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


"Perplexed in Peoria" <jimmenegay@xxxxxxxxxxxxx> wrote in
message news:disp5i$2nkn$1@xxxxxxxxxxxxxxxxxxxxxx
>
> "Anthony Cerrato" <tcerrato@xxxxxxxxxxxxx> wrote in
message news:dirg26$24jt$1@xxxxxxxxxxxxxxxxxxxxxx
> >
> > <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.
>
> Thanks for the links, Tony. However, I would be very
skeptical
> of any claims that atmospheric O2 levels had been
indirectly
> measured by directly measuring the ratio of C(12) to C(13)
> in ocean sediments. There are just too many dubious
assumptions
> involved in a chain of reasoning from the direct
measurement to
> the indirect inference. See, for example, all of the
different
> factors that go into conflicting interpretations of the
C(12)
> to C(13) ratio in the Snowball debates.

Yes, I agree!--In my haste, I just forgot to mention it
also. There seem to be several conflicting points
researchers have raised in general on any of the theories
proposed...a fertile field indeed for now. :)
....tonyC


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