11

Evolution of Oxygen in the Atmosphere

• “Early” reactions driven by high energy sunlight

H2O --- h< 240 nm ---> H + OH

CO2 --- h< 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!

22

To produce large amounts of O2 we need LIFE

BUG

+ 132 O2

Sunlight

• What regulates the global Oxygen concentration?

33

AtmosphereAtmosphereContinentalContinental

CrustCrust

OceanOcean

Global scale oxygen feedback loop

O2

44

AtmosphereAtmosphereContinentalContinental

CrustCrust

OceanOcean

Global scale oxygen feedback loop

O2

Deep, Cold, O2-rich Water

55

AtmosphereAtmosphereContinentalContinental

CrustCrust

OceanOcean

Global scale oxygen feedback loop

O2

Deep, Cold, O2-rich Water

Respiration

66

• “Correct” Atmospheric OxygenPhotosynthesis = Respiration

• Too Little Atmospheric OxygenPhotosynthesis >> Respiration

• Too Much Atmospheric OxygenPhotosynthesis << Respiration

Feedback on Atmospheric O2

77

• The rate of photosynthesis = respiration = 5 x 1015

mol/yr.

• The atmosphere is 20% oxygen, or 3.8 x 1019 mol.

How fast is O2 cycling in the environment?

88

• 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!

How fast is O2 cycling in the environment?

99

• 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

Nitrogen in the atmosphere

1010Source: http://www.esrl.noaa.gov/gmd/ccgg/trends/Source: http://www.esrl.noaa.gov/gmd/ccgg/trends/

1111

CO2

Mass in the atmosphere 700 GtMass in the atmosphere 700 Gt Annual flux to the atmosphere is about 150 GtAnnual flux to the atmosphere is about 150 Gt

Residence time = 5 yearsResidence time = 5 years Does this make sense with observed CODoes this make sense with observed CO22??

1212

• 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

1313

• 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)Temperature --> (oC)

1414