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Polycyclic aromatic hydrocarbons of pyrolytic origin in ancient sediments: evidence for Jurassic vegetation fires
Org. Geochem. Vol. 18, No. 1, pp. 1-7, 1992 Printed in Great Britain. All rights reserved
0146-6380/92$5.00+ 0.00 Copyright© 1992PergamonPresspic
Polycyclic aromatic hydrocarbons of pyrolytic origin in ancient sediments: evidence for Jurassic vegetation fires S. D. K I L L O P S 1 and M. S. MASSOUD2 ~Chemistry Department and 2Geology Department, Royal Holloway and Bedford New College, University of London, Egham Hill, Egham, Surrey TW20 0EX, England (Received 5 April 1991; returned for revision 30 May 1991; accepted 4 July 1991)
A ~ - - P o l y c y c l i c aromatic hydrocarbon (PAH) distributions in an Upper Jurassic succession from offshore Korea Bay Basin were consistent with a pyrolytic source, being dominated by non-alkylated species. Levels of these PAHs were significantly higher in samples containing abundant fusinite macerals, which are believed by some to be the products of combustion. The data were consistent with a PAH orion from periodic conflagration throughout the Upper Jurassic of vegetation adjacent to a tectonically formed lake. Distributions of PAHs exhibited some differences from those of combustion origin in Recent and some ancient sediments, and were characterized by the dominance of the highly peri-condensed compounds benzo[e]pyrene, benzo[g,h,i]peryleneand coronene. This characteristic reflects either preferential formation, or preferential preservation under the prevailing sedimentary conditions, of the highly peri-condensed structures. Key words--polycyclic aromatic hydrocarbons, peri-condensed PAH, pyrolytic PAH, vegetation fires, fusinite, Jurassic sediments
INTRODUCTION
fossil fuels (Blumer, 1975; Laflamme and Hites, 1978; Ramdahl, 1983; Sporstol et al., 1983; Van Vleet et al., 1984). While these PAHs provide evidence for combustion, they are not themselves combustion products, being formed under conditions of oxygen depletion by the action of heat resulting from the combustion process. We therefore refer to these compounds as pyrolytic products. Highly peri-condensed PAHs are more reacti~ce than their analogues exhibiting lower degrees of angular fusion, and so their high concentrations among pyrolytic products are attributed to rapid quenching by adsorption (through hydrogen bonding) on to particles of soot, which is itself a polycondensed PAH material (Biumer, 1975; Venkatesan and Dahl, 1989). Exposure to a variety of environmental agents, such as photo-oxidation and aqueous dissolution, during aeolian or fluvial transport appears to have little effect on the distributions of these particulate associated PAHs finally preserved in Recent sediments (Prahl and Carpenter, 1983; Readman et al., 1984). Consequently, the distributions of pyrolytic PAHs observed in a variety of Recent sedimentary environments are remarkably constant (Youngblood and Blumer, 1975; Laflamme and Hires, 1978; Wakeham et al., 1980a; Gschwend and Hites, 1981). It also appears that a recognizable PAH fingerprint of combustion can survive over geological time scales (Venkatesan and Dahl, 1989). We report here evidence for vegetation combustion in the form of pyrolytic PAH distributions in mudstones from an Upper Jurassic succession (2331-3016m depth interval) from offshore Korea Bay Basin. The data were obtained during evaluation
Polycyclic aromatic hydrocarbons (PAHs) of pyrolytic origin have been extensively studied in Recent sediments, but relatively little attention has been paid to the possible application of these molecular indicators in identifying palaeofires. It has been proposed that a background of pyrolytic PAHs exists in most ancient sediments, which has hitherto been overlooked (Radke, 1987). Palaeofires, primarily initiated by lightning strikes, are likely to have been important sources of pyrolytic PAHs, and have probably been a feature of terrestrial ecosystems from at least the Late Devonian (Chaloner, 1989; Scott, 1989). Temperatures required for the formation of PAHs can also occasionally arise in sedimentary environments as a result of igneous activity. Such an origin has been proposed for pyrolytic PAHs associated with ash, coal and wood fragments in sedimentary material from the Midland Valley of Scotland (Murchison and Raymond, 1989; Raymond et al., 1989). Similarly, pyrolytic PAHs in oil seeps in the Guaymas Basin have been attributed to in situ hydrothermal activity (Kawka and Simoneit, 1990). Pyrolytic PAHs are characterized by the dominance of non-alkylated molecules, especially highly peri-condensed compounds (i.e. compact structures resuting from extensive angular fusion, e.g. pyrene, benzopyrene, benzo[g,h,i]perylene and coronene; see Fig. 1). The presence of such PAH distributions in the aromatic hydrocarbon fractions of Recent sediments is generally considered to reflect inputs from the combustion of organic matter, such as wood and oo ts/t-A
1
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S.D. KILLOPSand M. S, MASSOUD
of the oil source rock potential of the succession, and full details of the organic geochemical assessment of this succession and its geological setting have been reported elsewhere (Killops et al., 1991). In brief, the succession represents a river delta prograding into a deep, freshwater lake, in a sub-tropical climate. Occasional turbidites were associated with a steep delta front. The lower part of the succession ( > 2 8 5 0 m depth) appears to have been associated with partly oxic, open water conditions and the dominance of terrestrially derived organic matter. Subsequently, deposition appears to have occurred within the delta. Here, sedimentary conditions were anoxic, and significant bacterial reworking of higher plant material occurred, leading to the accumulation of lipid-rich organic matter and the formation of a source rock for oil found in overlying Cretaceous sandstones. EXPERIMENTAL
Full experimental details for the Korea Bay Basin samples have been reported elsewhere (Killops et al., 1991). In summary, Upper Jurassic mudstone
samples took the form of cuttings, the larger being selected and any potential external contamination removed by washing with dichloromethane. Rock samples (10 g) were then milled to a fine powder and extracted ultrasonically for 20min with dichloromethane (50ml x 3). A total hydrocarbon fraction was obtained from the combined extracts by column chromatography on alumina (Merck, Brockman activity I) and elution with dichloromethane (3 x volume of alumina). Aromatic fractions were subsequently isolated by TLC on silica gel (Merck 60G) and elution with hexane (Rf ~< tridecylbenzene). PAH distributions were determined by G C - M S using a Carlo Erba Mega 5360 GC coupled to a Finnigan INCOS 50 quadrupole MS. On-column injection was used into a 25 m x 0.32 mm i.d. WCOT capillary column coated with 5%-phenylmethylsilicone, which fed directly into the MS ion source. Quantification was achieved by comparing the total ion current (TIC) peak area of an internal standard, squalane, with equivalent TIC peak areas of the PAHs. The latter were calculated from molecular ion responses and calibrated against TIC responses from
A
I[cd]Py
B[e]Py B[ghi]Pe
Co
Fig. 1. Structures of major PAHs detected in Upper Jurassic samples. P, phenanthrene; A, anthracene; PhN, 2-phenylnaphthalene; Fa, fluoranthene; lay, pyrene; B[a]A, benz[a]anthracene; C, chrysene; TPn, tripbenylene, BFa, benzo[b]fluoranthene + benzo[k]ttuoranthene; B[e]Py, benzo[e]pyrene; B[a]Py, benzo[a]pyrene; Pc, perylene; I[cd]Py, indeno[c,d]pyrene; l~ghi]Pe, benzo[g,h,i]perylene; Co, coronene; R, retene.
