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Re: "Oxygen Helped Mammals Grow, Study Finds"

On Thu, 29 Sep 2005 21:27:12 -0400 Jeff Hecht <jeff@jeffhecht.com>
> If you want to consider the impact on archosaurs of the supposed 
> oxygen levels, you really need to look at the plot in Figure 2 of 
> the Science paper. 
> The paper claims that oxygen concentration was at 10% only at the 
> start of the Jurassic, and by implication in the Triassic, although 
> the paper does not plot oxygen levels before the Jurassic. (The 
> curve is based on Bob Berner's work, but I think it's calculated 
> freshly in this paper using some new data.)

Large mass synapsids (active mega endotherms??) existed in the Triassic. 
And then there are those annoyingly puzzling Triassic red beds (Why are
there so many of them?  And why are they red?  Excess oxygen???).     I
don't know how (or if) either phenomenon fits into the discussion, but I
thought I'd bring them up anyway.

It's frustrating that the authors didn't also do the Triassic numbers.

> They show oxygen level 
> rising to about 17% in the early Jurassic, then dropping down to 12% 
> and rising to 19% at the end of the middle Jurassic. From the start 
> of the cretaceous through until the early Eocene, oxygen levels 
> range from 15% to 18% or so. 
> As a calibration point, air pressure at an altitude of 5 kilometers 
> is about half that at sea level, so air at that altitude has about 
> as much oxygen as sea-level air would have at 10% oxygen 
> concentration. 

Uhhh...yeah.  That part really freaks my freak.  If we model the system
using altitude as our test chamber, we find that there isn't a whole
lotta biologic diversity currently goin' on at 5 km elevation.  Even the
lichens and the marmots and the alpine firs haven't gone on a speciation
spree.  Of course it probably has more to do with decreased temperature
and a lower air pressure than it does with decreased O2 (see next

The authors' point about calibration with 5 km altitude is only a rough
analogy.  With increasing altitude, two *unrelated* atmospheric variables
affect critters:

1) decreased O2 (because total O2 mass per unit volume decreases), and

2) decreased atmospheric pressure (decreased O2 absorption efficiency per

The problem is that #2 complicates the modeling  of #1.  The vast
majority of the earth's Mesozoic biota probably didn't have to deal with

Here's another interesting corrolary.  I wonder if the authors considered
it as they were preparing their manuscript:

If the 10% O2 figure is correct (for SEA LEVEL), then the anoxic altitude
would have been vastly lower than it is today.  Meaning that even
moderate elevation mountains in the early Jurassic may have been totally
devoid of all animal life. (We REALLY need an O2 vs. altitude graph for
the early Jurassic, using as a starting point a 10% O2 concentration at
sea level (1 atm).  Is anyone up to the task?).