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 CO₂) as the "central external trigger for euxinia".