PAHs of pyrolytic origin samples in which PAH abundance was high and co-elutions did not occur. All samples were analysed at the same time under identical conditions. Fusinite was isolated from a sandstone from the Scalby Formation (Middle Jurassic; Yorkshire, England). The outer surface of the isolated fusinite was removed and discarded to limit the effects of any external contamination. A sample of powdered fusinite (500 mg) was extracted with dichloromethane (2 ml portions) as above, but no further fractionation was undertaken prior to GC-MS analysis of PAHs.
RESULTS AND DISCUSSION
Source o f P A H s in Upper Jurassic o f Korea Bay Basin
Gas chromatograms of the aromatic hydrocarbons of cutting samples from the deeper part of the succession (2847-3016m) were found to exhibit a distribution of PAHs dominated by highly peri-condensed structures [Figs 1, 2(b) and 2(c)]. The high ratio of parent to alkylated PAHs is more typical of a pyrolytic rather than a petroleum or coal source (e.g. Laflamme and Hites, 1978; Van Vleet et al., 1984). This part of the succession is believed to correspond to freshwater (probably deep), lacustrine deposition of mostly terrestrial material under partly oxic conditions, which favours preservation of the more refractory organic components (Killops et al., 1991). In the shallower part of the succession (2331-2846m), aromatic hydrocarbons were dominated by components typical of a potential oil source rock, particularly alkylnaphthalenes and alkylphenanthrenes. This part of the succession is thought to correspond to the thick, pro-delta muds of a prograding lacustrine delta and the development of anoxic sedimentary conditions (Killops et al., 1991). Closer examination by GC-MS showed that these bacterial lipid-rich sedimentary rocks also contained similar distributions of PAHs to the deeper samples, but at lower levels (ca 0.5 ppm of rock for 6 intervals over 2331-2846 m depth, cf. ca 25 ppm for 2 intervals over 2847-3016m). This suggests an input from a single source of PAHs throughout the whole of the Upper Jurassic succession (2331-3016 m). The variable rate of this PAH input to the sediments may reflect absolute changes in the input and/or dilution by varying inputs of other sedimentary material associated with the changing depositional environment. Although sedimentological evidence of igneous activity has been observed in some parts of Korea Bay Basin, there was no such evidence in the succession studied. However, a higher than average geothermal gradient was indicated by biomarker studies of the Upper Jurassic samples, suggesting that in situ temperatures probably exceeded 100°C (Killops et al., 1991). The oil seeps and associated PAHs in the Guaymas Basin, California, are believed to have been formed from contemporary organic matter (with a
3
significant terrestrial component) at temperatures in excess of 300°C by hydrothermal activity associated with the spreading centre in the Basin (Kawka and Simoneit, 1990). Such an origin for the PAHs in the Korea Bay Basin samples seems unlikely, as prolonged heating at these temperatures would have destroyed the oil-generating capacity exhibited by the Upper Jurassic succession. In addition, the thermal maturity of the indigenous organic matter appears only to correspond to the onset of oil generation (Killops et al., 1991). The petroleum hydrocarbons of the Guaymas seeps survive because they are able to migrate away from the heat source. Unfortunately, it is not possible to establish the proportion of PAHs in the Guaymas oil seeps generated by hydrous pyrolysis and the amount that may have resulted from an allochthonous input of vegetation combustion products to the sedimentary organic material. Evidence of a terrestrial higher plant input to the whole of the Upper Jurassic succession from Korea Bay Basin had previously been obtained from petrographic, sedimentological and biomarker studies (Killops et al., 1991). Levels of pyrolytic PAHs are recorded in Fig. 3, and apparently correlate with the relative abundance of terrestrial plant material. For example, the amount of C29 relative to C27 and C2s steranes, which probably reflects levels of allochthonous higher plant input relative to autochthonous sources of organic matter (Killops et al., 1991), shows similar depth trends to total pyrolytic PAH concentrations. In addition, the deeper part of the succession (2847-3016m), which exhibited the higher levels of PAHs, contained relatively abundant fusinites and semi-fusinites (up to 20% of total macerals). These macerals were absent from the shallower samples, which appeared to contain lesser amounts of small, disintegrated inerto-detrinite and exhibited lower PAH levels. Interestingly, fusinites and semi-fusinites have been proposed as products of vegetation fires (e.g. Scott, 1989). The apparent correlation of PAH concentrations with levels of higher plant material suggests that the PAHs arise from an allochthonous input of the products of periodic vegetation fires in the area adjacent to the lake. A combination of fluvial and aeolian transportation to the depositional site seems probable for this input. The relative amounts of alkylated homologues of the peri-condensed PAHs [Fig. 2(c)] is consistent with moderate to high combustion temperatures of 400-800°C, as established for contemporary vegetation (Youngblood and Blumer, 1975; Venkatesan and Dahl, 1989). The variation in depositional conditions throughout the Upper Jurassic succession in Korea Bay Basin does not seem to have noticeably affected the observed PAH distributions. This suggests a fixed, inherited distribution common to all samples, presumably attributable to stabilization of PAHs by particulate associations at the time of formation. The
4
S.D. KILLOPSand M. S. MASSOUD
stability of soot-bound PAHs towards aqueous leaching has been noted in Recent sediments (Prahl and Carpenter, 1983; Sochas and Carpenter, 1987). Pyrolytic PAHs have also been reported to be associated with charred woody material in low maturity samples (i.e. those which have not experienced significant heating by igneous activity) from the Midland Valley,
Scotland (Murchison and Raymond, 1989). It was not possible to test such an association for the Korea Bay basin samples because the disseminated fusinite could not be isolated. However, it was examined for large fragments of fusinite isolated from a sandstone, of fluvial or deltaic deposition, from the Middle Jurassic Scalby Formation, Yorkshire,
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Fig. 2. PAH distributions in (a) a Recent sediment exhibiting typical pyrolytic distributions (intertidal surface sediment, Bridgwater Bay, Severn Estuary, U.K.; after Killops and Howell, 1988) and (b and c) a representative Upper Jurassic sample (2931-3016 m depth and interval shown) from offshore Korea Bay Basin. Chromatograms (a) and (b) represent summed molecular ion responses from PAHs and (c) is a total ion current chromatogram from GC-MS analyses. See Fig. 1for key to abbreviations and structures. U, unidentifed PAH of mol. wt 302. Alkyl homologues of PAHs are represented by C~-C3 prefixes in (c). PAHs were identified by their mass spectra and retention indices (Lee et al., 1979).
