We only know about these events by their geologic fingerprints. Her money statement is: Vertical mixing of oceanic water had stopped during or soon after the mass extinction, causing widespread depletion of oxygen in the oceans. The restoration of the mixing of the layers of ocean certainly restored a habitat the destruction of which had decimated ocean fauna. Volcanic activity like that which over time created the Siberian Trapps put out plenty of nasty things CO2 is one of them.