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

Organic Geochemistry 77 (2014) 22–31

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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

´ ska Str. 60, 41-200 Sosnowiec, Poland Faculty of Earth Sciences, University of Silesia, Be˛dzin 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 b

a r t i c l e

i n f o

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

a b s t r a c t 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. 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 specific 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 compounds, with a significant predominance of unsubstituted, threeto 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 combustion of biomass and/or fossil fuel (e.g. Simoneit, 2002; Alves et al., 2011) and their characteristic distribution can survive for

⇑ Corresponding author. E-mail address: [email protected] (L. Marynowski). http://dx.doi.org/10.1016/j.orggeochem.2014.09.003 0146-6380/Ó 2014 Elsevier Ltd. All rights reserved.

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 unsubstituted PAHs vs. methyl derivatives and high concentration of specific compounds like anthracene, benz[a]anthracene and benzo[a]pyrene, which are generated almost exclusively during pyrolytic conditions (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 charcoalbearing coarse grained sediments, but results based on paleo-charcoal 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

L. Marynowski et al. / Organic Geochemistry 77 (2014) 22–31

composition of other organic compounds, it seems possible to estimate the temperature of paleo-wood burning and indirectly determine the type of wildfire (Scott, 2000). Up to now, the only method for paleo-wildfire temperature determination is fusinite reflectance measurement (see Scott, 2010 for review), but in some cases this method appears to be inaccurate (Kubik et al., in press). Here, we report the molecular composition of extracts from large fossil charcoal fragments from Upper Triassic clays and muds, to try to explain the complex factors which determine the occurrence and distribution of organic compounds in sedimentary rocks, especially their connection with ancient pyrolytic processes. This type of material has never been analyzed for the potential occurrence and composition of organic compounds, possibly due to the scarce occurrence of large, well preserved charcoal remnants in sedimentary rocks. Moreover, based on comparison of PAH concentrations from paleo-charcoals with biochars produced experimentally at specific heat treatment temperatures (Keiluweit et al., 2012), we suggest the possibility of using the method for paleo-wildfire temperature estimation.

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more details about the geology and paleontology of the formation refer also to Gruszka and Zielin´ski (2008), Dzik et al. (2008) and Sulej et al. (2012). 2.2. Methods 2.2.1. Extraction and separation The samples were crushed to 1–2 cm pieces which were ground after macroscopically identifiable jet-like fragments were removed. The fine samples were Soxhlet extracted for 72 h in pre-extracted cellulose thimbles with dichloromethane (DCM)/ MeOH) (80:20, v:v). Each extract was concentrated and separated into aliphatic, aromatic and polar fractions via modified column chromatography (Bastow et al., 2007). The silica gel had been activated at 110 °C for 24 h and was then added to a Pasteur pipette. The eluents for collection of the fractions were: (i) n-pentane (aliphatic), (ii) n-pentane and DCM (7:3 – aromatic), and (iii) DCM/ MeOH (1:1 – polar). All three fractions were analyzed using gas chromatography-mass spectrometry (GC–MS). 2.2.2. GC–MS GC–MS was carried out with an Agilent Technologies 6890 series II gas chromatograph and Agilent 5973 Network mass spectrometer. Two types of columns were used:

2. Material and methods 2.1. Samples Charcoal samples were collected from two Upper Triassic sites in the Zawiercie area (S. Poland; Fig. 1): Pore˛ba (POR – 7 samples) and Zawiercie-Marciszów (ZAW – 1 sample). They were partially rounded, ca. 4–15 cm in diameter and present as such in clays/ mudstones. They were mostly permineralized by calcite and much less by pyrite, marcasite, clay minerals and barite (Fig. 2). All the samples were macro- and microscopically similar, and differences according to extent and type of mineralization, as well as preservation, were insignificant. Only part of the PORHE sample was composed of non-charred gelified organic matter (OM; Fig. 2a). All samples were assigned to three different gymnosperm morphotypes (Kubik et al., in press). The host rocks of the charcoals are Upper Triassic (Norian) bone-bearing marly mudstones and sandstones called the Patoka member, which belongs to the recently defined Grabowa Formation (Szulc and Racki, in press). The maximal thickness of the Patoka member is 300 m and represents fluvial and pedogenic facies, as described by Szulc et al., (2006). For