PAHs of pyrolytic origin and coronene; Fig. 1) were more abundant than usual, while anthracene and 4,5-methylenephenanthrene, both pyrolytic PAHs (Youngblood and Blumer, 1975; Gschwend and Hites, 1981), were not detected. Comparison of the relative abundance of individual pyrolytic PAHs in the Korea Bay Basin samples with those typical of Recent sediments is shown in Fig. 4, using the amount of pyrene as an arbitrary reference level. The Korea Bay Basin samples are not alone, however, in exhibiting enhanced levels of coronene and related PAHs. Similar distributions of non-alkylated PAHs, believed to result from forest fires, have been reported for a Cretaceous/Tertiary boundary sample from Gubbio, Italy (Venkatesan and Dahl, 1989). The high relative abundance of phenanthrene (and its alkyl homologues) and chrysene + triphenylene in Fig. 2(b) probably results from the petroleum generating component within the succession, which would be anticipated to contribute phenanthrene and chrysene. It seems possible to distinguish two general types of pyrolytic PAH distributions in sediments and sedimentary rocks. One type is characterized by the usual Recent sediment distributions [as in Fig. 2(a)]. Further examples are combustion derived PAHs
England (Scott, 1989). Typical pyrolytic PAH distributions were recorded [similar to those in Fig. 2(a)], with a combined concentration of 3.7 ppm, which is significantly lower than the concentrations recorded for the deeper samples from Korea Bay Basin (ca 25 ppm). Consequently, it is not possible to estimate the proportions of PAHs likely to be associated with soot particles and with burnt/charred wood remains in the Korea Bay Basin samples. Retene (Fig. 1) was more abundant in the deeper than the shallower samples from Korea Bay Basin (e.g. ca 2.0ppm for 2931-3016m interval cf. ca 0.1 ppm for 2331-2424m interval), and while it may, therefore, derive from sedimentary diagenesis of gymnosperm resins (Simoneit, 1977; Laflamm¢ and Hites, 1978; Wakeham et al., 1980b), an origin from combustion of resinous trees cannot be discounted (Ramdahl, 1983). Variations in pyrolytic P A H distributions
The PAH distributions observed in the Upper Jurassic succession from Korea Bay Basin were not identical to those typically attributed to pyrolytic sources in Recent sediments [Fig. 2(a)]. The highly peri-condensed compounds sharing the basic coronene structure (benzo[e]pyrene, benzo[g,h,i]perylene
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Fig. 3. Plots vs depth of TOC, amount of C29 relative to C27---C29steranes (reflecting relative higher plant input) and approximate total pyrolytic PAH content for the Upper Jurassic succession from Korea Bay Basin. (Sample depths are mean values for each interval. Sterane ratios were calculated from m/z 218 GC-MS response for 20S and 20R isomers of C27-C295e,(H),14fl(H),17fl(H)-4-desmethylsteranes. Total pyrolytic PAHs were P, Fa, Py, C + TPn, BFa, B[e]Py, B[a]Py, Pc, I[cdlPY, B[ghi]Pe, Co; see Fig. l for key to abbreviations.)
6
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A Fig. 4. Relative distribution of individual PAHs in the Upper Jurassic succession from Korea Bay Basin compared with typical pyrolytic distributions in Recent sediments, based on samples in Fig. 2. Enrichment factors were calculated from molecular ion intensities for samples, taking pyrene as a reference level (arbitrary assignment or zero enrichment). associated with fusinites in the Middle Jurassic Scalby Formation (Yorkshire, England; discussed above), and hydrothermally generated PAHs in Quaternary turbidites of the Escanaba Trough (N.E. Pacific; Simoneit, 1990). The second type is characterized by enhanced levels of highly peri-condensed PAHs, as in the Korea Bay Basin samples, the combustion derived PAHs in the Cretaceous/Tertiary sample from Gubbio (Italy) and the hydrothermally derived PAHs in seep oils from the Guaymas Basin (California). Time alone is obviously not a decisive factor in determining observed PAH distributions. Variation in pyrolytic PAH distributions must be due to different pyrolysis conditions and/or differing stabilities of individual PAHs towards post-depositional conditions. The temperature of, and exposure time to, pyrolysis are probably the most important factors affecting distributions during PAH formation (Youngblood and Blumer, 1975; Venkatesan and Dahl, 1989). Ancient higher plants would not be expected to yield different combustion products from their contemporary counterparts under the same pyrolysis conditions, as the major organic tissue constituents are the same. For PAHs resulting from natural vegetation fires, post-depositional factors may be important in controlling the distributions observed in
sediments and rocks. One such factor is photooxidation, which occurs quite readily in anthracene at the 9 and 10 positions (Bjorseth and Eklund, 1979 and references therein), while coronene is relatively resistant towards oxidation (Campbell and Andrew, 1979). Post-depositional temperature may also be important. It has been suggested that highly pericondensed structures are preferentially removed by equilibration processes over long residence times at elevated temperatures in the sub-surface (Blumer, 1975). However, the Upper Jurassic succession in Korea Bay Basin has been buried quite deeply, in an area of above average heat flow, and appears to have experienced temperatures in excess of 100°C (Killops et al., 1991). The abundance of highly peri-condensed PAHs in this succession indicates that they can survive such thermal regimes. The effects on PAH distributions of water leaching in Recent sediments are extremely difficult to assess (e.g. Radke, 1987), due to the very low solubilities of the compounds concerned and the complication of adsorption processes. They are, therefore, not readily predictable for ancient sediments, although it would be anticipated that water leaching would be an important factor over geological time scales, particularly for hydrothermally generated PAHs.
