We are told that photosynthesis, developed a couple of billion years ago has produced all the atmospheric oxygen. I wonder, why did the process stop? Why do modern plants prefer to recover the entropy whereas ancient ones...

Actually, the ancient ones separated the carbon-hydrogen fuel from the oxygen. The modern trees consist of this fuel which re-joins with the oxygen afterlife. But why didn't this happen to the ancient plants? Where did the ancient carbohydrates go? If it is the oil and gas we can burn it. Why do you say that we do not have enough fossils to burn all the oxygen? Where are the ancient plant bodies deposited so securely that we say for sure that they cannot come back into the contact with the oxygen?

  • $\begingroup$ Well I have not heard that photosynthesis has stopped! We have a cyclical thing going with the composition of our atmosphere. The oceans, the soils and the plants use CO2 and their WASTE PRODUCT is Oxygen. Pretty stable system. I don't think there is any difference between ancient plant material and present day plant material. Photosynthesis is a pretty cool invention by bacteria and then the plants who fine tuned the process of using the energy of sunlight to make their own FOOD. Carbohydrates to be precise. Just because a resource is available should it be used up? $\endgroup$ – stormy Jul 7 '16 at 21:18
  • $\begingroup$ @stormy You contradict yourself. Saying that photosynthesis keeps increasing the O2 level, which stays stable, is self-defeating. Aside from your logorrhea, what is the problem to "use" the resources. I am asking, why cannot you burn out all the the ancient fossils and oxygen, separated 2 bln years ago, to restore 2 bln-old conditions? $\endgroup$ – Little Alien Jul 7 '16 at 21:28
  • $\begingroup$ I did say that there is a cyclical system for our atmosphere. Photosynthesis takes CO2 that the oceans, tilled soils, decomposing organic matter and of course man made CO2, MAKES its own food to store the energy and water and O2 are byproducts. Do you know what the conditions were 2 billion years ago? I give up, I've got LOGORRHEA after all. $\endgroup$ – stormy Jul 8 '16 at 3:10
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    $\begingroup$ None of the stuff you have posted so far makes any real sense. It is rambling and difficult to understand and you provide no evidence for it. If you want to have a serious academic conversation about this then rephrase your question. Otherwise this will just be closed as another junk question. $\endgroup$ – bon Jul 8 '16 at 9:58
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    $\begingroup$ Don't feed the troll. $\endgroup$ – David Hammen Jul 10 '16 at 15:52

Photosynthesis has not stopped. It happens all the time, splitting water and carbon dioxide, and producing oxygen and carbohydrates. Likewise, organic matter rots and decomposes all the time, requiring oxygen and releasing carbon dioxide. The Keeling curve actually illustrates this quite nicely: most of the land mass where this happens lies in the north, and during the summer photosynthesis dominates whereas in the winter, decomposition dominates. As a result, CO2 concentration in the atmosphere decreases in the northern summer and increases in the northern winter.

Your question about using up all of the oxygen in the atmosphere by burning all fossil fuels is a good one. If indeed you could get hold of all of the carbon that, over the past 2 billion years, has been separated from the oxygen by photosynthesis, then you could do that. But the carbon is not accessible: only a small fraction is actually in the form of fossil fuels. A much larger part is simply dispersed in the form of organic matter in the sediments of the earth (both on land and in the sea), and it may not be concentrated enough to actually burn by itself. For example, soil has a significant fraction of carbon, but it does not usually burn. More carbon has been subducted in sediments through plate tectonics and now resides dozens or hundreds of kilometers down in the Earth mantle. (A fraction of it will eventually come back out as carbon dioxide in volcanic eruptions).

In other words, most of the carbon that has resulted from photosynthesis is no longer accessible for burning, and consequently not available to react with the atmospheric oxygen.

