https://en.m.wikipedia.org/wiki/Anoxic_event

>An anoxic event describes a period wherein large expanses of Earth's oceans were depleted of dissolved oxygen (O2), creating toxic, euxinic (anoxic and sulfidic) waters.[1] Although anoxic events have not happened for millions of years, the geologic record shows that they happened many times in the past. Anoxic events coincided with several mass extinctions and may have contributed to them.[2] These mass extinctions include some that geobiologists use as time markers in biostratigraphic dating