The influence of microbial degradation and volcanic activity on a Carboniferous wood

The influence of microbial degradation and volcanic activity on a Carboniferous wood Anne C. Raymond, Susie Y. Liu”, Duncan Geoffrey H. Taylor*

G. Murchison

and

Organic Geochemistry Unit, Drummond Building, University of Newcastle, Newcastle upon Tyne, NE7 7RU, UK * Research School of Earth Sciences, Australian National University, Jnstitute of Advanced Studies, GPO Box 4, Canberra 2601, Australia (Received 7 June 7988)

The Midland Valley of Scotland was a region of widespread igneous activity throughout Carboniferous times. Organic matter, diverse in character, was incorporated in lavas, ashes, volcanic pipes and necks but frequently remained in a well preserved and relatively immature condition. This paper evaluates microscopical and geochemical data from wood enclosed in a hthic tuff found at Orrock Quarry, Central Fife, to the north of Edinburgh. Reflected-light microscopy reveals that the wood is of immature rank (R,iI=0.39%), with vacuolation, perhaps due to heating, possible micrinitization and leakage of bituminous material from cell walls. Two types of biodegradation are confirmed by transmission electron microscopy (TEM): one characterized by the intrusion of soft-rot fungi and with tissues relatively undisturbed, the second in which the tissues are much more disorganized and severely degraded by a variety of micro-organisms. Data from gc. and g.c.-m.s. analysis of the saturate fraction corroborate the microbial input revealed by TEM. The occurrence of S/?(H)-drimane in a suite of C,, to C,, bicyclics reflects microbial reworking and the 28,30 bisnorhopane may also be a marker for such activity. The presence of the tetracyclic diterpanes 16P(H)- and 16a(H)-phyllocladane points to a resin-bearing precursor and supports a gymnosperm origin for the wood. Biomarker ratios based on hopanes are consistent with low maturity. The high concentrations ofajp steranes are not but may be explained by matrix influences. The incidence of large quantities of unsubstituted polyaromatic hydrocarbons is due to the natural combustion of organic matter, testifying to the importance of volcanism in introducing such hydrocarbons to ancient sediments. (Keywords: dectmn microscopy; aromatic; hydrocarbon)

The Midland Valley of Scotland is an ancient graben structure in which several thousands of metres of predominantly lacustrine and deltaic sediments accumulated during Carboniferous times. Widespread igneous activity persisted throughout the period and the diverse nature of the products reflects the development of a complex igneous province I*’ . The principal intrusions are dolerite sills, sometimes accompanied by dykes, whilst lavas and a large number of volcanic necks testify to the extensive eruptive activity that also occurred. Tuffs and agglomerates are often associated with the vents and contain rare pieces of wood, and, more frequently, fragments of various sedimentary rocks, including oil shales and coals. Although wood fragments were reported in the pyroclastic deposits3, no systematic petrological or geochemical studies have been undertaken of such material from this area. Allan et ~1.~did, however, report geochemical data from the analyses of wood enclosed in a Tertiary conglomeratic tuff on the Isle of Rhum, Scotland, and Hamilton er al.’ have discussed the microflora and rank of Triassic coal fragments in volcanic necks near Sydney, Australia. This paper describes the organic petrology and geochemistry of a sample of wood (~20 x 6 x 0.5 cm), found in an ash-filled volcanic pipe, which cuts a Lower Carboniferous basaltic lava at Orrock Quarry in Central Fife, to the north of Edinburgh, Scotland (Figure I). The wood lies in a lithic tuff that contains abundant wood OOlf%2361/89/010066-08S3.00 @TJ 1989 Butterworth & Co. (Publishers) Ltd.

