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

Abstract C 4 and C 3 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 C 4 grasses is generally depleted in 13 C relative to the original plant, but the magnitude of the 13 C depletion varies with species from 4 grasses is, on the other hand, either depleted (by 0.1–3.5‰) or slightly enriched ( 13 C relative to the original grass, depending on species. In contrast, both smoke and ash produced from C 3 plants do not show any significant deviation in δ 13 C 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. C 4 grass appears to be more thermally stable than C 3 grass. The results have important implications for paleoecological or ecological studies based on 13 C signatures of black carbon (BC) or charcoal.