# Where did all the CO₂ come from that was deposited as coal during the Carboniferous?

During the Carboniferous, vast deposits of coal (or peat at that time) were created because – as far as I understand – decomposition of wood (white rot?) was not yet "invented" at that time. This means CO2 and finally carbon was being removed from the atmosphere by photosynthesis and hence removed from the carbon cycle.

This must have led to a depletion of CO2 in the Carboniferous atmosphere because the deposition of peat lasted for quite some time. And even today with wood decomposition, in the north summer the levels of CO2 are dropping as the Keeling curve is showing.

Was there enough CO2 in the atmosphere to support peat deposition over such a long time? Or was there some refuel mechanism like vulcanism or some other process(es)?

• the carbon stored in oil-coal-gass-biological matter is only a tiny fraction of our planets total carbon en.wikipedia.org/wiki/Carbon_cycle Feb 21 '20 at 14:12
• Yes, but is must have been "piped" through the atmosphere, because the wood, peat, coal was built from atmospheric CO₂+light by organisms (and maybe other stuff like water, that has been removed later, similar to the oxygen). Feb 21 '20 at 14:21
• this is related earthscience.stackexchange.com/questions/10114/… Feb 21 '20 at 16:29
• The original source of most of the CO2 in the atmosphere was volcanoes. As the answers in the links in other comments state, the amount of CO2 in the atmosphere then was 3000 ppm.
– Fred
Feb 21 '20 at 19:42
• You also have to think about the immense amounts of C/C02 that is stored in rocks like limestone & chalk (calcium carbonate: CaCO3) and other carbonate minerals. (And if emacs drives you nuts, why use it? There are far better editors out there.) Feb 22 '20 at 4:45

Prior to the carboniferous period, and until the human industrial era, the majority of carbon dioxide in the atmosphere was due to volcanoes.

For billions of years, geological processes like volcanic eruptions controlled the carbon concentration in the atmosphere, as volcanism is the major way that carbon rises from the mantle into the atmosphere. Most of the carbon stored in the mantle is in the form of carbonate (a salt of carbonic acid), but there are also huge stores of actual carbon dioxide sequestered deep within the mantle as a dissolved gas within the liquid rock.

We can measure the degassing of Mt. Etna extremely well, and find that it adds about 16,000 tons of CO2 to the atmosphere each day, or 5.8 million tons per year. This might not sound impressive, but it's only one volcano. Interestingly, it's not just smoking, actively erupting volcanoes that emit CO2, but seemingly inactive volcanoes around arcs and rift zones. Persistent degassing still occurs around a great number of volcanoes worldwide, on a continuous basis.

"Each part per million by volume of $$CO_2$$ in the atmosphere represents approximately 2.13 gigatonnes of carbon". Per a_donda's comment,"During the carboniferous, atmospheric $$CO_2$$ sank from ~1500 to ~250ppm", so, about 2.7 trillion tonnes. "There are an estimated 1.1 trillion tonnes of proven coal reserves worldwide." The latter number does not include coal already burnt, so the quantities are comparable.

However, there are other contributions:
1) Dissolved $$CO_2$$ would come out of the oceans.
2) Volcanos emit $$CO_2$$ over millions of years.
3) Coal "reserves" is defined as what can be mined. Unmineable quantities may be 20 times greater.
...

Starting from the beginning, the universe started with an explosion which generated, as the initial plasma cooled, mainly hydrogen and helium (The creation of hydrogen and helium).

The heavier elements such as carbon and oxygen were created later, as the first generation of stars collapsed and exploded in supernovae. It is thought that our Sun is a 3rd generation star (see "What is meant by the theory that the sun is a third generation star?"). Hence, we can say that the presence of carbon and oxygen in the solar system is caused by the stellar explosions which came before the formation of our Sun.

When the planets formed around the Sun, carbon and oxygen were part of the mix of elements that came together.

The processes leading to the formation of the early atmosphere, before life started on Earth are unclear, though it is certain that there were frequent meteor impacts and volcanic eruptions in this period (because the solar system was much more crowded and the Earth's core was much hotter -- see for example IFL Science).

If we look at nearby planets, we see that Mars has an atmosphere of 95% carbon dioxide, and Venus has 96%. On Earth it makes up only 0.04% of our atmosphere (Atmosphere of the Earth). The difference is due to life on Earth, which has taken CO2 out of the atmosphere.

Coal deposits were laid down in the Carboniferous, as stated in the question. At the time, atmospheric CO2 was at much higher concentrations, so the growth may have been denser, which could (I don't have a reference for this) have contributed to the fact that material got buried before it could rot (see "The Age of Oxygen" for some discussion).

The CO2 which formed the Carboniferous coal deposits came from the atmosphere. That's the only place it could have come from. Wood rotting organisms were in existence at the time, but the various vegetable matter which formed coal, mainly wood, formed a kind of peat which preserved it in anoxic and slightly acidic conditions. This is why you get well preserved human bodies thousands of years old in European peat bogs. Tollund Man, from Denmark, and Pete Marsh, from England, are well known examples.

This peat layer from dead trees built up over many centuries to form a peat marsh many metres thick. This was then buried and compressed by sediments and under great pressure over millions of years became coal. Coal and hydrocarbons were not the only things that devoured CO2 in vast quantities. The carbonate rocks, mainly calcium carbonate (limestone) did so too. So yes, there was enough CO2 in the atmosphere to support peat deposition over millions of years, with plenty to spare for limestone deposition as well.

• Michael, you need to stop making off-the-cuff posts that are in serious need of citations. The OP presumably knows that the immediate source of the carbon in the Carboniferous coal deposits was the Earth's atmosphere. In your first two sentences you are effectively implying that there was little or no vulcanism throughout the Carboniferous. Citation needed. The third sentence is an even more serious case of citation needed. (continued) Feb 21 '20 at 23:15
• Until recently, it was widely thought that one of the key reasons for the extensive Carboniferous coal fields was an evolutionary lag between the ability of plants to create wood and the ability of fungi / etc to consume wood. Note that I wrote "until recently". That's a hint to find a citation and incorporate it into your answer. Feb 21 '20 at 23:16
• The latter part of your first paragraph and all of the second read like a just-so story. Some citations would help here. Since you made so many different claims in this answer, you need a very large number of citations to back them up. Feb 21 '20 at 23:24
• all parts of this question is answered in the linked Q&A by myself and others with sources where one can find additional information. Feb 22 '20 at 6:44
• I don't know. Your answers are generally what I would expect from a first year undergraduate. They may be correct, but as you haven't cited any supporting evidence, it's hard to give you the benefit of the doubt. I have to conclude that either 1) you can't be bothered to find the evidence, or 2) you don't actually know what good evidence would be, and you're winging it.
– Will
Feb 23 '20 at 10:17