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Re: Atmosphere at K-T extinctions



Joe, would you like for me to scan and send you a JPEG of Dudley's graphs of
atmospheric oxygen and CO2 contents from the Cambrian up through the present?

I think the difference in his paper and Hedge's work may indicate that there is
still some uncertainty about the oxygen spike at the end of the Cretaceous.  I
personally don't have a clue who of the two is correct, but think the amber 
tests
may be pushing the limits of technology and statistics.

Incidentally, I looked the paper up (Atmospheric Oxygen, Giant Paleozoic Insects
and the Evolution of Aerial Locomotor Performance, R. Dudley, JExB), and Dudley
shows a high of about 35% just before the beginning of the Permian, with a rapid
decline to a low of about 13-14% near the beginning of the Triassic, then a 
small
spike at about 17% in mid Triassic, another drop to about 14-15% early in the
Jurassic, a sudden climb to about 21% by mid-Jurassic, then a gentle climb to
about 26% early in the Tertiary, and a rather constant, steaty decline to the
present 20.9%.  Please note that my memory failed me about the timing of the
latest oxygen decline (I was thinking about the CO2 curve which peaked at about
the first third of the Cretaceous, and has since declined steadily till 
recently),
but I was correct about Dudley's lack of an oxygen spike near the end of the
Cretaceous.

Seosamh wrote:

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

So is everyone else, I think.

> I do realize that the general trend in oxygen levels has been
> toward lower concentrations since before the Mesozoic.

Per Dudley, there was a steady climb from a low near 15% about 200M years ago to
about 26% 50M years ago.  I don't know if he was correct, but think he may have
based his work rather heavily on that of J.M. Robinson.  He also cites Watson,
Kump, Berner, Canfield, Makowski, Wignall, Twitchett, Hart, Holland, and 
Isozaki.

> Tony Hedges
> pointed out that there was a sudden spike in the oxygen
> concentration at the end of the Cretaceous:

Dudley doesn't show this, apparently based on Berner And Canfield.

> > 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,

Dudley - larger

> and
> > towards the end of the Cretacious the Oxygen content shot up by some 20 -
> > 23%. This is a significant increase.

Dudley - slow steady increase of about 25% from mid-Cretaceous to early 
Tertiary.

Cheers,

Jim