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On which time scale is the carbon stored in petroleum and black coal released back to the atmosphere under natural conditions? Exploiting fossil fuels and burning them releases the carbon today, but when would it have been released under natural conditions? Which processses does the coal go through on the carbon's long way back to atmosphere?

I have no idea of the orders of magnitude. Is it millions or billions of years? Being bound forever cannot be true, because nothing remains forever.

Primo Levi's chapter on Carbon didn't give me the answers I am looking for.

On the German Wikipedia page on the rock cycle, I read that the typical rock cycle takes approx. 200 million years. Is this the number I am looking for? Is this number reliable?

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  • $\begingroup$ 200 million years seems reasonable, but that's a nominal value, after all we have some examples of coals that have been dated to 400 million years, and oil formed in marine environments probably cycles much faster. $\endgroup$
    – Andy M
    Commented Apr 27, 2022 at 10:28
  • $\begingroup$ @AndyM. Thanks. When we have coal that is 400 million years old, it might take another 400 million years to come back to the surface and the atmosphere, so we deal with 800 million years, right? $\endgroup$
    – Hans-Peter Stricker
    Commented Apr 27, 2022 at 12:00

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What natural processes do you contemplate would result in fossilized carbon (coal, oil or gas) being returned to the atmosphere?

Judging from the picture below,

enter image description here

Some coal deposits are between 250 Ma & 325 Ma old. So this destroys the idea of 200 million year old rock cycle affecting fossilized carbon deposits. These deposits not being naturally depleted and won't be for a very long time, if ever.

Some of the oldest rock formations on Earth date back 3.8 Ga. Some parts of the world are geologically very inactive.

Some fossilized carbon may be returned to the atmosphere if they get caught up in a subduction trench.

Looking at the picture below, of the known coal deposits, globally, the deposits in Australia are possibly the most stable. Australia is in the middle of a tectonic plate & its coal & gas deposits are far from subduction zones. Similarly the deposits on the Eurasian land mass are also on very stable tectonic regions .

New Zealand and Indonesia are on the boundary of an active tectonic regions & their coal deposits might be affected by volcanic activity. Likewise some of the coal deposits of eastern Africa might be affected volcanic and tectonic activity associated with the East African Rift.

enter image description here

On the other hand, some coal seams that outcrop on the surface may be ignited by lightening that could result in a slow underground fire the burns for a long time. The oldest known underground coal fire is Australia's burning mountain. It is estimated to have been burning for 6000 years.

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  • $\begingroup$ I disagree with the view that because some coal is > 200 million years old that natural cycles can't be < 200 million years. Presumably much of the coal that was ever deposited has already been recycled, through tectonic processes and erosion. Once coal or oil is at the surface it will be oxidized - even without combustion - over geological timescales. $\endgroup$
    – Andy M
    Commented Apr 27, 2022 at 13:48

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