66

FUEL, 1989, Vol 68, January

fragments (Figure 2). This analysis of wood presents a rare opportunity to investigate a precursor in these ancient sediments of much of the terrestrially-derived organic matter within the Carboniferous succession in the Midland Valley. EXPERIMENTAL Organic

petrology

The wood tissues and their cell contents were examined by transmitted (TL) and reflected light (RL) microscopy and transmission electron microscopy (TEM). Transmitted light preparations were conventional 30pm slides. Particulate samples were mounted in cold-setting resin, before grinding and polishing to produce surfaces that were examined with oil immersion objectives. Average oil reflectances were measured at 546 nm. Ultra-thin sections were cut with diamond knives for TEM. Some sections were examined unstained, others after staining with uranyl acetate and lead citrate to enhance contrast by selectively increasing the electron density of some components. Organic

geochemistry

Soxhlet extraction of the wood with an azeotropic mixture of dichloromethane and methanol for 72 h yielded an extract that was then separated into saturated and aromatic hydrocarbon fractions by thin-layer

The influence

of microbial

degradation

and volcanic

activity

on a Carboniferous

wood: A. C. Raymond

et al

(Figure 3). The diffuse dark,grey areas on the right-hand

1Okm

/

Figure 1 area

Location

of Orrock

0

volcanic

I

sills

Quarry

neck

and geology

,

-

fold

-.-

fault

ax,s

side of several cells are, m reality. a brown organic staining of the mineral matter. The vacuolation of the cell contents, the development of what appears to be micrinite within some cell contents. which were probably originally resinous, and the release of brown, bituminous-like matter from cell walls, suggest some heating of the wood. However, the subsequent TEM study of the tissues showed no evidence for the formation of the vacuoles by heating, although there may have been some desiccation. The wood is clearly immature since the oil reflectance of the cell walls is only 0.39 per cent, well below the generally accepted threshold of vitrinite reflectance, R,, ~0.50;; for the ‘oil window’. In this case however, the wood has escaped the normal geochemical coalification process and does not have a vitrinite-like appearance. In longitudinal sections of the wood (Figure 6) there appear to be two principal types of structural zone: one in which long fibres of tissue are uncompressed and relatively unbroken, sometimes separated by the possible micrinitic/clay/‘humic-acid granular mixture, and a second in which the cell remains are much more disorganized and disturbed. Resolution of these remains in detail with the light microscope is difficult, but recourse to TEM allows a more precise assessment of the wood structure and its level of preservation.

of the surrounding

chromatography. G.1.c. of the saturates was performed using a Carlo Erba 4 160 g.c. equipped with an on-column injector and a 50 m x 0.32 mm i.d. column coated with OV-1. The temperature programming was from 40290°C at 4”Cmin-’ with hydrogen as the carrier gas. G.c.-m.s. of both saturates and aromatics was with a Hewlett Packard HP 5890 g.c. coupled to a 5970 series MSD quadrupole m.s. A 25mx0.2mm i.d. h.p. Ultra 1 column was used for the analysis of the saturates and a 25 m x 0.2 mm i.d. HP 5 (5 Y; phenylmethylsilicone) column for the aromatics. Split-splitless injection was employed for g.c.-m.s., with helium as the carrier gas and with the g.c. oven programmed from 4O”C-300°C at 4°C min-‘. Figure 2

Hand

specimen

of wood lying in lithic tuff

RESULTS AND DISCUSSION Organic petrology

The most striking features of the sample are its relatively unaltered state and the remarkable degree of preservation of certain of the cell structures and their contents, which is also true of wood fragments contained in the lithic tuff (Figure 3). In reflected light, depending upon the cut of the section, a range of cell structures was encountered. Figure 4 is a transverse section of the wood and shows the uncompressed character of cells with varied contents, which appear to be dominantly micrinitic, but which may also contain fine clay and granular humic material. There may be a contribution to the micrinitic appearance from pores and other submicron structures. In some of the cell contents, vacuoles or pores have developed, perhaps indicating the release of volatiles. Other uncompressed cells have their contents replaced with mineral matter, principally calcite, with only the thin, modified, lignified cell walls remaining

