[set fair (OP)]

Googling this, the answer given is theat it came from photosynthesis, originally from cynobacteria. My problem with this, is that if this were the case, then there should be enough oil & fossil fuel to use up all the oxygen and there doesn't seem to be anywhere near enough to do this.

[Kryptid]

Photosynthesis uses carbon dioxide and water. Before there was much oxygen in the Earth's atmosphere, there would still have been a lot of carbon dioxide and water present. Fossil fuels need not have any connection, especially since only a minority of every organism to ever live has ended up as fossil fuels. Much of the material has been recycled over the eons.

[paul cotter]

There are vast quantities of carbon sequestered in geology that is not economic to harvest.

[alancalverd]

Like the whole of the south of England, for instance. Chalk in the south, coal (now irrecoverable) in the north, oil under the sea. This island is all about carbon sequestration!

[paul cotter]

Chalk does not count as carbon sequestration in the OP's context as it is a carbonate which has not been reduced from co2. Another pedantic win!

[alancalverd]

But chalk is the sediment of plankton skeletons, and plankton ecology begins with photosynthetic phytoplankton, including cyanobacteria.

[paul cotter]

Yes Alan, you are 100% right on what you say but that is not the point. The OP was asking where is all the carbon, in reduced form, that would match the amount of oxygen in the atmosphere. co2 is carbon in oxidised form and the marine carbonate is produced directly from atmospheric co2 dissolved in seawater( unless I am greatly mistaken, not being a marine scientist, maybe BC would enlighten us with his opinion ).

[alancalverd]

But photosynthesis is the business of

CO₂ + H₂O  → C₆H₁₂O₆ + O₂

which is followed by all the processes of life, including the formation of carbonate skeletons. The CO₂ in sea or river water arrived by precipitation or direct absorption from the atmosphere.

This all makes sense because there is no other planet with free oxygen in the atmosphere, but there's plenty of CO2 around. Any free oxygen would be rapidly involved in further reactions, so what we have is the result of continuous replenishment by photosynthesis, and the carbon has to go somewhere!   

[paul cotter]

Ok Alan, one more try: the OP was enquiring where all the carbon in reduced form from the co2 originally in the atmosphere had gone. Photosynthesis releases oxygen and ties up the carbon in various forms and there will be one molecule of oxygen produced for every carbon atom and the carbon atoms may or may not go on to form fossil fuels but the co2 in carbonate has not been reduced. Therefore it does not count as carbon storage, post photosynthesis.

[alancalverd]

The OP made the error of assuming that all the carbon extracted from the atmosphere by photosynthesis ended up as oil or coal, so there should be enough fossil fuel to use up all the oxygen in the atmosphere if we burn it. But we know that

Chalk and limestone deposits account for a huge amount of carbon that was absorbed after the carboniferous period by photosynthetic plants and bacteria then eaten by animals and incorporated into their shells or skeletons as CaCO3 and can't be used as fuel.

Some recently-acquired CO2 ends up as vegetable humus or bits of dead animals, and soil depth generally increases at about 1 mm per 100 years if undisturbed.   

Living plants continue to exhale oxygen

If the atmospheric content of CO2 increases, plants grow faster and release more oxygen

So the probability is that even if there was enough flammable organic material to consume all the oxygen and it was all burned at once,  plant life would gradually replace the oxygen anyway, until there were enough animals to consume the oxygen and produce some temporary dynamic equilibrium, such as has been around for about a billion years.

Not sure what you mean by "reduced", or "storage". Carbon that was extracted from the atmosphere and is no longer part of the plant/animal exhalation cycle is, to my mind, "stored". Apart from coal and oil, there is very little
reduced CO2, i.e. from CO2 + 2 H2O → CH4 + 2O2 or CO2 + 2H2 → C + 2H2O,   in nature. 

[Bored chemist]

Lets look at it another way.
By burning fossil fuels we have added about 100 ppm of CO2 to the air.
In doing that we have removed something like 100 ppm of O2
(the actual amount depends on how much hydrogen was in the fuel).
We are talking about "peak oil" and such- and whether or not we have used up all the fossil fuels.

Let's assume that we are actually nowhere near that and we have actually used just 1% of it.

