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An anoxic event (AE) is an event in which parts of the ocean have become low in oxygen (O2) below the surface levels, or when all the oxygen has gone. They may be called oceanic anoxic events or deep ocean anoxic events.
Major anoxic events have happened, though not for millions of years. The geological record of organic-rich sediment (black shales) shows that they happened in the past. However, only "in rare, extreme cases, [did] euxinia lead to biotic crises. [The] hypothesis [is] best supported by evidence from the end-Permian mass extinction".
Anoxic events may have caused mass extinctions. These mass extinctions were so characteristic that they have been used by geologists as markers in biostratigraphic dating. Typically, oceanic anoxic events last for under half a million years, before a full recovery.
There are several places on earth today that show the features of anoxic events on a localized level. 'Dead zones' exist off the East Coast of the United States in the Chesapeake Bay, in the Scandinavian strait Kattegat, the Black Sea, in the northern Adriatic and off the coast of Louisiana.
Events which may have caused anoxic events
It is not known why anoxic events may have happened. Some ideas include:
- Global warm climate leading to a huge growth of biomass.
- High rainfall sweeping organic material down into the oceans.
- Deep water circulation between poles and the equator stopping.
- Oceanic oxygen being used up, and not being replaced fast enough.
- Poisonous hydrogen sulphide collecting in the oceans.
- Oceans becoming hostile to most forms of life
All of these events may have caused mass extinction in the seas, changing ways in which all animals which feed on sea creatures.
- Toarcian event 183 million years ago (mya)
- Aptian: mid-Aptian extinction event, 116/7 mya.
- Cenomanian–Turonian boundary event: black shale deposition in ocean basins. 91.5 (±8.6) mya.
- Meyer K.M.; Kump L.R. 2008. Oceanic euxinia in Earth history: causes and consequences. Annual Review of Earth and Planetary Sciences 36: 251–288 
- Courtillot, Vincent. 1999. Evolutionary catastrophes: the science of mass extinctions. Cambridge, Cambridge University Press.
- Kum, L.R; Pavlov A. and Arthur M.A. 2005. Massive release of hydrogen sulfide to the surface ocean and atmosphere during intervals of oceanic anoxia. Geology 33 (5): 397–400. .