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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.

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    $\begingroup$ I have put a link to this on my Twitter page $\endgroup$ – user889 Feb 19 '15 at 11:33
  • $\begingroup$ I am no biologist, but photosynthesis is not necessarily used to produce sugars. Chlorophyll absorbs photons of light and (in conjunction with other proteins) splits water into O2 and H+, which is what produces the O2 in the atmosphere, but the electron flow can also be used to generate ATP, which is the cells basic energy source. So photosynthesis can support celluar activity without producing sugars or respiration. The basic point is that production of sugars for respiration is one use of phyotosynthesis, there are others that still produce O2 as a by product (e.g. in stromatelites). $\endgroup$ – Dikran Marsupial Feb 21 '15 at 15:19
  • $\begingroup$ @DikranMarsupial can you write down a net reaction and/or reference (or just a suitable search term) for the metabolic pathways you mention that produce $\ce{O2}$ without fixing carbon? What happens to the H? $\endgroup$ – Nathaniel Feb 22 '15 at 0:42
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    $\begingroup$ @mtb-za based on the comments and answers so far, I think it's because biomass gets buried in anoxic conditions to form stuff like peat, some of which then eventually gets buried and fossilised into coal and oil, which consumers can't oxidise because it's physically separated from the oxygen in the atmosphere. (I'm fairly sure this is the right story, but I'm holding out for an answer from an expert who can explain it all in detail.) $\endgroup$ – Nathaniel Mar 2 '15 at 12:41
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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.

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  • $\begingroup$ That is a very interesting point - the potential long time difference between photosynthetic and respiration based lifeforms would have yielded a substantial amount of atmospheric oxygen, and, could be a mechanism to the oxygen level drop at 1.9 billion years ago, that you very nicely described here earthscience.stackexchange.com/questions/2965/… $\endgroup$ – user889 Feb 28 '15 at 9:12
  • $\begingroup$ I guess you mean aerobic respiration appeared after photosynthesis, right? (Respiration in the more general sense occurs in all organisms and I don't think a cell could exist without it, but it makes sense that aerobic respiration wouldn't evolve until there was oxygen around for it to use.) I'd be curious to know (a) what the net reaction(s) looked like for the forms of respiration dominant before the great oxidation, and (b) whether any form of anaerobic respiration contributes to maintaining the $\ce{O2}$ concentration today. $\endgroup$ – Nathaniel Feb 28 '15 at 11:33
  • $\begingroup$ (The latter isn't obvious. Something like methanogenesis will create products that then react with $\ce{O2}$, taking it out of the atmosphere after all. But perhaps there are respiratory pathways, for example with mineral electron acceptors, for which this is not the case.) Anyway +1. $\endgroup$ – Nathaniel Feb 28 '15 at 11:34
  • $\begingroup$ Yes, sorry, i meant aerobic respiration, I'll correct the answer. $\endgroup$ – plannapus Feb 28 '15 at 11:46
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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.

  1. 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)

  1. 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.

  1. 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.

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Some million years before the Great Oxygenation Event there was a large change in the biology of photosynthesis.

Bacterias started to do oxygenic photosynthesis.

"The first microbes to produce oxygen by photosynthesis were oceanic cyanobacteria. They evolved into tufted microbial mats more than 2.3 billion years ago, approximately 200 million years before the GOE (Great Oxydation Event)".

Source: Great Oxydation Event, Wikipedia.

The cyanobacteria were established at stromatolites and colonized most of biomes.

"Oxygen concentrations in the atmosphere remained around or below 1% of today's level until 2.4 Ga ago (the Great Oxygenation Event). The rise in oxygen may have caused a fall in the concentration of atmospheric methane, and triggered the Huronian glaciation from around 2.4 to 2.1 Ga ago. In this way, cyanobacteria may have killed off much of the other bacteria of the time."

Source: Cyanobacteria, Earth History, Wikipedia.

The presence of stromatolites shifted the equation you showed towards the formation of $\small\sf{O_2}$ and biomass: $$\begin{align} \ce{CO2 + H2O + h\nu} & \rightarrow \ce{O2 + CH2O(biomass)} \\ \end{align} $$ Atmospheric $\small\sf{O_2}$ formed was partially kidnapped by Banded Iron Formations (BIF).

$$\begin{align} \ce{Fe + Fe^2+ + O2} & \rightarrow \ce{Fe2O3 (hematite) + Fe3O4(magnetite)} \\ \end{align} $$

$\small\sf{O_2}$ has continued to concentrate at the atmosphere because of photosynthesis and $C$ sinks as coal.

enter image description here

Source: Wikipedia.

At Phanerozoic levels have fluctuated, but that, as explained in the question quoted, is due to extinction events affecting a growing general tendency balanced by the presence of respiration after Cambrian Explosion.

Graph depicting estimates of the percentage of oxygen in the atmosphere over the last 550 million years. Oxygen and Evolution, Robert A. Berner et al., Science 316, 557 (2007):

"The graph shows three marked drops in O2 levels, each corresponding to an extinction event. These are intervals 4, 9, and 11. Interval 9, the largest such drop, represents the end Permian event about 252 million years ago. Interval 11 corresponds to the end Triassic event, 201 million years ago." Causes of fluctuations in atmospheric oxygen in past 300 Mya

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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
enter image description herehttp://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 enter image description here 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?

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