Anoxic event
 nature.com A flow chart of magma sourcing trace metals, ocean fertilization, stratification, and anoxia. |
Oceanic anoxic events or anoxic events (anoxia conditions) refer to intervals in the Earth's past where portions of oceans become depleted in oxygen (O2) at depths over a large geographic area. During some of these events, euxinia, waters that contained H
2S hydrogen sulfide, developed. Although anoxic events have not happened for millions of years, the geological record shows that they happened many times in the past. Anoxic events coincided with several mass extinctions and may have contributed to them. These mass extinctions include some that geobiologists use as time markers in biostratigraphic dating. Many geologists believe oceanic anoxic events are strongly linked to slowing of ocean circulation, climatic warming, and elevated levels of greenhouse gases. Researchers have proposed enhanced volcanism (the release of CO2) as the "central external trigger for euxinia".
The concept of the oceanic anoxic event (OAE) was first proposed in 1976 by Seymour Schlanger (1927–1990) and geologist Hugh Jenkyns and arose from discoveries made by the Deep Sea Drilling Project (DSDP) in the Pacific Ocean. It was the finding of black carbon-rich shales in Cretaceous sediments that had accumulated on submarine volcanic plateaus (Shatsky Rise, Manihiki Plateau), coupled with the fact that they were identical in age with similar deposits cored from the Atlantic Ocean and known from outcrops in Europe - particularly in the geological record of the otherwise limestone-dominated Apennines chain in Italy - that led to the realization that these widespread similar strata recorded highly unusual oxygen-depleted conditions in the world ocean during several discrete periods of geological time.
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