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Oxygen-containing aromatic compounds in a Late Permian sediment
Organic Geochemistry Organic Geochemistry 36 (2005) 371–384 www.elsevier.com/locate/orggeochem
Oxygen-containing aromatic compounds in a Late Permian sediment Jonathan S. Watson a
a,*
, Mark A. Sephton a, Cindy V. Looy b, Iain Gilmour
a
Planetary and Space Sciences Research Institute, Open University, Milton Keynes, Buckinghamshire MK7 6AA, UK b Botanical Palaeoecology, Laboratory of Palaeobotany and Palynology, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands Received 5 November 2003; accepted 8 October 2004 (returned to author for revision 10 February 2004) Available online 9 December 2004
Abstract In northern Italy the base of the Tesero Oolite Horizon represents the lithological boundary between the Bellerophon and Werfen Formations and marks one of the most dramatic environmental perturbations in Phanerozoic time. The marl, which directly underlies the Tesero Horizon, contains many indicators of environmental change during the end-Permian mass extinction. At Vigo Meano, near Trento, the marl is relatively thermally immature and contains abundant terrestrially-sourced oxygen containing aromatic compounds such as xanthones, dibenzofurans and dibenzo-p-dioxin as well as abundant alkylated naphthalenes. Ó 2004 Elsevier Ltd. All rights reserved.
1. Introduction The end of the Permian approximately 250 million years ago was marked by the largest mass extinction of the Phanerozoic. Both the marine (Raup, 1979) and terrestrial (Maxwell, 1992) biospheres were affected. Significant changes in oceanic and atmospheric chemistry are reflected in dramatic variations in the carbon isotope ratios of carbonate rock and marine and terrestrial organic matter (Sephton et al., 2002 and references therein). These changes appear to occur within a period of 165,000 years (Bowring et al., 1998). Possible driving forces for these extreme changes include widespread volcanic activity (Renne et al., 1995), extraterrestrial impact
* Corresponding author. Tel.: +44 1908 652424; fax: +44 1908 858022. E-mail address: [email protected] (J.S. Watson).
(Becker et al., 2001), overturn of a stagnant ocean (Knoll et al., 1996) and rapid decomposition of a gas hydrate reservoir (Erwin, 1993). Recent organic geochemical studies of sections in northern Italy have discovered unusual abundances of oxygen-containing aromatic compounds close to the extinction horizon (Sephton et al., 1999a, 2001). These oxygen-containing aromatic compounds may reflect palaeoenvironmental conditions specific to the endPermian event. In this paper, we report data from a new end-Permian section in northern Italy that is characterised by relatively low thermal maturity. Several previously unreported Late Permian, oxygen containing aromatic compounds are abundant constituents of the rocks and their chemical structure relationship to previously detected components and palaeoenvironmental significance are considered. There are a number of possible sources for oxygen containing aromatic compounds. Both furans and
0146-6380/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2004.10.006
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dioxins can be produced during forest fire events (e.g. Velazquez et al., 1998). Furans can also be produced naturally by fungi (e.g. Radke, 2000) and they could also arise from dehydrated polysaccharides (e.g. Sephton et al., 1999a). Although dioxins are commonly considered to be anthropogenic in origin, in the modern environment they can be produced by organisms (e.g. Utkina et al., 2001) and have also been shown to be produced in peat bogs (Silk et al., 1997). Xanthones can also be generated by a number of organisms, including fungi and lower vascular plants (e.g. Peres et al., 2000).
2. Experimental 2.1. Samples In northern Italy, Late Permian sedimentary deposits were laid down on a proximal shallow shelf at a low northerly palaeolatitude in the western Tethys Ocean (Bosellini and Hardie, 1973; Scholger et al., 2000). Latest Permian sedimentary rocks outcrop in the Vincentian Alps, northern Italy and represent two lithostratigraphic units: the Bellerophon Formation and the Werfen Formation (Posenato, 1988). At Vigo Meano near Trento, the Permian Bellerophon Formation comprises limestone breccias overlain by 4–5 m of vuggy dolomites and a 80-cm thick organic-rich marl. The marl is immediately overlain by the oldest member of the Werfen Formation, the Tesero Oolite Horizon, which is a transgressive bed thought to have been deposited almost synchronously throughout the southern Alps. Historically, in the southern Alps, the Permian–Triassic boundary was considered to approximate the transition between the Bellerophon Formation and the Werfen Formation. It is now accepted that the first appearance of the conodont Hindeodus parvus should mark the onset of the Triassic (Yin et al., 1996), a protocol which consigns the mass extinction horizon in the southern Alps to Late Permian rocks. The samples (Table 1) were collected from the thick organic-rich Bellerophon marlat Vigo Meano. This marl and other latest Permian rocks contain the debris of land plants (Visscher and Brugman, 1986; Sephton et al., 1999a, 2001, 2002). Lateral equivalents of the marl display petrographic and chemical features reflecting a mostly terrestrial source (Sephton et al., 1999a).
2.2. Solvent extraction For each sample, ca. 2 g of crushed whole rock was sonicated in 3 ml of dichloromethane (DCM)/methanol (95:5) for 30 min, centrifuged for 15 min and the supernatant pipetted into a vial. This procedure was repeated 3 times. Sulfur was removed by adding activated copper turnings to the solvent extract and sonicating for 5 min. An apolar fraction was isolated by eluting the extract down a short silica column (Pasteur pipette with 2 cm bed depth) with 5 ml of DCM. A further aliquot of the crushed sediment was spiked with internal standards (squalane, Acros Organics and naphthalene-d8, Supelco) prior to extraction. Samples were extracted and an apolar fraction isolated as described above. This apolar fraction was further fractionated using silver ion chromatography prior to analysis using gas chromatography-mass spectrometry (GCMS) of the n-alkanes. Briefly, the aliphatic hydrocarbons were isolated by eluting the apolar fraction through a short column of 10% AgNO3 impregnated silica gel (Pasteur pipette with 2 cm bed depth) with 2 ml of hexane. The apolar fraction from one sample (V17-2) was further fractionated into hydrocarbons and non-hydrocarbons using the C18 solid phase extraction (SPE) method described in Bennett and Larter (2000). Briefly, the fraction was loaded on to a 3 ml 500 mg nonendcapped C18 SPE cartridge (International Sorbent Technology, UK) and the hydrocarbons were eluted with n-hexane and the non-hydrocarbons with DCM. 2.3. GC-MS GC-MS analysis was carried out using an Agilent Technology 6890 gas chromatograph coupled to a 5973 mass spectrometer. Separation was performed on a S.G.E. (U.K.) BPX5 column (30-m length, 0.25-mm i.d. and 0.25-lm film thickness) with He carrier gas at a constant column flow rate of 1.1 ml min 1. The GC oven temperature was held for 1 min at 50°C and ramped to 300°C at a rate of 5°C min 1 and then held for 9 min. The aromatic hydrocarbons were semi-quantified using GC-MS and the ions shown in Table 2. The n-alkanes, pristane and phytane were analysed as above using a J&W DB-5 column (30-m length, 0.25-mm i.d. and 0.25-lm film thickness) and quantified from the total ion current.