(i) Agilent J&W HP5-MS (60 m  0.32 mm  0.25 lm) with 95% polymethylsiloxane and 5% diphenylsiloxane. The GC oven program was: 40 °C (1 min) to 120 °C at 20 °C/min and then to 300 °C (held 35 min) at 3 °C/min (for aliphatic fraction). (ii) Agilent J&W DB35-MS (60 m  0.25 mm  0.25 lm) with 35% polymethylsiloxane and 65% diphenylsiloxane. The program was: 50 °C (1 min) to 120 °C at 20 °C/min and then to 300 °C (held 45 min) at 3 °C/min (for aromatic and polar fractions). The GC column outlet was connected directly to the ion source of the mass spectrometer. Spectra were recorded from m/z 45–550 (0–40 min) and m/z 50–700 da (> 40 min). The mass spectrometer was operated in the electron impact mode (ionization energy 70 eV). An Agilent Technologies Enhanced ChemStation (G1701CA ver. C.00.00) and the Wiley Registry of Mass Spectral

Fig. 1. Location of the sampling area, showing Pore˛ba and Zawiercie-Marciszów sites.

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L. Marynowski et al. / Organic Geochemistry 77 (2014) 22–31

Fig. 2. (a) Macroscopic picture of charcoal (PORHE) with the visible boundary between the unburned, gelified and charred parts, (b) charcoal with a visible knot in the central part (indicated with the arrow, PORHF), (c) scanning electron photomicrograph of a tangential section of well preserved, partially mineralized charcoal (PORHA) and (d) enlarged image. C, calcite; P, pyrite. Black cells represent empty channels.

Data (9th ed.) software were used for data collection and spectral processing. The concentration of the selected aromatic biomarkers and PAHs was calculated by comparison of the peak area for an internal standard (9-phenylindene) with those of the individual compounds obtained from the total ion current (TIC) chromatograms. Peak assignment was carried out by comparison of retention times with standards and by interpretation of fragmentation patterns of mass spectra. 2.2.3. Total organic carbon (TOC) Total carbon (TC) conent and total inorganic carbon (TIC) content were determined using an Eltra CS-500 IR-analyzer with a TIC module. TOC was calculated as the difference between TC and TIC. Calibration was by means of the Eltra standards. 2.2.4. Scanning electron microscopy (SEM) A Quanta 650 FEG microscope (15 kV tension and high vacuum), housed at the Wrocław Research Centre EIT + Ltd., was used for charcoal observation. 3. Results and discussion 3.1. Fractionation of extractable OM The polar fraction clearly dominated in the samples from Zawiercie-Marciszów and Pore˛ba. The sum of the aliphatic and aromatic compounds ranged only from 20% to 40%. The content (%) of the fractions from the charcoal extracts are presented in Table 1, which shows the predominance of the polar fraction. 3.2. n-Alkanes and isoprenoids Short chain n-alkanes with a maximum at C18 or C19 dominated in the samples from Zawiercie-Marciszów and Pore˛ba (Fig. 3). A similar distribution with a dominance of short chain homologues was noted for the Sołtyków charcoal-bearing sandstones, sandstones and shales from the Podole outcrop, but in some sequences (e.g. Gromadzice) long chain n-alkanes dominated (Marynowski

and Simoneit, 2009). However, when taking the distributions of long chain n-alkanes into account, an odd predominance (especially C25, C29, C31) was observed, resulting in carbon preference index (CPI) and CPI(25–31) values > 1.3 (Table 1). This demonstrates the minor contribution of OM from higher plant wax, characteristic of needles from gymnosperms and leaves of other plants (e.g. Peters et al., 2005). The pristane (Pr)/n-C17 and phytane (Ph)/n-C18 ratios, except for two cases, had values < 1 (Table 1), indicating a higher proportion of n-alkanes than isoprenoids (Fig. 3). Hopanes and steranes were present in very low amount in all the charcoal samples. 3.3. Aromatic biomarkers The most abundant compounds in the aromatic fractions (Fig. 4, Table 2) of the charcoal extracts were generally 1,2,5-trimethylnaphthalene (1,2,5-TMN) and 1,2,5,6-tetramethylnaphthalene (1,2,5,6-TeMN). Both are treated as terrestrial biomarkers (Püttmann and Villar, 1987; Strachan et al., 1988; Alexander et al., 1993; Radke et al., 1994; van Aarssen et al., 1999), but their origin may potentially also be connected with bacteria, because of their alternate formation from monoaromatic seco-hopanoids (Püttmann and Villar, 1987; Killops, 1991). Their concentration varied among the samples (1,2,5-TMN 0.04 to 25.5 lg/g TOC and 1,2,5,6-TeMN 0.04 to 10.9 lg/g TOC), but in most cases they were the dominant compounds in the aromatic fraction (Table 2). In this case, an origin for 1,2,5-TMN and 1,2,5,6-TeMN from bacteria seems less likely, because hopanes were trace constituents of the aliphatic fractions, and there was no other molecular evidence for extensive bacterial activity. Cadalene, another two-ring aromatic biomarker, was also present in all the charcoal samples, but its concentrations were lower than 1,2,5-TMN and 1,2,5,6-TeMN (0.02–1.41 lg/g TOC, Table 2). Other biomarkers associated with OM of terrestrial origin and found in the samples were dehydroabietane, simonellite and retene. They were present at relatively low concentration (0.01–0.04 lg/g TOC), with retene as the major compound of the three. The exception was PORHG, which was dominated by simonellite

