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Fossil charcoal, its recognition and palaeoatmospheric significance
Palaeogeography, Palaeoclimatology, Palaeoecology (Global and Planetary Change Section), 9"7(1991)39-50 Elsevier Science Publishers B.V., Amsterdam
39
Fossil charcoal, its recognition and palaeoatmospheric significance T i m o t h y P. J o n e s a n d W i l l i a m G. C h a l o n e r
Biology and Geology Departments, Royal Holloway and Bedford New College, London University, London, UK (Received June 13, 1991)
ABSTRACT Jones, T.P. and Chaloner, W.G., 1991. Fossil charcoal, its recognition and palaeoatmospheric significance. Palaeogeogr., Palaeoclimatol., Palaeoecol., (Global Planet. Change Sect.), 97: 39-50. Charcoal is produced by pyrolysisof plant material and its occurrence in the fossil record can be broadly equated with the incidence of palaeowildfire. The past record of such naturally occurring fire, and the availability of the biomass which represents its fuel, put two constraints on oxygen levels. For combustion of plant material to occur at all requires that the atmospheric oxygen did not drop below a threshold of 13%. Increasing inflammability of plant material at higher oxygen levels suggests that 35% would be a ceiling above which plant biomass would ignite and burn so readily as to be incompatible with sustained forest growth. As we have more or less continuous fossil evidence of forest trees from the Late Devonian onwards, and a similarly sustained record of fossil charcoal from that time to the present (Cope, 1984), this constrains oxygen levels between 13% and 35% over that period (Rabash and Langford, 1968; Watson etal., 1978). However, further experimental work is required to establish the validity of these oxygen values under appropriate conditions and also to sharpen the certainty by which we can discriminate between fusain produced by pyrolysis, and inert wood degradation products produced by other (? biogenic) means. We discuss experiments directed at attempting to establish the validity of physical parameters by which pyrolyticallyproduced fusain can be characterized. The most convincing evidence of pyrolysis hitherto recognised is the apparent homogenization of xylem cell walls, as seen under SEM. Work on charcoal from both wildfires and laboratory wood charring under controlled conditions confirms the homogenization as seen under both SEM and TEM. Controlled temperature experiments show that a further rise in temperature causes the cell walls, initially homogenized, to crack and separate along the site of the middle lamella, giving the charcoal a characteristic fibrous texture. Both of these distinctive phases of response to pyrolysis can be observed in fossil charcoals.
Introduction If we use the c o n t i n u o u s record of charcoal-like fusain in the fossil record since the late D e v o n i a n to establish m i n i m u m and m a x i m u m oxygen levels in palaeoatmospheres, it is of p a r a m o u n t importance that we can state b e y o n d reasonable doubt, that the fossil material is true charcoal, the endproduct of pyrolysis. F u t u r e research and experim e n t a t i o n might revalue the proposed levels of 13% and 35%, b u t the basic premise will stand: that below a certain ' m i n i m u m ' oxygen level, wildfire is not sustainable, and above a certain ' m a x i m u m ' oxygen level, the prevalence of wildfire and sustained plant growth are incompatible
( R a b a s h and Langford, 1968; W a t s o n e t a l . , 1978; Cope a n d Chaloner, 1980; Clark and Russell, 1981; Chaloner, 1989). A brief appraisal of m o d e r n wildfire is informative. T h e present atmospheric oxygen level of 21% falls approximately halfway b e t w e e n the proposed limits of 13% and 35%. This is shown in the "Fire W i n d o w " illustrated in Fig. 1. Recent naturally ignited wildfires, affecting such geographically and climatically diverse regions as Arctic t u n d r a (Racine et al., 1985) through to equatorial rain forest (Sanford et al., 1985), have b e e n extensively d o c u m e n t e d . Fires such as the great fire of B o r n e o (Johnson, 1984) and the Yellowstone fires of 1988 ( W u e r t h n e r , 1988; De-
0921-8181/9l/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
40
"F.P. JONES AND ~ . G . CHALONER
d.0 ci
33%
-
{}q
I[
0%
(
FIRE
(.
"))
.)
(--'x..,,,.¢-)
¢-.... P.A,I,. 21% 13%
10qc
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~tmosphericoxM~en Fig. 1. The oxygen "Fire Window" showing proposed minimum and maximum atmospheric oxygen levels, and present atmospheric level (P.A.L.)
spain et al., 1989) burnt vast tracts of land, generating huge amounts of charcoal. The sheer bulk of charcoal in the fossil record is easily explained. The origins of fusain have been debated since the early part of the 20th century. Readers are referred to Scott (1989) for the most recent re-
view of the history of the controversy. Comparisons between fossil charcoal and variously preserved fossil woods have been made by a number of workers (CantriU, 1919; Harris, 1958; Cope and Chaloner, 1980; Chaloner 1989; Scott, 1989) employing different macroscopic, microscopic and chemical characteristics. These are outlined in Table 1. Using scanning electron microscopy (SEM), McGinnes et al. (1971, 1974) studied charcoal which had been manufactured in an industrial kiln at temperatures ranging from 270 to 400°C. Of the features recorded, the apparent homogenization of the cell wall is significant. McGinnes et al. (1974, pp. 77) noted that after pyrolysis: "the original fibrillar arrangement of the cell wall has been replaced with a smooth 'amorphous-appearing' wall structure". Taking this apparent homogenization of the cell wall, along with other characteristics, as being indicative of fossil charcoal, many authors (Alvin, 1974; Cope and Chaloner, 1985; Scott, 1989) published papers on fusain, with SEM micrographs of homogenized cell walls. Other workers, notably Beck et al. (1982, pp. 69) observed that fusinized Callixylon, a Lower Mississippian wood, "In macro-
TABLE 1 Features of fusinite (charcoal) distinguishing it from non-pyrolyzed fossil wood. After Harris (1958, 1981): Cope and Chaloner (1985); Chaloner (1989). Charcoal
Fossil wood
Macroscopic features Fractures into cuboidal blocks. Twigs have low length-to-diameter ratio (typically < 2). Brittle: gives a black streak on paper. Shows porosity from open lumina of original cells, typically empty; wood grain and cellular layout perceptible. Silky lustre.
May show fracture of original wood grain or, if highly coalified, random conchoidal fracture. Twigs have high length-to-diameter ratio (> 30), No streak, or poor brownish streak. Wood more or less homogenous, On fracture surfaces cell lumina commonly closed; cellular texture not in evidence. Matt or glossy surface.
