Oceanic Euxinia
Oceanic anoxic events 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 develops - anoxic waters that contain high concentrations of hydrogen sulfide (H2S). Although anoxic events have not happened for millions of years, the geological record shows that they happened many times in the past, and coincide with, and may have contributed to, several mass extinctions. [1]