Table 1 Bulk data for Vigo Meano end-Permian marl samples (NEOC, non-extractable organic carbon, EOM, extractable organic matter) d13Corg (&)
Sample
Carbonate (%)
NEOC (mg/g)
EOM (mg/g)
Top
V17-1 V17-2 V17-3A
4.3 7.3 71.2
2.2 1.8 0.8
0.9 0.8 0.6
26.7 26.5 26.7
1.47 1.42 1.38
Bottom
V17-3B
72.2
0.8
0.9
26.5
1.39
Pr/Ph
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Table 2 Concentrations (lg/g) of selected compounds (ions given in table were used for quantification) Compound
V17-1 (lg/g sed)
V17-2 (lg/g sed)
V17-3A (lg/g sed)
V17-3b (lg/g sed)
Naphthalene (m/z 128) 2-Methylnaphthalene (m/z 142) 1-Methylnaphthalene (m/z 142) 2- + 1-Ethylnaphthalene (m/z 141 + 156) 2,6- + 2,7-Dimethylnaphthalene (m/z 141 + 156) 1,3- + 1,7-Dimethylnaphthalene (m/z 141 + 156) 1,6-Dimethylnaphthalene (m/z 141 + 156) 1,4- + 2,3-Dimethylnaphthalene (m/z 141 + 156) 1,5-Dimethylnaphthalene (m/z 141 + 156) 1,2-Dimethylnaphthalene (m/z 141 + 156) 1,3,7-Trimethylnaphthalene (m/z 155 + 170) 1,3,6-Trimethylnaphthalene (m/z 155 + 170) 1,4,6- + 1,3,5-Trimethylnaphthalene (m/z 155 + 170) 2,3,6-Trimethylnaphthalene (m/z 155 + 170) 1,2,7-Trimethylnaphthalene (m/z 155 + 170) 1,6,7-Trimethylnaphthalene (m/z 155 + 170) 1,2,6-Trimethylnaphthalene (m/z 155 + 170) 1,2,4-Trimethylnaphthalene (m/z 155 + 170) 1,2,5-Trimethylnaphthalene (m/z 155 + 170) 1,3,6,7-Tetramethylnaphthalene (m/z 169 + 184) 1,2,4,6- + 1,2,4,7- + 1,4,6,7-Tetramethylnaphthalene (m/z 169 + 184) 1,2,5,7-Tetramethylnaphthalene (m/z 169 + 184) 2,3,6,7-Tetramethylnaphthalene (m/z 169 + 184) 1,2,6,7-Tetramethylnaphthalene (m/z 169 + 184) 1,2,3,7-Tetramethylnaphthalene (m/z 169 + 184) 1,2,3,6-Tetramethylnaphthalene (m/z 169 + 184) 1,2,5,6- + 1,2,3,5-Tetramethylnaphthalene (m/z 169 + 184) Phenanthrene (m/z 178) Anthracene (m/z 178) 3-Methylphenanthrene (m/z 192) 2-Methylphenanthrene (m/z 192) 2-Methylanthracene (m/z 192) 9-Methylphenanthrene (m/z 192) 1-Methylphenanthrene (m/z 192) Flouranthrene (m/z 202) Pyrene (m/z 202) Benz[a]anthracene (m/z 228) Chrysene + triphenylene (m/z 228) Benzofluoranthenes (m/z 252) Benzo[e]pyrene (m/z 252) Benzo[a]pyrene (m/z 252) Dibenzothiophene (m/z 184) Xanthone (m/z 168 + 196) 1-Methylxanthone (m/z 181 + 210) 4-Methylxanthone (m/z 181 + 210) 2-Methylxanthone (m/z 181 + 210) 3-Methylxanthone (m/z 181 + 210) Dibenzofuran (m/z 139 + 168) 4-Methyldibenzofuran (m/z 181 + 182) 2- + 3-Methyldibenzofuran (m/z 181 + 182) 1-Methyldibenzofuran (m/z 181 + 182) Dibenzo-p-dioxin (m/z 184) n-C13 n-C14 n-C15 n-C16
2.85 0.91 0.87 0.18 0.65 0.62 1.27 0.35 0.41 1.04 0.32 0.35 0.31 0.31 0.11 0.12 2.83 0.09 3.43 0.18 0.24 0.41 0.04 0.35 0.04 0.03 5.06 1.55 0.23 0.08 0.15 0.11 0.18 0.24 0.35 0.21 0.13 0.08 0.12 0.01 0.01 0.33 3.07 0.52 1.82 0.18 0.04 2.97 0.79 1.55 0.40 0.47 0.06 0.16 0.74 1.55
3.05 0.75 0.75 0.17 0.58 0.59 1.16 0.34 0.31 0.96 0.31 0.36 0.32 0.32 0.10 0.27 2.78 0.08 3.47 0.19 0.27 0.44 0.05 0.37 0.04 0.30 5.83 1.51 0.23 0.08 0.15 0.12 0.16 0.24 0.36 0.22 0.11 0.07 0.10 0.02 0.02 0.32 3.32 0.55 1.83 0.18 0.04 2.76 0.76 1.48 0.40 0.42 0.03 0.06 0.10 0.33
0.25 0.13 0.11 0.05 0.12 0.13 0.18 0.07 0.06 0.13 0.07 0.08 0.07 0.07 0.03 0.08 0.33 0.02 0.47 0.05 0.05 0.07 0.01 0.08 0.01 0.07 0.88 0.45 0.04 0.03 0.05 0.02 0.05 0.05 0.14 0.07 0.03 0.03 0.03 0.01 0.00 0.06 0.49 0.09 0.35 0.05 0.01 0.41 0.14 0.24 0.06 0.05 0.10 0.30 0.89 1.93 (continued on
0.45 0.20 0.17 0.09 0.18 0.21 0.26 0.10 0.09 0.18 0.13 0.16 0.15 0.12 0.04 0.09 0.72 0.03 0.84 0.11 0.11 0.17 0.02 0.17 0.02 0.15 1.89 1.02 0.09 0.07 0.11 0.05 0.10 0.12 0.25 0.13 0.07 0.04 0.08 0.01 0.01 0.13 0.70 0.23 0.66 0.07 0.02 0.87 0.29 0.50 0.13 0.11 0.05 0.51 1.28 2.59 next page)
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Table 2 (continued) Compound
V17-1 (lg/g sed)
V17-2 (lg/g sed)
V17-3A (lg/g sed)
V17-3b (lg/g sed)
n-C17 Pristane n-C18 Phytane n-C19 n-C20 n-C21 n-C22 n-C23 n-C24 n-C25 n-C26 n-C27 n-C28 n-C29 n-C30
1.70 0.75 1.23 0.51 0.69 0.47 0.43 0.37 0.36 0.28 0.25 0.17 0.17 0.09 0.11 0.05
0.53 0.47 0.55 0.33 0.44 0.33 0.32 0.26 0.25 0.17 0.15 0.10 0.11 0.05 0.06 0.03
1.88 0.99 1.17 0.72 0.46 0.29 0.18 0.13 0.10 0.08 0.08 0.06 0.05 0.03 0.03 0.02
2.70 1.31 1.79 0.94 0.79 0.39 0.26 0.19 0.18 0.17 0.16 0.10 0.10 0.06 0.06 0.04
Identification of dibenzo-p-dioxin was based on comparison with retention time, mass spectra and by coinjection of an authentic standard (ChemService, USA). Co-injection was carried out on three different phases (BP1, BPX35 both S.G.E. and a J&W DB-5ms, all three columns 30-m length, 0.25-mm i.d. and 0.25lm film thickness), with GC-MS conditions as described above. Identification of other oxygen containing aromatic compounds was based on comparison of authentic standards, retention time, mass spectra and published reports (e.g. Sephton et al., 1999a; Radke, 2000; Oldenburg et al., 2002). 2.4. Pyrolysis-GC-MS Solvent-extracted crushed sediment samples (ca. 5 mg) were placed in quartz pyrolysis tubes plugged at either end with quartz wool. Samples were pyrolysed using a CDS Pyroprobe 1000 fitted with a 1500 valve interface held at 250°C (CDS Analytical, Oxford, PA) and coupled to a GC-MS for identification of the pyrolysis products (the pyrolysate). Following heating to 610°C at a rate of 20°C ms 1 the sample was held at this temperature for 15 s in a flow of helium. GC-MS conditions (using a BPX5 column) were as described above except that the GC was held isothermally at 30°C for 10 min and then ramped at 4°C min 1 to 300°C where it was held for 9 min. 2.5. Hydrous pyrolysis Hydrous pyrolysis was carried out as described in Sephton et al. (1999b). Briefly, samples of solvent extracted V17-2 (0.5 g) along with 0.4 ml deionised water were pyrolysed in duplicate in 1.8 ml (internal volume) bomblets. The bomblets were assembled from T316
stainless steel tube and sealed with Swagelock SS-600-C end caps. They were placed inside a 71-ml Parr bomb and heated for 24 h at 320 °C. The bomblets and contents were air dried and extracted as for the sediment samples. 2.6. Non-extractable organic carbon and d13Corg Solvent extracted samples were decalcified with 0.1 M through 1 M hydrochloric acid and washed with distilled water (until pH 7) followed by a second solvent extraction step. Carbonate-free carbon concentrations and isotope ratios were determined using an Elemental Analyser-Isotope Ratio Mass Spectrometer (PDZ Europa ANCA-SL) coupled to a continuous-flow stable isotope ratio mass spectrometer (PDZ Europa 20-20) in combustion mode.