Microscopic and chemical features Cell-wall substance homogenized; middle lamella lost as discernible feature of cell wall under SEM.
Cell-wall middle lamella recognizable under SEM.
Thin section typically totally opaque in transmitted light. Polished surface has high reflectance.
Thin section brown in transmitted light. Polished surface has lower reflectance.
Highly resistent to oxidative maceration in nitric acid with potassium chlorate (Schultze's solution). Attacked only after several days.
Attacked and oxidized by Schultze's solution within a few hours.
Low volatile content; glows on combustion.
High volatile content; burns with smoky flame.
FOSSIL CHARCOAL, ITS RECOGNITION AND PALAEOATMOSPHERIC SIGNIFICANCE
scopic features (it) cannot be distinguished from charcoal. Furthermore, in SEM sectional views, its cell walls have the distinctive appearance of those of charcoal--lacking, with few exceptions, any evidence of the discrete walls of contiguous cells". However, he goes on to argue for the possible origin of this preservation state by burial in organic sediments, under the action of anaerobic microorganisms. Beck et al. support their arguments with the work of Schmid (1967), whose transmitted electron microscopy (TEM) micrographs of Callixylon show internal cell wall structure. Transverse sections of tracheids show an electron-dense, compound middle [amella, markedly thickened at the cell corners, a thin electron-transparent S~ layer and a thick electron-dense S 2 layer. The example of Callixylon demonstrates that to take charcoal-like characteristics in isolation, even such distinctive characteristics as homogenized cell walls, is unsatisfactory. A number of criteria have been established to recognise 'true' fossil charcoal; homogenized cell walls, fidelity of anatomical preservation, chemical inertness and relatively high reflectance (Rr). Material which amply meets these requirements is abundant in the fossil record from the Lower Carboniferous onwards. This does not mean that material that does not fully meet these requirements did not necessarily undergo some degree of pyrolysis. A sizable proportion of the debris from modern wildfires is semi-charred (Scott, 1989). The history and taphonomy of this material is simply more ambiguous than that which meets all the criteria of typical fusain. Cope and Chaloner (1985) argue strongly for much semifusinite to be the result of "scorching', where the heating was insufficient to obliterate the middle lamella and microfibrillar detail. They note that remnant traces of a middle lamella are seen in the semifusinite portions of vitrinite-fusinite "transitional" material found in bituminous coals, and suggest that this is consistent with an origin as scorched material. The presence of semifusinite therefore, could also be taken as a probable record of palaeowildfire, with due implications for the palaeoatmospheric oxygen level. Jones et al. (1991) produced several series of
41
chars to observe the experimental conditions under which the homogenization of the cell wall occurs, and the reflectance (R r) of charcoals formed at different temperatures. This work has been extended to examine the transformation from wood to charcoal, properties of the charcoal, and final breakdown and combustion of the charcoal. Along with other physicochemical parameters the stable carbon isotope ratios of the wood-charcoal transformation are being measured, to establish whether charcoalification causes any fractionation, or merely freezes the isotopic ratios of living plants (DeNiro and Hastorf, 1984), with all the implications this could have for studies of fossil material. Material and methods
Preparation of the chars Charcoal was prepared from commercially dried Pinus sylvestris (Scots Pine) with pre-charring dimensions of 60 mm by 25 mm by 15 ram, with the grain along the longest axis. The wood was buried 10 mm under fine-grained quartz sand in a ceramic crucible with the wood grain horizontal and the crucible placed in a cold, Amalgams F29B, muffle furnace. This method was chosen to simulate the conditions of natural wildfire, where the wood has been subjected to heat, along with restricted oxygen. This is typically seen in the core of charred trunks, where the outer combusting layer inhibits the oxygen supply to the core. The furnace was switched on and the time required to reach the heat treatment temperature (H.T.T.) was recorded. Once the H.T.T. was reached the sample was left at this temperature for a controlled time. The muffle furnace was then switched off, the furnace door opened and the crucible left inside the furnace to cool overnight. This was done for both practical handling reasons and because the higher temperature samples, if removed from the hot sand, spontaneously ignite once in contact with unrestricted air. The range of temperatures and times is shown in Table 2. Only one furnace was used for the charring and this was calibrated and checked with Thermochrom heat-sensitive crayons. When
42
"r.P. J O N E S A N D W.G. C H A L O N E R
TABLE 2 Parameters of experimental charring of Pinus syh'estris. The time of exposure to the temperature given is 1 h in each case Temperature ( o C)
Heat-up time (rain)
180
20 20 20 20 20 20 20 20 20 20 25 25 25 30 30 35 35 40 45 50 55
190
20O 210 220 230 240 250 260 27O 28(I 290 300 34(I 37O 4O0 44(I 470 5OO 560 600
cooled to room temperature the sample was removed from the sand.
Preparation of fossil charcoal The rock samples containing the fossil charcoal were placed in 40% hydrofluoric acid in plastic bottles and left for 6 weeks. After a series of dilutions with distilled water, approximately 5 ml of residue-containing fluid was topped up to 50 ml with 60% hydrochloric acid, heated to boiling point and then allowed to cool overnight. This was again processed through a series of dilutions with distilled water until litmus paper showed the liquid to have a neutral pH. The residue was then sieved at 180 txm through nylon mesh. This residue was washed into a petri dish using distilled water and a few drops of phenol added as an antifungal agent. Fragments of charcoal were then hand-picked out of the petri dish using a fine paint brush.
Scanning electron microscopy Aluminium S.E.M. stubs were prepared for each temperature/time range. Each stub held three specimens approximately 1 mm by 1 mm by 1 mm allowing for T.E.M. after S.E.M. examination. The specimens were mounted on the stubs with double-sided adhesive tabs. They were then gold/palladium coated to a thickness of 26 nm using a $5100 sputterer and examined by scanning electron microscopy using a Cambridge S100 E.M. Micrographs showing longitudinal and transverse sections were taken on Ilford black and white Rollfilm. After examination by S.E.M. the specimens were washed off the aluminium S.E.M. stubs with ethyl acetate, the solvent for the adhesive tabs, and left to soak in 100% ethyl acetate for 24 h.
