Re: ?Evolutionary cause for oxygen peak ~50 Myrs. ago?


<stargene@xxxxxxxxxxxxx> wrote in message
news:diq3n5$1j6g$1@xxxxxxxxxxxxxxxxxxxxxx
> Thanks. However, I'm pretty sure that no snowball-earth
occurred in
> the time interval from ~201 Myr to ~50 Myr ago. I'll keep
on
> searching for info. I suppose it might even
> be something as indirect as the impact of
> giant reptiles which, in the mesozoic, would have severly
impacted the
> forests they lived in, similarly to the impact of
elephants today on
> their environment. Their absence in the cenozoic might
arguably have
> allowed the establishment
> of super-forests which might have injected
> massive amounts of O2 into the atmo-
> sphere. I'm probably way off, considering
> the greater importance of ocean plankton
> in the O2/CO2 budget.

Guess I completely overlooked your date criteria for the
peak--though arguably, the whole process of increase began
with the thaw of the 3d Snowball Earth, yielding the first
peak very much >21%. (It should be remembered that the
changes in the algae blooms and the resultant O2 increases
were not immediate even if the initiation of the process is
attributed due to Snowball thawing.) The answer to your
specific question is--it is still all
very controversial! Take a look at:

<http://www.newscientist.com/article.ns?id=dn8074>

Also:
<http://www.sciam.com/article.cfm?articleID=00065C28-A06A-13
3D-A06A83414B7F0000>

I didn't know there was actually a peak of 36% O2 purported
to have been found in core studies before finally declining
to the current value. Large mammals certainly could be a
factor in the more recent declines as would the growth and
loss of land plants, and trees in periodic large forest
fires. I'm sure much more work needs to be done in this
whole area, before we know the whole story with any
certainty.
Regards,
....tonyC


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