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L. Marynowski et al. / Organic Geochemistry 77 (2014) 22–31 Table 1 Amount (%) of organic fractions and parameters based on n-alkane and isoprenoid distributions. Sample

PORHA PORHB PORHC PORHD PORHE PORHF PORHG ZAWHA a b c d

FRACTION (%) AL

AR

POL

4 26 9 27 19 10 23 25

29 23 21 22 21 25 10 17

67 51 70 51 60 65 67 58

CPIa

CPI(25–31)b

Pr/Phc

Pr/n-C17

Ph/n-C18

SCh/LChd

1.72 1.57 1.45 1.46 1.41 1.52 1.58 1.47

1.35 1.60 1.59 1.55 1.49 1.48 1.35 1.60

0.54 0.72 0.59 0.59 0.64 0.72 0.75 0.78

0.78 0.74 0.98 0.76 0.76 0.82 0.81 0.75

1.10 0.80 1.32 0.87 0.83 0.88 0.83 0.74

16.76 25.78 8.48 15.97 14.25 13.58 10.50 13.21

Carbon preference index. (nC25 + nC27 + nC29) + (nC27 + nC29 + nC31)/2(nC26 + nC28 + nC30). Pristane/phytane. Short chain/long chain n-alkanes, (nC17 + nC18 + nC19)/(nC27 + nC28 + nC29).

Fig. 3. Distribution of aliphatic compounds from the Upper Triassic charcoal (sample PORHG).

Fig. 4. Total ion current (TIC) trace of the aromatic fraction from sample PORHE, showing the high concentrations of 1,2,5-TMN and 1,2,5,6-TeMN vs. high molecular weight PAHs. TMN, trimethyl naphthalene; TeMN, tetramethylnaphthalene; MP, methyl phenanthrenes; MA, methyl anthracenes; IS, internal standard (DB-35MS GC column).

(58.9%, Table 3). Biomolecules such as ferruginol or sugiol, characteristic for coniferous plants of the Cupressaceae, Podocarpaceae and Araucariaceae, are probably the main source of these compounds in the charcoal (Otto and Wilde, 2001; Otto and Simoneit, 2001; Marynowski et al., 2007), but these precursors are relatively less stable and should not be present. The alternative source of retene and simonellite could be phyllocladane (Otto and Simoneit, 2001), but this was not found in the charcoal.

All samples with the exception of ZAWHA contained perylene (Fig. 4), a biomarker for wood degrading fungi (e.g. Grice et al., 2009; Itoh et al., 2012; Marynowski et al., 2013). The concentration was extremely low (< 0.005 lg/g TOC), which is connected with the oxidative conditions during combustion and subsequent degradation (Marynowski et al., 2011b), rather than lack of wood decay. A number of the samples also exhibited charred fungal hyphae on tracheid walls (Kubik et al., in press), indicating dried-out wood

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L. Marynowski et al. / Organic Geochemistry 77 (2014) 22–31

Table 2 Total organic carbon (TOC) content and concentration of selected aromatic hydrocarbonsa (lg/g TOCb and ng/g of charcoalc). Sample

TOC (%)