Transmitted electron microscopy The samples for T.E.M. were transferred from 100% ethyl acetate to 100% propylene oxide, the link reagent for Spurrs Resin, and again left to soak for 24 h. The 100% propylene oxide was pipetted off and replaced with 50% Spurts Resin/50% propylene oxide and gently rotated for 48 h. The 50%/50% mixture was then pipetted off and replaced with 100% Spurrs Resin. The samples were then placed under vacuum and released back to atmospheric pressure several times in order to assist impregnation of the specimen with resin, and left on a rotator for 24 h. The Spurrs Resin was replaced with fresh resin, again vacuum treated and then rotated for 24 h. The specimens were then placed into individual beem capsules containing fresh Spurrs resin and polymerised at 70°C for 12 h. Ultra-thin sections 80-100 nm thick were cut on a Cambridge Huxley Ultra-microtome M2 using a glass knife. These were mounted on Gilder G200TH Cu/Pd grids and examined with a Hitachi H600 Transmission Electron Microscope. Micrographs were taken with Ilford "EM" black and white film.
Stable carbon isotopes The experimental charcoal samples, taken directly from the muffle furnace and without any
43
FOSSIL C H A R C O A L , ITS R E C O G N I T I O N A N D P A L A E O A T M O S P H E R I C S I G N I F I C A N C E
PLATE 1 SEM MICROGRAPHS OF EXPERIMENTAL CHARS.
DESCRIPTION.
CELL WALL STRUCTURE SEEN.
SEM MICROGRAPHS OF FOSSIL MATERIAL.
E. HOMOGENISED CELL WALLS. CARBONIFEROUS CHARCOAL FROM E KIRKTON, SCOTLAND. i
A. 210C, 1 hr.
10UM
220C
10UM
i
230C
SUDDEN APPARENT HOMOGENISATION OF CELL WALLS.
i
340C
FIRST APPEARANCE OF CRACKING ALONG SITE OF MIDDLE LAMELLA.
10UM
CRACKING BECOMES MORE PRONOUNCED AT EDGES OF SAMPLE. INDIVIDUAL CELLS PULL APART.
D. 560C, 1 hr.
10UM
F. CARBONIFEROUS CHARCOAL FROM E KIRKTON, SCOTLAND.
COMPLETE COMBUSTION
Scanning electron micrographs of experimental chars of P ~vh,estris, and comparison with mierographs of Carboniferous fossil material. Cracking ahmg the site of the middle lamella occurs in both early and late wood.
T.P. JONES AND W.G. CHALONER
44 PLATE I (continued)
i .................
!~
G. 200C, l hr.
] 10UM
[ H. 200C, 1 hr.
I'0UM
J. 260C, 1 hr.
10'1'1'1'1'1'1'1'1"~
i i ii Transmitted electron micrographs showing the homogenization of cell walls. Note the disappearance of the pit membrane in micrograph J.
FOSSIL CHARCOAL, ITS RECOGNITION AND PALAEOATMOSPHERIC SIGNIFICANCE
chemical treatment, were either first ground to a fine powder using a ceramic pestle and mortar, or small intact fragments were used. Between 200 #g and 350/zg were weighted out in quartz glass buckets and the buckets topped up with analar Cu(II)O. These were then placed inside 6 mm OD quartz glass tubes, the tubes were evacuated and sealed with a oxyhydrogen torch. The tubes were placed in a furnace for 10 h at 1000°C, then for 10 h at 600°C, and finally left overnight at 450°C to reabsorb any surplus oxygen. Once the combustion gases were released in the mass spectrometer preparation line by means of a flexible cracking joint, the CO~ was purified by cryo-distillation using liquid nitrogen and methanol slush. Measurements were taken using a PRISM mass spectrometer. This is an adaption of the method of Northfelt et al. (1981).
Results
Under the experimental conditions outlined, the apparent homogenization of the cell walls is seen to occur between 220°C and 230°C. SEM micrographs A and B of Plate I show detail of the cell wall in transverse section, before and after homogenization. The face for micrograph A was prepared by cutting the wood block with a razor blade, while the face for micrograph B is a fracture plane through the charcoal. TEM micrographs G to J of Plate II also illustrate the transition from structured to homogenized cell walls. Micrographs G and I show the change at the junction of 3 cells, as seen in transverse section. Micrographs H and J show the homogenization, as seen in sections through bordered pits; note that in micrograph J, while the walls are homogenized and the 3-dimensional shape of the bordered pit is retained, the pit membrane has disappeared. Sections through pit-pairs often miss one or both pit apertures, giving the false impression that the pit chambers are partially or fully enclosed. Both micrographs H and J give the impression of a partially closed chamber. With TEM, after the comparative ease of obtaining sections from non-homogenized material (micrographs G and H), extreme difficulties were expe-
45
rienced once the cell walls were homogenized. The excessive 'chattering' of the microtome knife, thickness of the sections and marks from imperfections in the glass knife (such as the vertical, darker band in micrograph I), can obscure fine structural detail. Accepting these limitations, cell wall structure is not apparent in micrographs I and J. In all cases where the cell wall appeared homogenized when viewed under SEM, the TEM micrographs of the same specimen confirmed the lack of internal wall structure. It is important to note here that this comparison was made of the same specimens, that is, after SEM, the fragments were washed off the SEM stub, embedded in resin, sectioned and then viewed under TEM. The technical difficulties experienced when attempting to cut ultra-thin sections of embedded charcoal, and the fragility of the sections obtained, with the electron beam tending to disintegrate the sections, may be considered as another physical characteristic of the charcoal. With temperatures rising from 230 to 340°C, above the level at which homogenization occurs, no further change is detectable in the cell walls under the SEM. Fracture faces are clean, often with concoidal fracture, and no internal cell wall structure is seen. Attempts to cut TEM sections were eventually abandoned as the charcoal became increasingly brittle. From 340°C onwards, cracking starts to appear along the site of the middle lamella, as can be seen in micrograph C of Plate I. This was observed in all specimens up to 600°C; above this range, complete combustion of the material occurred. Typically, the centre of the specimens in the range 340-600 °C still had uncracked, homogenized cell walls. Towards the edges, where the material was less insulated from heat and oxygen, cracking appears. This cracking progressively worsens, until the cells are completely isolated by cracks along the site of the middle lamella. In the final stages, the cells are seen as separate units. The extent of this breakdown was progressively more advanced with increase in temperature over the range 340-600°C. Measurement of the stable carbon isotopes ranging from unheated wood through to charcoal prepared at 600°C, showed that charcoalification does not "freeze" stable carbon isotopic ratios,
