Estimated fossil fuels are around 5 times as much as existing carbon biomass. If these estimates are appropriate, then probably before the oxygen was released it was not locked up in carbon compounds.

Oxygen could have been extracted from inorganic stuff and cycled through the biomass.

Which reduced minerals were left behind? Not silicates and carbonates. Little magnetite crystals?

Maybe there was more SiO₄ before (things like olivine), and living things tended to produce silicates with less oxygen (things like kaolin)? There isn't a whole lot of silica dissolved in seawater at any one time, but there's a whole lot of silica available and it could add up eventually. Silicates with more oxygen weathered away, and silicates with less oxygen were deposited?

Maybe there's so much free oxygen today because some H₂ got produced, and it rose right out of the atmosphere and got swept away. So there's less water and more oxygen than there used to be.

Maybe the biomass estimates, and the fossil fuel estimates, don't actually tell us about the carbon, and there is lots of reduced carbon elsewhere.

I'm speculating, does anybody have good answers?

大氧化事件是由光合作用的进化引起的，光合作用将二氧化碳和水转化为糖和氧气。所以氧气来自二氧化碳和水。请记住，CO2水平比今天高很多很多倍(~20倍)。氢气没有释放出来，它与碳和氧结合，形成糖

Oxygen is made by the splitting of water, either by photosynthesis or by UV-based photo-dissociation in the upper atmosphere.

In the case of photosynthesis, the reduced-carbon products must be buried in order for a net accumulation of oxygen to happen. In this case the oxygen effectively comes from water:

(1) 2H2O + vis. light. -> 2H2 + O2 (Water to hydrogen and oxygen) (2) 2H2 + CO2 -> H2CO + H2O (Carbon dioxide and Hydrogen to carbohydrate and water. Carbohydrate is buried)

And from UV-dissociation, we get

(1) 2H2O + uv -> 2H2 + O2 (Water to hydrogen and oxygen, hydrogen is lost to space)

The oceans weight over 10^21kg, most of which is oxygen, so there is plenty of mass there.

Edit:

Methane Photodissociation

The reaction is effectively (in an oxygen free atmosphere):

(3) CH4 + 2H2O + uv -> CO2 + 4H2 (lost to space)

Note that without hydrogen loss to space, there is a limit on the amount of oxygenation that can happen, because the reduced carbon generated in photosynthesis has to be buried geologically. Estimates for the amount of buried reduced carbon are hard to find (carbonates don't count for this purpose).

So the ultimate source of oxygen in the atmosphere would appear to be water, mostly.

我想补充其他答案。< / p >

Yes, it is correct that photosynthesis by cyanobacteria caused the rise of atmospheric oxygen. Some important points to talk about:

Estimated fossil fuels are around 5 times as much as existing carbon biomass. If these estimates are appropriate, then probably before the oxygen was released it was not locked up in carbon compounds.

Fossil fuels are irrelevant to the discussion. Fossil fuels only started forming in the Phanerozoic, about 500 million years ago. The rise of oxygen occurred more than 2 billion years ago.

Oxygen could have been extracted from inorganic stuff and cycled through the biomass. Which reduced minerals were left behind? Not silicates and carbonates. Little magnetite crystals?

Nothing, actually. The minerals found on the surface of the Earth formed as a response to the presence of atmospheric oxygen. This means that oxidised minerals on the surface only appeared after we had atmospheric oxygen. This means that something else had to be reduced. For example, sedimentary detrital pyrite was a thing! Not possible today with the presence of oxygen.

Maybe there was more SiO4 before (things like olivine), and living things tended to produce silicates with less oxygen (things like kaolin)?

Not accurate. Both olivine and kaolinite are stable over a very wide oxidation states. Doing the charge-balance math shows that the silica exists as Si⁴⁺ in all silicates. If anything, olivine is a reduced formed (it has Fe²⁺) and is not stable on the surface as it oxidises ("rusts").

Silicates with more oxygen weathered away, and silicates with less oxygen were deposited?

Not really.

The key point is that the oxygen came from photosynthesis, and reduction of H₂O and CO₂ to form organic matter. Carbon and hydrogen form the organic stuff, and the leftover O₂ is emitted to the atmosphere. But do we have enough carbon? You say:

was there that much CO2 before? Oxygen is supposedly the most common element in the earth's crust, about 466 parts per thousand by weight.

But, most of the oxygen is locked up in minerals and is completely irrelevant for this discussion as it does not participate in the reactions. Carbon is being constantly emitted by volcanism. It is then consumed by either mineral carbonation reactions (such as formation of olivine to magnesite: magnesium carbonate) on the surface, or by formation of organic matter. Therefore, CO₂ doesn't have the chance to accumulate in the atmosphere. However, oxygen isn't consumed by reactions, so it accumulates.

In the beginning, oxygen didn't actually accumulate. It also had a sink: dissolved Fe²⁺ in the oceans. Oxidation of that iron caused formation of insoluble Fe³⁺ oxides and hydroxides, which we see today as banded iron formations. Only once the iron was consumed, the oxygen could start accumulating in the atmosphere.

If all of the free oxygen came from water (and the hydrogen escaped to outer space), that isn't very different. That's around 10^18 liters of water lost from the oceans. The surface of the earth is about 5x10^14 square meters, one liter covers 10^-3 square meters, so sea level would have been somewhere vaguely around 2 meters higher. There's a whole lot of water in the world.

People say the atmosphere used to have 20% CO2 and that's where the oxygen came from. I didn't believe it because there is probably not that much reduced carbon in the world today. But it's possible that as the carbon was reduced, it was deposited on the sea floor and then it got subducted deep underground. It may still be there, waiting to get out.

Maybe there were other oxidized compounds that got reduced. The obvious candidates are nitrates and sulfates.

Today, bacteria mostly reduce nitrates when there is not enough free oxygen. In a series of steps they reduce it all the way to N2, getting energy at each step.

Other bacteria convert N2 to ammonia, using considerable energy. Often the ones that do this are photosynthetic. They have N2 and not enough nitrogen compounds to meet their needs, so they make what they need. O2 damages the enzyme.

Possibly there used to be a lot more oxidized nitrogen dissolved in the ocean, and the balance shifted, resulting in both increased N2 and increased O2.

Maybe there used to be a whole lot more sulfur in the ocean, in the form of sulfates etc. Most of the sulfur got reduced and deposited in the form of elemental sulfur or iron sulfide etc. Then it got subducted away, removing the evidence.

What was the pH of the ancient ocean? Various organic compounds can buffer pH differently according to concentration. H2CO2 (formic acid) acts different from H2 + CO2. Did the ocean used to be more acid? I don't know, and there were a lot of different buffers. Maybe H2CO3 (carbonic acid) was always the most important, if there was much more CO2 dissolved in the oceans than any other organic acid.

Just as this could happen for sulfur or iron, it could happen with any other good electron acceptor that happened to be dissolved in the ocean. Precipitate it out, and then subduction removes most of the evidence. But only things that had oxygen and then lost it, would contribute to free O2.

It was very different, and I can't even assume that the amounts of the various elements present were like today.

If it isn't clear what the pH of seawater was back then, I can't even be sure what the maximum amount of dissolved stuff would be. When I tried to guess at the maximum amount of dissolved iron, it turns out it varies with pH, with the amount of other dissolved chemicals, strongly with the amount of organic carbon compounds (because living things in today's ocean can't get enough iron and they scavenge it aggressively), etc. But that's today's ocean, where dissolved iron is quickly oxidized.

I expect there's room for a whole lot of uncertainty about all that. But experts can still know some things about it.