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
2H2 + O2 -----> 2H2O
• This was a minor process!
(H2 can be lost to space)
1
To produce large amounts of O2 we
need LIFE
106CO2 + 16HNO3 + H3 PO4 + 122H2O
Sunlight
(CH 2O)106 (NH3 )16 PO 4
+ 132 O2
BUG
• What regulates the global Oxygen concentration?
2
Global scale oxygen feedback loop
Atmosphere
Continental
Crust
O2
106CO2 + 16HNO3 + H3 PO 4 + 122H 2O Þ
Ocean
(CH 2O)106 (NH3 )16 PO 4 + 132O2
3
Global scale oxygen feedback loop
Atmosphere
Continental
Crust
O2
106CO2 + 16HNO3 + H3 PO 4 + 122H 2O Þ
Ocean
(CH 2O)106 (NH3 )16 PO 4 + 132O2
Deep, Cold,
O2-rich Water
4
Global scale oxygen feedback loop
Atmosphere
Continental
Crust
O2
106CO2 + 16HNO3 + H3 PO 4 + 122H 2O Þ
Ocean
(CH 2O)106 (NH3 )16 PO 4 + 132O2
106CO2
16HNO3
H 3PO 4
Deep, Cold,
O2-rich Water
Respiration
5
Feedback on Atmospheric O2
• “Correct” Atmospheric Oxygen
Photosynthesis = Respiration
• Too Little Atmospheric Oxygen
Photosynthesis >> Respiration
• Too Much Atmospheric Oxygen
Photosynthesis << Respiration
6
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.
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.
amount in reservoir
Residence time =
rate of inflow or outflow
• Residence time for oxygen = 7600 years
• What does this mean?? Oxygen is well mixed in
the atmosphere!
8
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
9
Source: http://www.esrl.noaa.gov/gmd/ccgg/trends/
10
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?
11
• 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
12
8
y = 6.087471E-05x3 + 4.598262E-04x2 + 5.315588E-02x + 5.951091E-01
R2 = 9.999823E-01
7
Pressure kPa
6
5
4
3
2
1
0
0
5
10
15
Temperature --> (oC)
20
25
30
35
40
45
temp 0C
• 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!
13
14