Evolution of Oxygen in the Atmosphere

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Evolution of Oxygen in the Atmosphere

• Early reactions driven by high energy sunlight
• H2O --- hn lt 240 nm ---gt H OH
• CO2 --- hn lt 240 nm ---gt CO O
• Recombination reactions
• 2OH -----gt H2O O
• 2O -- M ---gt O2
• 2H -- M --gt H2 (H2 can be lost to
  space)
• 2H2 O2 -----gt 2H2O
• This was a minor process!

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To produce large amounts of O2 we need LIFE
Sunlight
132 O2
BUG

• What regulates the global Oxygen concentration?

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Global scale oxygen feedback loop
Atmosphere
Continental Crust
O2
Ocean
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Global scale oxygen feedback loop
Atmosphere
Continental Crust
O2
Ocean
Deep, Cold, O2-rich Water
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Global scale oxygen feedback loop
Atmosphere
Continental Crust
O2
Ocean
Deep, Cold, O2-rich Water
Respiration
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Feedback on Atmospheric O2

• Correct Atmospheric Oxygen
• Photosynthesis Respiration
• Too Little Atmospheric Oxygen
• Photosynthesis gtgt Respiration
• Too Much Atmospheric Oxygen
• Photosynthesis ltlt Respiration

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How fast is O2 cycling in the environment?

• The rate of photosynthesis respiration
  5 x 1015 mol/yr.
• The atmosphere is 20 oxygen, or 3.8 x 1019
  mol.

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How fast is O2 cycling in the environment?

• The rate of photosynthesis respiration
  5 x 1015 mol/yr.
• The atmosphere is 20 oxygen, or 3.8 x 1019 mol.
• Residence time for oxygen 7600 years
• What does this mean?? Oxygen is well mixed in
  the atmosphere!

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Nitrogen in the atmosphere

• Nitrogen was outgassed early in Earth formation
  and constitutes 80 of the atmosphere. (1.4 x
  1020 moles)
• Two major sinks
• N2(g) O2(g) ----- gtT ----gt 2 NO(g)
• N2(g) 3H2(g) ---- gtT, catalyst ---gt 2 NH3(g)
• Total flux 4.6 x 1012 moles/year
• Residence time 3 x 107 years

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Source http//www.esrl.noaa.gov/gmd/ccgg/trends/
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CO2

• Mass in the atmosphere 700 Gt
• Annual flux to the atmosphere is about 150 Gt
• Residence time 5 years
• Does this make sense with observed CO2?

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• Major Constituents of the Atmosphere (atm.)
• N2 0.781
• O2 0.209
• Ar 0.0093
• CO2 0.00035
• What about WATER?
• Variable concentration 0 - 0.05 atm

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Temperature --gt (oC)

• p(H2O) p(saturation)(RH)/100
• Average amount of water 7 x 1014 mol.
• average flux 2.5 x 1016 mol/yr
• residence time 10 days
• Water is poorly mixed!

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