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Discrepancy between oxygen and carbon-dioxide levels

  1. Feb 9, 2012 #1
    Does anyone have explanation, or can point me to papers explaining this thread title?

    Personally, I did some reading through years, but I could never find an answer to this question.

    In short, here is what I learnt so far:

    - in free form, is a product of photosynthesis of green life on earth, from combination of CO2 and H2O source.
    - is short lived, about 50 thousand years in atmosphere, then it's gone due to oxidation.
    - some of O2 is a product of H2O breakdown, when hit by high energy photons, but probably not much.
    - 21% of atmosphere is oxygen by volume
    - there are some claims that current level of oxygen can be regenerated in 2,000 years!

    - comes mostly from original earths atmosphere, which probably was composed mainly of it.
    - in last 500 million years CO2 level is dropping slowly, from about 20 times current level to current level.
    - the drop in CO2 level is explained by continues sequestration of it from air to the ground.
    - 0.04% of atmosphere by volume

    The dilemma is with short lived oxygen in atmosphere. If all current O2 is younger than 50 thousand years, then there wasn't and isn't enough CO2 to recreate it, not even close. Other words, if all oxygen is gone now, all the green life would be able to produce only about 0.05% O2 from all the CO2 available, only a fraction of 21% we have right now.

    There is also a claim that states that O2 level was fluctuating between 20-35% in last 500 million years. How is it possible if CO2 volume fluctuated from 1-0.04% of atmosphere?

    ...or what am I missing?

    I can understand CO2 level and depletion through ages. I can't understand O2 level in a light of it's rather short life span in free form.
  2. jcsd
  3. Feb 10, 2012 #2
    The answer that springs to mind is that there are various mechanisms that remove CO2 form the atmosphere besides photosynthesis. Marine life is constantly depositing CACO3 that comes from H20 + C02 to form H2CO3 and then going through two ionizations to form 2H+ + CO3-2. The carbonate ion combines with a calcium ion to form calcite.
    Last edited: Feb 10, 2012
  4. Feb 10, 2012 #3
    The difficulty with this statement is that the atmosphere now is largely composed of nitrogen.

    Since there are limited known nitrogen transactions if this gas did not exist in the orignal atmosphere in large quanties, where did it come from.

    The Emerald Planet by David Berling

    Oxford Unibversity Press

    Has a good discussion of the history of the atmosphere, including oxygen.

    go well
  5. Feb 15, 2012 #4
    Yes, all the sequestration of CO2 from air can explain why there is so little of it after 4 billion years. My problem is that I can't find viable explanation why there is so much of oxygen in relation to CO2, the supposedly main source of it.
    Did anyone see any numbers explaining O2 production?
  6. Feb 15, 2012 #5
    It looks this way only if one considers that air density was always close to today's one. Most likely original atmosphere was few times denser, maybe even 10 times. This is not very far fetched, if we compare Earth with Venus, which still holds100 times more air.
    So, if our original atmosphere was 10 time denser, mostly CO2, then Nitrogen (if constant through ages in volume) was close to 7%. If it was 100 denser then Nitrogen would constitute not even one percent way back.
    Unfortunately so far these are just best guesses and speculations. I guess it is extremely hard to measure air pressure from billions years ago.

    Thanks for info, I will google David Berling tomorrow.
  7. Feb 15, 2012 #6
    I'm not 100% sure what the jist of the question is, but I would suggest that the ozone --> oxygen cycle helps support the constant capture and release of 02. Oxygen is not oxidized, per se, it is broken apart by ultraviolet light. Similarly, ozone can be broken apart by the same wavelengths of radiation to produce oxygen.

    Given this you can see that oxygen levels can be built up over long periods of time. It is not simply lost due to oxidation.
  8. Feb 21, 2012 #7
    You are treating CO2 as if it had no sources or sinks save sequestration by plants. If this were true plants would have long ago run out of CO2 and the global ecosystem would have collapsed. If we consider sources of CO2 which would not decline with a drastic reduction in O2 partial pressure, volcanic sources immediately come to mind. A quick calculation suggests this would take 10,000 years to restore the CO2. Anaerobic fermentation would release CO2, although it would decline as food sources were depleted. The death of reef-building organisms from lack of oxygen might cause outgassing of CO2 from the oceans.
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