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A ref that may be of interest to many on this list:
Fossil Plants and Global Warming at the Triassic-Jurassic Boundary.
McElwain, Beerling and Woodward
Science, 27th Aug, Vol 285, p 1386-1390
Abstract: The T-J boundary marks a major faunal mass extinction, but
records of accompanying environmental changes are limited. Paleobotanical
evidence indicates a fourfold increase in atmospheric carbon dioxide
concentration and suggests an associated 3 to 4 degree "greenhouse" warming
across the boundary. these environmental conditions are calculated to have
raised leaf temperatures above a highly conserved lethal limit, perhaps
contributing to the >95% species level turnover of T-J megaflora.
They attribute the CO2 increase to basaltic volcanicity associated with the
break-up of Pangea.
If this sort of increase in CO2 can cause the sort of extensive extinctions
seen at the T-J boundary (without, as I understand, any implication of
extraterrestrial impact factors?), does anyone know if any similar studies
have been done on flora across the K-T boundary?