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Re: "Oxygen Helped Mammals Grow, Study Finds"



My comments (below):

> >From Yahoo (around 2:00 PM EDT):
> 
> http://news.yahoo.com/s/ap/20050929/ap_on_sc/oxygen_mammals

> Today mammals, ranging from dogs and cats to elephants, dolphins and 
> people, 
> dominate the planet.


I'll bet that if insects, algae, and the bacteria were active subscribers
on the DML, they would disagree with that claim.  (maybe they're
lurkers?)


> By measuring the amount of
> carbon-13 in the samples
> they were able to estimate
> the amount of oxygen in the
> atmosphere at a particular time.


I assume (actually, hope) that Falkowski et. al took into account the
fact that C13 uptake in calcium carbonate shells is *highly* sensitive to
ambient water temperature.
(okay okay, I'll RTFP).

 
> They found that the air contained only about 10 percent oxygen at 
> the time 
> of the dinosaurs.

10%??  Oh come on....
Has anyone even bothered to TEST this hypothesis?  Construct an
atmospheric testing chamber with 10% O2 and record the activity level of
a gecko, a baby alligator, and a canary (one at a time).  Can they thrive
and live long enough to reproduce?

Bubble-oxygenate a fish tank with 10% atmospheric O2  and see how many
different fish and snail species can thrive and survive long enough to
reproduce.

I also wonder what a 10% atmospheric O2 level would do to the proportions
of aerobic bacteria vs anaerobic bacteria in the soils of the Mesozoic. 
If anaerobic bacteria became more abundant because of a much lower
atmospheric O2 %, then there would be an parallel increase in the out
gassing of methane from the Mesozoic soils.  Methane is the strongest
greenhouse gas.  Hmmm......

Anywhooo....
Besides, Falkowski et. al's conclusion is the polar opposite of some
conclusions from a few years ago!  Didn't  J. Keith Rigby Jr. et. al,
among other teams, claim that O2 levels were much *greater* during the
Mesozoic?



> By 50 million years ago the oxygen level had risen to 17 percent and 
> it was 
> 23 percent 40 million years ago, they reported. Currently the air 
> contains 
> about 21 percent oxygen.
> 
> The rise of oxygen "almost certainly contributed to evolution of 
> large 
> animals," the researchers reported. The oxygen needs of mammals and 
> birds 
> are three to six times as high as reptiles.



Reptiles in what sense?
And what about modern birds being dinosaurs?
And isn't a T. rex a "large animal"?



Alan Edels sez:
> This goes against earlier (somewhat flawed) studies that showed the 
> O2 
> levels were very much higher during the Mid to Late Jurassic.



Yeah, what Alan just said.  At any rate, I'm having a hard time wrapping
my mind around either the high O2 hypothesis or the low O2 hypothesis.

Here's my armchair scenario on how oxygen levels behave:

- Oxygen may have an intrinsic "optimum concentration" in earth's
atmosphere. Therefore, once it becomes established in abundance (by the
Silurian?, Ordovician?), the following two things can happen:

- If O2 % starts to fall (probably at the cost of increasing the CO2
level), the oceans' photosynthetic organisms will proliferate, gobbling
up the excess CO2 and pooping out O2, thereby reestablishing the O2
percentage to its former level.

and....

- If O2 % starts to become too abundant (probably at the cost of
decreasing CO2), an increase in the number and intensity of wild fires
and other oxidation processes occurs, which decreases the O2 and
increases the CO2.  This reestablishes the O2 percentage to its former
level.

Perhaps O2 is (and always has been) in rough equilibrium with the
atmosphere, and that the Mesozoic atmospheric O2 level was roughly the
same as today's O2 level.

So, can anyone point out weaknesses in this idea?

My main problem with a 10% O2 level in the Mesozoic is the following: 
Considering that eustatic sea levels were higher during much of the
Mesozoic (compared to today), there would have been an equally greater
ocean surface area.  Meaning that there was probably a sufficient number
of ocean plants available (algae, etc.) to gobble up CO2 and produce
adequate O2.

<pb>
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