Iridium, etc.

[keel@bildad.astr.ua.edu (William C. Keel)]

Brain Franczak (and previous posters) wrote:


In answer to my questions about the K-T strike, Dinogeorge@aol.com
wrote:

>> 1) Any estimates of the time between the impact and the time the      >> 
>> iridium layer was created?>>
> 
> A few hours, days, or weeks.
> 
>> 2) How close to the iridium layer are dinosaur fossils found? >>
> 
> Right up to it in the Deccan traps, according to Chatterjee, but never > 
> above it.

So within a few hours, days, or weeks ALL DINOSAURS (yeah, yeah, and
lotsa other things) were dead? ALL of them?

...

Just to be clear, that statement was not implicit in the previous posts.
In an impact model, the iridium would indeed have been precipitated
very rapidly (instantaneously on a geological timescale) after the
impact. However, the time resolution of the fossil record (or
indeed of the geological column except in very special instances)
is so coarse by comparison that finding no dinosaur (etc.) fossils
above the layer only equates to "too few survived by several millenia
to have a good shot at fossilization". The impact itself needn't
have directly killed all of them in order to have been the proximate
cause of the extinctions in the longer haul (and I won't get into
the sole-versus-multiple cause debate...)

Bill Keel
Astronomy, University of Alabama