oxygen catastrophe

Definitions

• An event during the oxygenation event of Earth's past, where the level of free oxygen in the environment exceeds the toleration levels of most extant anerobic life, becoming toxic to existing life, which prior to the mass production of oxygen, was an anerobic reducing environment. This is sometimes considered a mass extinction event, or the first mass extinction event, but as all life was unicellular, there is no fossil boundary to mark the event for conventional paleontology to recognize it.
• Such an event that theoretically occurs elsewhere, away from Earth.