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Atmosphere at K-T extinctions

In the thread on theories of extinction causes the discussion 
of atmospheric change came up. I'm unclear on what the changes 

I do realize that the general trend in oxygen levels has been 
toward lower concentrations since before the Mesozoic. Tony Hedges 
pointed out that there was a sudden spike in the oxygen 
concentration at the end of the Cretaceous:

> 3. Atmosphere.
> Studies of the atmospheric content contained within amber dated to the end
> of the Cretacious period has shown that towards the end of the last period
> of the dinosaurs reign contained a smaller ammount of Oxygen in the mix, and
> towards the end of the Cretacious the Oxygen content shot up by some 20 -
> 23%. This is a significant increase.

What are some of the other results of these studies of the atmospheric 
content? What are some other sources besides the amber available for study 
which can shed light on the make up of the late Cretaceous atmosphere? 
Generally speaking, how much such evidence is available?

Thanks again to all. At this rate I'll soon be able to keep up with my kids' 
questions <G>.