Carbon isotope evidence for early life

The Letter by Mojzsis et al. on evidence for life on Earth before 3,800 million years ago was accompanied by a discussion of the effects of prograde thermal metamorphism on carbon isotope ratios. This discussion included an analysis of the effect a Rayleigh distillation process might have in reducing the ^(13)C/^(12)C ratio of organic carbon that is residual to oxidation during diagenesis and metamorphism. The calculations presented in that discussion (including Fig. 3, its caption and text on page 58 of ref. 1) contained errors that contributed to the conclusion that it would have been physically impossible for such a process to produce the observed δ^(13)C values of approximately -35‰ (with respect to the PDB standard) from initial abiotic values of -10‰. Here we correct those errors and discuss the significance of the correct calculation.