Figure 3 Degraded elements: transmitted

wood fragment light, air

FUEL,

in lithic tuff showing

conducting

1989, Vol 68, January

67

The iniiuence

of microbial

degradation

and volcanic

activity

Figure 4 Uncompressed ceils in wood, mostly filled with probable micrinite-clay-granular humic mixture and some of the contents showing development of vacuoles: reflected light, oil immersion

Figure 6 Longitudinal section of wood illustrating degradation zone: reflected light, oil immersion:SR,soft-rot various micro-organisms

two types of fungi; VM,

on a Carboniferous

Figure

staining

wood:

A. C. Raymond

et al

Uncompressed cells with mineral-filled cavities, showing by bituminous humic exudate: reflected light. oil immersion

5

Figure 7 Soft-rot fungi penetrating wood in uncompressed TEM; H, hyphae; SH, orher hyphal forms; M, middle lamella

zone:

Figure 8 Various micro-organisms in relatively disturbed zone ofcells: TEM; C, microbial cells; M, middle lamella; F, long fibres; SW, secondary walls; P, phosphate mineral

Figure9 Various micro-organisms in relatively disturbed zone ofcells: TEM; C, microbial cells: H. tilamentous hyphae: G. granular humic acids; M, middle lameila: SF. stain fungi; P, phosphate mineral

TEM has already shown that biodegraded material and the remains of micro-organisms are widespread in Australian coals (Taylor et nly’j, Taylor and Liu’). From TEM analysis of the Orrock wood it appears that the two

structural zones reflect two principal types of biodegradation. The first is characterized by soft-rot fungi. These types of fungi normally attack wood under moist conditions and is abundant in the surface layers of

68

FUEL,

1989,

Vol 68, January

The influence

of microbial

degradation

and volcanic

activity

on a Carboniferous

Organic Saturated

hydrocarbons

Figure 10 Gaschromatogram of saturated hydrocarbons. Peaks 1 and 2 are bicyclic alkanes (see Figure JI)

In the wood the fungi appear as conical-ended segments of hyphae extending along the longitudinal direction of the wood (Figure 7). Soft-rot fungi attack the wood by depolymerizing cellulose and other polysaccharides, leaving lignin in the form of granular humic acids. The compound middle lamellae and, depending upon the species of wood and fungus, the semi-adjacent lignilied cellulosic layers, resist the attack of soft-rot fungi and may retain their form until the last stages of decay. This resistance is probably due to the high lignin content of these cell wall layers. The biodegradation processes that have occurred in the more disturbed zones (Figure 6), appear to be more complex than simply a stage in degradation produced by soft-rot fungi. Figure 8 shows the typical appearance of such areas by TEM, in which the higher plant cell walls are severely degraded. In the electron micrograph a band of severe degradation appears to have terminated against a middle lamella. More than one type of fungus was probably active in the wood degradation, for example, ‘stain-fungi’ acting on the cellulosic walls (Figure 9). There is also a prominent granular layer adjacent to bundles of long hlamentous hyphae. The small spherical bodies resemble coprolites of microfauna, although they are smaller than commonly reported in the literature. The varying degrees of granularity in hyphae and fungal-spore walls suggest that the fungal remains have been attacked by some other micro-organism, possibly bacteria. Two further features deserve comment. First, the regular spacing and repetition of bands where degradation has been severe may be related to a predisposing feature of the wood structure, for example, the presence of vessels offering a higher level of nutrition to micro-organisms, or the local absence of inhibiting chemical compounds. Second, all electron micrographs provide evidence of line crystals of inorganic matter. The crystals are akin to a phosphate mineral with the composition Ca,P,0,.2H,O (ASTM 28-233). All the crystals occur in cavities and not in the cell wall material, suggesting that mineralizing solutions may not have penetrated and certainly did not interact with the cell wall material. The deposition of calcium phosphate in cavities resulting from the activity of fungi indicates there was free movement of solutions of a composition, which would be consistent with the leaching of the tuff. peats.

