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 (C) 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 less than 0.5‰ to a maximum of 7.2‰. Ash derived from C₄ grasses, on the other hand, is either depleted (by 0.1 to 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 d¹³C 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 400-700⁰C. Multi-elemental thermo analysis of ash, smoke and original plant material reveal 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. These results have important implications for paleoecological or ecological studies based on ¹³C signatures of black C or charcoal.