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Evolution of Oxygen in the Atmosphere

Evolution of Oxygen in the Atmosphere. “ Early ” reactions driven by high energy sunlight H 2 O --- h n < 240 nm ---> H + OH CO 2 --- h n < 240 nm ---> CO + O Recombination reactions 2OH -----> H 2 O + O 2O -- M ---> O 2 2H -- M --> H 2 (H 2 can be lost to space)

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Evolution of Oxygen in the Atmosphere

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  1. Evolution of Oxygen in the Atmosphere • “Early” reactions driven by high energy sunlight H2O --- hn < 240 nm ---> H + OH CO2 --- hn < 240 nm ---> CO + O • Recombination reactions • 2OH -----> H2O + O • 2O -- M ---> O2 • 2H -- M --> H2 (H2 can be lost to space) • 2H2 + O2 -----> 2H2O • This was a minor process!

  2. To produce large amounts of O2 we need LIFE • What regulates the global Oxygen concentration? Sunlight + 132 O2 BUG

  3. Atmosphere Global scale oxygen feedback loop Continental Crust O2 Ocean

  4. Atmosphere Global scale oxygen feedback loop Continental Crust O2 Ocean Deep, Cold, O2-rich Water

  5. Atmosphere Global scale oxygen feedback loop Continental Crust O2 Ocean Deep, Cold, O2-rich Water Respiration

  6. Feedback on Atmospheric O2 • “Correct” Atmospheric Oxygen Photosynthesis = Respiration • Too Little Atmospheric Oxygen Photosynthesis >> Respiration • Too Much Atmospheric Oxygen Photosynthesis << Respiration

  7. 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.

  8. 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!

  9. 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) ----- >T ----> 2 NO(g) • N2(g) + 3H2(g) ---- >T, catalyst ---> 2 NH3(g) • Total flux = 4.6 x 1012 moles/year • Residence time = 3 x 107 years

  10. Source: http://www.esrl.noaa.gov/gmd/ccgg/trends/

  11. 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?

  12. 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

  13. 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! Temperature --> (oC)

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