3. Results and discussion 3.1. General organic composition of Vigo Meano samples Table 1 reveals that, from the bottom to the top of the thick organic-rich marl, carbonate content decreases, non-extractable organic carbon increases, while extractable organic matter (EOM) remains relatively constant. The d13C values do not vary through the section. Given that the end-Permian crisis culminates in a marked negative shift in d13Corg values, we conclude that the Vigo Meano samples must represent an interval before, but close to, the inferred worldwide changes in oceanic and atmospheric carbon chemistry. The solvent extracts contain abundant alkylated naphthalenes, with a C4 alkyl naphthalene generally being the most abundant aromatic compound present (Fig. 1; Table 2). Other aromatic compounds include
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Fig. 1. Total ion chromatograms of solvent extracts: , n-alkane (n-C12–25); N, naphthalene; MNs, methylnaphthalenes; DBF, dibenzofuran; DBD, dibenzo-p-dioxin; TMN, trimethylnaphthalene; MDBFs, methyldibenzofurans; TeMN, tetramethylnaphthalene; P, phenanthrene; X, xanthone; MX, methylxanthone.
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phenanthrene and alkylated homologues, fluoranthene and pyrene. Straight chain alkanes (C12 to C32) with a mode at n-C17 and the acyclic isoprenoids pristane and phytane were also present. Other abundant compounds include oxygen-containing aromatics such as dibenzofurans, xanthones and dibenzo-p-dioxin. The compound distribution does seem to change slightly through the sequence, with n-alkanes being relatively more abundant in the carbonate-rich samples lower in the section. The abundance of oxygen compounds appears to correlate with the aromatic compounds, suggesting that there are potentially two sources of organic material, one contributing n-alkanes and the other contributing to the aromatic hydrocarbons and oxygen containing aromatic compounds. 3.2. Alkyl naphthalenes The naphthalenes (Fig. 2) display a relatively immature distribution (cf. van Aarssen et al., 1999); 1- and 2methylnaphthalenes (MNs) are present in approximately equal amounts, dimethylnaphthalenes (DMNs) are dominated by the 1,6- and 1,2-isomers and the trimethylnaphthalenes (TMNs) are dominated by the 1,2,6- and 1,2,5-isomers. A peak due to tetramethylnaphthalenes (TeMN) is the most abundant; this component was tentatively identified as 1,2,5,6-TeMN, although 1,2,3,5-TeMN or a mixture of the two isomers is also a possibility (cf. van Aarssen et al., 1999). Its concentration increases towards the top of the marl (Table 2).
Land plants are a potential source of TMNs, yet due to the age of the Vigo Meano rocks (ca. 250 Ma) certain constraints can be placed on which types of land plants could have contributed. The 1,2,7-TMN isomer can be derived from compounds such as b-amyrin which is present in angiosperms; however as these plants did not appear until the Late Cretaceous period (Pu¨ttmann and Villar, 1987; Strachan et al., 1988) significant inputs from this source are improbable. The 1,2,5-TMN can be derived from bicylic diterpenoids and resins which are common in conifers. A further land plant precursor for 1,2,5-TMN could be a compound such as onocerane, which has been attributed to lower vascular plants, including ferns, horsetails and lycopods (Pearson and Obaje, 1999). Alternatively 1,2,5-TMN may have a bacterial source as it can be derived from pentacyclic triterpenoids via cleavage of the C ring. Irrespective of the source of the parent molecule, the 1,2,6-TMN isomer can be formed by isomerisation of the 1,2,5-TMN isomer by a 1,2 methyl shift (Strachan et al., 1988). However, due to the high abundance of the 1,2,6-TMN isomer and the relatively low thermal maturity of the sediment this compound could reflect an alternative source signal. Land plants are also a potential source of TeMN and again certain types of precursors can be discounted due to the age of the samples. As C3 oxygenated triterpenoids are commonly derived from angiosperms (Pu¨ttmann and Villar, 1987), they are an implausible source of 1,2,5,6-TeMN in end-Permian rocks. In C3 oxygenated triterpenoids the required methyl shift is mediated
Fig. 2. Partial mass chromatogram (m/z 128 + 142 + 156 + 170 + 184) displaying the naphthalenes in solvent extract of sample V17-2.
J.S. Watson et al. / Organic Geochemistry 36 (2005) 371–384
by a leaving group. Yet, under acidic conditions a leaving group may not be required (Pu¨ttmann and Villar, 1987, and references therein) and 1,2,5,6-TeMN could be potentially produced alongside 1,2,5-TMN from a biomarker such as onocerane; other candidates for precursors could include compounds such as arborene/fernene which can be abundant in other Permian sedimentary rocks (cf. Hauke et al., 1992). However, once again, 1,2,5,6-TeMN may have a bacterial source via cleavage of the C ring of pentacyclic triterpenoids under acidic conditions (Pu¨ttmann and Villar, 1987) or alternatively may be formed from bicyclanes (Bastow, 1998). In the context of the above discussions it is interesting to consider that at the end of the Permian, largescale volcanic emissions were occurring during the emplacement of the Siberian Traps. This episode of flood basalt eruption may have caused widespread destabilisation of terrestrial ecosystems. When a terrestrial ecosystem is in a stressed state, lower vascular plants are some of the first plants to recolonise an environment (e.g. DiMichele and Phillips, 2002) and a possible precursor for both 1,2,5,6-TeMN and 1,2,5-TMN could be a lower vascular plant biomarker. Recent palynological evidence confirms a pioneering role for lower vascular plants during the end-Permian crisis (Looy et al., 2001). Except for bacterially-derived hopanes, most other potential precursors (such as seco-hopanoids) for the alkyl naphthalenes appear to be absent from these sediments. Other potential sources for naphthalenes and other polycyclic aromatic hydrocarbons (PAHs) could include forest fires. However, if this was a significant source the parental aromatic hydrocarbons may be expected to dominate, especially three to four ring components (e.g. Jenkins et al., 1996; Hedberg et al., 2002). Also, there is also no evidence from the optical inspection of isolated organic matter for an input from burning (C.V. Looy, unpublished results). 3.3. Hopanes and steranes The distribution of hopane and sterane hydrocarbons implies that the organic matter in the Vigo Meano samples is relatively immature (Fig. 3), an observation consistent with the alkylated naphthalene distribution. The hopane hydrocarbons are dominated by Tm [17a(H)-22,29,30-trisnorhopane] and C29, C30 and C31 hopanes. Extended hopanes are not present, which is common for terrestrially-derived material where oxidation of the hopane side chain occurs. The presence of abundant ba isomers indicates that the sediment is relatively immature although the S/R isomers appear to be at equilibrium, indicating a 60.6 vitrinite reflectance equivalent. Sterane hydrocarbons are dominated by the C29 aaa 20R isomer which has the
377
biological configuration, again indicating that the sediment is relatively immature. The abundance of C29 steranes relative to the C27 and C28 steranes may indicate a terrestrial source from higher plants (e.g. Czochanska et al., 1988) or a possible algal input (e.g. Grantham, 1986). 3.4. Aromatic oxygen compounds The distribution of dibenzofurans and xanthones in sample V17-2 is displayed in Fig. 4. Dibenzofurans are known to occur only within a specific thermal window and they become selectively concentrated as maturity increases until 250°C, above which they are destroyed (Siskin and Kitritzky, 1991). Abundant dibenzofurans have been identified before in Late Permian sediments and are thought to be derived mainly from land plant polysaccharides (Sephton et al., 1999a), although contributions from other sources can not be ruled out. For example, dibenzofurans can also be produced by plants as antimicrobial agents (Dixon, 2001) and are found in lichens (e.g. Culberson, 1969; Radke, 2000). Xanthone precursors are also produced by higher plants where they act as pigments and antimicrobial agents (e.g. Peres et al., 2000). Functionalised xanthones are also found in fungi, lichens and lower vascular plants (e.g. Berti and Bottari, 1968; Culberson, 1969; Chexal et al., 1975; Devon and Scott, 1975). Previous reports of xanthones in fossil organic matter include that by Oldenburg et al. (2002) who reported their presence in a sediment with a terrestrial input and in crude oils. Xanthones have also been produced during the pyrolysis of a Late Permian wood (Sephton et al., 1997) and have been reported to be produced in small quantities from wood burning (Fine et al., 2001). To our knowledge this is the first report of dibenzop-dioxin in geological samples. Only the parent molecule is present and alkylated homologues appear to be absent. The source of dibenzo-p-dioxin is unclear but is most probably related to the xanthones and dibenzofurans. Dibenzo-p-dioxin was identified by co-injection using three different column phases (Fig. 5). There are a number of potential sources of dioxins (see Green et al., 2004, and references therein). Polychlorinated dibenzo-p-dioxins (PCDDs) have been detected in peat bogs where they are thought to be produced by microbial dimerisation of chlorophenols (Silk et al., 1997) and they have also been reported to occur in clays (Ferrario et al., 2002; Green et al., 2004). In addition, they can be produced during forest fires but only at very low levels (e.g. Martı´nez et al., 2000; Gabos et al., 2001) and they have also been shown to be produced during peat burning (Meharg and Killham, 2003). It is not unreasonable to consider that, during diagenesis if PCDDs are present, dechlorination would occur,
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Fig. 3. Partial mass chromatograms displaying hopane (m/z 191) and sterane (m/z 217) hydrocarbons in solvent extract from sample V17-2.
