[CliffordK (OP)]

Ok,

The explanations I've heard is that somewhere around 650 million  years ago, plants generated the oxygen that we now have in the atmosphere.  And, I thought it was from using carbon dioxide.

I was comparing these two charts from Wikipedia.
http://en.wikipedia.org/wiki/File:Phanerozoic_Carbon_Dioxide.png
http://en.wikipedia.org/wiki/File:OxygenLevel-1000ma.svg

Atmospheric Oxygen (O₂) vs Atmospheric Carbon Dioxide (CO₂)

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Unfortunately, the CO₂ chart stops at about 575 million years ago.  However, since at least 400 million years ago, the estimates are that the atmospheric CO₂ was less than 8000 ppm, or less than 0.8%.  Today it is supposed to be approaching 400ppm, or 0.04%.

At the same time, the estimate of oxygen was about 15% (5% less than today's level, and half the peak levels).

So,

How did the oxygen levels increase somewhere between 5% and 20%, while the CO₂ levels decreased by less than 1%.

Ahhh,

Maybe I'm missing half the equation.
Every carbon requires 2 hydrogen atoms (on average).

So,
2H₂O + 2CO₂ --> 3O₂ + (CH₂)₂

So, the expected change in oxygen would be 1.5x the change in carbon dioxide.

Still, with a less than 1% change in atmospheric carbon dioxide, that leaves us quite a bit short of the estimates for absolute change, as well as fluctuation in oxygen levels.

[Ethos_]

From what I understand about this, the oceanic plankton contributes the majority of oxygen to our atmosphere. So the carbon dioxide which is used by plant life on land is only part of the equation.

[CliffordK (OP)]

Thanks,

I see as much as 50x as much CO₂ is held in the oceans as in the atmosphere (at equilibrium).  I had forgotten that it was so much.

One of the problems we are having now is that it takes a while for the CO₂ to diffuse through the whole ocean depth, so it is somewhat concentrated near the surface.

So, a ½% change in atmospheric CO₂ could in fact correspond to a much greater change in atmospheric O₂.

[Spacetectonics]

http://researchnews.osu.edu/archive/plankatmo.htm

Having said that!

http://www.scientificamerican.com/article.cfm?id=origin-of-oxygen-in-atmosphere

But still one problem!

The ability of cyanobacteria to perform oxygenic photosynthesis is thought to have converted the early reducing atmosphere into an oxidizing one, which dramatically changed the composition of life forms on Earth by stimulating biodiversity and leading to the near-extinction of oxygen-intolerant organisms.

"extinction of oxygen-intolerant organisms"

now we may ask which one came first!!:) chicken or egg!

How much we "don't know?"!!
)