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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.
Frank Bliss
MS Biostratigraphy
Weston, Wyoming
On Sep 20, 2004, at 1:33 PM, Tommy Tyrberg wrote:

An interesting paper I don't think have been mentioned on the DML before:

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 Leiopelma).

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 small animals.
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).

Tommy Tyrberg