<p>The assumptions you state are:</p>
<ul>
<li>Oxygen only exists in the atmosphere, as either O2 or CO2</li>
<li>Carbon only exists in the atmosphere as CO2 and above surface in the form of organic matter</li>
<li>Oxidization of organic matter and photosynthesis are the only ways in which oxygen and carbon move through their respective reservoirs.</li>
</ul>
<p>But all of these assumptions are, in essence, wrong:</p>
<ul>
<li>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). (<a href="https://worldoceanreview.com/en/wor-1/ocean-chemistry/co2-reservoir/#:%7E:text=The%20ocean%2C%20with%20around%2038%2C000,before%20people%20began%20to%20drastically" rel="nofollow noreferrer">Source</a>) There is far far more carbon even in the crust and the mantle: About <span class="math-container">$10^{8}$</span> and <span class="math-container">$10^{10}$</span> gigatons, respectively. (<a href="https://en.wikipedia.org/wiki/Geochemistry_of_carbon#:%7E:text=Earth%27s%20mantle%20is%20a%20significant,of%20100%20between%20different%20parts." rel="nofollow noreferrer">Source</a>)</li>
<li>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.</li>
<li>Oxygen, too, is bound up elsewhere. An example are oxidized iron rocks -- <a href="https://en.wikipedia.org/wiki/Banded_iron_formation" rel="nofollow noreferrer">banded iron formations</a> -- in which oxygen was drawn out of the atmosphere-ocean system and sequestered into rocks.</li>
</ul>

CC BY-SA 4.0
2023-03-22T00:14:40.667Z
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.
John
CC BY-SA 4.0
2023-03-22T03:05:29.200Z
@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.
Wolfgang Bangerth
CC BY-SA 4.0
2023-03-22T09:37:38.433Z
"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.
Alessandro Power
CC BY-SA 4.0
2023-03-23T19:11:22.023Z
@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.
Wolfgang Bangerth
CC BY-SA 4.0
2023-03-23T19:15:49.800Z
@AlessandroPower Banded iron formations are the result of the oxidization of the ocean by photosynthesizing organisms. They predate the Carboniferous era by ~3 billion years, of course, but still provide an example of an oxygen sink other than carbon dioxide. The latter was the point I wanted to make: That there are large sinks and sources for both oxygen and carbon that have nothing to do with the O2 <-> CO2 exchange.
Wolfgang Bangerth
CC BY-SA 4.0
2023-03-25T03:29:03.030Z
@Universal_learner You can call it "outgassing of the mantle", but the question is "does that just reduce the amount of carbon in the mantle, or is this carbon that was only recently brought from the surface down into the mantle". The latter is the case, just like the water that volcanoes emit: It's the sediment layer on top of oceanic plates that consists of CaCO3 saturated with water.
Wolfgang Bangerth
CC BY-SA 4.0
2023-03-21T23:17:20.510Z
2023-03-22T03:08:26.103Z
2023-03-22T03:08:26.103Z
Wolfgang Bangerth
<h2>It doesn't go anywhere, you don't need CO2 production to loose oxygen.</h2>
<p>photosynthesis is water + CO<sub>2</sub> into O<sub>2</sub> 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.</p>
<p><strong>You don't need CO<sub>2</sub> production to get oxygen loss.</strong>
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 CO<sub>2</sub> 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.</p>
<p><a href="https://www.nsf.gov/news/news_summ.jsp?cntn_id=124570" rel="nofollow noreferrer">https://www.nsf.gov/news/news_summ.jsp?cntn_id=124570</a></p>
<p><a href="https://www.sciencedirect.com/science/article/pii/S0009254120302047" rel="nofollow noreferrer">https://www.sciencedirect.com/science/article/pii/S0009254120302047</a></p>
<p><a href="https://www.nature.com/articles/ncomms14379" rel="nofollow noreferrer">https://www.nature.com/articles/ncomms14379</a></p>

CC BY-SA 4.0
2023-03-21T11:11:55.613Z
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
Alessandro Power
CC BY-SA 4.0
2023-03-22T00:02:13.520Z
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.
John
CC BY-SA 4.0
2023-03-21T00:10:14.513Z
2023-03-22T19:58:30.223Z
2023-03-22T19:58:30.223Z
John
<p>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 <em>consumer</em> of O<sub>2</sub> is the oxidation of carbon (e.g. through respiration or combustion), in which the consumption of an O<sub>2</sub> molecule requires the production of a CO<sub>2</sub> molecule. Therefore, without an external source or sink of CO<sub>2</sub>, any increase in O<sub>2</sub> requires an equivalent decrease in CO<sub>2</sub>, and vice versa.</p>
<p>We know that O<sub>2</sub> levels rose to ~30% of the total atmosphere during the Carboniferous era. Thus, O<sub>2</sub> levels fell by 9% in the 350 million years that followed. This appears to require a concomitant <em>increase</em> in CO<sub>2</sub> of 9%, by the assumptions above. Given that the current concentration of CO<sub>2</sub> is merely 0.04%, <strong>where did all the CO<sub>2</sub> go?</strong></p>

<atmosphere><co2><paleoclimatology><carbon-cycle><oxygen>
How could oxygen levels have ever been higher if there is so little carbon dioxide present?
Alessandro Power
# How could oxygen levels have ever been higher if there is so little carbon dioxide present?

(Asked by: username_0 on 2023-03-20)

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?**

**Tags:** atmosphere, co2, paleoclimatology, carbon-cycle, oxygen


---

## 2 answers

### Answer by username_1 on 2023-03-21. Score: 1

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](https://worldoceanreview.com/en/wor-1/ocean-chemistry/co2-reservoir/#:%7E:text=The%20ocean%2C%20with%20around%2038%2C000,before%20people%20began%20to%20drastically)) There is far far more carbon even in the crust and the mantle: About $10^{8}$ and $10^{10}$ gigatons, respectively. ([Source](https://en.wikipedia.org/wiki/Geochemistry_of_carbon#:%7E:text=Earth%27s%20mantle%20is%20a%20significant,of%20100%20between%20different%20parts.))
* 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](https://en.wikipedia.org/wiki/Banded_iron_formation) -- in which oxygen was drawn out of the atmosphere-ocean system and sequestered into rocks.

> **Comments:**
> **username_2**: 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.  
> **username_1**: @username_2 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.  
> **username_0**: "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.  
> **username_1**: @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.  
> **username_1**: @username_0 Banded iron formations are the result of the oxidization of the ocean by photosynthesizing organisms. They predate the Carboniferous era by ~3 billion years, of course, but still provide an example of an oxygen sink other than carbon dioxide. The latter was the point I wanted to make: That there are large sinks and sources for both oxygen and carbon that have nothing to do with the O2 <-> CO2 exchange.  
> **username_1**: @Universal_learner You can call it "outgassing of the mantle", but the question is "does that just reduce the amount of carbon in the mantle, or is this carbon that was only recently brought from the surface down into the mantle". The latter is the case, just like the water that volcanoes emit: It's the sediment layer on top of oceanic plates that consists of CaCO3 saturated with water.  

---

### Answer by username_2 on 2023-03-21. Score: 1

## 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>

> **Comments:**
> **username_0**: 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  
> **username_2**: 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.  

---