1,2,5TMN

1,2,5,6TeMN

Cad

Ph

Antr

Fl

Py

BaA

Chr & Triph

BeP

BbFl

BaP

B[ghi]Pe

Cor

Pe

PAHs 1

PAHs 2

PORHA

23.87

25.5b 20253c

10.9 8706

1.19 950

1.56 1238

0.41 328

0.87 693

1.66 1322

0.14 109

0.07 58

0.03 24

0.05 36

0.02 17

0.01 8

0.002 2

0.005 3

0.24 231

2.62 2525

PORHB

21.17

1.91 1349

0.84 595

0.10 71

0.31 223

0.08 58

0.09 64

0.14 99

0.02 15

0.02 12

0.01 7

0.01 5

0.01 5

0.005 3

0.001 0.5

0.001 1

0.05 32

0.26 172

PORHC

29.77

18.3 18112

6.75 6694

1.41 1478

0.86 853

0.22 217

0.81 799

1.37 1359

0.14 140

0.05 52

0.02 19

0.05 46

0.02 24

0.01 11

0.002 2

0.002 2

0.23 344

2.27 3310

PORHD

25.38

0.50 493

1.10 1074

0.03 29

0.23 223

0.06 56

0.15 151

0.19 187

0.01 13

0.02 19

0.01 13

0.01 13

0.01 5

0.01 10

0.001 1

0.001 1

0.04 45

0.37 361

PORHE

28.39

PORHF

26.71

1.76 1895 6.70 6673

0.77 827 2.66 2653

0.10 107 0.56 559

0.34 362 0.47 473

0.10 111 0.14 138

0.16 168 0.32 322

0.22 237 0.61 604

0.03 35 0.07 65

0.02 26 0.03 34

0.01 16 0.02 19

0.01 16 0.03 27

0.01 16 0.02 16

0.01 9 0.01 14

0.002 3 0.003 2

0.002 3 0.003 2

0.07 86 0.14 129

0.43 433 0.99 970

PORHG

22.63

0.89 777

0.64 556

0.15 130

0.88 765

0.21 186

0.30 260

0.38 331

0.02 14

0.01 10

0.02 14

0.01 11

0.01 3

0.01 12

0.001 1

0.001 1

0.05 43

0.71 615

ZAWHA

18.55

0.04 25

0.04 26

0.02 13

0.18 120

0.02 16

0.05 33

0.05 31

0.004 2

0.01 4

0.002 1

0.002 1

0.001 0.2

0.001 0.1

0.001 0.1

0.0001 0.1

0.01 5

0.11 65

a TMN, trimethylnaphthalene; TeMN, tetramethylnaphthalene; Cad, cadalene; Ph, phenanthrene; Fl, fluoranthene; Py, pyrene; BaA, benz[a]anthracene; Chr, chrysene; Triph, triphenylene; BeP, benzo[e]pyrene; BbFl, benzo[b]fluoranthene; BaP, benzo[a]pyrene; B[ghi]Pe, benzo[ghi]perylene; Cor, coronene; Antr, anthracene; Pe, perylene; PAHs 1 = BaA + BbFl + BeP + BaP + Cor (Marynowski and Simoneit, 2009); PAHs 2 = Fl + Py + BbFl + BaP + B[ghi]P + Cor (Finkelstein et al., 2005). b Aromatic hydrocarbon concentrations as ug/g TOC. c Aromatic hydrocarbon concentrations in italics as ng/g of charcoal.

Table 3 Geochemical parameters based on aromatic hydrocarbon distributions and content (%) of aromatic biomarkers in charcoal samples.

a b c d e f g

Sample

P/Aa

MP/MAb

MP/Pc

MPI1d

Rc (%)

DEHe (%)

SIMf (%)

RETg (%)

PORHA PORHB PORHC PORHD PORHE PORHF PORHG ZAWHA

3.77 3.86 3.92 3.98 3.27 3.44 3.72 7.71

4.44 3.37 2.73 10.68 4.22 2.71 3.01 10.28

0.83 0.63 1.13 0.86 0.69 0.89 0.20 0.48

0.29 0.38 0.31 0.28 0.19 0.33 0.34 0.30

0.57 0.63 0.59 0.57 0.52 0.60 0.60 0.58

5.2 16.4 14.6 5.8 20.4 17.9 9.0 8.9

9.5 6.1 4.5 15.1 6.1 11.3 58.9 1.4

85.3 77.5 80.9 79.1 73.5 70.8 32.1 89.7

Phenanthrene/anthracene. Methyl phenanthrenes/methyl anthracenes. Methyl phenanthrenes/phenanthrene. Methyl phenanthrene index, MPI1 = 1.5([2-MP] + [3-MP])/([P] + [1-MP] + [9-MP]) (Radke and Welte, 1983). Dehydroabietane. Simonellite. Retene.