  • $\begingroup$ Do you mean just one thing, the limestone when speak about inaccessible sediments at the bed of the ocean? I do not understand why soil carbon is inaccessible. The soil of full of life, of bacteria. How can it be if they cannot burn the food? $\endgroup$ – Little Alien Jul 8 '16 at 7:03
  • $\begingroup$ I also learn the human logic a bit. It seems that other members of the community won't receive the answer if they do not hear about ongoing photosynthesys. People seem necessary to speak about they things they know very well about. When I say about O2 cycle's inability to close the ancient carbohydrates, I do not mean that there is not O2 cycle anymore. Why is it so necessary to pollute the matter with the kid stuff? I would not ask about burning the bodies of ancient plants if I did not realize the reality of O2 cycle. $\endgroup$ – Little Alien Jul 8 '16 at 7:03
  • $\begingroup$ Look at the Keeling curve yourself. It says that carbon burns naturally. You get increase of CO2 at autumn because the leaves burn, despite "we cannot do this because of low concentrations". The microorganisms can do this if they are lucky. Otherwise, the nature smoulders the carbohydrates itself. The reaction is spontaneous. I am not aware of the chemical phenomenon that says that there is a minimal amount of C and H that is subject to oxidation. All the H and C burns itself as long as the oxygen is available. Your curve proves that. $\endgroup$ – Little Alien Jul 8 '16 at 22:00
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    $\begingroup$ @LittleAlien -- no need to argue so forcefully. I'm happy to answer everything calmly. You ask about what kinds of sediments. Limestone may not be the best example because it's inorganic matter. But sandstones and in particular shales often have significant amounts of organic matter in them. $\endgroup$ – Wolfgang Bangerth Jul 28 '16 at 18:30
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    $\begingroup$ Regarding accessibility: for microorganisms to decompose the organic and convert the carbon in it again to CO2, you need to have 3 things in one place: organic matter, microorganisms, and oxygen. Even in sediments several hundred meters below the surface (or the sea floor), you typically have microorganisms that could do the job, but you don't have oxygen. The best microorganisms can do in these environments is, for example, to convert the carbon into methane. But, ultimately, most of the carbon that is not oxidized when buried deep, will remain inaccessible. $\endgroup$ – Wolfgang Bangerth Jul 28 '16 at 18:35

If I understand it right, you are assuming that in the beginning we had CO2, which was then split to organic carbon and O2 via photosynthesis. And now you are asking if it's possible to reverse all of that by burning all organic carbon, so that it consumes the O2.

Your assumption is not quite right. Oxygen is the most abundant element in Earth's crust and mantle, locked up in rocks. Rocks are made out of mostly oxygen. Atmospheric oxygen is negligible, when looked up in the context of the entire Earth. Oxygen is constantly being consumed and produced by reactions between the atmosphere and the solid Earth.

Not only that, there are processes in the Earth that take sediments and pull it down deep into the Earth, in subduction zones. Anything can be subducted: both organic unoxidised carbon and oxidised carbon in the form of carbonates (e.g. limestone). This goes down to tens to several hundreds of kilometres, far away from any interaction with the atmosphere.

As an example, let's say that you have a forest which that consumes a certain amount of CO2 and H2O, generating organic carbon-hydrogen and releasing the excess O2 to the atmosphere. Now you have carbon (and hydrogen) locked up in solids. Some of this organic material burns, decomposes, or otherwise reacts with atmospheric O2 to generate CO2 and H2O all over again. However, some of it is buried deep in the Earth's crust (that's how we get coal, gas and oil). So in theory, we can extract and mine everything, and then burn it. However, some of it is buried even deeper and is carried down by subduction to the Earth's mantle. In there, it is far away from our reach and there is no way we can extract it to burn it. So the oxygen generated while producing this organic material lingers in the atmosphere, but the organic material itself is deep below in the mantle, unable to react with any oxygen (as O2, any redox reactions are a different story).

To sum it up, your point of view is too simplistic as it does not take into account the Earth as a whole, but only top soil and atmosphere.

  • $\begingroup$ You overlook that I try to put the official dogma straight. From the very first lines it says that O2 that we enjoy today was produced by Photosynthetic prokaryotic organisms. Your statement that we have pure O2 all over the Earth crust (shadowing a cast of doubt upon the photosynthetic origins of the oxygen) contradicts to fact that we have got it in the atmosphere only 2 bln years ago, right with first photosysthesis. $\endgroup$ – Little Alien Jul 8 '16 at 9:11
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    $\begingroup$ There is no pure O2 in the Earth's crust. As I said, it's locked up in rocks. As to your question of where are the "coal and hydrogen", I already mentioned in the answer that it goes down in subduction zones. I will edit the answer to clarify it even further. $\endgroup$ – Gimelist Jul 8 '16 at 11:56
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    $\begingroup$ @LittleAlien -- Re your reading of that wikipedia article that "From the very first lines it says that O2 that we enjoy today was produced by photosynthetic prokaryotic organisms." That wikipedia article most certainly does not say that. Moreover, wikipedia is not the be all and end all. Some wikipedia articles are well written, others are not. Wikipedia is not the "official dogma." This question is based on your erroneous reading that photosynthesis has stopped. It has not. Look outside and you'll see plants with green leaves due to chlorophyl; they are photosynthesizing right now. $\endgroup$ – David Hammen Jul 8 '16 at 15:19
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    $\begingroup$ @LittleAlien no, I am not confusing CO2 and O2. To put it short, CO2 is being used up in photosynthesis to make organic carbon and O2. Then O2 goes to atmosphere, organic carbon gets subducted and is unavailable for burning. I'm done here. $\endgroup$ – Gimelist Jul 8 '16 at 19:46
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    $\begingroup$ @LittleAlien Organic carbon is buried as carbohydrates, not carbonate (limestone). Thus, leaving the excess oxygen in the atmosphere. $\endgroup$ – Gimelist Jul 8 '16 at 21:08

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