Great Oxygenation Event

The Great Oxidation Event (GOE), sometimes also called the Great Oxygenation Event, Oxygen Catastrophe, Oxygen Crisis, Oxygen Holocaust, or Oxygen Revolution, was when Earth's atmosphere and the shallow ocean experienced a rise in oxygen around 2.4 billion years ago (2.4 Ga) to 2.1-2.0 Ga during the Paleoproterozoic era. Geological, isotopic, and chemical evidence suggests that biologically induced molecular oxygen (dioxygen, O2) started to accumulate in Earth's atmosphere and changed Earth's atmosphere from a weakly reducing atmosphere to an oxidizing atmosphere. The causes of the event remain unclear. A chronology of oxygen accumulation suggests that free oxygen was first produced by prokaryotic and then later eukaryotic organisms in the ocean that 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+) are primary factors 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 a total of 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 by the difference between global sources and sinks. The oxygen sinks include reducing 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 reducing 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 reducing 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), so the weathering sink on oxygen would have been negligible compared to that from reducing gases and dissolved iron in ocean. Dissolved iron in oceans is an example of the O2 sinks. Free oxygen produced during this time was chemically captured by dissolved iron, converting iron Fe {displaystyle {ce {Fe}}} and Fe 2 + {displaystyle {ce {Fe^2+}}} to magnetite ( Fe 2 + Fe 2 3 + O 4 {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. It took 50 million years or longer to deplete the oxygen sinks. The rate of photosynthesis and associated rate of organic burial also affects 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. This residence time is relatively short compared to geological time, so in the Phanerozoic, there must have been feedback processes that kept the atmospheric O2 level within bounds suitable for animal life.