13C-depleted charcoal from C4 grasses and the role of occluded carbon in phytoliths

The δ13C values of plants and corresponding charred materials from wood, C3 and C4 grasses, derived from natural burning and laboratory combustion were obtained to determine whether there was a significant difference in δ13C of grass-derived char (C3 and C4) compared with wood-derived (C3) char. Our data showed that there is an up to 8‰ 13C-depletion in C4-derived chars from natural burning but there was no significant isotopic change in chars from wood or C3 grasses. We suggest that this 13C-depletion in C4-derived chars is due to protected organic matter in silicate structures (phytoliths), which were found to be depleted by up to 9‰. Analysis of this protected carbon by Py-GCMS indicated the presence of low relative amounts of n-alkanes. However, 13C-NMR data suggested that a significant portion of the phytolith-occluded material was composed of simple carbohydrates (O-alkyl carbon) and that alkyl carbon (lipid material) constituted a minor fraction. These isotopic and spectroscopic data have important implications for the calculation of the proportions of C3- versus C4-derived charred organic matter in modern as well as geological studies.