And then let's imagine that - regardless of any possible climate change- we decide to extract and burn all that fuel.
We burn the other 99%
If the 1% we have used raised the CO2 concentration by 100 ppm then the remaining 99%  will raise it 99 times as much
We will increase the CO2 by something like 9900 ppm i.e. about 0.99%


And we would reduce the oxygen concentration by about 0.99% from about 21% to about 20%

Now, you can play with chalk until the cows come home, but you won't make a difference to the oxygen that is left.

Where did this excess oxygen come from?

[Bored chemist]

Not sure what you mean by "reduced",

A change in oxidation state from nominally (IV) to (0)
Converting CO2 to C is a reduction.

[alancalverd]

My point precisely. Carbon has not been directly sequestered from the atmosphere as an element.

Wikipedia has an interesting paradox: some of the reduced carbon actually resulted from high atmospheric oxygen levels that supported extensive forest fires leaving charcoal residues.

There is some volcanic graphite and occasional diamonds, but most geological carbon is found as carbonates.

[Bored chemist]

Carbon has not been directly sequestered from the atmosphere as an element.

It has; we call it coal.


But feel free to answer what I think was the OP's original question.

Where did this excess oxygen come from?

[alancalverd]

Coal was not directly sequestered as elemental carbon, but is the result of photosynthesis of atmospheric CO2 to carbohydrates and suchlike (trees) followed millions of years later by pyrolysis and hydrolysis.

The concept of "excess" oxygen is either meaningless or temporarily inevitable. In cosmic terms, all the free atmospheric O2  is "excessive" as it is highly reactive and absent from every other planet. It is (as the OP said) entirely the waste product of photosynthesis, which AFAIK has not been observed elsewhere (though it might have happened briefly on Mars). Therefore you could classify it as entirely "excess" or entirely the intermediate constituent of a series of reactions that has not reached equilibrium.   

[Bored chemist]

Coal was not directly sequestered as elemental carbon, but is the result of photosynthesis of atmospheric CO2 to carbohydrates and suchlike (trees) followed millions of years later by pyrolysis and hydrolysis.

Can you point out the post where you think anyone said otherwise?

In cosmic terms,

Can you point out the post which you think implies that we are talking about anywhere except the planet earth?

[Bored chemist]

I think the question is "Why is there more oxygen on earth than can be explained by the conversion of CO2 into "C" and O2, based on how much "C" we apparently have?"

(Some of the "C" is coal, some is oil, some cellulose or methane - whatever).

We know that photosynthesis did it.
But where did it put the electrons?
Making oxides into oxygen involves taking the electrons off the oxide ions.
Biology commonly addresses this by sticking them onto (formally) C4+ ions and making C(0) (Actually, it generally goes the whole hog and converts them to (formal) C4- ions, but that's beside the point).
It can also do it by sticking them onto H+ ions and making H2
If that happened on a large scale and the H2 got into the atmosphere and escaped (because it's too light for gravity to trap it) then that would explain the surplus of O2 that we now see.
A huge stock of undiscovered oil-shale or some such might also explain it.

I really don't know what the answer is.
At the moment I'm just wondering if Alan understood the question.

[alancalverd]

Quote from: alancalverd on Today at 11:49:30
Coal was not directly sequestered as elemental carbon, but is the result of photosynthesis of atmospheric CO2 to carbohydrates and suchlike (trees) followed millions of years later by pyrolysis and hydrolysis.
Can you point out the post where you think anyone said otherwise?

er, yes:

Quote from: alancalverd on Today at 09:22:19
Carbon has not been directly sequestered from the atmosphere as an element.

It has; we call it coal.

[alancalverd]

I think the question is "Why is there more oxygen on earth than can be explained by the conversion of CO2 into "C" and O2, based on how much "C" we apparently have?"

And it has been partially answered, I think, by reference to sedimentary carbonate rocks and soil. One thing we have all ignored is the quantity of "live carbon" in the form of existing plant and animal material - literally the elephant in the room!

There are a few biochemical reactions that release hydrogen and methane (particularly in the animal gut) but the key processes of photosynthesis incorporate it into the -CH₂- skeleton of carbohydrates and other complex molecules. See reply #7 above.

[alancalverd]

Can you point out the post which you think implies that we are talking about anywhere except the planet earth?

It's important to realise that this planet appears to be unique in its chemical complexity and surface history. Hence we need to consider a large number of processes that have led to the capture of CO2 and the release of O2 over geological time. 

If there is a god, its name is the hydrogen bond.