fuel prior to burning. Both observations demonstrate that the wood was probably dead for some time before charring, leaving enough time for biological decay of the wood. 3.4. PAHs and alkyl derivatives A dominance of three- and four-ring PAHs was observed, but they were in almost all cases much less abundant than 1,2,5TMN and 1,2,5,6-TeMN (Table 2). Phenanthrene, pyrene and fluoranthene had the highest concentration in PORHA (1.56, 1.66 and 0.87 lg/g TOC, respectively). A relatively high concentration was also observed in PORHC (0.86, 1.37 and 0.81 lg/g TOC, respectively), while the lowest amount was noted for ZAWHA (0.18, 0.05 and 0.05 lg/g TOC, respectively; Table 2). Such concentrations are rather low vs. some, recently described, charcoal-bearing sedimentary rocks (Marynowski and Simoneit, 2009; Marynowski and Zaton´, 2010). For example, phenanthrene in the Sołtyków section was in the range of 1.88–449.15 lg/g TOC and for the Kamien´ Pomorski borehole 0.33–53.51 lg/g TOC, with only one sample < 2 lg/g TOC (Marynowski and Simoneit, 2009). On the other

hand, the PAH concentrations were similar to those from Middle Jurassic sequences in Argentina, which were described as partially weathered (Marynowski et al., 2011a). However, we exclude the possibility of extensive weathering of the samples here, due to the occurrence of methyl naphthalenes, which are sensitive to such a process (Marynowski et al., 2011c). In contrast to the charcoalbearing sediments, both the distributions and concentrations of PAHs in the charcoal samples were comparable with those described from wood biochar samples. The distributions of lower molecular weight (two- to four- ring) PAHs clearly dominated (Figs. 4 and 5), while the abundances of five- to seven-ring PAHs were much lower (Fig. 5; see Freddo et al., 2012; Hilber et al., 2012; Kloss et al., 2012; Quilliam et al., 2013; Fabbri et al., 2013; Bucheli et al., 2014). The concentrations of PAHs differ between samples due to the heterogenic character of biochar (Bucheli et al., 2014) and differences in charring temperature (e.g. Keiluweit et al., 2012). Despite this, the individual PAH concentrations from Upper Triassic charcoal samples were substantially similar to those in biochar samples (Fig. 6; see Keiluweit et al., 2012; Hilber et al., 2012; Fabbri et al., 2013).

L. Marynowski et al. / Organic Geochemistry 77 (2014) 22–31

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Fig. 5. Total concentrations of major aromatic compounds in charcoal samples. Only three samples were chosen for clarity. For explanation of abbreviations see Table 2.

Fig. 6. Comparison of unsubstituted PAH concentrations in biochars produced by heat treatment at discrete temperatures (after Keiluweit et al., 2012) and in Upper Triassic charcoal samples.

The distributions of the three- to five-ring PAHs were characteristic for high temperature formation (Fig. 4). Elevated concentrations for compounds such as anthracene, 4H-cyclopenta[def]phenanthrene, benz[a]anthracene and benzo[a]pyrene vs. the other PAHs were noted. All these compounds are typical products of combustion of OM (Killops and Massoud, 1992; Marynowski and Simoneit, 2009; Nabbefeld et al., 2010) and their co-occurrence is a good indicator of pyrolytic processes. Moreover, the above-mentioned PAHs are characteristic products of wood combustion (e.g. Fine et al., 2001, 2002; Bari et al., 2009, 2011). Values for the methyl phenanthrene index (MPI1) varied from 0.19 to 0.38 (Table 3). Considering the fact that MPI1 was created as a maturity parameter (Radke and Welte, 1983), the values (calculated vs. theoretical vitrinite reflectance, Rc) did not correspond to the actual values of the vitrinite reflectance. The latter values for the Upper Triassic sediments from the Zawiercie-Marciszów area, measured on fossil wood fragments, were between 0.22% to 0.36% (Marynowski, unpublished data), similar to those obtained from MPI1. Because the extractable OM from sedimentary charcoal is a mixture genetically connected with both pyrolytic and diagenetic processes, the MPI1 values were averaged, and thus not

