<p>The Carboniferous was when the growth of woody plants took off. Non-plant life had not yet evolved the ability to consume lignins, the key chemical components that makes woody plants "woody". Lignins are rather hard to decompose. Despite high volcanic activity, carbon dioxide levels fell by a factor of over four during the Carboniferous, from over sixteen times preindustrial levels at the start of the period to less than four times preindustrial levels at the end of the period.</p>

<p>The end result was a gradual increase in oxygen levels and huge deposits of then non-digestible materials that eventually became coal. </p>

CC BY-SA 4.0
2019-03-16T14:57:46.877Z
I have heard this argument, but I still don't understand how this has any effect since carbon dioxide levels are less than 1% of the atmosphere.  How did so little carbon dioxide give rise to so much oxygen?
Neil G
CC BY-SA 4.0
2019-03-16T15:21:34.557Z
Vascular plants [first evolved during the Silurian Period](http://palaeos.com/plants/tracheophyta/cooksonia.html) and [became widespread during the Devonian Period](http://www.devoniantimes.org/opportunity/forests.html). There are even [Devonian coal deposits](https://www.sciencedirect.com/science/article/abs/pii/0166516289900529).
Spencer
CC BY-SA 4.0
2019-03-16T20:29:25.700Z
@Spencer - I addressed your comment. It was woody plants, an offshoot of vascular plants, that led to the huge drawdown of $\text{CO}_2$ in the Carboniferous.
David Hammen
CC BY-SA 4.0
2019-03-17T04:50:05.007Z
@Universal_learner  This is a good answer.   However, the idea of carbon being sequestered by plants is already in the question I asked.  The missing information is where all the oxygen came from, which is not answered in this answer.  This is a classic example of saying a lot of true things without answering the question that was asked.
Neil G
CC BY-SA 4.0
2019-03-17T07:50:08.117Z
@NeilG - Perhaps too briefly, I did answer your question with the phrase "despite high volcanic activity". Until the latter part of the 19th century, volcanos spewed more carbon dioxide than did humanity. (Human activity now spews over a hundred times as much carbon dioxide than do volcanos.)
David Hammen
CC BY-SA 4.0
2019-03-17T07:58:04.667Z
Oh you're right!  Sorry, I didn't catch that.  I guess I'm the classic example of an inattentive reader.
Neil G
CC BY-SA 4.0
2019-03-16T14:53:31.030Z
2019-03-16T20:28:14.447Z
2019-03-16T20:28:14.447Z
David Hammen
<p>To complement @DavidHammen answer and address the point "where did so much oxygen come from?" I will elaborate on David's final remark</p>

<blockquote>
  <p>The end result was a gradual increase in oxygen levels</p>
</blockquote>

<p>The short answers to "where did so much oxygen come from?" is: mostly from volcanos in the form of <span class="math-container">$\ce{CO2}$</span>.</p>

<p>To understand this, we have to consider that the amount of atmospheric <span class="math-container">$\ce{CO2}$</span> is controlled by sources and sinks. At geological time scales the main source is volcanic activity and the main sink is <a href="https://en.wikipedia.org/wiki/Carbonate%E2%80%93silicate_cycle" rel="noreferrer">silicate weathering</a>.
A key point to consider in this case the that the source is independent of <span class="math-container">$\ce{CO2}$</span> concentration while the sink is proportional to <span class="math-container">$\ce{CO2}$</span> concentration (and other factors like temperature or surface of exposed silicate rocks).</p>

<p>Therefore, if you have intense photosynthetic activity like the one described by @DavidHammen you can lower atmospheric <span class="math-container">$\ce{CO2}$</span> concentration, therefore reducing the intake by natural silicate weathering sinks, and at the same time increasing the atmospheric oxygen concentration. While this takes place, volcanoes keep putting <span class="math-container">$\ce{CO2}$</span> into the atmosphere and plants keep turning it into oxygen. If you keep this going on for a long enough time, you can rise oxygen levels as high as you want. Although, at some point other feedbacks will kick in to keep the oxygen level at bay. For example, wildfires will be more common and extensive in an oxygen-rich atmosphere, providing a stabilizing feedback that keeps a balance between <span class="math-container">$\ce{O2}$</span> and <span class="math-container">$\ce{CO2}$</span>.</p>

<p>Effectively, plants in such scenario would have replaced part of the contribution of the weathering sink of <span class="math-container">$\ce{CO2}$</span>. With the notable difference that the oxygen instead of getting washed to the deep ocean (and eventually subducted), was getting piled into the atmosphere as <span class="math-container">$\ce{O2}$</span>, thus, slowly rising its atmospheric concentration.</p>

