Why does Earth have abundant oxygen in the atmosphere?

Question

Because of photosynthesis, obviously. But then it's not actually that obvious after all, because photosynthesis is mostly balanced by respiration.

We can summarise the processes of photosynthesis and respiration like this: $$\begin{align} \ce{CO2 + H2O + h\nu} & \rightarrow \ce{O2 + CH2O(biomass)} \\ \ce{O2 + CH2O(biomass)} & \rightarrow \ce{CO2 + H2O} \end{align} $$ If these two reactions were always balanced then the concentration of $\ce{O2}$ would never have changed. If I understand correctly, there are abiotic processes that deplete $\ce{O2}$ to form iron oxides, and so in order for there to be persistent $\ce{O2}$ in the Earth's atmosphere, the biosphere as a whole must continually produce an excess of it. In order for this to happen, there must be some processes that consume biomass at a non-neglible rate without removing a corresponding amount of $\ce{O2}$ from the atmosphere.

My question is, what are the processes primarily responsible for this? Is it just due to the burial of unoxidised organic matter (to form coal, oil, etc.), or is there more to it than that?

If they are different processes, I'm interested both in what caused the excess of oxygen during the "great oxidation" event, and what maintains the excess of oxygen on the modern Earth.

Answer 1

This is only a partial answer as it doesn't explain why the excess of O₂ stayed but one thing you have to appreciate is the fact that aerobic respiration appeared almost half a billion years after photosynthesis, so we can't really say that photosynthesis and respiration have always balanced each other.

Cyanobacteria (and with them photosynthesis) are believed to have appeared somewhen between 3 and 2.5Ga (e. g. Altermann & Kazmierczak 2003, Brocks et al. 2003) while Mitochondria (and with them aerobic respiration) are thought to have evolved somewhen between 2 and 1.5Ga (see e. g. Hedges et al. 2004).

There was therefore at least half a billion year during which cyanobacteria were producing O₂ with no one able to breathe it.

Answer 2

My answer goes a little beyond the evidence -- there isn't much evidence.

The question

There's a lot of free oxygen now. This oxygen did not suddenly come from underground or from space. So it used to be attached to something. The question is, what was it attached to, and what happened to what it was attached to?

Background

First off, there's every reason to think that bacteria already did oxidative phosporylation before there was any free oxygen. It does not require free oxygen, it requires an electron acceptor. O₂ provides the most energy, but many other reactants provide some energy, and the bacterial kingdom uses a bewildering variety of them. To use a different electron acceptor requires only one different enzyme -- the rest of the pathway can be the same.

So before there was free oxygen, prokaryotes did arbitrage -- they evolved to use the best electron donor/electron acceptor pair they could find under whatever conditions they faced at the moment. And everything they used for that had to be created in a cycle. Anything which they used up that was not replaced, would quickly be gone.

Before there was free oxygen, photosynthesis still got done. Some bacteria used H₂S as an electron donor, creating S which could later be reduced for energy. Some created H₂. Some could reduce CO₂ so that they could use the carbon and oxygen to make structures they needed. Some could not, but could still get energy from light. One way to do that was almost the same as oxidative phosphorylation, it used the absorbed light energy for electron transport instead of using redox energy, in practically the same pathway.

Cycles. Every reaction must be undone, or its products will increase to the point that the reaction gets very slow. The exception is products that change state. A solid or gas product won't inhibit the reaction much.

So, a bewildering variety of electron donors and acceptors. Photosynthesis creates energetic pairs of molecules or ions. Then redox creates less-energetic pairs of molecules or ions.

What was it that was bound to oxygen in the old days, that was not bound later?

Some possibilities

1. Hydrogen. One form of photosynthesis creates H₂. Maybe, at some times a whole lot of H₂ was created which was lost to space. Just as much O and less H leaves room for O₂ left over.

2. Sulfur. Maybe most of the sulfur in the oceans was stored as SO₃.

H₂ SO₃ -> S + H₂O + O₂.

Sulfur can be oxidized -- SO₃. Or it can replace Oxygen as an oxidizer -- H₂O -> H₂S etc.

So by changing states it could affect free O₂. And solid sulfur could wind up on the ocean bottom where it would be subducted.

3. Heavy metals + sulfur. Maybe there was a lot of iron and nickel etc dissolved in the ancient ocean, along with a lot of sulfur. Iron sulfate is fairly soluble at room temperature, nearly 300 grams/liter. Provided it is not alkaline. But iron sulfide is not soluble. Lots of other heavy metals behave similarly.