wood:

A. C. Raymond

et al.

geochemistry

Alkanes. The gc. (Figure 10) shows

of the saturated hydrocarbon fraction a smooth, heavily skewed n-alkane distribution, which maximizes at nCr3. The lack of nalkanes above nC,,, coupled with no odd-over-even predominance amongst lower homologues, may be partially attributed to an absence of waxy cuticular material. Numerous studies have shown that n-alkanes of plant waxes impart a marked odd carbon number terrestrially-derived immature, preference in materials-‘o. The dominance of the n-alkanes over the C,,-C,, range suggests a bacterial contribution, since extant bacteria contain relatively large quantities of straightchain hydrocarbons and fatty acids with comparable carbon number distributions, which often show no oddover-even preference 11-14. Iso and anteiso alkanes are also prominent within the same range as the major nalkanes and, in association with the ‘hump’ of unresolved components, provide further evidence of the bacterial activity”, which was clearly demonstrated by TEM examination. Terracyclic diterpanes. The two peaks nC,, and nC,, on the saturate g.c. were

eluting between identified as the tetracyclic diterpanes 16fl(H)- and 16a(H)-phyllocladane, based on the similarities of their mass spectra and retention times with those of Noble et a1.“-16 Phyllocladanes have been recognized in a variety of geological materials: hard coals, sediments and oils’ 5-1*, lignites’ 9 and, in the case of the 16a(H)-epimer, as the mineral bombiccite”. However, in biological materials

Bicyclic alkanes

”

10000-

z

2

o

r.

10000-

11

:: 2 0

o 2

10000-

z i

o 0

20

Peak assignments

22

24

26

26

30

for bicyclic alkanes

1

C,3 bicyclic alkane

5

Cts bicyclic alkane

9

C,6 bicyclic alkane

2

C,,

6

6LWdrimane

10

C,g bicvclic alkane

3

C,, bicyclic alkane

7

C,,

bicfclicrlkane

11 6&H)-homodrimane

4

Cts biwclic

6

C,,

bicvclicalkane

12

bicyclic alkane

alkane

WHI-homodrimane

Figure 11 Mass fragmentograms of C,,-C,, bicyclic alkanes. Peak 2 corresponds to the C,, bicyclic alkane of Vorob’eva et al.“; peaks 3-5 are the C,, and C,, bicyclics of Bendoraitis2* and peaks 6-12 were identified by comparing mass spectra and retention times with data from Philp et 01.2s, Alexander er ~1.‘~ and Noble”

FUEL, 1989, Vol 68, January

99

The influence Q

of microbial

bicyclic alkane (peak

degradation

and volcanic

165

180 JI,.

Mb,

100

,111" ,I,

_t

150

200

Mass/Char94

Figure 12 Mass spectrum of C,, bicyclic alkane

Hopanes

m/z 191 9

10000-

8

5

6’

sooo-

nl*bh , 58

o56

Peak assignments

: 3 4 : ; 9 10 11 12 13 14 15 16 17

I 62

for m/t

1 64

I 66

1 70

68

191 fragmentogram

18fx(H),22.29,30-trisnorneohopane Us) 17cf(H),22.29,30-trisnorhopane (Tm) 17fi(H),22,29,30_trisnorhopane 28.30bisnorhopane 17a(H),21~(H),30_norhopane 17fi(H),21a(H),30-norhopane 17a(H),21P(H)-hopane unknown norhopane 17fi(H),21fi(H),30-norhopane 17fi(H),21dH)-hopane 17o!(H).2lp(H)-homohopane (22s) 17a(H).21fi(H)-homohopane (22R) 17P(H),2lp(H)-hopane 17@(H).2lcr(H)-homohopane (22s + 22R) 17rx(H),2lfl(H)-bishomohopane (22s) 17cY(H),2lfl(H)-bishomohopane (22R) (+unknown?) 17fl(H),2lfl(H)-homohopane

