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.
A possible scenario
It is not known for certain what the causes were of AEs. A possible course of events is:
- Global warm climate lead to a huge growth of biomass.
- High rainfall sweeps organic material down into the oceans.
- Deep water circulation between poles and the equator stopped.
- Oceanic oxygen gets used up, and is not replaced fast enough.
- Poisonous hydrogen sulphide collects in the oceans.
- Oceans become hostile to most forms of life
- Result: mass extinctions in the seas, with knock-on effects to all animals which feed on sea creatures.
This is not yet well-established science. It is a speculation by scientists interested in palaeoecology and climate change. However, the fact is that these anoxic events did occur, and did have causes which might be in operation today.
- 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.
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