Re: Chicxulub impact effects

[frank bliss <frank@blissnet.com>]

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