Since the pioneering work by Nier and Gulbransen (1939) it is known that the incorporation of inorganic carbon into living systems entails sizeable fractionations of the stable carbon isotopes. Consequently, it became firmly established that the observed bias in favour of the light isotope (12C) characteristic of biogenic substances derives, for the most part, from the isotope-discriminating properties of the principal carbon-fixing enzyme (ribulose-1,5-bisphosphate carboxylase) that is operative in the main chemosynthetic and photosynthetic pathways, channelling most of the carbon transfer from the nonliving to the living world. It is known, furthermore, that biological carbon isotope fractionations are basically retained when organic carbon is incorporated in sediments, the enzymatic isotope effect thus having been propagated into the rock section of the carbon cycle over 3.5, if not 3.8 Ga, of recorded Earth history.
Postulating a universality of biological principles in analogy to the proven universality of the laws of physics and chemistry, we may expect enzymatic reactions in exobiological systems to be beset with similar kinetic fractionation effects. Hence, the retrieval from the oldest Martian sediments of isotopic fractionations between reduced and oxidized (carbonate) carbon may substantially constrain current conjectures on the possible existence of former life on Mars.