Hopane

ratios cz2 W22Sl

c,, W22S)

Trll q

8 60

= 4.24

= 0.36 c,, Q$(22S+22R

1

= 0.29 C3,cY~(22S+22R~

Figure 13 Hopanes (m/z 191 fragmentogram) with associated ratios

the precursor, phyllocladene, is restricted to the resins of conifers*l***. The presence of the saturated derivatives of phyllocladene point to the wood being a resin-bearing precursor of the conifers, possibly Cordaites, a widespread primitive genus in the Carboniferous. Noble’ ’ and Noble et al. ’ 6 have reported diterpenoids varying in structural type in coals and sediments of widely differing ages. It seems that phyllocladane and kaurane are the two diterpenoids dominant in samples older than the Mesozoic, although the oldest sediments in which Noble” finds these diterpanes are of Permian age. The present paper cites the first known occurrence of these diterpanes in a Carboniferous sample, although only trace amounts of the kaurane epimers are present. None of the other diterpenoid classes are represented in the

70

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on a Carboniferous

wood:

A. C. Raymond

et al

wood and this restricted range may reflect an evolutionary trend. The wider range of diterpenoid types present in post-Palaeozoic sediments seems to mirror the increased diversity of flora in Mesozoic and later times and possibly also the ability of individual ‘higher’ gymnosperms to synthesize a greater diversity of diterpenoids as the resin-bearing systems of the plants became more advanced.

1)

loo-

activity

Bicycfic sesquiterpenoids. G.c.-m.s confirms the presence of a series of bicyclic sesquiterpenoids of which the C 14-C1 6 members have previously been reported (see Figure Il. The mass spectrum of peak 1 (Figure 12) is consistent with a CL3 bicyclic sesquiterpenoid, which has not to our knowledge been previously recorded in the literature. The widespread occurrence of bicyclic sesquiterpanes in crude oils and sediments, where there has been a terrestrial contribution, has often led to those compounds being used as markers for land-plant input25*27 In addition to those samples known to have been terrestrially influenced, Alexander et a1.26*28 have reported S/I(H)-drimane in marine samples, some of which are of Lower Palaeozoic age and clearly predate the development of land plants. The ubiquity of 8/3(H)drimane would therefore seem to require derivation from similarly widespread precursors and these authors have proposed the microbial degradation of hopanoids as being a feasible mechanism. Noble” has also suggested an origin through direct bacterial synthesis. Although there may be more than one source of bicyclic alkanes in the geosphere, the presence of S/?(H)-drimane and the associated homodrimanes (peaks 11 and 12, Figure I I ), in the wood corroborate the microbial input and reworking previously suggested. Steranes and hopanes. The presence of 17/?(H), 2 la(H) hopanes in the m/z 191 fragmentogram (Figure 13), in with the hopane ratios, is further conjunction confirmation of the immaturity of the wood. The 28,30bisnorhopane has often been recorded in the literature29-34. Since derivation from high carbonnumbered hopanoids is unfavourable on kinetic grounds3’, direct input from a specific precursor organism or formation in a restricted environment may account for the existence of 28,30-bisnorhopane3*. The incidence of this hopane in this sample would seem to preclude any algal affinities and, combined with microscopical evidence, favours either direct microbial synthesis or microbial reworking of the terrestrial substrate as a source. From the sterane (m/z 2 17) fragmentogram (Figure 14) and associated ratios (Tab/e I), the high concentration of aP/3 steranes (most notably the C,, components), is inconsistent with the low maturity level of the wood, Coelution of 28,30_bisnorhopane with the Czs aaa S isomer also results in erroneously high maturation indices. Clarification is achieved through the use of selective metastable ion monitoring (SMIM). The m/z

T&e 1 Sterane ratios C,, c,,

217 trace SMIM M/r

0.24 0.08

KKK

(20s)

Cz9 Kflfi (20s + 2OR)

