Example of the enrichment of carbon-13 in coals in the process of coalification

δ13 C measurements on a series of coals from the Mahakam delta (Kalimantan, Indonesia) show a progressive increase from ≈ ? 29.5% at the peat-lignite transition to ≈ ? 27.5% at a reflectance of R₀ ≈ 0.9. Given the very good homogeneity of the series, the shift is likely to be due to coalification. However, a coalification model based on this trend and the release of C0₂, H₂0 and CH₄ indicates δ13 C values of C0₂ which are very low as compared to currently accepted figures.