Carbon Isotope Ratios–some inconvenient facts about fossil fuels

There is a general belief that the measured increase in the amount of the lighter Carbon isotope C12 in the earth’s atmosphere is due to the burning of carbon from fossil fuels, petroleum and coal. Hydrocarbons are believed to be accumulations of fossils, whether floral or faunal based on the scientific fact that the biosphere preferentially extracts C12 out of the atmosphere and buries it as petroleum or coal. Hydrocarbons are assumed biogenic because the C12/C13 ratio is high, and that the increase of this ratio in the atmosphere proves that it’s due to humans burning oil and coal.

Unfortunately the belief that the C12/C13 isotope ratio can distinguish between biogenic carbon and non biogenic is plainly wrong.

Experiments involving the passage of methane through a column of rock representative of the earth’s crust showed unequivocally that methane becomes progressively enriched in C12 as it passes through the column.

Columbo, Gazzarini, and Gonfiantini demonstrated conclusively, by a simple experiment the results of which admitted no ambiguity, that the carbon isotope ratios of methane change continuously along its transport path, becoming progressively lighter with distance traveled. Colombo et al. took a sample of natural gas and passed it through a column of crushed rock, chosen to resemble as closely as possible the terrestrial environment.2 Their results were definitive: The greater the distance of rock through which the sample of methane passes, the lighter becomes its carbon isotope ratio.

The reason for the result observed by Colombo et al. is straightforward: there is a slight preference for the heavier isotope of carbon to react chemically with the rock through which the gas passes. Therefore, the greater the transit distance through the rock, the lighter becomes the carbon isotope ratio, as the heavier is preferentially removed by chemical reaction along the transport path. This result is not surprising; contrarily, and is entirely consistent with the fundamental requirements of quantum mechanics and kinetic theory.

Pertinent to the matter of any claim that a light carbon isotope ratio might be indicative of a biological origin, the results demonstrated by Colombo et al. establish that such a claim is insupportable. Methane which might have originated from carbon material from the remains of a carbonaceous meteorite in the mantle of the Earth, and possessing initially a heavy carbon isotope ratio, would have that ratio diminished, along the path of its transit into the crust of the Earth, to a value comparable to common biological material. Source

But because of the aprioristic belief that coal and oil are buried recycled biosphere mass, and thus associated with the very high C12/C13 ratios as measured, that any increase in atmospheric C12 is due to burning of recycled biomass cannot be supported by invoking C12/C13 ratios as proof.

Put simply, C12 enriched hydrocarbons, whether oil or coal, cannot be deduced to have been derived from biological material simply on the value of the C12/C13 isotope ratio.

High C12/C13 ratios do not imply a biogenic origin for the carbon compound.

About Louis Hissink

Retired exploration geologist
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