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Assumptions: Essentially all oxygen that's ever been present in the atmosphere originated from oxygenic photosynthesis. The production of an oxygen molecule during photosynthesis leads to the consumption of a carbon dioxide molecule. Likewise, we assume that the principal consumer of O2 is the oxidation of carbon (e.g. through respiration or combustion), in which the consumption of an O2 molecule requires the production of a CO2 molecule. Therefore, without an external source or sink of CO2, any increase in O2 requires an equivalent decrease in CO2, and vice versa.

We know that O2 levels rose to ~30% of the total atmosphere during the Carboniferous era. Thus, O2 levels fell by 9% in the 350 million years that followed. This appears to require a concomitant increase in CO2 of 9%, by the assumptions above. Given that the current concentration of CO2 is merely 0.04%, where did all the CO2 go?

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Where did all the CO2 go?

Shell organisms stored it in the sediments and rocks in the form of CaCO3. This is known as the main sink of CO2.

There are other C sinks on the Earth, but lithosphere is the larger by far:

enter image description here

Source: NOAA

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    $\begingroup$ Do you have a citation? It seems surprising that shell organisms alone can be responsible for such a significant consumption of CO2 $\endgroup$
    – Alessandro Power
    Commented Mar 20, 2023 at 20:27
  • $\begingroup$ @Alessandro Power I wrote that too much quickly. I realized the CO2 disolves bellow the Calcite Compensation Depth level. But the CO2 levels are correlated with Wilson Cycle I think, or that's what I learned in University. The larger reservoir of CO2 are rocks. Maybe this graph is of interest $\endgroup$
    – user28185
    Commented Mar 20, 2023 at 21:09
  • $\begingroup$ @Alessandro Power And most of limestones are maden by shell organisms, the sea is plentifull. $\endgroup$
    – user28185
    Commented Mar 20, 2023 at 21:23
  • $\begingroup$ So, combining both answers, the solution is that 1) there are non-oxidizing sinks of CO2, such as limestone formation; and 2) there are other ways for oxygen to oxidize away, such as weathering of newly exposed volcanic rocks. (I'm not convinced that hydrocarbons are relevant -- see my comment below.) $\endgroup$
    – Alessandro Power
    Commented Mar 21, 2023 at 11:24
  • $\begingroup$ If you combine that part of John's answer into your's, I'll happily mark it as accepted. Side note: I didn't know that limestone was mostly made by shell organisms. Fun fact. $\endgroup$
    – Alessandro Power
    Commented Mar 21, 2023 at 11:25
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The assumptions you state are:

  • Oxygen only exists in the atmosphere, as either O2 or CO2
  • Carbon only exists in the atmosphere as CO2 and above surface in the form of organic matter
  • Oxidization of organic matter and photosynthesis are the only ways in which oxygen and carbon move through their respective reservoirs.

But all of these assumptions are, in essence, wrong:

  • Carbon dioxide dissolves in the ocean where it is then sequestered via CaCO3 into sedimentary rocks and from there into the Earth mantle. There were ~600 gigatons of carbon in the atmosphere in the pre-industrial era, about 4 times that much in the biosphere (organic carbon), and about 60 times that much in the oceans (about 38,000 gigatons of carbon). (Source) There is far far more carbon even in the crust and the mantle: About $10^{8}$ and $10^{10}$ gigatons, respectively. (Source)
  • A certain fraction of the carbon is also moved from the atmosphere via photosynthesis into rocks by way of hydrocarbons (oil, gas, and coal). And then, of course, some of that is released again as well: By way of us burning fossil fuels, but also by way of direct methane and oil leaks from underground reservoirs to the ocean or the atmosphere; by oxidation of these fossil fuels by bacteria if oxygen can reach these reservoirs; or by coal seam fires.
  • Oxygen, too, is bound up elsewhere. An example are oxidized iron rocks -- banded iron formations -- in which oxygen was drawn out of the atmosphere-ocean system and sequestered into rocks.
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  • $\begingroup$ it is true there are many factors but the ocean and land plants abord co2 at very similar rates. also petroleum originates from biotic carbon but is not normally counted in biosphere numbers, so your not really comparing them fairly. $\endgroup$
    – John
    Commented Mar 22, 2023 at 0:14
  • $\begingroup$ @John I'm not actually comparing any exchange rates between reservoirs at all. All I'm saying is that on geologic time scales, much larger carbon and oxygen reservoirs come into play than just the atmosphere and organic matter as suggested by the original question. I have augmented my answer for the hydrocarbon reservoir route into rocks. $\endgroup$
    – Wolfgang Bangerth
    Commented Mar 22, 2023 at 3:05
  • $\begingroup$ Your assumption CaCO3 rises the Mantle is almost wrong. Bellow the Calcite Compensation Depth, CaCO3 dissolves so when the plate subducts most of CO2 have returned to ocean water yet. $\endgroup$
    – user28185
    Commented Mar 22, 2023 at 6:32
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    $\begingroup$ "An example are oxidized iron rocks" My understanding is that nearly all banded iron formed before the great oxidation event -- certainly well before the Carboniferous at least. My question is specific to the post-Carboniferous era. The rest of the answer looks good though. $\endgroup$
    – Alessandro Power
    Commented Mar 22, 2023 at 9:37
  • $\begingroup$ @Universal_learner "when the plate subducts most of CO2 have returned to ocean water yet": That is just not right. A few hundred km from subduction zones, there are large volcanos (example: the Cascade volcanoes in the Western US) that produce large quantities of CO2. That CO2 is the result of subducted carbon, coming from depths of a few hundred km off the top of subducted plates. $\endgroup$
    – Wolfgang Bangerth
    Commented Mar 23, 2023 at 19:11
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It doesn't go anywhere, you don't need CO2 production to loose oxygen.

photosynthesis is water + CO2 into O2 and sugar. In this case the sugar is mostly cellulose. what happened in the carboniferous? Lignin bound massive cellulose (wood) evolved but almost nothing could digest it leading to burial being far more likely than today. At the same time mountain building features of Pangea make burial of peat and swamps a continuous process. Carbon gets buried Oxygen gets released. That is where the oxygen itself came from.

You don't need CO2 production to get oxygen loss. Oxygen level fall naturally as it well, oxidizes stuff, mostly weathered of minerals and volcanics. There needs to be a continuous production of new oxygen to replace that. Today this effect is minimal but the continuous evolution of things that can live on land, and in dryer and dryer places would have noticeably increased this as weathering compared to today. Modern sediment source rock is much more oxidized as a whole that is why today the exchange of CO2 and oxygen biotically has a bigger effect. How much sediment is reworked stuff that is already oxidized increases over time but it is still fairly low at the time. The carbon/oxygen cycle is much more complex than just the two exchanging with each other.

https://www.nsf.gov/news/news_summ.jsp?cntn_id=124570

https://www.sciencedirect.com/science/article/pii/S0009254120302047

https://www.nature.com/articles/ncomms14379

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    $\begingroup$ I'm not sure that the first part of this answer is relevant. Carbon burial explains oxygen increases, but the question is about decrease. And the corollary -- that the decrease in oxygen since the Carboniferous is due to greater exposure and oxidation of hydrocarbons -- would require a 9% increase in CO2, leaving us back where we started. The second part is interesting though. I was under the impression that nearly everything that could be oxidized had already been oxidized since the Great Oxidation Event, but I will check out those articles $\endgroup$
    – Alessandro Power
    Commented Mar 21, 2023 at 11:11
  • $\begingroup$ the first part is the literal answer to the question, were the carbon from making the oxygen went. the second part is why you your assumption is wrong, the decrease in oxygen after the carboniferous has nothing to do with oxidizing hydrocarbons. weathered rock absorbs oxygen. and thanks to the evolution of land plants terrestrial weathering rises dramatically. also oxidization of strata is not a hard drop it changes as mineral exposure changes and reduces as more and more minerals are recycled already oxidized minerals. $\endgroup$
    – John
    Commented Mar 22, 2023 at 0:02

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