Molecular composition of fossil charcoal and relationship with incomplete combustion of wood

Upper Triassic charcoal extracts were studied using gas chromatography-mass spectrometry (GC-MS) to recognize their molecular composition. Extractable compounds were divided into: (i) biomarkers, i.e. diagenetically changed primary wood components and (ii) products of combustion. Major compounds in the first group were: 1,2,5-trimethylnaphthalene and 1,2,5,6-tetramethylnaphthalene, cadalene, dehydroabietane, simonellite and retene. All of these are derived from resins. Moreover, propyl phenols, butyl acetophenones and pentyl acetophenones, as products of lignin breakdown, as well as fatty acids with a predominance of palmitic acid, typical constituents of wood, were also detected. Polycyclic aromatic hydrocarbons (PAHs), as well as ketones and aryl phenols, considered as high temperature combustion products, occurred at relatively low concentration in the samples due to their enhanced solubility in gelified, non-charred wood fragments, and vaporization of the major part of the burn products. Despite the low PAH concentrations, their distribution, with a significant contribution from typical pyrolytic compounds such as anthracene, 4H-cyclopenta[def]phenanthrene, benz[a]anthracene and benzo[a]pyrene was typical for rapid combustion. We propose to estimate paleo-wildfire temperature based on the PAH concentrations in the paleo-charcoal samples. The presence of thermally less stable organic compounds and low PAH abundances indicates a temperature < 400 °C. High PAH amounts seem to be characteristic for charring between 400 and 500 °C. Above these temperatures PAH concentrations again decrease, but less stable compounds are absent.