The last scenario talks about, if I recall correctly, the oceans becoming more welcoming to bacteria that produce hydrogen sulfide. Sorry, I don't remember the details, but will update tonight from the book at home if I remember.
Thumbnail sketch of the event: something (possibly massive volcanic eruptions near in time to a significant meteorite impact) deoxygenated the water enough to cause a substantial marine life die-off. The death of so much marine life in such a short time encouraged sulfate-reducing bacteria, which would have released significant amounts of hydrogen sulfide into the atmosphere. Hydrogen sulfide would have killed plant (and, therefore, animal) life and weakened the ozone layer, exposing the suriving flora and fauna to further UV radiation.
"96% of all marine species and 70% of terrestrial vertebrate species becoming extinct."
There is some concern regarding a risk (I have no idea how big the risk is) that anthropogenic global warming could "tip the scales" of the global ecology towards the conditions that allow for a Permian-Triassic-style cascade.