useful for estimating fire temperature as well as the diagenesis extent of rock complexes where charcoal occurs. The ratio of methyl phenanthrenes to phenanthrene (MP/P) could be used as a parameter for distinguishing pyrogenic from petrogenic PAH sources. Values < 1 are typical for pyrogenic sources, while those > 2 are characteristic for fossil fuels and unburned sedimentary OM (Keiluweit et al., 2012 and citations therein). Values of the above ratio for the Upper Triassic charcoal samples ranged from 0.2 to 1.13 (Table 3) and are more typical for combustion processes, with an admixture of non-burned OM indicated for most of the samples (values from 0.6 to 1.13; Table 3). 3.5. Polar compounds In the polar fraction of the samples from the Zawiercie area, 39 heteroatomic compounds were identified, many for the first time in sedimentary OM (e.g. hydroxyxanthones and their methyl derivatives). However, our assignmentss based on mass spectra as well as retention time are tentative and require confirmation using authentic standards. Oxygenated PAHs like ketones, aldehydes and phenols dominated, but saturated fatty acids (FAs, n-C16 preponderance) were also detected. Differences in distributions

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Table 4 Main polar compounds (by elution order on DB-35 column) and proportion (%) in charcoal samples. No.

Compound

Mol. mass

Characteristic fragments m/z (%)

Abundance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

Phenylmethanol Ethylphenol (3 isomers) Hydroxymethylbenzaldehyde (3 isomers) n-Nonanoic acid Ethylmethylphenol (4 isomers) 3-Methylphenylethanone 4-Methylphenylethanone 2-Undecanone Propyl phenol (3 isomers) n-Decanoic acid 1-(2.4.6-Trimethylphenyl)ethanone Indan-1-one n-Undecanoic acid Methylindan-1-one (4 isomers) n-Dodecanoic acid Dimethylindan-1-one (8 isomers) Butylacetophenone (3 isomers) Hydroxybiphenyl (2 isomer) n-Tridecanoic acid Pentylacetophenone (3 isomers) Trimethylindan-1-one (5 isomers) 9H-Fluoren-9-one n-Hexadecanoic acid Carbazole Anthrone Xanthone 9.10-Phenanthrenedione Methylcarbazole (3 isomers) Hydroxyfluorene (5 isomers) Methylxanthone (4 isomers) Hydroxyxanthone (2 isomers) 1H-Benz[de]isoquinoline-1,3(2H)-dione Phenanthrenol and/or anthrol (2 isomers) Methylhydroxyxanthone and/or methoxy-xanthone and/or dihydroxyanthracenone (4 isomers) Phenylindenol (3 isomers) 5H-Phenanthro[4,5-bcd]pyran-5-one 6(5H)-Phenanthridinone 9(10H)-Acridinone Hydroxypyrene and/or hydroxyfluoranthene (2 isomers)

108 122 136 158 136 134 134 170 136 172 162 132 186 146 200 160 176 170 214 190 174 180 256 167 194 196 194 181 182 210 212 197 194 226

79(100), 108(92), 77(90), 107(83), 51(40), 105(35) 107(100), 122(45), 77(42) 135(100), 136(85), 107(50) 60(100), 73(93), 115(35), 129(33), 158(3) 121(100), 136(58), 107(10), 91(10) 119(100), 91(98), 134(35), 65(33) 119(100), 91(80), 134(35), 65(30) 43(100), 58(98), 71(30), 170(4) 107(100), 136(45), 77(30) 60(98), 73(100), 129(70), 115(30), 143(25), 172(6) 147(100), 119(70), 162(35) 104(100), 132(98), 103(40), 131(20) 60(100), 73(98), 129(30), 143(28), 157(10), 186(10) 117(100), 146(80) 60(100), 73(98), 129(35), 157(30), 200(20) 160(100), 117(98), 131(30), 145(15) 161(100), 176(20), 91(18), 105(10) 170(100), 141(25), 115(20) 60(100), 73(98), 129(35), 171(30), 214(25) 175(100), 190(25), 91(14), 105(8) 174(100), 131(80), 159(55), 145(10) 180(100), 152(50) 60(100), 73(98), 129(40), 213(20), 256(20) 167(100), 139(20) 194(100), 165(95) 196(100), 168(70), 139(55) 165(100), 194(80) 181(100), 180(90), 152(20) 182(100), 152(40), 165(15) 210(100), 196(100), 181(60), 139(60), 168(50), 152(30) 212(100), 184(25), 128(20) 197(100), 153(80), 126(60) 194(100), 165(65) 226(100), 197(30)

0.5 0.1 0.1 0.3 0.3 0.0 0.1 0.1 0.5 0.8 0.2 1.4 1.0 4.3 0.2 2.6 2.4 1.0 0.8 2.2 1.4 0.9 1.2 1.7 0.4 100 0.5 0.2 2.1 6.6 3.6 3.3 2.5 1.7

