Where does Earth carbon come from?

D,Anton

The importance of the element carbon in planetary and geological processes has been sometimes underestimated. In the Solar System carbon is found in unoxidized form in rhe outer giant planets such as Jupiter, Saturn, Uranus and Neptune. In those planets it appears as methane, ethane and other hydrocarbons. In Venus and Mars carbon is oxidized into CO² representing above 95% of the atmospheric masses. In the Earth, carbon is found in unoxidized forms (coal, crude oil, natural gas, etc) and in oxidized forms as CO² in the atmosphere (400 ppm) and as carbonates in limestones and other calcaire geological formations.

The question thar arises is the following: Where does the carbon on earth come from? The answers are several but two possibilities are prevalent. It may be an original compound from the moments in which the planet.was formed or resulted from the ascent of methane (and other hydrocarbons) through the crust. The first theory relates to the biogenic theory of petroleum formation and the second to the abiotic hypothesis. Thomas Gold was a energetic supporter of the second theory. Here we reproduce his views on the carbon cycle on earth.

Clues about Earth carbon in the carbonate record

T.Gold

"The surface and subsurface sediments on the earth contan approximately one hundred timesas much of the element carbon as would have derived from the grinding up of the basementrocksthat contributed to the sediments. This surface is thus enormoously enriched in carbon. This enrichment requires an explanatiion.

The total quantity of carbon contained in the sediments and on the surface isestimated to average about 200 tons per each square meter of the earth surface area.

One-fifth of all this carbon is in unoxidized form, including various forms of coal, crude ols, kerogen (carbonaceous compounds diffusely distributed in the rocks), and natural gas, either as free gas or in the form of methane hydrate ices. In addition, there is the thin veneer of living and not-yet-decomposed biological material. This latter category- in my opinion the only demonstrably biological component-represents only a very small fraction of the total unoxidized carbon.

The other four-fifthof the carbon is the oxidized form, mostly limestone (calcium carbonate), and dolomite (a blend of calcium and magnesium carbonate). Much of this carbonate was deposited in oceansm, having derived the carbon from the atmospheeric-oceanic pool of carbon dioxide. Carbonate precippitates naturally out of the water column from dissolved carbon dioxide and calcium and magnesium oxides. It can also be precipitated out of the water biologically, by organisms that build carbonate shells or skeletons."

Reproduced from "The Deep Hot Biosphere", Thomas Gold,  2001.
More information in "Inexhaustible? Petroleum and Natural Gas",  D.Anton, Piriguazu Ediciones