leading to the formation of the parent molecule, dibenzo-p-dioxin, detected in these Permian samples. The high abundances of the dibenzofurans, xanthones and dibenzo-p-dioxin (Table 2) are very unusual. In a similar fashion to the concentration of TeMN, but in opposition to the concentrations of n-alkanes, the abundances of xanthones (and DBF, DBD) are higher in the upper part of the section. Further fractionation of the solvent extract from V17-2 using a C18 SPE cartridge enabled the identification of benzophenone, fluorenone and the tentative identification of 7H-benzo[de]anthracen-7-one (Fig. 6). These compounds are relatively minor when compared
to the xanthones. Alkylated fluorenones appear to be absent or at very low concentration. This confirms that the xanthones and related compounds can be isolated from the apolar fraction using the method described in Bennett and Larter (2000). 3.5. Pyrolysis The main products released during on-line ‘‘flash’’ Py-GC-MS of solvent extracted, crushed samples were benzene, toluene, xylene, naphthalenes, phenols and benzofurans (Fig. 7). Other more minor components include n-alkanes, dibenzofurans and dibenzo-p-dioxin.
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Fig. 4. Partial mass chromatograms displaying dibenzofurans (m/z 168 + 182 + 196) and xanthones (m/z 196 + 210 + 226) in solvent extract from sample V17-2.
The distribution of products appears to be similar throughout the sequence. In each of the Vigo Meano samples only low levels of xanthones were detected in the pyrolysis products. This relatively low abundances could be caused by the loss of a keto-group during thermal degradation, leaving behind a dibenzofuran moiety (e.g. de Champlain and de Mayo, 1972). Off-line hydrous pyrolysis of a solvent extracted sample V17-2 liberated compounds similar to those in the solvent extract (Fig. 8). This indicates that some of the naphthalenes and oxygen containing compounds in the solvent extracts may have been cleaved from the high molecular weight kerogen when the organic matter approached the oil window. The distribution of compounds in the hydrous pyrolysate is different from the
on-line Py-GC-MS products. The main discrepancy is due to the loss of volatile components such as BTX (benzene, toluene and xylene), phenols and benzofurans during the off-line solvent removal step following pyrolysate extraction. The distribution of compounds in the hydrous pyrolysate suggests a lower organic maturity than for the solvent extract. In the hydrous pyrolysate, for example, anthracene is much more abundant than phenathrene and 1,2,6-TMN is not as dominant as in the extract, indicating a limited extent of maturity-driven 1,2 methyl shifts. In contrast to the more acutely temperature sensitive compounds, the relative abundance and distribution of the xanthones is similar in both the solvent extract and hydrous pyrolysate. No detectable dibenzo-p-dioxin was liberated by hydrous pyrolysis.
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Fig. 5. Total ion chromatgrams of solvent extract of V17-1, V17-1 + dibenzo-p-dioxin and dibenzo-p-dioxin on DB-5ms (J&W), BP1 and BPX35 (both S.G.E.) columns: DBD, dibenzo-p-dioxin, DBF, dibenzofuran; MDBF, methyldibenzofuran; TMN, trimethylnaphthalene.
Fig. 6. Non-hydrocarbon fraction (DCM eluate from a C18 SPE cartridge) from sample V17-2.
3.6. Palaeoenvironmental significance It is interesting to note that there is a large and global fungal spike in the palynological record at the end of the Permian (Visscher et al., 1996; Steiner et al., 2003), which suggests that the terrestrial ecosystem was undergoing ecological stress brought about by the effects of massive volcanism associated with the emplacement of the Siberian Traps. This volcanic event was accompanied by widespread acidification (Visscher et al., 1996) and a period of increased UV-B flux following emission of volcanic organohalogen compounds (Visscher et al., 2004). The most probable 1,2,5-TMN precursor-
containing plants that could recolonise a stressed ecosystem are lower vascular plants such as ferns, horsetails and club mosses. A similar recolonisation of a stressed terrestrial ecosystem by lower vascular plants has been proposed as an explanation for the abrupt disappearance of late Cretaceous angiosperm pollen taxa, followed by an anomalous fern spore spike at the CretaceousTertiary boundary (Wolfe and Upchurch, 1986). Fungal material itself may also be a contributor to the organic input. Antifungal plant toxins and pigments can be based on the xanthone skeleton and dibenzofurans can also be structural units of antifungal agents produced by plants (e.g. Kokubub and Harborne, 1995).
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Fig. 7. Py-GC-MS total ion chromatograms: , n-alkane (n-C11–17); B, benzene; T, toluene; EB, ethylbenzene; X, xylene; P, phenol; BF, benzofuran; MP, methyphenol; MBFs, methylbenzofurans; N, naphthalene; DMBFs, dimethylbenzofurans; MN, methylnaphthalene; DMNs, dimethylnaphthalenes, DBF, dibenzofuran; DBD, dibenzo-p-dioxin; MDBFs, methyldibenzofurans.
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Fig. 8. Total ion chromatogram of hydrous pyrolysate from sample V17-2: , n-alkane (n-C16–25); TMN, trimethylnaphthalene; DBF, dibenzofuran; MDBFs, methydilbenzofurans; TeMN, tetramethylnaphthalene; A, anthracene; X, xanthone; MX, methylxanthone.
As abundances of both alkyl aromatic and oxygencontaining compounds increase relative to carbonate and n-alkane contents towards the top of the section it would appear that the flux of land-derived material to the marine setting was gathering pace. Hence, the overall palaeoenvironmental signal provided by alkyl naphthalene and oxygen-containing aromatic compounds could be the result of progressive opportunistic expansion of fungi and lower vascular plants during a period of ecological stress. 4. Conclusions An end-Permian marl from the Vigo Meano section in northern Italy contains an unusual distribution of aromatic and oxygen-containing compounds. Xanthones and dibenzofurans are present alongside numerous alkylated naphthalenes. Furthermore, dibenzo-p-dioxin has been detected for the first time in geological samples. The literature reveals that potential sources for xanthones and dibenzofurans include lower vascular plants, fungi and lichens. The most likely source for our unusual molecular assemblage is land-derived material mobilised as a consequence of a destabilised terrestrial ecosystem during the end-Permian mass extinction.
Acknowledgements This work was supported by PPARC. Charlie Pearce is thanked for d13Corg analysis. The authors also thank Barry Bennett, Heinz Wilkes and Lorenz Schwark for constructive reviews which improved this paper.
