1. Multiple Equilibria in Atmospheric Oxygen: Archean, Proterozoic,
Phanerozoic.
Tom Laakso & Dan Schrag
Goldschmidt Geochemistry
June 13, 2014

2. Multiple Equilibria in pO2
Kump 2008

3. Multiple Equilibria model: Proterozoic / Phanerozoic
3-box ocean/atmosphere model
oxygen, carbon, sulfur, iron cycles
first order kinetics
oxygen-sensitive organic carbon burial efficiency
oxygen-sensitive recycling of sedimentary P
oxygen-sensitive riverine P flux

4. Multiple Equilibria model: Proterozoic / Phanerozoic
Laakso & Schrag 2014

5. The Great Oxidation Event
Kump 2008

6. Archean redox budget: prebiotic world
hydrogen
outgassing
serpentinization
Archean redox budget: prebiotic world

7. Archean redox budget: prebiotic world
mantle
hydrogen
source
Archean redox budget: prebiotic world

8. Archean redox budget: prebiotic world
hydrogen escape
mantle
hydrogen
source
Archean redox budget: prebiotic world

9. Archean redox budget: early life
hydrogen escape
mantle
hydrogen
source
chemoautotrophy:
2 H2 + CO H2O + CH2O
Archean redox budget: early life

10. Archean redox budget: early life
hydrogen escape
mantle
hydrogen
source
chemoautotrophy:
2 H2 + CO H2O + CH2O
organic
burial
(< nutrient input)
Archean redox budget: early life

11. Archean redox budget: oxygenic photosynthesis
hydrogen escape
mantle
hydrogen
source
oxygenic photosynthesis
CH2O + O CO2 + H2O
chemoautotrophy:
2 H2 + CO H2O + CH2O
organic
burial
(< nutrient input)
Archean redox budget: oxygenic photosynthesis

12. Archean redox budget: oxygenic photosynthesis
hydrogen escape
oxidation:
atmosphere
mantle
hydrogen
source
oxygenic photosynthesis
CH2O + O CO2 + H2O
chemoautotrophy:
2 H2 + CO H2O + CH2O
oxidation:
seafloor,
aqueous,
continental
organic
burial
(< nutrient input)
Archean redox budget: oxygenic photosynthesis

13. Archean equilibrium redox budget
Hydrogen: Oxygen:
mantle H2 source = nutrient source =
hydrogen escape aqueous & crustal oxidation +
atmospheric chemistry atmospheric chemistry

14. Is this model consistent with the Proterozoic?
Hydrogen: Oxygen:
mantle H2 source = nutrient source =
hydrogen escape aqueous & crustal oxidation +
atmospheric chemistry atmospheric chemistry
Across the GOE:

15. Is this model consistent with the Proterozoic?
Hydrogen: Oxygen:
mantle H2 source = nutrient source =
hydrogen escape aqueous & crustal oxidation +
atmospheric chemistry atmospheric chemistry
Across the GOE:
1. crustal oxidation rises => atmospheric sink must decrease

16. Is this model consistent with the Proterozoic?
Hydrogen: Oxygen:
mantle H2 source = nutrient source =
hydrogen escape aqueous & crustal oxidation +
atmospheric chemistry atmospheric chemistry
Across the GOE:
1. crustal oxidation rises => atmospheric sink must decrease
2. Slower atmos. chemistry, increasing O2 => decreasing H2

17. Is this model consistent with the Proterozoic?
Hydrogen: Oxygen:
mantle H2 source = nutrient source =
hydrogen escape aqueous & crustal oxidation +
atmospheric chemistry atmospheric chemistry
Across the GOE:
1. crustal oxidation rises => atmospheric sink must decrease
2. Slower atmos. chemistry, increasing O2 => decreasing H2
3. Decreasing H2 => decreased H2 escape

18. Is this model consistent with the Proterozoic?
Hydrogen: Oxygen:
mantle H2 source = nutrient source =
hydrogen escape aqueous & crustal oxidation +
atmospheric chemistry atmospheric chemistry
Across the GOE:
1. crustal oxidation rises => atmospheric sink must decrease
2. Slower atmos. chemistry, increasing O2 => decreasing H2
3. Decreasing H2 => decreased H2 escape
4. Hydrogen budget cannot be balanced in the Proterozoic!