(20s + 2OR)

Total regular Cz9 steranes

KKK

0.58 0.56

The influence

Steranes

degradation

and volcanic

56

56

62

60

64

Sterane peak assignments a 12&H).17cxfHl-diacholestane

(20s) b 12&H),17a(H)-diacholestane (2OR) c 12~~(H).176(H)-diacholestane (20s) d 13cu(H).176IH)-diacholestane (2OR) e 24-methyl-l?#(H),17cY(H)-diacholestane (20s)’ f 24.methyl-136(H).17a(Hl-diacholestane (20R)’ 6 24.methyl-lWH).17P(H)-diacholestane (20s) + 14cx(H),17cx(H)-cholestane (20s) h 24.ethyLl~(H),17a(H)-diacholestane (20s) i 14cY(H).17cY(H)-cholestane (20R) j 24.ethyl-12#H),17crfH)-diacholestane (20R) k 24-eth~l-l3a(H),17~IH)~diacholestane (20s) I 24.methyl-l&(H),17cr(H)-cholestane (205) m 24-ethyl-13a(H).176lH)-diacholestane (20R) + 24.methyl-145(H),17P(H)-cholestane (20R) n 24.methyl-146(H),176(H)cholestane (20s) o 24.methyl-l&(H),17ol(H)-cholestane (20R) p 24.ethyl-14WH).17(Y(H)-cholestane (20s)” q 24.ethyl-14&H),176(H)-cholestane (20R) r 24.ethyl-lq(H).176(H)-cholestane 120s) s 24.ethyl-14a(H),17tY(H)-cholestane (20R) l

l

possibly isomeric at C-24 * enhanced due to coelution

of 28.30 bisnorhopane

activity

on a Carboniferous

wood:

et al.

A. C. Raymond

of afl/? steranes36.37*39, although the mode of occurrence of the wood renders this suggestion highly unlikely. An alternative explanation may lie with the volcanoclastic matrix, which has been highly altered to a montmorillonite-chlorite mixture set in a matrix of calcite. Catalytic isomerization and rearrangement reactions at acid sites in the clay minerals, perhaps enhanced when the matrix was still warm, may have promoted the formation of c$P steranes from suitable precursor sterob. This catalytic effect of the matrix could also account for the high concentration of diasteranes, particularly the Cz9 compounds which, along with the large quantities of Cz9 regular steranes, presumably reflect the terrestrial origin of the sample.

m/z 217

54

52

of microbial

in m/z 217 trace

Figure 14 Steranes (m/z 217 fragmentogram)

SMIM for C29 steranes m/z 400-217

Aromatics. Complex mixtures of polyaromatic hydrocarbons (PAH) are found in soils, recent and ancient sediments, coals and petroleums (see Radke40). The PAH can be broadly divided into two groups, namely alkylated and non-alkylated (parent) PAH. Most unsubstituted PAH originate through natural or anthropogenic combustion processes, while the alkylated species result from the diagenetic transformation of biogenic precursors and perhaps also parent PAH4im4’. Alkylated PAH may, in part, be pyrolytically or combustion-derived, possibly at temperatures lower than those required for the formation of parent PAH41*48. The total ion current (TIC) of the aromatic fraction (Figure 16~) of the wood reveals that in addition to the commonly occurring naphthalene and phenanthrene and their alkylated homologues (peaks 1 to 6), there is also a significant contribution from unsubstituted aromatics (peaks 7 to 22). The major alkylated and non-alkylated PAH are more clearly delineated by adding the relevant molecular ions (Table 2) to form a composite fragmentogram (Figure 16b).