4(3
FP. JONES AND W.G. CHALONER
can be considered as fully charcoalified. In this phase the cell wall is apparently homogenized, structure is anatomically preserved, pit membranes have usually disappeared, the tissue is more chemically inert, the reflectance (R r) is moderate to high and the charcoal has undergone gl3C isotopic fractionation. In the third phase the material is still charcoal, but is breaking-up. In this phase the cell wall is cracking along the site of the middle lamella, structural preservation is becoming blurred, tissue is chemically inert, reflectance (R r) is high and the charcoal is 6~3C isotopically fractionated. This information is summarised in Table 3. All three of these phases are responses to pyrolysis, and the fossilization potential and likely diagenesis of each of these phases can be reliably predicted. The first phase, a response to 'scorching' or 'charring' certainly changes the nature of the wood. This information is not new; primitive man hardened wooden arrow points in open fires and modern man expects to hit a baseball a few more yards if his wooden baseball bat is 'flame tempered'. The vitrinite-fusinite 'transitional' material found in bituminous coals (Cope and Chaloner, 1985), has an intermediate layer of semifusinite with remnant traces of a middle lamella. This material had sufficient preservation-
but rather results in an isotopic fractionation, favouring the lighter isotopes, Fig. 2. Untreated whole tissue gave a value averaging 613C = - 24.5, charred wood gave an average value of 613C = -24.3, charcoal prepared at temperatures of 300-500°C gave values averaging 6~3C ~ -24.9, and charcoal prepared at 600°C gave an average result of gl3c = -25.3. The spread of data for each temperature range is believed to be due to the inherently heterogeneous nature of the wood. Results at each temperature level were much closer for the finely powdered charcoal, than corrcsponding results obtained from whole fragments. Discussion
The experimental material is seen to have three distinctive phases of response to pyrolysis, with the three important controlling parameters being temperature, time and availability of oxygen. The first phase results from 'scorching' or 'charring' of the tissue. In this phase the cell wall has internal structure, structure is anatomically preserved, pit membranes are intact, tissue is not chemically inert, reflectance (R,) is low and the material retains the ~13C isotopic ratio of the unheated wood. In the second phase the material
-24.0
I
I
I
1
[
I
-24.5
O
D ?
Z -25.0 0
m i
-25.5
-26.0
I OC
I 200C
I 3OOC
I 400C
I 500C
I 600C
TEMPERATURE OF FORMATION Fig. 2. Plot of stable carbon isotopes 8C against temperature of formation °C for experimental chars of Psyh'estris.
FOSSIL CHARCOAL, ITS RECOGNITIONAND PALAEOATMOSPHERICSIGNIFICANCE
potential to resist being converted to vitrinite, but is not fully fossil charcoal, but rather is fossil charred wood. The charring of the plant tissue has clearly made it more resistant to biodegradation and diagenetic alteration, and it has survived as a fossil with relatively little alteration. However, regardless of whether we consider the charcoal or the charred wood, both are the product of wildfire, and if there was a palaeowildfire, the oxygen level constraints on the palaeoatmosphere must stand. This argument does not include charcoal or charred wood found in association with volcanic rocks or other signs of active volcanism, which clearly could have undergone pyrolysis regardless of palaeoatmospheric oxygen levels. This is an important proviso in any general statements about global oxygen levels based on charcoal occurrences. Cope and Chaloner (1985) responded to Beck et al.'s suggestion (1982) that Callixylon fusain could have had a non-pyrolysis origin, by suggesting that the Callixylon shows features consistent with charring produced by wildfire conditions. The experiments described in this paper strongly support this suggestion, and they place limits on the degree of pyrolysis that the Callixylon must have undergone. It is proposed that the material
47
lies across the boundary between phases 1 and 2 of the experimental material; that is, that the Callixylon material is transitional between charred wood and charcoal. This suggestion is supported by the presence of both homogenized and structured cell walls, and the preservation of pit membranes as seen under TEM. As already quoted, Beck et al. (1982) states that under SEM the cells lack any evidence of discrete walls of contiguous cells, but importantly adds "with few exceptions". Schmid (1967, p. 723), while understandably concentrating on the more interesting structured walls, does record "Other micrographs show little or no layering in the walls". With the experimental chars, without exception, where the wall had internal structure, this was seen under both SEM and TEM; where the walls appeared homogenized, this again was seen under both SEM and TEM. It should be remembered that the same fragment was used for both SEM and TEM. It is suggested that the "few exceptions" of Beck et al. are the structured cell wails of Schmid, and the homogenized walls of Beck are the "no layering in the walls" of Schmid. Research is currently underway to clarify this. The argument for charring is further supported by the presence of preserved pit membranes,
TABLE 3 Physical and chemical parameters of phases 1, 2 and 3 of pyrolysis of xylem tissue Phase 1
Phase 2
Phase 3
Charred wood.
Charcoal
Charcoal breaking-up.
Experimental chars "180 ° C-220 o C".
Experimental chars "230 ° C-340 ° C".
Experimental chars "3411 o C-6110 ° C".
Internal cell wall structure seen.
Cell walls apparently homogenised.
Cracking along site of middle lamella.
Preservation potential moderate, usually compressed.
Preservation potential excellent. Anatomic preservation of high fidelity.
Preservation potential poor. Very fragile, typically smashed to minute pieces.
Pit membranes intact.
Pit membranes usually disappeared.
Pit membranes disappeared.
Attacked and oxidised by Schultze's solution within a few hours.
Highly resistant to oxidation by Schultze's solution.
Highly resistant to oxidation by Schultze's solution.
Reflectance Rr _<"0.4%".
Reflectance R r ="0.4%-1.2%".
Reflectance Rr >-"1.2~".
Stable carbon isotopes same as parent plant whole tissue.
Stable carbon isotopes fractionated. exp' chars ---"0.5" lighter.
In
Stable carbon isotopes fractionated. In exp' chars ~"1.0" lighter.
Seen in fossil record as semifusinite.
Seen in fossil record as fusain, fusinite and semifusinite.
Seen in fossil record as fusain, inertodetrinite.