19. Is this model consistent with the Proterozoic?
Hydrogen: Oxygen:
mantle H2 source = nutrient source =
hydrogen escape aqueous & crustal oxidation +
atmospheric chemistry atmospheric chemistry
Across the GOE:
1. crustal oxidation rises => atmospheric sink must decrease
2. Slower atmos. chemistry, increasing O2 => decreasing H2
3. Decreasing H2 => decreased H2 escape
4. Hydrogen budget cannot be balanced in the Proterozoic!
5. …unless hydrogen escape increases, despite falling H2

20. Oxygen-sensitive hydrogen escape
H2 escape depends on the temperature of the thermosphere.
Temperature depends on O2 absorption of UV radiation.

21. Oxygen-sensitive hydrogen escape
H2 escape depends on the temperature of the thermosphere.
Temperature depends on O2 absorption of UV radiation.
Hydrodynamic model with Jeans boundaries
Temperature boundary related to O2 through thermosphere energy balance model (Bougher & Roble 1991)

22. Oxygen-sensitive hydrogen escape

23. Hydrogen escape and the Great Oxidation
Archean: high H2, low O2
Oxygen controlled by reaction with free H2
Hydrogen controlled by oxidation and escape

24. Hydrogen escape and the Great Oxidation
Archean: high H2, low O2
Oxygen controlled by reaction with free H2
Hydrogen controlled by oxidation and escape
Great Oxidation: large perturbation in pO2
Thermosphere warms, increasing H2 escape
pH2 falls, slowing reaction with O2
Oxygen remains elevated

25. Hydrogen escape and the Great Oxidation
Archean: high H2, low O2
Oxygen controlled by reaction with free H2
Hydrogen controlled by oxidation and escape
Great Oxidation: large perturbation in pO2
Thermosphere warms, increasing H2 escape
pH2 falls, slowing reaction with O2
Oxygen remains elevated
3. Proterozoic: higher O2, lower H2
Oxygen controlled by weathering and respiration
Hydrogen controlled by escape from a hot thermosphere

26. Archean / Proterozoic biogeochemical model
Escape:
Hydrodynamic model with Jeans boundaries
Temperature boundary related to O2 through thermosphere energy balance model
Atmospheric chemistry:
Photochemical model of Pavlov et al. (2001)
Solid phase oxidation:
Pyrite: linear in pO2 up to 10-4 PAL
Organic carbon: linear in pO2
Inputs:
P: 20% modern
H2: 1010 cm2 s-1

27. Archean / Proterozoic biogeochemical model

28. Archean / Proterozoic biogeochemical model

29. Glaciation and oxidation
Hoffman & Schrag 2002

30. Glaciation and oxidation

31. Glaciation and oxidation

32. Glaciation and oxidation

33. Glaciation and oxidation

34. Glaciation and oxidation

35. Glaciation and oxidation

36. Multiple equilibria: a history of pO2

37. Multiple equilibria: a history of pO2

38. Multiple equilibria: a history of pO2

39. Summary and Conclusions
The hydrogen escape rate are not diffusion limited for less-than-modern levels of pO2, but are in the Jeans regime. The escape rate varies strongly with pO2.
In our simple model, the Archean atmosphere is stabilized at low oxygen levels by the reaction kinetics between O2 and H2 in the atmosphere. Escape from the cold thermosphere is a secondary term in the H2 budget.
Hydrogen levels are suppressed at high pO2 by efficient escape from a hot thermosphere. This allows for a second equilibrium at high pO2: the weathering-dominated regime of the Proterozoic and Phanerozoic.
Transient slowing of atmospheric reactions during a >250,000 year glaciation pumps enough oxygen into the atmosphere to flip the atmosphere between its low- and high-oxygen states.