I'm surmising that it all came from the air at one time or another. From a simplistic notion, hydrocarbons are a byproduct of photosynthetic respiration where C02+H20--->sugars and thus hydrocarbons, no? So all the fossil fuel carbon that is buried in the earth came originally from the atmosphere. Why is this assertion right or wrong? Anyone in the know, know?
That is correct. Here is the carbon cycle: Interestingly the big arrow to the right are volcanoes, which are the only way C02 can go back to the atmosphere from marine sediments.
It came from exploding stars, as did all of the heavier elements. Carbon isn't formed by anything on the earth, so all of the carbon came from space, and it just gets recycled around into different forms. Tom
As you implied, "fossil fuels" generally come from plants. The plants live in environments where CO2 is present, either in the air or dissolved in water. The plants take up the CO2 through photosynthesis. When the plants die, they are gradually covered with sediment, and eventually end up buried deep beneath the surface, where the pressure and heat eventually converts carbohydrates to hydrocarbons, etc.
The wikipedia article on this is quite informative: http://en.wikipedia.org/wiki/Petroleum Atmospheric CO2 concentrations decreased markedly during the Carboniferous, which was about when the majority of coal and oil deposits were formed http://en.wikipedia.org/wiki/Carbon_dioxid...ion_in_the_past I once did a back of the envelope calculation that the earth's fossil carbon stores were much more than that removed from the atmosphere (back then). This was either a math error, or suggestive that additional CO2 was injected into the atmosphere by volcanoes during that 'draw-down' of atmospheric CO2.
"Exploding stars" may sound like a little bit of an obtuse answer, but the OP's notion of "origin" was framed in an open-ended way and so it really is the correct answer. Elements cannot be changed from one to another except by nuclear reactions, and until very recently, no biological organism was capable of instigating nuclear fission or fusion, so that rules out biology as a source for elemental carbon. Yes, biology is responsible for combining some carbon with oxygen and putting that in the atmosphere as CO2. Some biology is also responsible for taking CO2 from the atmosphere and storing the carbon, while releasing the oxygen, but biological and atmospheric carbon are only 2 of a large number of places that carbon is hidden on our planet, so I don't think it's possible that all carbon from all fossils was ever in the atmosphere at one point in Earth's history. Even if it was all in the atmosphere at once in the form of CO2, I don't think that would be a good argument in favor of releasing all fossil carbon BACK into the atmosphere
Volcanoes will have been the original source but CO2 is also released to atmosphere through the action of acid rain on limestone, now that we have limestone. The CO2 has been a part of our atmosphere longer than O2, it was the cyanobacteria and then the plants that split the carbon out of the atmosphere and so boosted oxygen from a few % to the current levels. That was a big event in Earth history, more important than the extinction of the dinosaurs (all respect to them). So the coal and oil get the carbon from plants and animals, they get it from the atmosphere and the atmosphere got it from the rocks, and they came from asteroids and they in turn from exploding stars. It all took quiet a few years.