F₃O₄ + 4 H₂SO₃ -> F₃S₄ + 6 O₂ + 4 H₂0 (and many other combinations)

4. Nitrogen. Nitrogen can be reduced to NH₄ or oxidized to NO₂ or NO₃.

2 NO₂ + 4 H+ = N₂ + O₂ + 2 H₂O

Perhaps there was a time when the atmosphere had only 75% as much N₂ as it does now, and no O₂. This reaction would imply the oceans used to be more acidic than they are now. But there could be other reactions that would counteract that, and there were lots of other buffers.

5. Silicon. Silicon is usually oxidized, SiO₂. It doesn’t have to have that ratio. Other silicon compounds are possible, eg carborundum SiC, and perryite Fe₅Si₂ which is stable in acid water, silicon nitride Si₃N₄, silicon phosphide SiP₂ etc . Siloxanes can have ratio Si:O of 1:1. Low molecular weight siloxanes are produced by modern anaerobic biogas fermenters, though it could possibly be eukaryotes producing them. There is potentially a tremendous amount of SiO₂ available to release oxygen, although only a little of it is soluble at any one time.

Or bacteria could have metabolized silicic acid into insoluble compounds that had less oxygen. This is entirely hypothetical since only a little carborundum has been discovered, and the rest would have had to be subducted.

Banded iron formations do though have layers of amorphous quartz. Silicon dioxide was being removed from the water. Perhaps it was alternately iron being removed while soluble siloxanes accumulated in water, and then the siloxanes were converted to silica and removed while iron accumulated in water.

It was a complicated web of reactions. Photosynthesis provided the energy to create energy-rich chemicals. Meanwhile every organism that was not photosynthetic was busy finding its best mix of catalyzing chemical reactions that provided energy, versus creating the molecules needed to grow and reproduce. Strange things happened.

Answer 3

I will explore the answer admitting that the Earth's temperature in the ancient past was much higher than we now assume as possible. The current standard model is the 'Snowball Earth' which leaves many unsolved problems - see 'Faint Young Sun Paradox'.

I will assume that the total amount of $\ce{H_2O}$, $\ce{CO_2}$, $\ce{O_2}$ is substantially constant throughout the entire history of the earth.

All the water of the oceans would stay in the atmosphere layer provided the temperature was high enough. The percentage of oxygen would be $\approx 0$ because it would be masked by the massive presence of $\ce{H_2O}$.

Later, when the temperature dropped, the water has migrated to the ocean floor and the percentage of $\ce{O}$ exploded to current levels. This fact is called the 'Great Oxygenation Event' and it is not very well understood under the standard model.

About the free Oxygen origin: (Copy / Paste)

"There are two known natural ways of producing oxygen: by UV dissociation of H2O and by photosynthesis. In the ECM, the former process was initially much more efficient than at present because of the huge amount of water vapour and of the greater intensity of UV radiation occurring then. The biological production of oxygen would have been important during the Archean, which ended at 2.5 Ga, therefore earlier than the GOE. The usual approach is to consider that the biologically produced oxygen was first combined with dissolved iron in oceans and only significantly released to the atmosphere later – a way to explain the delay between the presumed time of the biological production of oxygen and the rise of its atmospheric level. Here, there is no need to consider a delay – the biologically produced oxygen was dissolved in water and absorbed by the huge atmosphere without significantly changing its composition; the oxygen level increased only when the atmosphere significantly decreased due to the condensation of water vapour. One can now consider that oxygen was produced in large amounts since the beginning, first from UV dissociation, at a rate that decreased over time, and very soon followed by biological production. In this case, there was an important amount of free oxygen in the atmosphere long before its atmospheric level started to increase significantly; and some potential evidences of it can be identified (for a review see Yamaguchi, 2005)."

ECM stands for Evolving Climate Model where this new concept was brought to life in a formal document (if you wish to explore further you can follow the 2nd link in my profile, not peer-reviewed)

EDIT add:
Although no one wants to peer review the document of ECM,(*) its graph on fig.9 provides modeled data that is aligned with the most recent measurement of Oxigen.
Measures goes against all of the accepted models ( 5 times more Oxigen at 815Ma), as seen on fig 4 on this news
http://motherboard.vice.com/en_uk/read/scientists-release-air-that-has-been-trapped-for-800-million-years

On the contrary the Fig. 9 of ECM Fig. 9. The variation of Oxygen volumetric ratio in the ECM1 due to the variation of the size of the atmosphere, holding the amount of Oxygen constant (thick blue line). The grey lines represent the limits for the oxygen volumetric ratio (partial pressure in the atmosphere) estimated in Holland (2006)

ECM is not peer-reviewed but it predicted the data to be seen (it is Data-reviewed) !

(*) ECM is a creation of an engineer, and scientists assume that science is not for engineers.

note: This new measurement enable to say: Oxygen first, Life second, rendering erroneous most of the answers one can find around.

Why so many down votes (5) if it is notorious that none could find an error in the document?