. 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 15.8 ± 3.3 T mol (1 T mol = 10¹² moles) 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. The oxygen sinks include reduced gases and minerals from volcanoes, metamorphism and weathering. 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. For the weathering mechanisms, 12.0 ± 3.3 T mol 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. On the other hand, 5.7 ± 1.2 T mol of O₂ per year today oxidizes reduced gases in the atmosphere through photochemical reaction. 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  and  to magnetite () 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. It took 50 million years or longer to deplete the oxygen sinks. 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. Today, the average time that an O₂ molecule spends in the air before it is consumed by geological sinks is about 2 million years. 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. 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.
  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 availa
caption: Timeline of glaciations, shown in blue.
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