How is the equilibrium of 21% oxygen in Earth's atmosphere established?

Question

The atmosphere is 21% oxygen. I assume that there must be an equilibrium between processes that produce oxygen (e.g. photosynthesis) and those that consume oxygen (e.g. aerobic respiration).

Moreover, there should be a self-regulatory negative feedback which restores the equilibrium if the system is driven away from it. If the amount of oxygen were increased, something that would subsequently cause it to decrease over time, and if the amount of oxygen were decreased, something to cause it to be replenished.

What processes are responsible for this negative feedback?

Answer 1

I'm not sure I can give a good technical answer. I don't think the amount of oxygen in the Earth's atmosphere is due to equilibrium but more of a consequence of the formation of the solar system, Earth chemistry and biology.

If you look at the formation, for inner planets, much of the gas and ices were blown off due to their inner orbits and the planets being too hit by very active Coronal mass ejections in the early solar system. There are 3 likely types of close to the sun planets: 1) small rocky planets (Earth, Mercury, Venus, Mars), 2) hot jupiters, which are large enough to trap hydrogen even though they're close to the sun and hot and 3) super-earths, which have enough gravity to trap hydrogen.

An abundance of hydrogen in a planetary atmosphere would likely make oxygen formation impossible. The Oxygen would bind with the hydrogen.

The Earth didn't have the gravity to trap much hydrogen and some of the early atmosphere would have been blown off by large meteor strikes and by coronal mass ejections, which were much more common when the sun was young.

A clue to the Earth's atmosphere's formation is the formation of the oceans, because oceans would be impossible without an atmosphere. The oceans formed 3.8 billion years ago, so we had to have an atmosphere, at least for about 3.8 billion years.

http://people.chem.duke.edu/~jds/cruise_chem/oceans/ocean1.html

The young Earth's atmosphere was mostly methane (CH₄), ammonia (NH₃), water vapor (H₂O), and carbon dioxide (CO₂) as per the link above, as soon as the Earth was cool enough, the water vapour turned into liquid water (rain) and the oceans began to form.

Fast forward to the Great Oxygenation event when cyanobacteria, pulled CO₂ from the air and release O₂. Early on, the O₂ binded with iron in the ocean and some of it likely reacts with CH₄ and NH₃ in the atmosphere when there is lightning. Over time, CO₂ was pulled from the air, replaced by O₂ and O₂ reacted with CH₄ and NH₃, producing more CO₂, H₂0, and N₂ - the common elements we have today.

Oxygen levels in planets with cyanobacteria, likely depends on how much iron is dissolved in the oceans and how much hydrogen is in the atmosphere. If there's not enough CO₂ to produce enough O₂ to saturate the iron and hydrogen, the planet probably never gets much O₂ in it's atmosphere - so it's all about the ratio of early elements. Super-earths might never get oxygen atmospheres - too much hydrogen.

You also get factors like the development of lignin (got that from Neil deGrass' Cosmos) - http://evolution.about.com/od/Cosmos/fl/Cosmos-A-Spacetime-Odyssey-Recap-Episode-109.htm

Lignin made trees possible, but nothing could eat trees, so trees captured more and more carbon, so CO₂ levels fell and O₂ levels rose. About 100 million years later, termites evolved and the digestion of trees released much of this captured CO₂ became possible and Oxygen and CO₂ leveled out.

Shellfish, to make their shells, take more O₂ out of the air than CO₂ (calcium carbonate has lots of oxygen in it). That's a slow process, but over time it takes some O₂ out of the air, so over tens of millions of years, parts of the atmosphere are effectively sequestered and trapped in the Earths crust, but over time, some of this is also returned to the atmosphere with volcanic activity - so the type of life forms present is also a factor and the extent of tectonic activity is a factor as is, as is the presence of a Jupiter and how many comets are likely to hit the planet and the size of the planet.

It's more chance, the solar system and planetary factors that lead to a planet's atmosphere.

To answer your question:

So if we increase the amount of oxygen, it will decrease over time and if we decrease it, it will be replenished.

Tough call regarding oxygen. CO₂ is easier. Because there's a relatively small percentage of CO₂ in the air, more CO₂ will lead to some of it dissolving in the ocean as carbolic acid, though the warming of the ocean might also slow the oceanic absorption of CO₂. Because there's so much more Oxygen in the Earth, 21% vs 0.04% of CO₂, a measurable increase, say 21% to 22%, you'd probably see a slight increase in dissolved O₂ in the ocean, but beyond that, I don't think you'd see that extra oxygen go away any time soon. My guess is, if such an increase was done, it would last a long time, hundreds of thousands of years if not millions, because oxygen sequestering is pretty slow. Insects would get slightly bigger pretty quick. That might be the most noticeable effect.

If you significantly increase O₂ levels, say from 21% to 30%, things would become noticeably more flammable and over time, our lungs would probably get smaller and there would be other effects, probably. But I don't believe there's any sort of equilibrium that would fairly quickly bring it back to 21%.

Answer 2

Atmospheric oxygen is not in an equilibrium of 21%, it just changes very slowly. For instance, oxygen has decreased by 0.7% over the past 800 thousand years, likely due to increased erosion (which exposes more rock that can be oxidized) and cooler oceans (which can then absorb more oxygen). So, while it is a slow process on geologic timescales, the amount of atmospheric oxygen does vary and is not in true equilibrium.

Oxygen was not abundant in the Earth's early atmosphere and it is chemically reactive so it can be difficult to accumulate. It was mainly created by lifeforms and was only able to accumulate significantly over geologic time for a few reasons:

• Most of the hydrogen in the atmosphere has escaped, so oxygen doesn't easily react in the atmosphere
• decreases in volcanic activity (which produce sulfur that can react with oxygen) also allow more accumulation of oxygen.
• once there was enough oxygen to allow the ozone layer to form, more lifeforms were able to develop which were able to produce more oxygen

It is important to note that the idea of a negative feedback has been postulated before. Namely, that wildfire could limit the maximum atmospheric oxygen possible. Since combustion occurs more easily in high oxygen environments, the idea was that if atmospheric oxygen levels increased much more than 21%, wildfire occurrences would be extreme enough to burn up the excess oxygen (and thereby reduce oxygen production from trees). However, there is evidence that atmospheric oxygen was up to 35% in the late paleozoic era, which doesn't support the idea of wildfire as a negative feedback mechanism. Furthermore, this paper by Wildman et al., says:

Theoretical models suggest that atmospheric oxygen reached concentrations as high as 35% O2 during the past 550 m.y. Previous burning experiments using strips of paper have challenged this idea, concluding that ancient wildfires would have decimated plant life if O2 significantly exceeded its present level of 21 %. New thermochemistry and flame-spread experiments using natural fuels contradict these results and indicate that sustained burning of forest fuels at moisture contents common to living plants does not occur between 21% and 35% 02. Therefore, the fires under atmospheres with high oxygen concentrations would not have prevented the persistence of plant communities. Times of high O2 also agree with observations of concurrent fire-resistant plant morphology, large insects, and high concentrations of fossil charcoal.