# Why did the carboniferous period have so much atmospheric oxygen?

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?

• Hi. Please give a source for your assertion in the first sentence. – Spencer Mar 16 '19 at 13:08
• @spencer A source that the atmosphere is 21% oxygen? – Neil G Mar 16 '19 at 13:58
• No, your assertion that it would still be 21% if all of the CO2 got sequestered. – Spencer Mar 16 '19 at 15:01
• @Spencer I never asserted that. I asked three questions. – Neil G Mar 16 '19 at 15:18
• What do you mean by "so much"? How much oxygen do you believe was there in the carboniferous? What's your source? – Camilo Rada Mar 16 '19 at 17:18

To complement @DavidHammen answer and address the point "where did so much oxygen come from?" I will elaborate on David'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. 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 @DavidHammen 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.

• That explains my misunderstanding! Thanks! – Neil G Mar 16 '19 at 23:16
• I did not understand the question. Now I understand your complement Camilo, but I think it should be clarified O as an atom and not as diatomic. – user12525 Mar 17 '19 at 14:44
• @Universal_learner why and where do you think that clarification is needed. In some cases it should be $\ce{O2}$. – Camilo Rada Mar 17 '19 at 17:47
• Well yes you know my english is poor and I may have some missunderstanding, but I think if you say O2 it is plants that made it. But if you do the O16/18 then that single O atoms should come from volcanoes as I think you are notizing here. – user12525 Mar 17 '19 at 17:49
• @Universal_learner Does that makes a difference in this case? Fell free to explain your point in Spanish if you rather. – Camilo Rada Mar 17 '19 at 19:57

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.

• 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 Mar 16 '19 at 14:57
• Vascular plants first evolved during the Silurian Period and became widespread during the Devonian Period. There are even Devonian coal deposits. – Spencer Mar 16 '19 at 15:21
• @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 Mar 16 '19 at 20:29
• Despite Camilo nice speach I feel more confortable with this answer. Carnoniferous is previous to the break of Pangea, so I feel like volcanic activity is not the main reason for O2 increase. Also David and Camilo know I posted a graph about CO2 showing a fall at Silurian-Devonian and specially Carboniferous. commons.wikimedia.org/wiki/File:Phanerozoic_Carbon_Dioxide.png There was a great volcanic activity sure if Camilo post it but lignins is a key here is my guess. A big biological event @Neil G – user12525 Mar 17 '19 at 1:59
• Also @Camilo Rada: I can be wrong, but I said CO2 is not a main factor for plants growing at my previous answer doing wrongdoings with nasa, so CO2 increase could increase a bit biomass, but if I am not wrong not a lot and Carboniferous...well a fall by a factor of four (totaly trusting it being the source...), big words in my humble opinion to attribute it to volcanoes. earthscience.stackexchange.com/questions/7627/… – user12525 Mar 17 '19 at 2:04