Associate Editor—L. Schwark
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Oxygen-containing aromatic compounds in a Late Permian sediment Jonathan S. Watson a
a,*
, Mark A. Sephton a, Cindy V. Looy b, Iain Gilmour
a
Planetary and Space Sciences Research Institute, Open University, Milton Keynes, Buckinghamshire MK7 6AA, UK b Botanical Palaeoecology, Laboratory of Palaeobotany and Palynology, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands Received 5 November 2003; accepted 8 October 2004 (returned to author for revision 10 February 2004) Available online 9 December 2004
Abstract In northern Italy the base of the Tesero Oolite Horizon represents the lithological boundary between the Bellerophon and Werfen Formations and marks one of the most dramatic environmental perturbations in Phanerozoic time. The marl, which directly underlies the Tesero Horizon, contains many indicators of environmental change during the end-Permian mass extinction. At Vigo Meano, near Trento, the marl is relatively thermally immature and contains abundant terrestrially-sourced oxygen containing aromatic compounds such as xanthones, dibenzofurans and dibenzo-p-dioxin as well as abundant alkylated naphthalenes. Ó 2004 Elsevier Ltd. All rights reserved.
1. Introduction The end of the Permian approximately 250 million years ago was marked by the largest mass extinction of the Phanerozoic. Both the marine (Raup, 1979) and terrestrial (Maxwell, 1992) biospheres were affected. Significant changes in oceanic and atmospheric chemistry are reflected in dramatic variations in the carbon isotope ratios of carbonate rock and marine and terrestrial organic matter (Sephton et al., 2002 and references therein). These changes appear to occur within a period of 165,000 years (Bowring et al., 1998). Possible driving forces for these extreme changes include widespread volcanic activity (Renne et al., 1995), extraterrestrial impact
* Corresponding author. Tel.: +44 1908 652424; fax: +44 1908 858022. E-mail address: [email protected] (J.S. Watson).
(Becker et al., 2001), overturn of a stagnant ocean (Knoll et al., 1996) and rapid decomposition of a gas hydrate reservoir (Erwin, 1993). Recent organic geochemical studies of sections in northern Italy have discovered unusual abundances of oxygen-containing aromatic compounds close to the extinction horizon (Sephton et al., 1999a, 2001). These oxygen-containing aromatic compounds may reflect palaeoenvironmental conditions specific to the endPermian event. In this paper, we report data from a new end-Permian section in northern Italy that is characterised by relatively low thermal maturity. Several previously unreported Late Permian, oxygen containing aromatic compounds are abundant constituents of the rocks and their chemical structure relationship to previously detected components and palaeoenvironmental significance are considered. There are a number of possible sources for oxygen containing aromatic compounds. Both furans and
0146-6380/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2004.10.006
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dioxins can be produced during forest fire events (e.g. Velazquez et al., 1998). Furans can also be produced naturally by fungi (e.g. Radke, 2000) and they could also arise from dehydrated polysaccharides (e.g. Sephton et al., 1999a). Although dioxins are commonly considered to be anthropogenic in origin, in the modern environment they can be produced by organisms (e.g. Utkina et al., 2001) and have also been shown to be produced in peat bogs (Silk et al., 1997). Xanthones can also be generated by a number of organisms, including fungi and lower vascular plants (e.g. Peres et al., 2000).
2. Experimental 2.1. Samples In northern Italy, Late Permian sedimentary deposits were laid down on a proximal shallow shelf at a low northerly palaeolatitude in the western Tethys Ocean (Bosellini and Hardie, 1973; Scholger et al., 2000). Latest Permian sedimentary rocks outcrop in the Vincentian Alps, northern Italy and represent two lithostratigraphic units: the Bellerophon Formation and the Werfen Formation (Posenato, 1988). At Vigo Meano near Trento, the Permian Bellerophon Formation comprises limestone breccias overlain by 4–5 m of vuggy dolomites and a 80-cm thick organic-rich marl. The marl is immediately overlain by the oldest member of the Werfen Formation, the Tesero Oolite Horizon, which is a transgressive bed thought to have been deposited almost synchronously throughout the southern Alps. Historically, in the southern Alps, the Permian–Triassic boundary was considered to approximate the transition between the Bellerophon Formation and the Werfen Formation. It is now accepted that the first appearance of the conodont Hindeodus parvus should mark the onset of the Triassic (Yin et al., 1996), a protocol which consigns the mass extinction horizon in the southern Alps to Late Permian rocks. The samples (Table 1) were collected from the thick organic-rich Bellerophon marlat Vigo Meano. This marl and other latest Permian rocks contain the debris of land plants (Visscher and Brugman, 1986; Sephton et al., 1999a, 2001, 2002). Lateral equivalents of the marl display petrographic and chemical features reflecting a mostly terrestrial source (Sephton et al., 1999a).
2.2. Solvent extraction For each sample, ca. 2 g of crushed whole rock was sonicated in 3 ml of dichloromethane (DCM)/methanol (95:5) for 30 min, centrifuged for 15 min and the supernatant pipetted into a vial. This procedure was repeated 3 times. Sulfur was removed by adding activated copper turnings to the solvent extract and sonicating for 5 min. An apolar fraction was isolated by eluting the extract down a short silica column (Pasteur pipette with 2 cm bed depth) with 5 ml of DCM. A further aliquot of the crushed sediment was spiked with internal standards (squalane, Acros Organics and naphthalene-d8, Supelco) prior to extraction. Samples were extracted and an apolar fraction isolated as described above. This apolar fraction was further fractionated using silver ion chromatography prior to analysis using gas chromatography-mass spectrometry (GCMS) of the n-alkanes. Briefly, the aliphatic hydrocarbons were isolated by eluting the apolar fraction through a short column of 10% AgNO3 impregnated silica gel (Pasteur pipette with 2 cm bed depth) with 2 ml of hexane. The apolar fraction from one sample (V17-2) was further fractionated into hydrocarbons and non-hydrocarbons using the C18 solid phase extraction (SPE) method described in Bennett and Larter (2000). Briefly, the fraction was loaded on to a 3 ml 500 mg nonendcapped C18 SPE cartridge (International Sorbent Technology, UK) and the hydrocarbons were eluted with n-hexane and the non-hydrocarbons with DCM. 2.3. GC-MS GC-MS analysis was carried out using an Agilent Technology 6890 gas chromatograph coupled to a 5973 mass spectrometer. Separation was performed on a S.G.E. (U.K.) BPX5 column (30-m length, 0.25-mm i.d. and 0.25-lm film thickness) with He carrier gas at a constant column flow rate of 1.1 ml min 1. The GC oven temperature was held for 1 min at 50°C and ramped to 300°C at a rate of 5°C min 1 and then held for 9 min. The aromatic hydrocarbons were semi-quantified using GC-MS and the ions shown in Table 2. The n-alkanes, pristane and phytane were analysed as above using a J&W DB-5 column (30-m length, 0.25-mm i.d. and 0.25-lm film thickness) and quantified from the total ion current.
