Actually, this was before dinosaurs, so my vote for the change was the development of trees and climax forests as opposed to marshes and ferns. It is generally recognized that climax forests use more CO2 than grasslands or other lower growing landscapes. I agree the Siberian traps were big, but might they have been started by a giant meteor strike?
Despite lots of searching, they've yet to find any signs of an asteroid impact coinciding with that extinction event. ie, no iridium layer, etc. Dinosaur extinction event, yes.