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
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. It is believed 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 CO
2) as the "central external trigger for euxinia".