Oxygenation of the earth’s atmosphere

It is now generally accepted that the earth’s atmosphere first had free oxygen around 2300 million years (m.y.) ago (Kasting, 2001), and there is no doubt that this change in conditions changed the course of evolution.  Prior to that time there might have been small pockets of free oxygen – just as there are localized oxygen-free environments now - but there was no widespread occurrence of oxygen - as the gas O₂ - in the atmosphere.

It is also reasonably well accepted that there has been life on the earth in one form or another since as early as 3800 to 3900 m.y. ago, and that photosynthetic (oxygen-producing) cyanobacteria have been with us for at least 2700 m.y. – possibly for as long as 3500 m.y. 

The photosynthetic process involves the fissioning of water into O₂ and H₂.  The O₂ is released and the H₂ then combines with CO₂ to form organic compounds which the organism can use.  When cyanobacteria first started engaging in this process the oxygen released was quickly used up in reactions involving the oxidation of chemicals in the oceans (such as ferrous iron), organic matter and minerals in the crust (such as pyrite and olivine).  Geologist have long argued that it was just a matter of time, while these reservoirs of reductants were being used up, before the oxygen level of the atmosphere could rise, but this view has recently been challenged by some NASA researchers.

Catling et al. (2001) suggest that, in fact, we owe our vital oxygen-bearing atmosphere to those creatures of dank and smelly places – the methanogens.  The methane-producing organisms started to increase in numbers and productivity late in the Archean (just prior to 2500 m.y.) and this led to an increase in the methane content of the atmosphere - to as much as 2000 ppmv (as compared with just under 2 ppmv today).

The crucial point to consider here is that the vast majority of the hydrogen on earth is tied up in water.  Other hydrogen-bearing phases include free hydrogen (H₂) methane (CH₄) and ammonia (NH₃) but methane is the only one of these that has ever been present at significant levels in our atmosphere.  Water vapour concentrations decrease dramatically with altitude in the atmosphere because of the decreasing temperature.  The upper troposphere (approximately 18 km up) has a temperature of around -60º C, and there is virtually no water above this altitude.   Methane, on the other hand, is not “frozen out” by the low temperatures of the troposphere, and it can diffuse into the upper atmosphere.  When methane molecules are broken apart at this altitude, the hydrogen can escape because the high velocity of the tiny hydrogen atoms can exceed the earth’s escape velocity. 

When methane is produced down on earth, the reaction is something like this:

The free oxygen produced may combine with something else, for the time-being, but if the methane molecule makes it into the upper atmosphere it is likely that the hydrogen atoms will be lost into space.  This process makes the earth more oxygenated, because while the oxygen atoms are still here, and still available to become free atmospheric oxygen, the hydrogen atoms are gone forever. 

Before the methanogens got going there was not enough atmospheric methane for significant escape of hydrogen to space and, according to Catling et al., the potential for oxygenation of our atmosphere did not exist.  Now that the atmosphere is oxygenated, methane cannot exist in significant amounts in the atmosphere (methane quickly breaks down in an oxygen-rich environment) so the rate of hydrogen escape has dropped back to a very low level.

References

Kasting, J., The rise of atmospheric oxygen, Science, V. 293, p. 819-820 (August 2001)

Catling, D., Zahnle, K. and McKay C., Biogenic methane, hydrogen escape and the irreversible oxidation of early earth, Science, V. 293, p. 819-820 (August 2001)

Steven Earle, 2001. Return to Earth Science News