PAHs of pyrolytic origin CONCLUSIONS An Upper Jurassic succession of lacustrine mudstones, representing a prograding delta (ca 500 m thick), in Korea Bay Basin was found to contain a range of non-alkylated PAHs typical of pyrolytic origins. The source of this input is probably periodic vegetation fires. The highest levels of pyrolytic PAHs (ca 25 ppm) were recorded for samples which were found to contain fusinite and semi-fusinite, proposed products of vegetation combustion. The PAHs produced during combustion are likely to be associated with soot and possibly also with charred woody remains. The PAH distributions in the Korea Bay Basin samples differed from those usually found in Recent sediments and in some ancient sedimentary rocks in having enhanced levels of the highly peri-condensed components coronene, benzo[g,h,i]perylene and benzo[e]pyrene. However, other examples of this type of distribution are known for samples of varying geological age. It is not clear how the two types of PAH distributions observed in sedimentary material arise; a number of factors may be responsible. Temperature and time during pyrolysis are likely to be important factors in PAH formation, while after deposition geochemical factors, such as water leaching, may be important, particularly for ancient sediments. It is not possible to determine whether formation conditions or post-depositional conditions resulted in the enhanced levels of highly peri-condensed PAHs in the Korea Bay Basin samples. Examination of further samples of known sedimentary history is required to establish a framework for evaluating the factors controlling observed PAH distributions. Acknowledgements--The authors are grateful to Dr M.
Gamali of Topeco for permission to publish these results, and to Dr A. C. Scott of the Geology Department, Royal Holloway and Bedford New College, for the sample of Scalby Formation fusinite. REFERENCES
Bjorseth A. and Eklund G. (1979) Analysis of polynuclear aromatic hydrocarbons by glass capillary gas chromatography using simultaneous flame ionisation and electron capture detection. J. High Resolut. Chromatogr. Chromatogr. Commun. 22-26. Blumer M. (1975) Curtisite, Idrialite and Pendletonite, polycyclic aromatic hydrocarbon minerals: their composition and origin. Chem. Geol. 16, 245-256. Campbell N. and Andrew H. F. (1979) Polycyclicaromatic compounds with four or more (usually six-memlmred) fused carbocyclic ring systems. In Rodd's Chemistry of Carbon Compounds. Vol. IlL Aromatic Compounds, Part H (Edited by Coffey S.), p. 310. Elsevier Scientific,
Amsterdam. Chaloner W. G. (1989) Fossil charcoal as an indicator of palaeoatmospheric oxygen level. J. Geol. Soc. London 146, 171-174, Gschwend P. M. and Hites R. A. (1981) Fluxes of polycyclic aromatic hydrocarbons to marine and lacustrine sediments in the northern United States. Geoehim. Cosmochim. Acta 45, 2359-2367. Kawka O. E. and Simoneit B. R. T. (1990) Polycyclic aromatic hydrocarbons in hydrothermal petroleums from
the Guaymas Basin spreading center. Appl. Geochem. 5, 17-27. Killops S. D. and Howell V. J. (1988) Sources and distributions of hydrocarbons in Bridgwater Bay (Severn Estuary, U.K.) intertidal surface sediments. Estuarine Coastal Shelf Sci. 27, 237-261. Killops S. D., Massoud M. S. and Scott A. C. (1991) Biomarker characterisation of an oil and its possible source rock from offshore Korea Bay Basin. Appl. Geochem. 6, 143-157. Laflamme R. E. and Hites R. A. (1978) The global distribution of polycyclic aromatic hydrocarbons in recent sediments. Geochim. Cosmochim. Acta 42, 289-303. Lee M. L., Vassilaros D. L., White C. M. and Novotny M. (1979) Retention indices from programmed-temperature capillary-column gas chromatography of polycyclic aromatic hydrocarbons. Anal. Chem. 51, 768-773. Murchison D. G. and Raymond A. C. (1989) Igneous activity and organic maturation in the Midland Valley of Scotland. Int. J. Coal Geol. 14, 47-82. Prahl F. G. and Carpenter R. (1983) Polycyclic aromatic hydrocarbons (PAH)-phase associations in Washington coastal sediment. Geochim. Cosmochim. Acta 47, 1013-1023. Radke M. (1987) Organic geochemistry of aromatic hydrocarbons. In Advances in Petroleum Geochemistry (Edited by Brooks J. and Welte D.), Vol.2, pp. 141-207. Academic Press, New York. Ramdahl T. (1983) Retene--a molecular marker of wood combustion in ambient air. Nature 306, 580-582. RaymondA. C., Liu S. Y., MurchisonD. G. and Taylor G. H. (1989) The influence of microbial degradation and volcanic activity on a Carboniferous wood. Fuel 68, 65-73. Readman J. W., Mantoura R. F. C. and Rhead M. M. (1984) The physico-chemicalspeciation of polycyclicaromatic hydrocarbons (PAH) in aquatic systems. Fresenius Z. Anal. Chem. 319, 126-131. Scott A. C. (1989) Observations on the nature and origin of fusain. Int. J. Coal Geol. 12, 443-475. Simoneit B. R. T. (1977) Diterpenoid compounds and other lipids in deep-sea sediments and their geochemical significance. Geochim. Cosmochim. Acta 41, 463-476. Simoneit B. R. T. (1990) Petroleum generation, an easy and widespread process in hydrothermal systems: an overview. Appl. Geochem. 5, 3-15. Sochas S. B. and Carpenter R. (1987) Factors affectingpore water hydrocarbon concentrations in Puget Sound sediments. Geochim. Cosmochim. Acta 51, 1273-1284. Sporstol S., Gjos N., Lichtenhaler R. G., Gustavsen K. O., Urdal K., Oreld F. and Skei J. (1983) Source identification of aromatic hydrocarbons in sediments using GC/MS. Environ. Sci. Technol. 17, 282-286. Van Vleet E. S., Pierce R. H., Brown R. C. and Reinhardt S. B. (1984) Sedimentary hydrocarbons from a subtropical marine estuary. In Advances in Organic Geochemistry 1983 (Edited by Schenck P. A., de Leeuw J. W. and Lijmbach G. W. M.). Org. Geochem. 6, 249--257. Pergamon Press, Oxford. Venkatesan M. I. and Dahl J. (1989) Organic geochemical evidence for global fires at the Cretaceous/Tertiary boundary. Nature 338, 57~50. Wakeham S. G., Schaffner C. and Giger W. (1980a) Polycyclic aromatic hydrocarbons in Recent lake sediments. I. Compounds having anthropogenic origins. Geochim. Cosmochim. Acta 44, 403-413. Wakeham S. G., Schaffner C. and Giger W. (1980b) Polycyclic aromatic hydrocarbons in Recent lake sediments. II. Compounds derived from biogenic precursors during early diagenesis. Geochim. Cosmochim. Acta 44, 415-429. Youngblood W. W. and Blumer M. (1975) Polycyclic aromatic hydrocarbons in the environment: homologous series in soils and recent marine sediments. Geochim. Cosmochim. Acta 39, 1303-1314.
