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

Abstract

The δ 13C 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 δ 13C 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.