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Re: Chicxulub impact effects
I haven't read the paper but did they take different oxygen levels
(percentages) in the Cretaceous into account for Ignition thresholds?
I seem to remember reading somewhere that the percentage of Oxygen
present in the atmosphere was higher in the Cretaceous.
On Sep 20, 2004, at 1:33 PM, Tommy Tyrberg wrote:
An interesting paper I don't think have been mentioned on the DML
Durda, D. D. & Kring, D. A. 2004. Ignition threshold for
impact-generated fires. Journal of Geophysical Research 109 E08004.
They study three energy levels 51 kWm-2 for 2 minutes (ignites wood
under any circumstances), 20 kWm-2 for 20 minutes (ignites wood in the
presence of an ignition source) and 28 kWm-2 for 1 minute (ignites
foliage, rotten wood and dry litter, which would provide an ignition
source). Calculates that the threshold for continent-wide and
world-wide ignition of woody material equates to c. 10^15 and 10^16
kilograms of ejecta respectively, which implies crater diameters of c.
85 and 135 km respectively.
This means that Chicxulub is the only Phanerozoic crater certainly in
the "worldwide" category, though the Bedout Structure would probably
also qualify if it is really an impact crater. Definitely
"continent-wide" impacts would include at least Manicougan, Popigai
and Chesapeake Bay.
An interesting feature is that in most cases the effects in the
antipodal area are worse than near the impact (the ejecta landing
there have much more energy). Anybody know where the antipodal points
of Popigai and Chesapeake Bay were in the Eocene?
India and environs must have been very badly hit by Chicxulub. Might
explain why there are no land vertebrates with definitely Mesozoic
antecedents on Madagascar, while New Zealand which was almost
completely flooded in the Oligocene still has two (Sphenodon and
They do not address the direct effect on animals, but even the lowest
studied energy level would generate temperatures close to 500 degrees
centigrade on an exposed surface and certainly kill any unprotected
animal quite quickly. Incidentally this implies that there is probably
a rather wide range of irradiation that would kill unprotected animals
but not ignite tree trunks, so treeholes might work as protection for
Neither is there any discussion of the effect of heavy cloud or snow
which would certainly give some protection (in the case of snow
probably a lot because of its very high albedo and thermal inertia).