h

100 1

Table 2

I

Peak assignments for aromatics

60-

Molecular weight

Peak

Assignment

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Naphthalene Methylnaphthalenes Dimethylnaphthalenes Trimethylnaphthalenes Phenanthrene Methylphenanthrenes Fluoranthene Pyrene Benzo(a)fluorene Benzo(b)lluorene Methylfluoranthenes or pyrenes Dimethylfluoranthenes or pyrenes” Benzo(ghi)fluoranthene Benz(a)anthracene Chrysene (+ triphenylene?) Trimethyhluoranthenes or pyrenesb Methylchrysenes and/or methylbenz(a)anthracenes Dimethylchrysenes and/or dimethylbenz(a)anthracenes” Benzo(j)tluoranthene + benzo(k)tluoranthene Benzo(e)pyrene Benzo(a)pyrene Perylene

60-

40-

OJ

I

53:20

I

1

1:04:00

1:14:40

1:25:20

1:36:00

Figure 15 SMIM for C1, steranes (m/z 4QO+2 17)

transition (C,, steranes) is shown in Figure 15. This pattern is similar to that reported by other workers (cf. Ten Haven et al. 36*37) for samples displaying anomalously high sterane ratios: there is an abundance of the C,, aaa Rand C,, a&? S and R isomers with a virtual absence of the Cz9 aaa S isomer. Such large amounts of aj?fl steranes, given the low rank of the sample, contradict the traditionally accepted view that maturation-induced isomerization is the source of a/3/l steranes38. Input from specific precursor sterols, particularly in hypersaline environments, has been proposed as an alternative source 400-217

18 19 20 21 22

Peaks l-6 may Peaks 7-21 are ‘Could also be bCould also be

128 142 156 170 178 192 202 202 216 216 216 230 226 228 228 244 242 256 252 2.52 252 252

be partly combustion-derived typically combustion-derived ethyl ethyl methyl or propyl

FUEL, 1989, Vol 68, January

71

The influence of microbial degradation and volcanic activity on a Carboniferous

wood: A. C. Raymond

et al.

Thanks are also due to Dr D. M. Jones of the Organic Geochemistry Unit for helpful discussions. MS C. Jeans of the Department of Geology, University of Newcastle, prepared the text figures for this paper and to her the authors offer sincere thanks as they do to Mrs Y. Hall for preparation of the text. Grateful acknowledgement is made to NERC for the award of a research studenship (GR4/83/GS/72) to one of the authors (ACR).

Aromatic hydrocarbons a

TIC 10000 1

REFERENCES 1

b

Composite

10000

fragmentogram

5 I

1

i 6

5000-

2 3

6910

14 13.15 lll2_l

1

4 5

20

30

40

50

60

6 7 8

70

9 Figure 16 a, Total ion current for aromatic hydrocarbons; composite fragmentogram for aromatic hydrocarbons

b, 10 11

Forest fires have been advocated as a source of unsubstituted PAH in soils and recent sediments41*49. Although there is no direct evidence of burning or even charring of the wood, there is the conflicting occurrence of relatively high-temperature combustion products in a sample which, on the basis of all previously discussed evidence, has experienced only a mild thermal history. The presence of unsubstituted PAH may be due to their absorption by the porous woody tissues in an environment where the burning of organic materials presumably occurred to varying degrees as a consequence of volcanic activity.

12 13 14 15 16 17 18

19

20

CONCLUSIONS Although the wood contains combustion-derived aromatics, the overall impression is of very gentle heating of a probable early gymnospermous wood. The heating was only sufficient to mobilize the more labile resins and bituminous material, but was insufficient to have caused any radical alteration of the line structure of cell tissues. The principal agents of modification were fungal and bacterial micro-organisms, which have caused degradation of the tissues. The presence of these microorganisms was detected by TEM, but a contribution from their own lipids and their reworking of the woody substrate have also left a recognizable geochemical imprint.

21 22 23 24

25 26 27 28 29 30

ACKNOWLEDGEMENTS 31

The authors thank Mr M. A. E. Browne of the British Geological Survey, Murchison House, Edinburgh, for providing the wood sample. They are also most appreciative of the assistance from Mr N. Telnaes of Norsk Hydro, Bergen, for provision of SMIM data.

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