48
beautifully seen in Schmid's TEM sections of Callixylon. It is hard to envisage a biotic process which homogenizes cell walls; indeed, a modern (biotic) equivalent has yet to be found. It is even harder to imagine anaerobic microorganisms which not only homogenize cell walls, but preserve pit membranes as well. If we allow that the CalFtxylon was charred, then this dilemma is removed. Not only is the pit membrane retained in the proposed phase l, but the charring enhances the preservation potential of the membrane as well. A final consideration is 'selectivity' of preservation. A biotic process is hard to reconcile with the observed gradient of charcoal to uncharted material. This problem is again removed if we invoke a palaeowildfire origin, with some of the tissue pyrolysed to charcoal and the rest sufficiently insulated to remain only charred. The experimental charring also roughly support the temperature of charring of the Call~,lon ~ 250°C suggested by Cope and Chaloner (1985). The changeover from phase 1-2 was seen to occur between 220°C and 230°C, under the experimental conditions outlined. That is, heating from cold, one hour at H.T.T., then allowed to cool overnight. With slightly higher temperatures the same effect would have been reached more quickly, and conversely, slightly lower temperatures would have required longer. Allowing for a small degree of variation, the pyrolysis temperatures would still have been significantly above average diagenetic temperatures of 10°C/kin (Winkler, 1965; Rolfe and Brett, 1969). Material physicochemically identical to the phase 2 experimental charcoal is abundant in the fossil record from the Lower Carboniferous onwards. Fossil charcoal is so abundant, that in many sedimentary sequences it is the dominant fossil, and occasionally the only fossil recovered. It is found in coal seams and sedimentary sequences of many different lithologies. Little imagination is required to visualise fires sweeping through the Carboniferous coal-swamps during phases of drought, or Jurassic conifer forests. Such a process is mirrored today in the wildfires of the North American Everglades (Cohen et al., 1987), or conifer forests of Yellowstone National Park, USA (Wuerthrer, 1988; Despain et al.,
T.P. JONES A N D W.G, CHALONER
1989). We can test and confirm that this fossil material meets all the requirements of phase 2 experimental charcoal, with the exception of the fractionation of the 8~3C isotopes. Certainly we can measure the ~13C ratios of conifer fossil charcoal, and the readings are approximately as expected; however, since we do not know the original isotopic ratio, the fractionation can only be assumed. Modern conifer xylem cell walls are composed of two main components, cellulose and lignin, on average 2/3 cellulose and 1/3 lignin. As a result of the plant's photosynthetic and ensuing biochemical pathways lignin is isotopically lighter than cellulose (Degens, 1967; Schidlowski, 1983, 1984; DeNiro and Hastorf, 1984; Benner et al., 1987). Unfortunately, the two distinct end products of fossil charcoal and vitrinite (compression fossil) both tend to be isotopically fractionated towards the lighter isotope, even though the fractionation processes are quite different. In the formation of fossil charcoal, cellulose is removed before the [ignin by pyrolysis (Beall et al., 1974), while in vitrinite formation degradation has preferentially removed the cellulose. Some confirmation of the fractionation may be found in sampling across the fusinite, semifusinite to vitrinite, 'transition' material found in bituminous coals. The third phase recognised in the experimental charring, with the charcoal breaking-up, is also seen in the fossil record, micrograph F, Plate I. However, such very fragile material is not commonplace, especially in charcoal which has undergone transportation. The 'fibrous' nature of charcoal has been recognised for a long time (Stopes, 1919). However there are three separate reasons why charcoal can appear fibrous when examined macroscopically. Firstly the wood, anatomically preserved, could appear fibrous as a result of the essentially elongate nature of its xylem cellular structure. Secondly, a very common phenomenon seen in fossil charcoal is 'bogen-struktur' (Stach, 1927a, b), where as a result of compression after burial cell walls are fractured, usually perpendicularly across the weakest or thinnest part of the wall and lengthwise down the cells. Once the containing matrix or any diagenetic cementation is removed, such
FOSSIL CHAR('OAL, ITS RECOGNITION AND PALAEOATMOSPHERIC SIGNIFICANCE
charcoal tends to disaggregate into separate tiny spikes or "fibres". Thirdly, the fossil charcoal may have cracks along the site of the middle lamella, as seen in phase three of the experimental chars. The maceral inertodetrinite, composed of masses of tiny, granular, high reflecting particles, could be partially composed of smashed and broken minute charcoal fragments, corresponding to the break-up material of phase 3 of the experimental chars. Conclusion Xylem tissue has three distinctive phases of response to pyrolysis. These phases can be physically and chemically defined, and approximately equated to formation temperatures. Fossil equivalents of these phases are seen from the late Devonian onwards. The presence of this fossil charcoal record indicates a history of palaeowildfire, and this places upper and lower constraints on oxygen levels in palaeoatmospheres during this period. Material which is categorically fossil charcoal, corresponding to phases 2 and 3 is abundant in the fossil record from the early Carboniferous onwards. Fossil material equivalent to phases 1-2 can be found from the late Devonian onwards. Although the pyrolysis origin of some of the latter material is less certain, evidence is increasingly pointing towards a 'charred wood' origin. Charring enhances the preservationpotential of plant tissue, and can be seen in the fossil record as the maceral semifusinite. Further experimental work is required to increase the certainty of this pyrolysis origin. Doubts remain about the pyrolysis origin of some of the more problematic semifusinites, but the origins of 'typical' fusain are evidently pyrolytic. Acknowledgements We would like to thank the staff of the Electron Microscope Unit, R.H.B.N.C. and Dr D. Mattey, Stable Isotope Laboratory, Geology Dept., R.H.B.N.C.T.J gratefully acknowledges the receipt of a NERC studentship.
