Where does the oxygen come from?

Two formulations of the same question. I sent the latter one to the „Last Word“ column of the New Scientist magazine.

Why hasn’t the amount of O2 in the atmosphere decreased significantly by mankind burning large amounts of carbon hydrates in form of oil, coal and gas? The amount of CO2 in the atmosphere is reported to have been greatly increased because of human activity. But for every molecule of CO2 to be created, a molecule of O2 must be taken out of the atmosphere. If the amount of O2 in the atmosphere is far too great for burning carbon and carbon hydrates having a significant influence on it, what geological process does the O2 originate from? Common theory has it that it got produced through photosynthesis in a massive environmental change known as the „Oxygen revolution“. But if the process of photosynthesis is responsible for most of the O2 currently located in the atmosphere, then a similar amount of carbon and carbon hydrates created by this process of photosynthesis must be found somewhere on earth. Where is it?

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Where does the molecular oxygen (O2) in the atmosphere come from?According to prevailing opinion, photosynthesis is the main source of molecular oxygen in the atmosphere. This reaction creates sugars and molecular oxygen (O2) from water and carbon dioxide (CO2). For every molecule of carbon dioxide (CO2) one molecule of oxygen (02) gets produced. The single atom of carbon is stored in sugars or other matter, which the sugars get processed into, like all kinds of carbon hydrates or coal. Only when the produced carbon-containing matter gets burned, the carbon will be processed back to carbon dioxide (CO2), but not without consuming the molecule of oxygen (O2), which got produced before in the photosynthetic process. Therefore, if photosynthesis is the main source of molecular oxygen in the atmosphere, then for almost every molecule of oxygen there must be a corresponding atom of carbon elsewhere, which could consume the oxygen by burning it.

This source lists all deposits of carbon on earth with their presumed amount of carbon in them measured in billions of tonnes. Those deposits which contain carbon which was produced by the photosynthetic process are terrestrial vegetation (610), soils and organic matter (1600), dissolved organic carbon (700), marine organisms (3), coal deposits (3000) and oil and gas deposits (300). Thus the total amount of bound carbon, produced from photosynthesis, can’t exceed 610+1600+700+3+3000+300=6213 Gigatons, according to this source. The mass of a carbon atom is approx. 12 u, therefore no more than 2*10^41 atoms of carbon can be bound in those deposits, which got produced by photosynthesis.

The mass of the earth’s atmosphere is estimated to be 5*10^18 kg, of which no less than one fifth is molecular oxygen. The molar mass of molecular oxygen is 32 g/mol, therefore one kg of oxygen contains 6*10^26/32 molecules, therefore there must be no less than 1.875*10^43 molecules of O2 in the atmosphere. So, there is far more molecular oxygen in the atmosphere than there is carbon-containing matter which got produced by photosynthesis. Where does the molecular oxygen (O2) in the atmosphere come from then, if not from photosynthesis?