Anoxic event
Anoxic events occur when the Earth's oceans become completely depleted of oxygen (O2) below the surface levels. They may be called Oceanic anoxic events or Deep ocean anoxic events.
Anoxic events have happened. Although anoxic events have not happened for millions of years, the geological record shows that they happened many times in the past.[1]
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.[2] It is believed oceanic anoxic events are strongly linked to climate warming and greenhouse gases.
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. Typically, oceanic anoxic events last for under half a million years, before a full recovery.
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[change] Historic examples
[change] Jurassic period
- Toarcian event 183 million years ago (mya)
[change] Lower Cretaceous period
- Aptian: mid-Aptian extinction event, 116/7 mya.
[change] Upper Cretaceous period
- Cenomanian–Turonian boundary event: black shale deposition in ocean basins. 91.5 (±8.6) mya.
[change] Cainozoic
- Palaeocene–Eocene Thermal Maximum (PETM), 55.8 mya.
