Anoxic Event - Occurrence

Occurrence

Oceanic anoxic events most commonly occurred during periods of very warm climate characterized by high levels of carbon dioxide (CO₂) and mean surface temperatures probably in excess of 25 °C (77 °F). The Quaternary levels, our current period, are just 13 °C (55 °F) in comparison. Such rises in carbon dioxide may have been in response to a great outgassing of the highly flammable natural gas (methane) some have christened an "oceanic burp". Vast quantities of methane are normally locked into the Earth's crust on the continental plateaus in one of the many deposits consisting of compounds of methane hydrate, a solid precipitated combination of methane and water much like ice. Because the methane hydrates are unstable, save at cool temperatures and high (deep) pressures, scientists have observed smaller "burps" due to tectonic events. Studies suggest the huge release of natural gas could be a major climatological trigger, methane itself being a greenhouse gas many times more powerful than carbon dioxide. However, anoxia was also rife during the Hirnantian (late Ordovician) ice age.

Oceanic anoxic events have been recognized primarily from the already warm Cretaceous and Jurassic Periods, when numerous examples have been documented, but earlier examples have been suggested to have occurred in the late Triassic, Permian, Devonian (Kellwasser event(s)), Ordovician and Cambrian.

The Paleocene–Eocene Thermal Maximum (PETM), which was characterized by a global rise in temperature and deposition of organic-rich shales in some shelf seas, shows many similarities to oceanic anoxic events.

Typically, oceanic anoxic events last for under half a million years, before a full recovery.