Anoxic event

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:
All of these events may have caused mass extinction in the seas, changing ways in which all animals which feed on sea creatures.