Four theoretical causes of the Permian extincion event.
To what extent could the first event have triggered one or more of the other three effects?
Hydrogen sulfide is found at depth in the oceans, either as a product of inorganic processes deep in the Earth, or as a biological byproduct. A large ocean impact (or a series of impacts from a former single object that broke up) would release that. However, with large impacts, that wouldn't be the only problem. :')