0146-6380/92$5.00+ 0.00 Copyright© 1992PergamonPresspic
Polycyclic aromatic hydrocarbons of pyrolytic origin in ancient sediments: evidence for Jurassic vegetation fires S. D. K I L L O P S 1 and M. S. MASSOUD2 ~Chemistry Department and 2Geology Department, Royal Holloway and Bedford New College, University of London, Egham Hill, Egham, Surrey TW20 0EX, England (Received 5 April 1991; returned for revision 30 May 1991; accepted 4 July 1991)
A ~ - - P o l y c y c l i c aromatic hydrocarbon (PAH) distributions in an Upper Jurassic succession from offshore Korea Bay Basin were consistent with a pyrolytic source, being dominated by non-alkylated species. Levels of these PAHs were significantly higher in samples containing abundant fusinite macerals, which are believed by some to be the products of combustion. The data were consistent with a PAH orion from periodic conflagration throughout the Upper Jurassic of vegetation adjacent to a tectonically formed lake. Distributions of PAHs exhibited some differences from those of combustion origin in Recent and some ancient sediments, and were characterized by the dominance of the highly peri-condensed compounds benzo[e]pyrene, benzo[g,h,i]peryleneand coronene. This characteristic reflects either preferential formation, or preferential preservation under the prevailing sedimentary conditions, of the highly peri-condensed structures. Key words--polycyclic aromatic hydrocarbons, peri-condensed PAH, pyrolytic PAH, vegetation fires, fusinite, Jurassic sediments
INTRODUCTION
fossil fuels (Blumer, 1975; Laflamme and Hites, 1978; Ramdahl, 1983; Sporstol et al., 1983; Van Vleet et al., 1984). While these PAHs provide evidence for combustion, they are not themselves combustion products, being formed under conditions of oxygen depletion by the action of heat resulting from the combustion process. We therefore refer to these compounds as pyrolytic products. Highly peri-condensed PAHs are more reacti~ce than their analogues exhibiting lower degrees of angular fusion, and so their high concentrations among pyrolytic products are attributed to rapid quenching by adsorption (through hydrogen bonding) on to particles of soot, which is itself a polycondensed PAH material (Biumer, 1975; Venkatesan and Dahl, 1989). Exposure to a variety of environmental agents, such as photo-oxidation and aqueous dissolution, during aeolian or fluvial transport appears to have little effect on the distributions of these particulate associated PAHs finally preserved in Recent sediments (Prahl and Carpenter, 1983; Readman et al., 1984). Consequently, the distributions of pyrolytic PAHs observed in a variety of Recent sedimentary environments are remarkably constant (Youngblood and Blumer, 1975; Laflamme and Hires, 1978; Wakeham et al., 1980a; Gschwend and Hites, 1981). It also appears that a recognizable PAH fingerprint of combustion can survive over geological time scales (Venkatesan and Dahl, 1989). We report here evidence for vegetation combustion in the form of pyrolytic PAH distributions in mudstones from an Upper Jurassic succession (2331-3016m depth interval) from offshore Korea Bay Basin. The data were obtained during evaluation
Polycyclic aromatic hydrocarbons (PAHs) of pyrolytic origin have been extensively studied in Recent sediments, but relatively little attention has been paid to the possible application of these molecular indicators in identifying palaeofires. It has been proposed that a background of pyrolytic PAHs exists in most ancient sediments, which has hitherto been overlooked (Radke, 1987). Palaeofires, primarily initiated by lightning strikes, are likely to have been important sources of pyrolytic PAHs, and have probably been a feature of terrestrial ecosystems from at least the Late Devonian (Chaloner, 1989; Scott, 1989). Temperatures required for the formation of PAHs can also occasionally arise in sedimentary environments as a result of igneous activity. Such an origin has been proposed for pyrolytic PAHs associated with ash, coal and wood fragments in sedimentary material from the Midland Valley of Scotland (Murchison and Raymond, 1989; Raymond et al., 1989). Similarly, pyrolytic PAHs in oil seeps in the Guaymas Basin have been attributed to in situ hydrothermal activity (Kawka and Simoneit, 1990). Pyrolytic PAHs are characterized by the dominance of non-alkylated molecules, especially highly peri-condensed compounds (i.e. compact structures resuting from extensive angular fusion, e.g. pyrene, benzopyrene, benzo[g,h,i]perylene and coronene; see Fig. 1). The presence of such PAH distributions in the aromatic hydrocarbon fractions of Recent sediments is generally considered to reflect inputs from the combustion of organic matter, such as wood and oo ts/t-A
1
2
S.D. KILLOPSand M. S, MASSOUD
of the oil source rock potential of the succession, and full details of the organic geochemical assessment of this succession and its geological setting have been reported elsewhere (Killops et al., 1991). In brief, the succession represents a river delta prograding into a deep, freshwater lake, in a sub-tropical climate. Occasional turbidites were associated with a steep delta front. The lower part of the succession ( > 2 8 5 0 m depth) appears to have been associated with partly oxic, open water conditions and the dominance of terrestrially derived organic matter. Subsequently, deposition appears to have occurred within the delta. Here, sedimentary conditions were anoxic, and significant bacterial reworking of higher plant material occurred, leading to the accumulation of lipid-rich organic matter and the formation of a source rock for oil found in overlying Cretaceous sandstones. EXPERIMENTAL
Full experimental details for the Korea Bay Basin samples have been reported elsewhere (Killops et al., 1991). In summary, Upper Jurassic mudstone
samples took the form of cuttings, the larger being selected and any potential external contamination removed by washing with dichloromethane. Rock samples (10 g) were then milled to a fine powder and extracted ultrasonically for 20min with dichloromethane (50ml x 3). A total hydrocarbon fraction was obtained from the combined extracts by column chromatography on alumina (Merck, Brockman activity I) and elution with dichloromethane (3 x volume of alumina). Aromatic fractions were subsequently isolated by TLC on silica gel (Merck 60G) and elution with hexane (Rf ~< tridecylbenzene). PAH distributions were determined by G C - M S using a Carlo Erba Mega 5360 GC coupled to a Finnigan INCOS 50 quadrupole MS. On-column injection was used into a 25 m x 0.32 mm i.d. WCOT capillary column coated with 5%-phenylmethylsilicone, which fed directly into the MS ion source. Quantification was achieved by comparing the total ion current (TIC) peak area of an internal standard, squalane, with equivalent TIC peak areas of the PAHs. The latter were calculated from molecular ion responses and calibrated against TIC responses from
A
I[cd]Py
B[e]Py B[ghi]Pe
Co
Fig. 1. Structures of major PAHs detected in Upper Jurassic samples. P, phenanthrene; A, anthracene; PhN, 2-phenylnaphthalene; Fa, fluoranthene; lay, pyrene; B[a]A, benz[a]anthracene; C, chrysene; TPn, tripbenylene, BFa, benzo[b]fluoranthene + benzo[k]ttuoranthene; B[e]Py, benzo[e]pyrene; B[a]Py, benzo[a]pyrene; Pc, perylene; I[cd]Py, indeno[c,d]pyrene; l~ghi]Pe, benzo[g,h,i]perylene; Co, coronene; R, retene.
