Molecular composition of fossil charcoal and relationship with incomplete combustion of wood Leszek Marynowski a,⇑ , Rafał Kubik a,b , Dieter Uhl c , Bernd R.T. Simoneit d a Faculty of Earth Sciences, University of Silesia, Be ˛dzin ´ska Str. 60, 41-200 Sosnowiec, Poland b Wrocław Research Centre EIT + Ltd., Stabłowicka 147, 54-066 Wrocław, Poland c Senckenberg Forschungsinstitut und Naturmuseum Frankfurt, Senckenberganlage 25, 60325 Frankfurt am Main, Germany d Department of Chemistry, Oregon State University, Corvallis, OR 97331, USA article info Article history: Received 27 April 2014 Received in revised form 10 August 2014 Accepted 4 September 2014 Available online 16 September 2014 Keywords: Charcoals PAHs Biomarkers Incomplete combustion Wood Temperature of wildfires abstract 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. dia- genetically 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, dehydroa- bietane, simonellite and retene. All of these are derived from resins. Moreover, propyl phenols, butyl ace- tophenones 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 geli- fied, 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 com- pounds 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 com- pounds and low PAH abundances indicates a temperature < 400 °C. High PAH amounts seem to be char- acteristic for charring between 400 and 500 °C. Above these temperatures PAH concentrations again decrease, but less stable compounds are absent. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Fossil charcoal is a common constituent of sedimentary rocks as old as the Upper Silurian (Glasspool et al., 2004) and, due to its spe- cific origin, is a useful proxy for wildfires that occurred on land (Scott, 2000, 2010). The occurrence of charcoal is usually closely connected with characteristic distributions of aromatic com- pounds, with a significant predominance of unsubstituted, three- to seven-ring polycyclic aromatic hydrocarbons (PAHs) in the host sediment (e.g. Venkatesan and Dahl, 1989; Killops and Massoud, 1992; Finkelstein et al., 2005; Marynowski and Simoneit, 2009; Scott et al., 2010; Marynowski et al., 2011a; Denis et al., 2012). These PAHs formed during high temperature incomplete combus- tion of biomass and/or fossil fuel (e.g. Simoneit, 2002; Alves et al., 2011) and their characteristic distribution can survive for hundreds of millions of years in sedimentary rocks as indicators of paleowildfires and, in some exceptional cases, other pyrolytic processes (e.g. Simoneit and Fetzer, 1996; Rospondek et al., 2009). The most characteristic feature of their distribution, when formed during combustion, is a high predominance of unsubstitut- ed PAHs vs. methyl derivatives and high concentration of specific compounds like anthracene, benz[a]anthracene and benzo[a]pyr- ene, which are generated almost exclusively during pyrolytic con- ditions (Killops and Massoud, 1992; Simoneit, 2002; Marynowski and Simoneit, 2009). It would be expected that PAHs and other organic compounds could be adsorbed predominantly on the extensive surface of charcoal fragments, especially in charcoal- bearing coarse grained sediments, but results based on paleo-char- coal are lacking. However, recent studies of biochar show diverse, and in some cases significant, concentrations of PAHs associated with charred residues produced by incomplete combustion of wood (e.g. Keiluweit et al., 2012; Hale et al., 2012; Hilber et al., 2012). Therefore, based on the PAH concentrations and http://dx.doi.org/10.1016/j.orggeochem.2014.09.003 0146-6380/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: leszek.marynowski@us.edu.pl (L. Marynowski). Organic Geochemistry 77 (2014) 22–31 Contents lists available at ScienceDirect Organic Geochemistry journal homepage: www.elsevier.com/locate/orggeochem