The isotopic fractionation of carbon in geological processes

Stability of the isotopic composition of carbon in endogenic terrestrial substances, as well as in meteorites, consistently lower in C¹³ than the biogenic marine carbonates, suggests both presence and stability of a certain zone under the crust of the earth in which the C systems are maintained at certain equilibria, at different levels, typical of certain geological processes operative therein. Isotopic exchanges and recurrent fractionations of the C isotopes, in the course of the migrations of carbon, are indicated by the available evidence, the net result of which is an impoverishment of C¹³ in living substance and in its derivatives (oils, coals, etc.), and its enrichment in the biogenic residual carbonates. – IGR Staff.