49
References
Alvin, K.L., 1974. Leaf anatomy of Weichselia based on fusainized material. Palaeontology, 17: 587-598. Beall, F.C., Blankenhorn, P.R. and Moore, G.R., 1974. Carbonized Wood. Physical properties and use as a S.E.M. preparation. Wood Sci., 6: 212-219. Beck, C.B., Coy, K. and Schmid, R., I982. Observations on the fine structure of Callixylon wood. Am. J. got., 69: 54-76. Benner, R., Fogel, M.L., Sprague, E.K. and Hodson, R.E.. 1987. Depletion of 13C in lignin and its implications for stable carbon isotopes studies. Nature, 329: 708-71(I. Cantrill, T.C., 1919. Some Chemical Characteristics of Ancient Charcoals. Cambrian Archaeological Ass., "Archaeologia Cambrensis". 1919. Chaloner, W.G., 1989. Fossil charcoal as an indicator of palaeoatmospheric oxygen level. J. Geol. Soc. London, 146: 171-174. Clark, F.R,S. and Russell, D.A., 1981. Fossil charcoal and the palaeoatmosphere. Nature, 29(1: 428. Cohen, A.D., Spackman, W. and Raymond, R. Jr., 1987. Interpreting the characteristics of coal seams from chemical, physical and petrographic studies of peat deposits. In: A.C. Scott (Editor), Coal and Coal-bearing Strata: Recent Advances. Blackwell, Oxford, Geol Soc. Spec. Pubk, 32: 107-125. Cope M.J., 1984. Some Studies of the Origin, Nature and Occurrence of Charcoalified Plant Fossils. Ph.D. Thesis, Univ. London, U.K., (unpubl.). Cope M.J. and Chaloner, W.G., 1980. Fossil charcoal as evidence of past atmospheric composition. Nature, 283: 647-649. Cope, M.J. and Chaloner, W.G., 1985. Wildfire, an interaclion Of biological and physical processes. In: B.H. Tiffney (Editor), Geological Factors and the Evolution of Plants. Yale University Press, Hartford, CT, pp. 257 277. Degens, E.T., 1967. Biochemistry of stable carbon isotopes. In: G. Eglinton and M.T.J. Murphy (Editors), Organic Geochemistry, Methods and Results. Longman. DeNiro, M.J. and Hastorf, C.A., 1984. Alteration of 15N/14N and 13C/12C Ratios of Plant Matter during the Initial Stages of Diagenesis: Studies Ulilizing Archaeological Specimens from Peru. Geochim. Cosmochim. Acta, 49: 97-115. Despain, D., Rodman, A., SchulIer~, P and Shovic., 1989. Burned area survey of Yellowstone National Park: The fires of 1988. Division of Research and Geographic Information Systems laboratory. Yellowstone National Park, USA. Harris, T.M., 1958. Forest fire in the Mesozoic. J. Ecol., 46: 447-453. Harris, T.M., 1981. Burnt Ferns from lhe English Wealden. Proc. Geol. Assoc. London, 92:47 58. Johnson, B., 1984. The Great Fire of Borneo. World Wild-life Fund, Godalming, Surrey, 24 pp. Jones, T.P., Scott, A.C. and Cope, M.J., In press. Reflectance Measurements and the Temperature of Formation of
5(}
Modern Charcoals and Implications for Studies of Fusain. Bull. Soc. Go61, Fr., 162(2): 193-200. McGinnes, E.A., Kandeel, S.A. and Szopa, P.S., 1971. Some structural changes observed in the structure of wood. W~x)d Fibre 3: 77-83, McGinnes, E.A., Szopa, P.S. and Phelps, J.E., 1974. Use of Scanning Electron Microscope in Studies of Wood Charcoal Formation. Scanning Electron Microsc., 1974, Part II, ITT Res. Inst. Chicago, pp. 469-476. Northfelt. D.W., DeNiro. M.J. and Epstein, S., 1981. Hydrogen and carbon isotopic ratios of the cellulose nitrate and saponifiable lipid fractions prepared from annual growth rings of a Californian redwood. Geochim. Cosmochim. Acta, 45:1895 1898. Rabash, D.J. and Langford, B., 1968. Burning of wood in atmospheres of reduced oxygen concentrations. Combust. Flame, 12: 33-4(), Racine, C.H,, Dennis, J.G. and Patterson, W.A. the 3rd., 1985. Tundra fire regimes in the Noatak River watershed, Alaska: 1956-83. ARCTIC, 38: 194-200. Rolfe, W.D.I. and Brett, D.W., 1969. Fossilization process. In: G. Egliston and M.T,J. Murphy (Editors), Organic Geochemistry. Longman. Sanford. R.L. Jr,, Saldarriage, J., Clark, K.E., Uhl, C. and Herrera. R., 1985. Amazon rain-forest fires. Science, 227: 53 55.
T.P. JONES AND W.G C H A L O N E R
Schidlowski, M., 1983. Organic matter in sedimentary rocks: "The dust we tread upon was once alive". Terra Cognita, 4(1): 45-49. Schidlowski, M., 1984. Biological modulation of the Terrestrial Carbon cycle: Isotope clues to early organic evolution. Adv. Space. Res., 4 (12): 183-193. Schmid, R., 1967. Electron Microscopy of wood of Callixylon and Cordaites Am. J. Bot., 54: 720-729. Scott, A.C.,1989. Observations on the nature and origin of fusain. Int. J. Coal Geol., 12: 443-475. Stach, E., 1927a. Der Kohlenreliefschliff, ein neues Hilfsmittel fiir die angewandte Kohlenpetrographie. Mitt. Abt. Gestein, Erz, Kohle und Salzunters. Preuss. Geol. Landesanst. Berlin, 2: 75-94. Stach, E., 1927b. Zur Enstehung des Fusits. Gluckauf, 43: 759-763. Stopes, M.C., 1919. On the four visible ingredients in banded bituminous coal: Studies in the composition of coal. Proc. R. Soc., Ser. B., 90: 480-487. Watson, A., Iawelock. J.E. and Margulis, L., 1978. Methanogenesis, fires and the regulation of atmospheric oxygen. Biosystems, 10(1978): 293-298. Winkler, H.G.F., 1965. Petrogenesis of Metamorphic Rocks. Springer, Berlin. Wuerthner, G., 1988. Yellowstone and the Fires of Change. Haggis House, Salt Lake City, Utah, 64 pp.