PAHs of pyrolytic origin samples in which PAH abundance was high and co-elutions did not occur. All samples were analysed at the same time under identical conditions. Fusinite was isolated from a sandstone from the Scalby Formation (Middle Jurassic; Yorkshire, England). The outer surface of the isolated fusinite was removed and discarded to limit the effects of any external contamination. A sample of powdered fusinite (500 mg) was extracted with dichloromethane (2 ml portions) as above, but no further fractionation was undertaken prior to GC-MS analysis of PAHs.
RESULTS AND DISCUSSION
Source o f P A H s in Upper Jurassic o f Korea Bay Basin
Gas chromatograms of the aromatic hydrocarbons of cutting samples from the deeper part of the succession (2847-3016m) were found to exhibit a distribution of PAHs dominated by highly peri-condensed structures [Figs 1, 2(b) and 2(c)]. The high ratio of parent to alkylated PAHs is more typical of a pyrolytic rather than a petroleum or coal source (e.g. Laflamme and Hites, 1978; Van Vleet et al., 1984). This part of the succession is believed to correspond to freshwater (probably deep), lacustrine deposition of mostly terrestrial material under partly oxic conditions, which favours preservation of the more refractory organic components (Killops et al., 1991). In the shallower part of the succession (2331-2846m), aromatic hydrocarbons were dominated by components typical of a potential oil source rock, particularly alkylnaphthalenes and alkylphenanthrenes. This part of the succession is thought to correspond to the thick, pro-delta muds of a prograding lacustrine delta and the development of anoxic sedimentary conditions (Killops et al., 1991). Closer examination by GC-MS showed that these bacterial lipid-rich sedimentary rocks also contained similar distributions of PAHs to the deeper samples, but at lower levels (ca 0.5 ppm of rock for 6 intervals over 2331-2846 m depth, cf. ca 25 ppm for 2 intervals over 2847-3016m). This suggests an input from a single source of PAHs throughout the whole of the Upper Jurassic succession (2331-3016 m). The variable rate of this PAH input to the sediments may reflect absolute changes in the input and/or dilution by varying inputs of other sedimentary material associated with the changing depositional environment. Although sedimentological evidence of igneous activity has been observed in some parts of Korea Bay Basin, there was no such evidence in the succession studied. However, a higher than average geothermal gradient was indicated by biomarker studies of the Upper Jurassic samples, suggesting that in situ temperatures probably exceeded 100°C (Killops et al., 1991). The oil seeps and associated PAHs in the Guaymas Basin, California, are believed to have been formed from contemporary organic matter (with a
3
significant terrestrial component) at temperatures in excess of 300°C by hydrothermal activity associated with the spreading centre in the Basin (Kawka and Simoneit, 1990). Such an origin for the PAHs in the Korea Bay Basin samples seems unlikely, as prolonged heating at these temperatures would have destroyed the oil-generating capacity exhibited by the Upper Jurassic succession. In addition, the thermal maturity of the indigenous organic matter appears only to correspond to the onset of oil generation (Killops et al., 1991). The petroleum hydrocarbons of the Guaymas seeps survive because they are able to migrate away from the heat source. Unfortunately, it is not possible to establish the proportion of PAHs in the Guaymas oil seeps generated by hydrous pyrolysis and the amount that may have resulted from an allochthonous input of vegetation combustion products to the sedimentary organic material. Evidence of a terrestrial higher plant input to the whole of the Upper Jurassic succession from Korea Bay Basin had previously been obtained from petrographic, sedimentological and biomarker studies (Killops et al., 1991). Levels of pyrolytic PAHs are recorded in Fig. 3, and apparently correlate with the relative abundance of terrestrial plant material. For example, the amount of C29 relative to C27 and C2s steranes, which probably reflects levels of allochthonous higher plant input relative to autochthonous sources of organic matter (Killops et al., 1991), shows similar depth trends to total pyrolytic PAH concentrations. In addition, the deeper part of the succession (2847-3016m), which exhibited the higher levels of PAHs, contained relatively abundant fusinites and semi-fusinites (up to 20% of total macerals). These macerals were absent from the shallower samples, which appeared to contain lesser amounts of small, disintegrated inerto-detrinite and exhibited lower PAH levels. Interestingly, fusinites and semi-fusinites have been proposed as products of vegetation fires (e.g. Scott, 1989). The apparent correlation of PAH concentrations with levels of higher plant material suggests that the PAHs arise from an allochthonous input of the products of periodic vegetation fires in the area adjacent to the lake. A combination of fluvial and aeolian transportation to the depositional site seems probable for this input. The relative amounts of alkylated homologues of the peri-condensed PAHs [Fig. 2(c)] is consistent with moderate to high combustion temperatures of 400-800°C, as established for contemporary vegetation (Youngblood and Blumer, 1975; Venkatesan and Dahl, 1989). The variation in depositional conditions throughout the Upper Jurassic succession in Korea Bay Basin does not seem to have noticeably affected the observed PAH distributions. This suggests a fixed, inherited distribution common to all samples, presumably attributable to stabilization of PAHs by particulate associations at the time of formation. The
4
S.D. KILLOPSand M. S. MASSOUD
stability of soot-bound PAHs towards aqueous leaching has been noted in Recent sediments (Prahl and Carpenter, 1983; Sochas and Carpenter, 1987). Pyrolytic PAHs have also been reported to be associated with charred woody material in low maturity samples (i.e. those which have not experienced significant heating by igneous activity) from the Midland Valley,
Scotland (Murchison and Raymond, 1989). It was not possible to test such an association for the Korea Bay basin samples because the disseminated fusinite could not be isolated. However, it was examined for large fragments of fusinite isolated from a sandstone, of fluvial or deltaic deposition, from the Middle Jurassic Scalby Formation, Yorkshire,
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Fig. 2. PAH distributions in (a) a Recent sediment exhibiting typical pyrolytic distributions (intertidal surface sediment, Bridgwater Bay, Severn Estuary, U.K.; after Killops and Howell, 1988) and (b and c) a representative Upper Jurassic sample (2931-3016 m depth and interval shown) from offshore Korea Bay Basin. Chromatograms (a) and (b) represent summed molecular ion responses from PAHs and (c) is a total ion current chromatogram from GC-MS analyses. See Fig. 1for key to abbreviations and structures. U, unidentifed PAH of mol. wt 302. Alkyl homologues of PAHs are represented by C~-C3 prefixes in (c). PAHs were identified by their mass spectra and retention indices (Lee et al., 1979).