CC BY-SA 4.0
2019-03-16T23:16:40.127Z
That explains my misunderstanding!   Thanks!
Neil G
CC BY-SA 4.0
2019-03-17T17:47:10.823Z
@Universal_learner why and where do you think that clarification is needed. In some cases it should be $\ce{O2}$.
Camilo Rada
CC BY-SA 4.0
2019-03-17T19:57:04.610Z
@Universal_learner Does that makes a difference in this case? Fell free to explain your point in Spanish if you rather.
Camilo Rada
CC BY-SA 4.0
2019-07-17T22:42:08.200Z
This video seems relevant: https://www.youtube.com/watch?v=4ONwQV26L-k
Neil G
CC BY-SA 4.0
2019-03-16T23:14:53.227Z
2019-03-16T23:26:23.947Z
2019-03-16T23:26:23.947Z
Camilo Rada
<p>Even if all the carbon dioxide (which makes up less than 1% of the atmosphere) in the air were sequestered by plants, would the atmosphere not remain about 21% oxygen? Why did the carboniferous period have 35% atmospheric oxygen?</p>

CC BY-SA 4.0
2019-03-16T13:08:29.520Z
Hi. Please give a source for your assertion in the first sentence.
Spencer
CC BY-SA 4.0
2019-03-16T13:58:48.180Z
@spencer A source that the atmosphere is 21% oxygen?
Neil G
CC BY-SA 4.0
2019-03-16T15:01:45.553Z
No, your assertion that it would still be 21% if all of the CO2 got sequestered.
Spencer
CC BY-SA 4.0
2019-03-16T15:18:59.067Z
@Spencer I never asserted that.  I asked three questions.
Neil G
CC BY-SA 4.0
2019-03-16T15:28:19.353Z
You asked your first question in a way that indicates you assume it is true. What do you base that on?
Spencer
CC BY-SA 4.0
2019-03-16T15:34:37.030Z
@Spencer I don't assume anything, which is why I have only asked questions.
Neil G
CC BY-SA 4.0
2019-03-16T17:18:10.853Z
What do you mean by "so much"? How much oxygen do you believe was there in the carboniferous? What's your source?
Camilo Rada
CC BY-SA 4.0
2019-03-16T20:01:07.280Z
@NeilG Regarding *I asked three questions* -- That's bad form for this and almost all other stackexchange network sites. A question at these sites should ask one single question, and that one single question should ideally have an answer that is unambiguously the correct and the best answer. That ideal is not always possible, but that is the ideal.
David Hammen
CC BY-SA 4.0
2019-03-16T22:17:59.177Z
@DavidHammen I understand.  I've removed the additional question.  The first two questions are really just one question.
Neil G
CC BY-SA 4.0
2019-03-16T22:19:52.230Z
@CamiloRada There are lots of sites on the internet that list the oxygen levels during the Carboniferous at 35%.  I didn't want to specify since I'm not interesting in the precise value, but my question is where did so much oxygen come from.
Neil G
<atmosphere><geophysics><paleoclimatology><carbon-cycle><oxygen>
Why did the carboniferous period have so much atmospheric oxygen?
Neil G
# Why did the carboniferous period have so much atmospheric oxygen?

(Asked by: username_0 on 2019-03-16)

Even if all the carbon dioxide (which makes up less than 1% of the atmosphere) in the air were sequestered by plants, would the atmosphere not remain about 21% oxygen? Why did the carboniferous period have 35% atmospheric oxygen?

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

> **Comments:**
> **username_1**: Hi. Please give a source for your assertion in the first sentence.  
> **username_0**: @username_1 A source that the atmosphere is 21% oxygen?  
> **username_1**: No, your assertion that it would still be 21% if all of the CO2 got sequestered.  
> **username_0**: @username_1 I never asserted that.  I asked three questions.  
> **username_1**: You asked your first question in a way that indicates you assume it is true. What do you base that on?  
> **username_0**: @username_1 I don't assume anything, which is why I have only asked questions.  
> **username_2**: What do you mean by "so much"? How much oxygen do you believe was there in the carboniferous? What's your source?  
> **username_3**: @username_0 Regarding *I asked three questions* -- That's bad form for this and almost all other stackexchange network sites. A question at these sites should ask one single question, and that one single question should ideally have an answer that is unambiguously the correct and the best answer. That ideal is not always possible, but that is the ideal.  
> **username_0**: @username_3 I understand.  I've removed the additional question.  The first two questions are really just one question.  
> **username_0**: @username_2 There are lots of sites on the internet that list the oxygen levels during the Carboniferous at 35%.  I didn't want to specify since I'm not interesting in the precise value, but my question is where did so much oxygen come from.  