39
Fossil charcoal, its recognition and palaeoatmospheric significance T i m o t h y P. J o n e s a n d W i l l i a m G. C h a l o n e r
Biology and Geology Departments, Royal Holloway and Bedford New College, London University, London, UK (Received June 13, 1991)
ABSTRACT Jones, T.P. and Chaloner, W.G., 1991. Fossil charcoal, its recognition and palaeoatmospheric significance. Palaeogeogr., Palaeoclimatol., Palaeoecol., (Global Planet. Change Sect.), 97: 39-50. Charcoal is produced by pyrolysisof plant material and its occurrence in the fossil record can be broadly equated with the incidence of palaeowildfire. The past record of such naturally occurring fire, and the availability of the biomass which represents its fuel, put two constraints on oxygen levels. For combustion of plant material to occur at all requires that the atmospheric oxygen did not drop below a threshold of 13%. Increasing inflammability of plant material at higher oxygen levels suggests that 35% would be a ceiling above which plant biomass would ignite and burn so readily as to be incompatible with sustained forest growth. As we have more or less continuous fossil evidence of forest trees from the Late Devonian onwards, and a similarly sustained record of fossil charcoal from that time to the present (Cope, 1984), this constrains oxygen levels between 13% and 35% over that period (Rabash and Langford, 1968; Watson etal., 1978). However, further experimental work is required to establish the validity of these oxygen values under appropriate conditions and also to sharpen the certainty by which we can discriminate between fusain produced by pyrolysis, and inert wood degradation products produced by other (? biogenic) means. We discuss experiments directed at attempting to establish the validity of physical parameters by which pyrolyticallyproduced fusain can be characterized. The most convincing evidence of pyrolysis hitherto recognised is the apparent homogenization of xylem cell walls, as seen under SEM. Work on charcoal from both wildfires and laboratory wood charring under controlled conditions confirms the homogenization as seen under both SEM and TEM. Controlled temperature experiments show that a further rise in temperature causes the cell walls, initially homogenized, to crack and separate along the site of the middle lamella, giving the charcoal a characteristic fibrous texture. Both of these distinctive phases of response to pyrolysis can be observed in fossil charcoals.
Introduction If we use the c o n t i n u o u s record of charcoal-like fusain in the fossil record since the late D e v o n i a n to establish m i n i m u m and m a x i m u m oxygen levels in palaeoatmospheres, it is of p a r a m o u n t importance that we can state b e y o n d reasonable doubt, that the fossil material is true charcoal, the endproduct of pyrolysis. F u t u r e research and experim e n t a t i o n might revalue the proposed levels of 13% and 35%, b u t the basic premise will stand: that below a certain ' m i n i m u m ' oxygen level, wildfire is not sustainable, and above a certain ' m a x i m u m ' oxygen level, the prevalence of wildfire and sustained plant growth are incompatible
( R a b a s h and Langford, 1968; W a t s o n e t a l . , 1978; Cope a n d Chaloner, 1980; Clark and Russell, 1981; Chaloner, 1989). A brief appraisal of m o d e r n wildfire is informative. T h e present atmospheric oxygen level of 21% falls approximately halfway b e t w e e n the proposed limits of 13% and 35%. This is shown in the "Fire W i n d o w " illustrated in Fig. 1. Recent naturally ignited wildfires, affecting such geographically and climatically diverse regions as Arctic t u n d r a (Racine et al., 1985) through to equatorial rain forest (Sanford et al., 1985), have b e e n extensively d o c u m e n t e d . Fires such as the great fire of B o r n e o (Johnson, 1984) and the Yellowstone fires of 1988 ( W u e r t h n e r , 1988; De-
0921-8181/9l/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
40
"F.P. JONES AND ~ . G . CHALONER
d.0 ci
33%
-
{}q
I[
0%
(
FIRE
(.
"))
.)
(--'x..,,,.¢-)
¢-.... P.A,I,. 21% 13%
10qc
0q
~tmosphericoxM~en Fig. 1. The oxygen "Fire Window" showing proposed minimum and maximum atmospheric oxygen levels, and present atmospheric level (P.A.L.)
spain et al., 1989) burnt vast tracts of land, generating huge amounts of charcoal. The sheer bulk of charcoal in the fossil record is easily explained. The origins of fusain have been debated since the early part of the 20th century. Readers are referred to Scott (1989) for the most recent re-
view of the history of the controversy. Comparisons between fossil charcoal and variously preserved fossil woods have been made by a number of workers (CantriU, 1919; Harris, 1958; Cope and Chaloner, 1980; Chaloner 1989; Scott, 1989) employing different macroscopic, microscopic and chemical characteristics. These are outlined in Table 1. Using scanning electron microscopy (SEM), McGinnes et al. (1971, 1974) studied charcoal which had been manufactured in an industrial kiln at temperatures ranging from 270 to 400°C. Of the features recorded, the apparent homogenization of the cell wall is significant. McGinnes et al. (1974, pp. 77) noted that after pyrolysis: "the original fibrillar arrangement of the cell wall has been replaced with a smooth 'amorphous-appearing' wall structure". Taking this apparent homogenization of the cell wall, along with other characteristics, as being indicative of fossil charcoal, many authors (Alvin, 1974; Cope and Chaloner, 1985; Scott, 1989) published papers on fusain, with SEM micrographs of homogenized cell walls. Other workers, notably Beck et al. (1982, pp. 69) observed that fusinized Callixylon, a Lower Mississippian wood, "In macro-
TABLE 1 Features of fusinite (charcoal) distinguishing it from non-pyrolyzed fossil wood. After Harris (1958, 1981): Cope and Chaloner (1985); Chaloner (1989). Charcoal
Fossil wood
Macroscopic features Fractures into cuboidal blocks. Twigs have low length-to-diameter ratio (typically < 2). Brittle: gives a black streak on paper. Shows porosity from open lumina of original cells, typically empty; wood grain and cellular layout perceptible. Silky lustre.
May show fracture of original wood grain or, if highly coalified, random conchoidal fracture. Twigs have high length-to-diameter ratio (> 30), No streak, or poor brownish streak. Wood more or less homogenous, On fracture surfaces cell lumina commonly closed; cellular texture not in evidence. Matt or glossy surface.
Microscopic and chemical features Cell-wall substance homogenized; middle lamella lost as discernible feature of cell wall under SEM.
Cell-wall middle lamella recognizable under SEM.
Thin section typically totally opaque in transmitted light. Polished surface has high reflectance.
Thin section brown in transmitted light. Polished surface has lower reflectance.
Highly resistent to oxidative maceration in nitric acid with potassium chlorate (Schultze's solution). Attacked only after several days.
Attacked and oxidized by Schultze's solution within a few hours.
Low volatile content; glows on combustion.
High volatile content; burns with smoky flame.