PAHs of pyrolytic origin and coronene; Fig. 1) were more abundant than usual, while anthracene and 4,5-methylenephenanthrene, both pyrolytic PAHs (Youngblood and Blumer, 1975; Gschwend and Hites, 1981), were not detected. Comparison of the relative abundance of individual pyrolytic PAHs in the Korea Bay Basin samples with those typical of Recent sediments is shown in Fig. 4, using the amount of pyrene as an arbitrary reference level. The Korea Bay Basin samples are not alone, however, in exhibiting enhanced levels of coronene and related PAHs. Similar distributions of non-alkylated PAHs, believed to result from forest fires, have been reported for a Cretaceous/Tertiary boundary sample from Gubbio, Italy (Venkatesan and Dahl, 1989). The high relative abundance of phenanthrene (and its alkyl homologues) and chrysene + triphenylene in Fig. 2(b) probably results from the petroleum generating component within the succession, which would be anticipated to contribute phenanthrene and chrysene. It seems possible to distinguish two general types of pyrolytic PAH distributions in sediments and sedimentary rocks. One type is characterized by the usual Recent sediment distributions [as in Fig. 2(a)]. Further examples are combustion derived PAHs
England (Scott, 1989). Typical pyrolytic PAH distributions were recorded [similar to those in Fig. 2(a)], with a combined concentration of 3.7 ppm, which is significantly lower than the concentrations recorded for the deeper samples from Korea Bay Basin (ca 25 ppm). Consequently, it is not possible to estimate the proportions of PAHs likely to be associated with soot particles and with burnt/charred wood remains in the Korea Bay Basin samples. Retene (Fig. 1) was more abundant in the deeper than the shallower samples from Korea Bay Basin (e.g. ca 2.0ppm for 2931-3016m interval cf. ca 0.1 ppm for 2331-2424m interval), and while it may, therefore, derive from sedimentary diagenesis of gymnosperm resins (Simoneit, 1977; Laflamm¢ and Hites, 1978; Wakeham et al., 1980b), an origin from combustion of resinous trees cannot be discounted (Ramdahl, 1983). Variations in pyrolytic P A H distributions
The PAH distributions observed in the Upper Jurassic succession from Korea Bay Basin were not identical to those typically attributed to pyrolytic sources in Recent sediments [Fig. 2(a)]. The highly peri-condensed compounds sharing the basic coronene structure (benzo[e]pyrene, benzo[g,h,i]perylene
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Fig. 3. Plots vs depth of TOC, amount of C29 relative to C27---C29steranes (reflecting relative higher plant input) and approximate total pyrolytic PAH content for the Upper Jurassic succession from Korea Bay Basin. (Sample depths are mean values for each interval. Sterane ratios were calculated from m/z 218 GC-MS response for 20S and 20R isomers of C27-C295e,(H),14fl(H),17fl(H)-4-desmethylsteranes. Total pyrolytic PAHs were P, Fa, Py, C + TPn, BFa, B[e]Py, B[a]Py, Pc, I[cdlPY, B[ghi]Pe, Co; see Fig. l for key to abbreviations.)
6
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A Fig. 4. Relative distribution of individual PAHs in the Upper Jurassic succession from Korea Bay Basin compared with typical pyrolytic distributions in Recent sediments, based on samples in Fig. 2. Enrichment factors were calculated from molecular ion intensities for samples, taking pyrene as a reference level (arbitrary assignment or zero enrichment). associated with fusinites in the Middle Jurassic Scalby Formation (Yorkshire, England; discussed above), and hydrothermally generated PAHs in Quaternary turbidites of the Escanaba Trough (N.E. Pacific; Simoneit, 1990). The second type is characterized by enhanced levels of highly peri-condensed PAHs, as in the Korea Bay Basin samples, the combustion derived PAHs in the Cretaceous/Tertiary sample from Gubbio (Italy) and the hydrothermally derived PAHs in seep oils from the Guaymas Basin (California). Time alone is obviously not a decisive factor in determining observed PAH distributions. Variation in pyrolytic PAH distributions must be due to different pyrolysis conditions and/or differing stabilities of individual PAHs towards post-depositional conditions. The temperature of, and exposure time to, pyrolysis are probably the most important factors affecting distributions during PAH formation (Youngblood and Blumer, 1975; Venkatesan and Dahl, 1989). Ancient higher plants would not be expected to yield different combustion products from their contemporary counterparts under the same pyrolysis conditions, as the major organic tissue constituents are the same. For PAHs resulting from natural vegetation fires, post-depositional factors may be important in controlling the distributions observed in
sediments and rocks. One such factor is photooxidation, which occurs quite readily in anthracene at the 9 and 10 positions (Bjorseth and Eklund, 1979 and references therein), while coronene is relatively resistant towards oxidation (Campbell and Andrew, 1979). Post-depositional temperature may also be important. It has been suggested that highly pericondensed structures are preferentially removed by equilibration processes over long residence times at elevated temperatures in the sub-surface (Blumer, 1975). However, the Upper Jurassic succession in Korea Bay Basin has been buried quite deeply, in an area of above average heat flow, and appears to have experienced temperatures in excess of 100°C (Killops et al., 1991). The abundance of highly peri-condensed PAHs in this succession indicates that they can survive such thermal regimes. The effects on PAH distributions of water leaching in Recent sediments are extremely difficult to assess (e.g. Radke, 1987), due to the very low solubilities of the compounds concerned and the complication of adsorption processes. They are, therefore, not readily predictable for ancient sediments, although it would be anticipated that water leaching would be an important factor over geological time scales, particularly for hydrothermally generated PAHs.