Table 1 Bulk data for Vigo Meano end-Permian marl samples (NEOC, non-extractable organic carbon, EOM, extractable organic matter) d13Corg (&)
Sample
Carbonate (%)
NEOC (mg/g)
EOM (mg/g)
Top
V17-1 V17-2 V17-3A
4.3 7.3 71.2
2.2 1.8 0.8
0.9 0.8 0.6
26.7 26.5 26.7
1.47 1.42 1.38
Bottom
V17-3B
72.2
0.8
0.9
26.5
1.39
Pr/Ph
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Table 2 Concentrations (lg/g) of selected compounds (ions given in table were used for quantification) Compound
V17-1 (lg/g sed)
V17-2 (lg/g sed)
V17-3A (lg/g sed)
V17-3b (lg/g sed)
Naphthalene (m/z 128) 2-Methylnaphthalene (m/z 142) 1-Methylnaphthalene (m/z 142) 2- + 1-Ethylnaphthalene (m/z 141 + 156) 2,6- + 2,7-Dimethylnaphthalene (m/z 141 + 156) 1,3- + 1,7-Dimethylnaphthalene (m/z 141 + 156) 1,6-Dimethylnaphthalene (m/z 141 + 156) 1,4- + 2,3-Dimethylnaphthalene (m/z 141 + 156) 1,5-Dimethylnaphthalene (m/z 141 + 156) 1,2-Dimethylnaphthalene (m/z 141 + 156) 1,3,7-Trimethylnaphthalene (m/z 155 + 170) 1,3,6-Trimethylnaphthalene (m/z 155 + 170) 1,4,6- + 1,3,5-Trimethylnaphthalene (m/z 155 + 170) 2,3,6-Trimethylnaphthalene (m/z 155 + 170) 1,2,7-Trimethylnaphthalene (m/z 155 + 170) 1,6,7-Trimethylnaphthalene (m/z 155 + 170) 1,2,6-Trimethylnaphthalene (m/z 155 + 170) 1,2,4-Trimethylnaphthalene (m/z 155 + 170) 1,2,5-Trimethylnaphthalene (m/z 155 + 170) 1,3,6,7-Tetramethylnaphthalene (m/z 169 + 184) 1,2,4,6- + 1,2,4,7- + 1,4,6,7-Tetramethylnaphthalene (m/z 169 + 184) 1,2,5,7-Tetramethylnaphthalene (m/z 169 + 184) 2,3,6,7-Tetramethylnaphthalene (m/z 169 + 184) 1,2,6,7-Tetramethylnaphthalene (m/z 169 + 184) 1,2,3,7-Tetramethylnaphthalene (m/z 169 + 184) 1,2,3,6-Tetramethylnaphthalene (m/z 169 + 184) 1,2,5,6- + 1,2,3,5-Tetramethylnaphthalene (m/z 169 + 184) Phenanthrene (m/z 178) Anthracene (m/z 178) 3-Methylphenanthrene (m/z 192) 2-Methylphenanthrene (m/z 192) 2-Methylanthracene (m/z 192) 9-Methylphenanthrene (m/z 192) 1-Methylphenanthrene (m/z 192) Flouranthrene (m/z 202) Pyrene (m/z 202) Benz[a]anthracene (m/z 228) Chrysene + triphenylene (m/z 228) Benzofluoranthenes (m/z 252) Benzo[e]pyrene (m/z 252) Benzo[a]pyrene (m/z 252) Dibenzothiophene (m/z 184) Xanthone (m/z 168 + 196) 1-Methylxanthone (m/z 181 + 210) 4-Methylxanthone (m/z 181 + 210) 2-Methylxanthone (m/z 181 + 210) 3-Methylxanthone (m/z 181 + 210) Dibenzofuran (m/z 139 + 168) 4-Methyldibenzofuran (m/z 181 + 182) 2- + 3-Methyldibenzofuran (m/z 181 + 182) 1-Methyldibenzofuran (m/z 181 + 182) Dibenzo-p-dioxin (m/z 184) n-C13 n-C14 n-C15 n-C16
2.85 0.91 0.87 0.18 0.65 0.62 1.27 0.35 0.41 1.04 0.32 0.35 0.31 0.31 0.11 0.12 2.83 0.09 3.43 0.18 0.24 0.41 0.04 0.35 0.04 0.03 5.06 1.55 0.23 0.08 0.15 0.11 0.18 0.24 0.35 0.21 0.13 0.08 0.12 0.01 0.01 0.33 3.07 0.52 1.82 0.18 0.04 2.97 0.79 1.55 0.40 0.47 0.06 0.16 0.74 1.55
3.05 0.75 0.75 0.17 0.58 0.59 1.16 0.34 0.31 0.96 0.31 0.36 0.32 0.32 0.10 0.27 2.78 0.08 3.47 0.19 0.27 0.44 0.05 0.37 0.04 0.30 5.83 1.51 0.23 0.08 0.15 0.12 0.16 0.24 0.36 0.22 0.11 0.07 0.10 0.02 0.02 0.32 3.32 0.55 1.83 0.18 0.04 2.76 0.76 1.48 0.40 0.42 0.03 0.06 0.10 0.33
0.25 0.13 0.11 0.05 0.12 0.13 0.18 0.07 0.06 0.13 0.07 0.08 0.07 0.07 0.03 0.08 0.33 0.02 0.47 0.05 0.05 0.07 0.01 0.08 0.01 0.07 0.88 0.45 0.04 0.03 0.05 0.02 0.05 0.05 0.14 0.07 0.03 0.03 0.03 0.01 0.00 0.06 0.49 0.09 0.35 0.05 0.01 0.41 0.14 0.24 0.06 0.05 0.10 0.30 0.89 1.93 (continued on
0.45 0.20 0.17 0.09 0.18 0.21 0.26 0.10 0.09 0.18 0.13 0.16 0.15 0.12 0.04 0.09 0.72 0.03 0.84 0.11 0.11 0.17 0.02 0.17 0.02 0.15 1.89 1.02 0.09 0.07 0.11 0.05 0.10 0.12 0.25 0.13 0.07 0.04 0.08 0.01 0.01 0.13 0.70 0.23 0.66 0.07 0.02 0.87 0.29 0.50 0.13 0.11 0.05 0.51 1.28 2.59 next page)
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Table 2 (continued) Compound
V17-1 (lg/g sed)
V17-2 (lg/g sed)
V17-3A (lg/g sed)
V17-3b (lg/g sed)
n-C17 Pristane n-C18 Phytane n-C19 n-C20 n-C21 n-C22 n-C23 n-C24 n-C25 n-C26 n-C27 n-C28 n-C29 n-C30
1.70 0.75 1.23 0.51 0.69 0.47 0.43 0.37 0.36 0.28 0.25 0.17 0.17 0.09 0.11 0.05
0.53 0.47 0.55 0.33 0.44 0.33 0.32 0.26 0.25 0.17 0.15 0.10 0.11 0.05 0.06 0.03
1.88 0.99 1.17 0.72 0.46 0.29 0.18 0.13 0.10 0.08 0.08 0.06 0.05 0.03 0.03 0.02
2.70 1.31 1.79 0.94 0.79 0.39 0.26 0.19 0.18 0.17 0.16 0.10 0.10 0.06 0.06 0.04
Identification of dibenzo-p-dioxin was based on comparison with retention time, mass spectra and by coinjection of an authentic standard (ChemService, USA). Co-injection was carried out on three different phases (BP1, BPX35 both S.G.E. and a J&W DB-5ms, all three columns 30-m length, 0.25-mm i.d. and 0.25lm film thickness), with GC-MS conditions as described above. Identification of other oxygen containing aromatic compounds was based on comparison of authentic standards, retention time, mass spectra and published reports (e.g. Sephton et al., 1999a; Radke, 2000; Oldenburg et al., 2002). 2.4. Pyrolysis-GC-MS Solvent-extracted crushed sediment samples (ca. 5 mg) were placed in quartz pyrolysis tubes plugged at either end with quartz wool. Samples were pyrolysed using a CDS Pyroprobe 1000 fitted with a 1500 valve interface held at 250°C (CDS Analytical, Oxford, PA) and coupled to a GC-MS for identification of the pyrolysis products (the pyrolysate). Following heating to 610°C at a rate of 20°C ms 1 the sample was held at this temperature for 15 s in a flow of helium. GC-MS conditions (using a BPX5 column) were as described above except that the GC was held isothermally at 30°C for 10 min and then ramped at 4°C min 1 to 300°C where it was held for 9 min. 2.5. Hydrous pyrolysis Hydrous pyrolysis was carried out as described in Sephton et al. (1999b). Briefly, samples of solvent extracted V17-2 (0.5 g) along with 0.4 ml deionised water were pyrolysed in duplicate in 1.8 ml (internal volume) bomblets. The bomblets were assembled from T316
stainless steel tube and sealed with Swagelock SS-600-C end caps. They were placed inside a 71-ml Parr bomb and heated for 24 h at 320 °C. The bomblets and contents were air dried and extracted as for the sediment samples. 2.6. Non-extractable organic carbon and d13Corg Solvent extracted samples were decalcified with 0.1 M through 1 M hydrochloric acid and washed with distilled water (until pH 7) followed by a second solvent extraction step. Carbonate-free carbon concentrations and isotope ratios were determined using an Elemental Analyser-Isotope Ratio Mass Spectrometer (PDZ Europa ANCA-SL) coupled to a continuous-flow stable isotope ratio mass spectrometer (PDZ Europa 20-20) in combustion mode.
