If it weren't for man oil, coal, and natural gas would just accumulate until eventually all atmospheric carbon became fixed underground?
By nature, not all of it stays underground. Oil and gas seep out and some of it is consumed by microbes. Coal is exposed by erosion and some if it burns in wildfires. But those processes don't happen at the same rate we dig it up.
Coal formation has been quite low through time, after the peak about 350 million years ago when land plants were getting big. Oil formation has been less variable, but I can't offer any specific studies about that without doing some looking. On geologic timescales, the major source of CO2 to the atmosphere is volcanic exhalation. The major sinks are silicate mineral weathering and limestone formation. Some of the secondary (carbon-containing) minerals formed are subducted as oceanic crust goes under continental crust. After a few hundreds of millions of years, the CO2 comes out of volcanoes again. The amounts of fossil (organic) carbon are large indeed, but much smaller than the inorganic forms mentioned above. Numbers of course are available, this is just the qualitative picture. Methane hydrates in the oceans are in the news again lately; a small amount compared to the geological (inorganic carbon) pools, but larger than fossil fuel stores. They are susceptible to rapid release to the atmosphere with sufficient oceanic warming. This type of sudden release apparently contributed to the Earth's largest extinction event 250 million years ago, and probably also to two other wild rides that the Earth has had in the geologic past.
Wouldn't the plant population of the planet be the major CO2 sink, since they aspirate CO2 and exhale O2?
Indeed, terrestrial plant photosynthesis and decomposition are quite well balanced at present. About 63 and 60 petagrams carbon per year, respectively. The difference is net uptake, and this contributes to why atmospheric CO2 is increasing only half as fast (5 Pg C/yr) as fossil fuel combustion (10 Pg C/yr).
It's amazing that multicellular life managed to hang on for five hundred fifty million years until we came along :_>
Doug, Was there a release of these Methane hydrates during the very warm period about 55 Mya? How exactly do we know how they played a role in the geologic past?
Tripp, marine methane degassing during PETM seems likely, but I have not read much that has been published about it. There are sevetral competing hypotheses as well, but frankly none of them describes a world where we would enjoy life much. Biogenic methane hydrates are very isotopically "light" (13/12 C about -60 per mille) and that is how people search for evidence of their release. The CH4 becaomes CO2 in the atmosphere and then gets incorporated into biogenic limestone and waits for somebody to look at it. The limestone is isotopically dated by other means. Mass spectrometry is a fabulous tool that has oopened these and many other doors. Conceptually they are not to tough to understand either. If you are comfortable with the idea that a light car can corner more sharply than a heavy one you are halfway there
