Chemical and carbon isotopic characteristics of ash and smoke derived from burning of C3 and C4 grasses

C4 and C3 grasses were subjected to burning in the laboratory to determine whether there was any significant fractionation of carbon isotopes between plant material and corresponding ash and smoke produced from burning. The results show that smoke produced from C4 grasses is generally depleted in 13C relative to the original plant, but the magnitude of the 13C depletion varies with species from <0.5‰ to a maximum of 7.2‰. Ash derived from C4 grasses is, on the other hand, either depleted (by 0.1–3.5‰) or slightly enriched (<1‰) in 13C relative to the original grass, depending on species. In contrast, both smoke and ash produced from C3 plants do not show any significant deviation in δ13C signature from that of the original plant material. Our data also show that the C isotope fractionation between ash and smoke and the original plant material depends not only on plant species and plant type but also on burning temperature. The weight percentage of C in ash and smoke decreases with increasing burning time in the temperature range 400–700 °C. Multi-elemental thermo analysis of ash, smoke and original plant material reveals distinctly different chemical characteristics for these materials. Ash is preferentially enriched in compounds with higher thermal stability whereas smoke contains a wide spectrum of compounds with different stability in comparison with the original plant material. C4 grass appears to be more thermally stable than C3 grass. The results have important implications for paleoecological or ecological studies based on 13C signatures of black carbon (BC) or charcoal.