3. Results and discussion 3.1. General organic composition of Vigo Meano samples Table 1 reveals that, from the bottom to the top of the thick organic-rich marl, carbonate content decreases, non-extractable organic carbon increases, while extractable organic matter (EOM) remains relatively constant. The d13C values do not vary through the section. Given that the end-Permian crisis culminates in a marked negative shift in d13Corg values, we conclude that the Vigo Meano samples must represent an interval before, but close to, the inferred worldwide changes in oceanic and atmospheric carbon chemistry. The solvent extracts contain abundant alkylated naphthalenes, with a C4 alkyl naphthalene generally being the most abundant aromatic compound present (Fig. 1; Table 2). Other aromatic compounds include
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Fig. 1. Total ion chromatograms of solvent extracts: , n-alkane (n-C12–25); N, naphthalene; MNs, methylnaphthalenes; DBF, dibenzofuran; DBD, dibenzo-p-dioxin; TMN, trimethylnaphthalene; MDBFs, methyldibenzofurans; TeMN, tetramethylnaphthalene; P, phenanthrene; X, xanthone; MX, methylxanthone.
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phenanthrene and alkylated homologues, fluoranthene and pyrene. Straight chain alkanes (C12 to C32) with a mode at n-C17 and the acyclic isoprenoids pristane and phytane were also present. Other abundant compounds include oxygen-containing aromatics such as dibenzofurans, xanthones and dibenzo-p-dioxin. The compound distribution does seem to change slightly through the sequence, with n-alkanes being relatively more abundant in the carbonate-rich samples lower in the section. The abundance of oxygen compounds appears to correlate with the aromatic compounds, suggesting that there are potentially two sources of organic material, one contributing n-alkanes and the other contributing to the aromatic hydrocarbons and oxygen containing aromatic compounds. 3.2. Alkyl naphthalenes The naphthalenes (Fig. 2) display a relatively immature distribution (cf. van Aarssen et al., 1999); 1- and 2methylnaphthalenes (MNs) are present in approximately equal amounts, dimethylnaphthalenes (DMNs) are dominated by the 1,6- and 1,2-isomers and the trimethylnaphthalenes (TMNs) are dominated by the 1,2,6- and 1,2,5-isomers. A peak due to tetramethylnaphthalenes (TeMN) is the most abundant; this component was tentatively identified as 1,2,5,6-TeMN, although 1,2,3,5-TeMN or a mixture of the two isomers is also a possibility (cf. van Aarssen et al., 1999). Its concentration increases towards the top of the marl (Table 2).
Land plants are a potential source of TMNs, yet due to the age of the Vigo Meano rocks (ca. 250 Ma) certain constraints can be placed on which types of land plants could have contributed. The 1,2,7-TMN isomer can be derived from compounds such as b-amyrin which is present in angiosperms; however as these plants did not appear until the Late Cretaceous period (Pu¨ttmann and Villar, 1987; Strachan et al., 1988) significant inputs from this source are improbable. The 1,2,5-TMN can be derived from bicylic diterpenoids and resins which are common in conifers. A further land plant precursor for 1,2,5-TMN could be a compound such as onocerane, which has been attributed to lower vascular plants, including ferns, horsetails and lycopods (Pearson and Obaje, 1999). Alternatively 1,2,5-TMN may have a bacterial source as it can be derived from pentacyclic triterpenoids via cleavage of the C ring. Irrespective of the source of the parent molecule, the 1,2,6-TMN isomer can be formed by isomerisation of the 1,2,5-TMN isomer by a 1,2 methyl shift (Strachan et al., 1988). However, due to the high abundance of the 1,2,6-TMN isomer and the relatively low thermal maturity of the sediment this compound could reflect an alternative source signal. Land plants are also a potential source of TeMN and again certain types of precursors can be discounted due to the age of the samples. As C3 oxygenated triterpenoids are commonly derived from angiosperms (Pu¨ttmann and Villar, 1987), they are an implausible source of 1,2,5,6-TeMN in end-Permian rocks. In C3 oxygenated triterpenoids the required methyl shift is mediated
Fig. 2. Partial mass chromatogram (m/z 128 + 142 + 156 + 170 + 184) displaying the naphthalenes in solvent extract of sample V17-2.
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by a leaving group. Yet, under acidic conditions a leaving group may not be required (Pu¨ttmann and Villar, 1987, and references therein) and 1,2,5,6-TeMN could be potentially produced alongside 1,2,5-TMN from a biomarker such as onocerane; other candidates for precursors could include compounds such as arborene/fernene which can be abundant in other Permian sedimentary rocks (cf. Hauke et al., 1992). However, once again, 1,2,5,6-TeMN may have a bacterial source via cleavage of the C ring of pentacyclic triterpenoids under acidic conditions (Pu¨ttmann and Villar, 1987) or alternatively may be formed from bicyclanes (Bastow, 1998). In the context of the above discussions it is interesting to consider that at the end of the Permian, largescale volcanic emissions were occurring during the emplacement of the Siberian Traps. This episode of flood basalt eruption may have caused widespread destabilisation of terrestrial ecosystems. When a terrestrial ecosystem is in a stressed state, lower vascular plants are some of the first plants to recolonise an environment (e.g. DiMichele and Phillips, 2002) and a possible precursor for both 1,2,5,6-TeMN and 1,2,5-TMN could be a lower vascular plant biomarker. Recent palynological evidence confirms a pioneering role for lower vascular plants during the end-Permian crisis (Looy et al., 2001). Except for bacterially-derived hopanes, most other potential precursors (such as seco-hopanoids) for the alkyl naphthalenes appear to be absent from these sediments. Other potential sources for naphthalenes and other polycyclic aromatic hydrocarbons (PAHs) could include forest fires. However, if this was a significant source the parental aromatic hydrocarbons may be expected to dominate, especially three to four ring components (e.g. Jenkins et al., 1996; Hedberg et al., 2002). Also, there is also no evidence from the optical inspection of isolated organic matter for an input from burning (C.V. Looy, unpublished results). 3.3. Hopanes and steranes The distribution of hopane and sterane hydrocarbons implies that the organic matter in the Vigo Meano samples is relatively immature (Fig. 3), an observation consistent with the alkylated naphthalene distribution. The hopane hydrocarbons are dominated by Tm [17a(H)-22,29,30-trisnorhopane] and C29, C30 and C31 hopanes. Extended hopanes are not present, which is common for terrestrially-derived material where oxidation of the hopane side chain occurs. The presence of abundant ba isomers indicates that the sediment is relatively immature although the S/R isomers appear to be at equilibrium, indicating a 60.6 vitrinite reflectance equivalent. Sterane hydrocarbons are dominated by the C29 aaa 20R isomer which has the
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biological configuration, again indicating that the sediment is relatively immature. The abundance of C29 steranes relative to the C27 and C28 steranes may indicate a terrestrial source from higher plants (e.g. Czochanska et al., 1988) or a possible algal input (e.g. Grantham, 1986). 3.4. Aromatic oxygen compounds The distribution of dibenzofurans and xanthones in sample V17-2 is displayed in Fig. 4. Dibenzofurans are known to occur only within a specific thermal window and they become selectively concentrated as maturity increases until 250°C, above which they are destroyed (Siskin and Kitritzky, 1991). Abundant dibenzofurans have been identified before in Late Permian sediments and are thought to be derived mainly from land plant polysaccharides (Sephton et al., 1999a), although contributions from other sources can not be ruled out. For example, dibenzofurans can also be produced by plants as antimicrobial agents (Dixon, 2001) and are found in lichens (e.g. Culberson, 1969; Radke, 2000). Xanthone precursors are also produced by higher plants where they act as pigments and antimicrobial agents (e.g. Peres et al., 2000). Functionalised xanthones are also found in fungi, lichens and lower vascular plants (e.g. Berti and Bottari, 1968; Culberson, 1969; Chexal et al., 1975; Devon and Scott, 1975). Previous reports of xanthones in fossil organic matter include that by Oldenburg et al. (2002) who reported their presence in a sediment with a terrestrial input and in crude oils. Xanthones have also been produced during the pyrolysis of a Late Permian wood (Sephton et al., 1997) and have been reported to be produced in small quantities from wood burning (Fine et al., 2001). To our knowledge this is the first report of dibenzop-dioxin in geological samples. Only the parent molecule is present and alkylated homologues appear to be absent. The source of dibenzo-p-dioxin is unclear but is most probably related to the xanthones and dibenzofurans. Dibenzo-p-dioxin was identified by co-injection using three different column phases (Fig. 5). There are a number of potential sources of dioxins (see Green et al., 2004, and references therein). Polychlorinated dibenzo-p-dioxins (PCDDs) have been detected in peat bogs where they are thought to be produced by microbial dimerisation of chlorophenols (Silk et al., 1997) and they have also been reported to occur in clays (Ferrario et al., 2002; Green et al., 2004). In addition, they can be produced during forest fires but only at very low levels (e.g. Martı´nez et al., 2000; Gabos et al., 2001) and they have also been shown to be produced during peat burning (Meharg and Killham, 2003). It is not unreasonable to consider that, during diagenesis if PCDDs are present, dechlorination would occur,
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Fig. 3. Partial mass chromatograms displaying hopane (m/z 191) and sterane (m/z 217) hydrocarbons in solvent extract from sample V17-2.
