Isotopic fractionation between coexisting organic carbon—carbonate pairs in Precambrian sediments

δ¹³C_org and δ¹³C_carb values of 58 coexisting organic carbon-carbonate pairs covering the whole Precambrian have yielded means of ?24.7 ± 6.0%. PDB] and +0.9 ± 2.7%. PDB], respectively. Accordingly, isotopic fractionation between inorganic and organic carbon in Precambrian sediments is about the same as in geologically younger rocks (Δδ ? 25%.), a slight increase displayed by the Early Precambrian pairs (Δδ ? 28%.) being probably biassed by an over-representation in this age group of samples from one single locality (nevertheless, this value still lies within the range permitted for a possible deviation). It is reasonable to assume, therefore, that the overall isotope fractionation factor governing biological fixation of inorganic carbon has been virtually constant since some 3.3 × 10⁹ yr ago.