---

## 2 answers

### Answer by username_2 on 2019-03-16. Score: 6 | Accepted answer

To complement @username_3 answer and address the point "where did so much oxygen come from?" I will elaborate on username_3's final remark

> The end result was a gradual increase in oxygen levels

The short answers to "where did so much oxygen come from?" is: mostly from volcanos in the form of $\ce{CO2}$.

To understand this, we have to consider that the amount of atmospheric $\ce{CO2}$ is controlled by sources and sinks. At geological time scales the main source is volcanic activity and the main sink is [silicate weathering](https://en.wikipedia.org/wiki/Carbonate%E2%80%93silicate_cycle).
A key point to consider in this case the that the source is independent of $\ce{CO2}$ concentration while the sink is proportional to $\ce{CO2}$ concentration (and other factors like temperature or surface of exposed silicate rocks).

Therefore, if you have intense photosynthetic activity like the one described by @username_3 you can lower atmospheric $\ce{CO2}$ concentration, therefore reducing the intake by natural silicate weathering sinks, and at the same time increasing the atmospheric oxygen concentration. While this takes place, volcanoes keep putting $\ce{CO2}$ into the atmosphere and plants keep turning it into oxygen. If you keep this going on for a long enough time, you can rise oxygen levels as high as you want. Although, at some point other feedbacks will kick in to keep the oxygen level at bay. For example, wildfires will be more common and extensive in an oxygen-rich atmosphere, providing a stabilizing feedback that keeps a balance between $\ce{O2}$ and $\ce{CO2}$.

Effectively, plants in such scenario would have replaced part of the contribution of the weathering sink of $\ce{CO2}$. With the notable difference that the oxygen instead of getting washed to the deep ocean (and eventually subducted), was getting piled into the atmosphere as $\ce{O2}$, thus, slowly rising its atmospheric concentration.

> **Comments:**
> **username_0**: That explains my misunderstanding!   Thanks!  
> **username_2**: @Universal_learner why and where do you think that clarification is needed. In some cases it should be $\ce{O2}$.  
> **username_2**: @Universal_learner Does that makes a difference in this case? Fell free to explain your point in Spanish if you rather.  
> **username_0**: This video seems relevant: https://www.youtube.com/watch?v=4ONwQV26L-k  

---

### Answer by username_3 on 2019-03-16. Score: 8

The Carboniferous was when the growth of woody plants took off. Non-plant life had not yet evolved the ability to consume lignins, the key chemical components that makes woody plants "woody". Lignins are rather hard to decompose. Despite high volcanic activity, carbon dioxide levels fell by a factor of over four during the Carboniferous, from over sixteen times preindustrial levels at the start of the period to less than four times preindustrial levels at the end of the period.

The end result was a gradual increase in oxygen levels and huge deposits of then non-digestible materials that eventually became coal.

> **Comments:**
> **username_0**: I have heard this argument, but I still don't understand how this has any effect since carbon dioxide levels are less than 1% of the atmosphere.  How did so little carbon dioxide give rise to so much oxygen?  
> **username_1**: Vascular plants [first evolved during the Silurian Period](http://palaeos.com/plants/tracheophyta/cooksonia.html) and [became widespread during the Devonian Period](http://www.devoniantimes.org/opportunity/forests.html). There are even [Devonian coal deposits](https://www.sciencedirect.com/science/article/abs/pii/0166516289900529).  
> **username_3**: @username_1 - I addressed your comment. It was woody plants, an offshoot of vascular plants, that led to the huge drawdown of $\text{CO}_2$ in the Carboniferous.  
> **username_0**: @Universal_learner  This is a good answer.   However, the idea of carbon being sequestered by plants is already in the question I asked.  The missing information is where all the oxygen came from, which is not answered in this answer.  This is a classic example of saying a lot of true things without answering the question that was asked.  
> **username_3**: @username_0 - Perhaps too briefly, I did answer your question with the phrase "despite high volcanic activity". Until the latter part of the 19th century, volcanos spewed more carbon dioxide than did humanity. (Human activity now spews over a hundred times as much carbon dioxide than do volcanos.)  
> **username_0**: Oh you're right!  Sorry, I didn't catch that.  I guess I'm the classic example of an inattentive reader.  

---