leading to the formation of the parent molecule, dibenzo-p-dioxin, detected in these Permian samples. The high abundances of the dibenzofurans, xanthones and dibenzo-p-dioxin (Table 2) are very unusual. In a similar fashion to the concentration of TeMN, but in opposition to the concentrations of n-alkanes, the abundances of xanthones (and DBF, DBD) are higher in the upper part of the section. Further fractionation of the solvent extract from V17-2 using a C18 SPE cartridge enabled the identification of benzophenone, fluorenone and the tentative identification of 7H-benzo[de]anthracen-7-one (Fig. 6). These compounds are relatively minor when compared
to the xanthones. Alkylated fluorenones appear to be absent or at very low concentration. This confirms that the xanthones and related compounds can be isolated from the apolar fraction using the method described in Bennett and Larter (2000). 3.5. Pyrolysis The main products released during on-line ‘‘flash’’ Py-GC-MS of solvent extracted, crushed samples were benzene, toluene, xylene, naphthalenes, phenols and benzofurans (Fig. 7). Other more minor components include n-alkanes, dibenzofurans and dibenzo-p-dioxin.
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Fig. 4. Partial mass chromatograms displaying dibenzofurans (m/z 168 + 182 + 196) and xanthones (m/z 196 + 210 + 226) in solvent extract from sample V17-2.
The distribution of products appears to be similar throughout the sequence. In each of the Vigo Meano samples only low levels of xanthones were detected in the pyrolysis products. This relatively low abundances could be caused by the loss of a keto-group during thermal degradation, leaving behind a dibenzofuran moiety (e.g. de Champlain and de Mayo, 1972). Off-line hydrous pyrolysis of a solvent extracted sample V17-2 liberated compounds similar to those in the solvent extract (Fig. 8). This indicates that some of the naphthalenes and oxygen containing compounds in the solvent extracts may have been cleaved from the high molecular weight kerogen when the organic matter approached the oil window. The distribution of compounds in the hydrous pyrolysate is different from the
on-line Py-GC-MS products. The main discrepancy is due to the loss of volatile components such as BTX (benzene, toluene and xylene), phenols and benzofurans during the off-line solvent removal step following pyrolysate extraction. The distribution of compounds in the hydrous pyrolysate suggests a lower organic maturity than for the solvent extract. In the hydrous pyrolysate, for example, anthracene is much more abundant than phenathrene and 1,2,6-TMN is not as dominant as in the extract, indicating a limited extent of maturity-driven 1,2 methyl shifts. In contrast to the more acutely temperature sensitive compounds, the relative abundance and distribution of the xanthones is similar in both the solvent extract and hydrous pyrolysate. No detectable dibenzo-p-dioxin was liberated by hydrous pyrolysis.
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Fig. 5. Total ion chromatgrams of solvent extract of V17-1, V17-1 + dibenzo-p-dioxin and dibenzo-p-dioxin on DB-5ms (J&W), BP1 and BPX35 (both S.G.E.) columns: DBD, dibenzo-p-dioxin, DBF, dibenzofuran; MDBF, methyldibenzofuran; TMN, trimethylnaphthalene.
Fig. 6. Non-hydrocarbon fraction (DCM eluate from a C18 SPE cartridge) from sample V17-2.
3.6. Palaeoenvironmental significance It is interesting to note that there is a large and global fungal spike in the palynological record at the end of the Permian (Visscher et al., 1996; Steiner et al., 2003), which suggests that the terrestrial ecosystem was undergoing ecological stress brought about by the effects of massive volcanism associated with the emplacement of the Siberian Traps. This volcanic event was accompanied by widespread acidification (Visscher et al., 1996) and a period of increased UV-B flux following emission of volcanic organohalogen compounds (Visscher et al., 2004). The most probable 1,2,5-TMN precursor-
containing plants that could recolonise a stressed ecosystem are lower vascular plants such as ferns, horsetails and club mosses. A similar recolonisation of a stressed terrestrial ecosystem by lower vascular plants has been proposed as an explanation for the abrupt disappearance of late Cretaceous angiosperm pollen taxa, followed by an anomalous fern spore spike at the CretaceousTertiary boundary (Wolfe and Upchurch, 1986). Fungal material itself may also be a contributor to the organic input. Antifungal plant toxins and pigments can be based on the xanthone skeleton and dibenzofurans can also be structural units of antifungal agents produced by plants (e.g. Kokubub and Harborne, 1995).
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Fig. 7. Py-GC-MS total ion chromatograms: , n-alkane (n-C11–17); B, benzene; T, toluene; EB, ethylbenzene; X, xylene; P, phenol; BF, benzofuran; MP, methyphenol; MBFs, methylbenzofurans; N, naphthalene; DMBFs, dimethylbenzofurans; MN, methylnaphthalene; DMNs, dimethylnaphthalenes, DBF, dibenzofuran; DBD, dibenzo-p-dioxin; MDBFs, methyldibenzofurans.
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Fig. 8. Total ion chromatogram of hydrous pyrolysate from sample V17-2: , n-alkane (n-C16–25); TMN, trimethylnaphthalene; DBF, dibenzofuran; MDBFs, methydilbenzofurans; TeMN, tetramethylnaphthalene; A, anthracene; X, xanthone; MX, methylxanthone.
As abundances of both alkyl aromatic and oxygencontaining compounds increase relative to carbonate and n-alkane contents towards the top of the section it would appear that the flux of land-derived material to the marine setting was gathering pace. Hence, the overall palaeoenvironmental signal provided by alkyl naphthalene and oxygen-containing aromatic compounds could be the result of progressive opportunistic expansion of fungi and lower vascular plants during a period of ecological stress. 4. Conclusions An end-Permian marl from the Vigo Meano section in northern Italy contains an unusual distribution of aromatic and oxygen-containing compounds. Xanthones and dibenzofurans are present alongside numerous alkylated naphthalenes. Furthermore, dibenzo-p-dioxin has been detected for the first time in geological samples. The literature reveals that potential sources for xanthones and dibenzofurans include lower vascular plants, fungi and lichens. The most likely source for our unusual molecular assemblage is land-derived material mobilised as a consequence of a destabilised terrestrial ecosystem during the end-Permian mass extinction.
Acknowledgements This work was supported by PPARC. Charlie Pearce is thanked for d13Corg analysis. The authors also thank Barry Bennett, Heinz Wilkes and Lorenz Schwark for constructive reviews which improved this paper.
Associate Editor—L. Schwark
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