Biological modulation of the terrestrial carbon cycle: Isotope clues to early organic evolution

The characteristic difference of some 20 to 30 ‰ in the ¹³C/¹²C rations of reduced (organic) and oxidized (carbonate) carbon as observed in the contemporary environment can be traced back over 3.5 (if not 3.8) Ga of recorded Earth history. There is little doubt that the enrichment of isotopically light carbon in fossil organic matter ultimately derives from the bias in favor of ¹²C during photosynthetic carbon fixation (notably the RuBP carboxylase reaction of the Calvin cycle). Accordingly, the mainstream of the sedimentary δ ¹³C_org record can be most reasonably explained as geochemical evidence of the isotope-discriminating properties of the principal CO₂-fixing enzymatic reaction of the assimilatory pathway, thus giving eloquent testimony to an extreme degree of evolutionary conservatism in the biochemistry of autotrophic carbon fixation.