208 220 195 195 218

208(100), 220(100), 195(100), 195(100), 218(100),

1.0 1.5 0.9 0.9 0.8

35 36 37 38 39

between samples were minor and in all cases the dominant compound was xanthone (Table 4; Fig. 7). Among the nitrogen containing compounds small amounts of carbazoles, as well as phenanthridinone and acridinone, were found (Table 4). Some polar compounds were primary, being biomolecules (i.e. FAs) derived directly from wood or formed via diagenetic transformation of primary OM. Secondary compounds, like aldehydes and ketones, constituted > 80% of the total polar fractions and probably formed during wood combustion. Oxygenated aromatic compounds like phenols, ketones and aldehydes are commonly observed in the ambient air of big cities and industrial areas (e.g. Schauer et al., 1996; Marynowski et al., 2004; del Rosario Sienra, 2006), suggesting a pyrolytic origin, but phenols are a compound class which can be classified into both source groups.

4. Implications 4.1. Pyrolytic vs. diagenetic products The compounds in the charcoal can be divided into two groups: (i) biomarkers, i.e. diagenetically changed primary wood components, and (ii) combustion products. The major compounds in the first group are: 1,2,5-TMN, 1,2,5,6-TeMN, cadalene, dehydroabietane, simonellite and retene. The propyl phenols, butyl acetophenones and pentyl acetophenones (Table 4) are probably products of lignin breakdown (Nolte et al., 2001) and subsequent diagenetic transformation, while the FAs, with the preponderance of n-hexa-

178(35), 163(40), 167(30), 167(40), 189(75),

152(20) 192(15) 139(20) 139(20) 94(40), 109(10)

decanoic (palmitic) acid, are typical constituents of wood (e.g. Simoneit et al., 2000; Rencoret et al., 2007). The second group is comprised of pyrolytic PAHs and some oxyand hydroxy-PAHs. Many of these other compounds could be generated by both the above processes (e.g. Wilkes et al., 1998; Bennett and Larter, 2000; del Rosario Sienra, 2006). The very high abundance of xanthone is exceptional (Fig. 7). It has been reported in polluted estuarine and marine waters (Bester and Theobald, 2000), atmospheric aerosols (Marynowski et al., 2004; del Rosario Sienra, 2006; Delhomme et al., 2008), contaminated coking plant soil (Biache et al., 2013), products from wood burning (Fine et al., 2001) and in self-combusting coal dumps (Misz-Kennan and Fabian´ska, 2011). All this suggests a pyrolytic origin, which was confirmed by experiments on the thermal, oxidative decomposition of selected PAHs (Onwudili and Williams, 2006; DeCoster et al., 2007). However, in all these cases xanthone was found in small quantity in relation to the other organic constituents. Xanthone and its methyl derivatives have been reported in crude oil (Oldenburg et al., 2002) and sedimentary rocks (Watson et al., 2005), where their source designation is more complex. Watson et al. (2005) suggested a terrestrial, non-combustion origin, due to the relatively immature character of the OM and with no evidence of burning. Because biomolecules with the xanthone skeleton are ubiquitous components of higher plants, fungi or lichens (e.g. Peres and Nagem, 1997 and references therein; Patten et al., 2010; Siridechakorn et al., 2014), biological precursors are likely. A similar distribution of methyl naphthalenes to that reported here, with a dominance of 1,2,5-TMN and 1,2,5,6-TeMN

L. Marynowski et al. / Organic Geochemistry 77 (2014) 22–31

29

Fig. 7. Total ion current (TIC) trace of polar fraction from sample PORHC, showing the high predominance of xanthone. For clarification of the numbers see Table 4. MX, methyl xanthone isomers (identification after Oldenburg et al., 2002).