PAHs of pyrolytic origin CONCLUSIONS An Upper Jurassic succession of lacustrine mudstones, representing a prograding delta (ca 500 m thick), in Korea Bay Basin was found to contain a range of non-alkylated PAHs typical of pyrolytic origins. The source of this input is probably periodic vegetation fires. The highest levels of pyrolytic PAHs (ca 25 ppm) were recorded for samples which were found to contain fusinite and semi-fusinite, proposed products of vegetation combustion. The PAHs produced during combustion are likely to be associated with soot and possibly also with charred woody remains. The PAH distributions in the Korea Bay Basin samples differed from those usually found in Recent sediments and in some ancient sedimentary rocks in having enhanced levels of the highly peri-condensed components coronene, benzo[g,h,i]perylene and benzo[e]pyrene. However, other examples of this type of distribution are known for samples of varying geological age. It is not clear how the two types of PAH distributions observed in sedimentary material arise; a number of factors may be responsible. Temperature and time during pyrolysis are likely to be important factors in PAH formation, while after deposition geochemical factors, such as water leaching, may be important, particularly for ancient sediments. It is not possible to determine whether formation conditions or post-depositional conditions resulted in the enhanced levels of highly peri-condensed PAHs in the Korea Bay Basin samples. Examination of further samples of known sedimentary history is required to establish a framework for evaluating the factors controlling observed PAH distributions. Acknowledgements--The authors are grateful to Dr M.
Gamali of Topeco for permission to publish these results, and to Dr A. C. Scott of the Geology Department, Royal Holloway and Bedford New College, for the sample of Scalby Formation fusinite. REFERENCES
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Amsterdam. Chaloner W. G. (1989) Fossil charcoal as an indicator of palaeoatmospheric oxygen level. J. Geol. Soc. London 146, 171-174, Gschwend P. M. and Hites R. A. (1981) Fluxes of polycyclic aromatic hydrocarbons to marine and lacustrine sediments in the northern United States. Geoehim. Cosmochim. Acta 45, 2359-2367. Kawka O. E. and Simoneit B. R. T. (1990) Polycyclic aromatic hydrocarbons in hydrothermal petroleums from
the Guaymas Basin spreading center. Appl. Geochem. 5, 17-27. Killops S. D. and Howell V. J. (1988) Sources and distributions of hydrocarbons in Bridgwater Bay (Severn Estuary, U.K.) intertidal surface sediments. Estuarine Coastal Shelf Sci. 27, 237-261. Killops S. D., Massoud M. S. and Scott A. C. (1991) Biomarker characterisation of an oil and its possible source rock from offshore Korea Bay Basin. Appl. Geochem. 6, 143-157. Laflamme R. E. and Hites R. A. (1978) The global distribution of polycyclic aromatic hydrocarbons in recent sediments. Geochim. Cosmochim. Acta 42, 289-303. Lee M. L., Vassilaros D. L., White C. M. and Novotny M. (1979) Retention indices from programmed-temperature capillary-column gas chromatography of polycyclic aromatic hydrocarbons. Anal. Chem. 51, 768-773. Murchison D. G. and Raymond A. C. (1989) Igneous activity and organic maturation in the Midland Valley of Scotland. Int. J. Coal Geol. 14, 47-82. Prahl F. G. and Carpenter R. (1983) Polycyclic aromatic hydrocarbons (PAH)-phase associations in Washington coastal sediment. Geochim. Cosmochim. Acta 47, 1013-1023. Radke M. (1987) Organic geochemistry of aromatic hydrocarbons. In Advances in Petroleum Geochemistry (Edited by Brooks J. and Welte D.), Vol.2, pp. 141-207. Academic Press, New York. Ramdahl T. (1983) Retene--a molecular marker of wood combustion in ambient air. Nature 306, 580-582. RaymondA. C., Liu S. Y., MurchisonD. G. and Taylor G. H. (1989) The influence of microbial degradation and volcanic activity on a Carboniferous wood. Fuel 68, 65-73. Readman J. W., Mantoura R. F. C. and Rhead M. M. (1984) The physico-chemicalspeciation of polycyclicaromatic hydrocarbons (PAH) in aquatic systems. Fresenius Z. Anal. Chem. 319, 126-131. Scott A. C. (1989) Observations on the nature and origin of fusain. Int. J. Coal Geol. 12, 443-475. Simoneit B. R. T. (1977) Diterpenoid compounds and other lipids in deep-sea sediments and their geochemical significance. Geochim. Cosmochim. Acta 41, 463-476. Simoneit B. R. T. (1990) Petroleum generation, an easy and widespread process in hydrothermal systems: an overview. Appl. Geochem. 5, 3-15. Sochas S. B. and Carpenter R. (1987) Factors affectingpore water hydrocarbon concentrations in Puget Sound sediments. Geochim. Cosmochim. Acta 51, 1273-1284. Sporstol S., Gjos N., Lichtenhaler R. G., Gustavsen K. O., Urdal K., Oreld F. and Skei J. (1983) Source identification of aromatic hydrocarbons in sediments using GC/MS. Environ. Sci. Technol. 17, 282-286. Van Vleet E. S., Pierce R. H., Brown R. C. and Reinhardt S. B. (1984) Sedimentary hydrocarbons from a subtropical marine estuary. In Advances in Organic Geochemistry 1983 (Edited by Schenck P. A., de Leeuw J. W. and Lijmbach G. W. M.). Org. Geochem. 6, 249--257. Pergamon Press, Oxford. Venkatesan M. I. and Dahl J. (1989) Organic geochemical evidence for global fires at the Cretaceous/Tertiary boundary. Nature 338, 57~50. Wakeham S. G., Schaffner C. and Giger W. (1980a) Polycyclic aromatic hydrocarbons in Recent lake sediments. I. Compounds having anthropogenic origins. Geochim. Cosmochim. Acta 44, 403-413. Wakeham S. G., Schaffner C. and Giger W. (1980b) Polycyclic aromatic hydrocarbons in Recent lake sediments. II. Compounds derived from biogenic precursors during early diagenesis. Geochim. Cosmochim. Acta 44, 415-429. Youngblood W. W. and Blumer M. (1975) Polycyclic aromatic hydrocarbons in the environment: homologous series in soils and recent marine sediments. Geochim. Cosmochim. Acta 39, 1303-1314.