and high abundance of xanthone, was presented for Late Permian marls from Northern Italy (Watson et al., 2005). Therefore, the interpretation of a primary, terrestrial character for these compounds (including xanthone) seems possible and can be adopted for our samples. Products with biological substitution patterns, like 1,2,5-TMN and 1,2,5,6-TeMN, significantly dominated among the PAHs in the charcoal samples, as did xanthone among the polar compounds. Furthermore, the less stable compound, dehydroabietane, which generally becomes aromatized to simonellite and retene during early diagenesis (Simoneit, 1998), survived in the charcoal (Table 3). Gelified, non-charred fragments are even macroscopically visible in some charcoal samples (Fig. 2a), suggesting incomplete charring of wood. The possible reason for incomplete charring could be a relatively low burning temperature, in most cases not exceeding 350–450 °C, but fusinite reflectance measurements provided very diverse results depending on the measured section (Kubik et al., in press). On the other hand, the temperature regime during the incomplete burning of wood can also move resin from the unburned parts of the wood and migrate it to the charred part. In summary, both primary and pyrolytic compounds can be found in fossil charcoal, and in the case of the samples analyzed here, the natural biomarker structures predominated. 4.2. PAH concentration in charcoal The surprisingly low content of PAHs (Table 2) could be connected with such factors as: (i) greater solubility of primary noncharred OM from gelified fragments in charcoal, and (ii) vaporization of the major part of the volatile burn products. The predominance of the primary compounds is also manifested in a high proportion of the polar fractions vs. the aromatic and aliphatic fractions (Table 1), which is typical for immature OM with a dominance of aromatic biomarkers as discussed above. Moreover, partial oxidation, especially of PAHs, and formation of polar compounds (ketones and aryl phenols; Bennett and Larter, 2000; Simoneit et al., 2007) could be another, but probably less important reason for the low PAH concentration. All this suggests that the concentration of PAHs in sedimentary rocks is not always directly related to wildfire intensity, assuming

that the highest intensity is directly near the burned wood. Therefore, the PAH distribution, with the occurrence of typical pyrolytic compounds, instead of just their concentration, seems to be a crucial indicator for burning in the geological past. A final factor which could influence the primary distributions is early diagenetic mineralization of charcoal, possibly connected with hydrothermal processes (more details are given by S´rodon´ et al., 2014). However, based on the preservation of the less stable compounds, we believe that such processes were minor or absent. Despite the generally lower PAH concentration in the charcoals than in charcoal-bearing sediments (e.g. Marynowski and Simoneit, 2009; Marynowski and Zaton´, 2010; Marynowski et al., 2011a), they are quite high compared with data for experimentally produced biochars and charring simulations (Keiluweit et al., 2012; Hale et al., 2012). The concentrations of total PAHs from the PORHA and PORHC samples correspond approximately to the PAHs measured for biochar produced at 500 °C (Fig. 6). Other charcoal samples contained lower amounts of PAHs, which could be connected with lower charring temperature. Although at higher charring temperature the PAH concentration also decreased (Keiluweit et al., 2012), the occurrence of thermally less stable compounds in all these samples excludes such a possibility. Based on the unsubstituted PAH abundances in the Upper Triassic charcoals compared with the char temperature series, we estimate burn temperatures between 300 and 400 °C for the PORHB, PORHD, PORHE and ZAWHA samples, and between 400 and 500 °C for the other samples (Fig. 6). We realize, that many factors besides temperature can control the PAH content of paleo-charcoals (e.g. type of starting material, heterogeneity of the charcoal, diagenesis, secondary processes), but a temperature estimate based on PAH concentration may in some cases be a useful tool and could be considered as a complementary method. 5. Conclusions The predominance of 1,2,5-TMN, 1,2,5,6-TeMN and xanthone in charcoal extracts from the Upper Triassic of the Zawiercie-Marciszów and Pore˛ba areas in southern Poland suggests a major origin from biologically related compounds and a lesser one from

30

L. Marynowski et al. / Organic Geochemistry 77 (2014) 22–31

burn products. The concentration of PAHs was low compared with the charcoal-bearing sedimentary rocks, but the distribution, with a significant content of typical pyrolytic compounds like anthracene, 4H-cyclopenta[def]phenanthrene, benz[a]anthracene and benzo[a]pyrene, confirms their combustion source. The concentration of PAHs present directly in paleo-charcoal can be used as a tool for palaeo-wildfire temperature estimation. The occurrence of thermally less stable organic compounds and low PAH amount indicate temperatures < 400 °C, while high PAH abundances are characteristic for temperatures between 400 and 500 °C. Above this temperature level the PAH concentration again decreased, but thermally less stable compounds were absent. Such temperature estimates for fires may be useful for cases of immature to low maturity sedimentary rock sequences. Acknowledgements This work was partially supported by NCN Grant: 2011/01/B/ ST10/01106 (to L.M.) and Wrocław Research Centre EIT+Ltd., project NanoMat task 5.4. We benefited from constructive comments by two anonymous reviewers of the manuscript.

Associate Editor—G.D. Abbott

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