FOSSIL CHARCOAL, ITS RECOGNITION AND PALAEOATMOSPHERIC SIGNIFICANCE
scopic features (it) cannot be distinguished from charcoal. Furthermore, in SEM sectional views, its cell walls have the distinctive appearance of those of charcoal--lacking, with few exceptions, any evidence of the discrete walls of contiguous cells". However, he goes on to argue for the possible origin of this preservation state by burial in organic sediments, under the action of anaerobic microorganisms. Beck et al. support their arguments with the work of Schmid (1967), whose transmitted electron microscopy (TEM) micrographs of Callixylon show internal cell wall structure. Transverse sections of tracheids show an electron-dense, compound middle [amella, markedly thickened at the cell corners, a thin electron-transparent S~ layer and a thick electron-dense S 2 layer. The example of Callixylon demonstrates that to take charcoal-like characteristics in isolation, even such distinctive characteristics as homogenized cell walls, is unsatisfactory. A number of criteria have been established to recognise 'true' fossil charcoal; homogenized cell walls, fidelity of anatomical preservation, chemical inertness and relatively high reflectance (Rr). Material which amply meets these requirements is abundant in the fossil record from the Lower Carboniferous onwards. This does not mean that material that does not fully meet these requirements did not necessarily undergo some degree of pyrolysis. A sizable proportion of the debris from modern wildfires is semi-charred (Scott, 1989). The history and taphonomy of this material is simply more ambiguous than that which meets all the criteria of typical fusain. Cope and Chaloner (1985) argue strongly for much semifusinite to be the result of "scorching', where the heating was insufficient to obliterate the middle lamella and microfibrillar detail. They note that remnant traces of a middle lamella are seen in the semifusinite portions of vitrinite-fusinite "transitional" material found in bituminous coals, and suggest that this is consistent with an origin as scorched material. The presence of semifusinite therefore, could also be taken as a probable record of palaeowildfire, with due implications for the palaeoatmospheric oxygen level. Jones et al. (1991) produced several series of
41
chars to observe the experimental conditions under which the homogenization of the cell wall occurs, and the reflectance (R r) of charcoals formed at different temperatures. This work has been extended to examine the transformation from wood to charcoal, properties of the charcoal, and final breakdown and combustion of the charcoal. Along with other physicochemical parameters the stable carbon isotope ratios of the wood-charcoal transformation are being measured, to establish whether charcoalification causes any fractionation, or merely freezes the isotopic ratios of living plants (DeNiro and Hastorf, 1984), with all the implications this could have for studies of fossil material. Material and methods
Preparation of the chars Charcoal was prepared from commercially dried Pinus sylvestris (Scots Pine) with pre-charring dimensions of 60 mm by 25 mm by 15 ram, with the grain along the longest axis. The wood was buried 10 mm under fine-grained quartz sand in a ceramic crucible with the wood grain horizontal and the crucible placed in a cold, Amalgams F29B, muffle furnace. This method was chosen to simulate the conditions of natural wildfire, where the wood has been subjected to heat, along with restricted oxygen. This is typically seen in the core of charred trunks, where the outer combusting layer inhibits the oxygen supply to the core. The furnace was switched on and the time required to reach the heat treatment temperature (H.T.T.) was recorded. Once the H.T.T. was reached the sample was left at this temperature for a controlled time. The muffle furnace was then switched off, the furnace door opened and the crucible left inside the furnace to cool overnight. This was done for both practical handling reasons and because the higher temperature samples, if removed from the hot sand, spontaneously ignite once in contact with unrestricted air. The range of temperatures and times is shown in Table 2. Only one furnace was used for the charring and this was calibrated and checked with Thermochrom heat-sensitive crayons. When
42
"r.P. J O N E S A N D W.G. C H A L O N E R
TABLE 2 Parameters of experimental charring of Pinus syh'estris. The time of exposure to the temperature given is 1 h in each case Temperature ( o C)
Heat-up time (rain)
180
20 20 20 20 20 20 20 20 20 20 25 25 25 30 30 35 35 40 45 50 55
190
20O 210 220 230 240 250 260 27O 28(I 290 300 34(I 37O 4O0 44(I 470 5OO 560 600
cooled to room temperature the sample was removed from the sand.
Preparation of fossil charcoal The rock samples containing the fossil charcoal were placed in 40% hydrofluoric acid in plastic bottles and left for 6 weeks. After a series of dilutions with distilled water, approximately 5 ml of residue-containing fluid was topped up to 50 ml with 60% hydrochloric acid, heated to boiling point and then allowed to cool overnight. This was again processed through a series of dilutions with distilled water until litmus paper showed the liquid to have a neutral pH. The residue was then sieved at 180 txm through nylon mesh. This residue was washed into a petri dish using distilled water and a few drops of phenol added as an antifungal agent. Fragments of charcoal were then hand-picked out of the petri dish using a fine paint brush.
Scanning electron microscopy Aluminium S.E.M. stubs were prepared for each temperature/time range. Each stub held three specimens approximately 1 mm by 1 mm by 1 mm allowing for T.E.M. after S.E.M. examination. The specimens were mounted on the stubs with double-sided adhesive tabs. They were then gold/palladium coated to a thickness of 26 nm using a $5100 sputterer and examined by scanning electron microscopy using a Cambridge S100 E.M. Micrographs showing longitudinal and transverse sections were taken on Ilford black and white Rollfilm. After examination by S.E.M. the specimens were washed off the aluminium S.E.M. stubs with ethyl acetate, the solvent for the adhesive tabs, and left to soak in 100% ethyl acetate for 24 h.
Transmitted electron microscopy The samples for T.E.M. were transferred from 100% ethyl acetate to 100% propylene oxide, the link reagent for Spurrs Resin, and again left to soak for 24 h. The 100% propylene oxide was pipetted off and replaced with 50% Spurts Resin/50% propylene oxide and gently rotated for 48 h. The 50%/50% mixture was then pipetted off and replaced with 100% Spurrs Resin. The samples were then placed under vacuum and released back to atmospheric pressure several times in order to assist impregnation of the specimen with resin, and left on a rotator for 24 h. The Spurrs Resin was replaced with fresh resin, again vacuum treated and then rotated for 24 h. The specimens were then placed into individual beem capsules containing fresh Spurrs resin and polymerised at 70°C for 12 h. Ultra-thin sections 80-100 nm thick were cut on a Cambridge Huxley Ultra-microtome M2 using a glass knife. These were mounted on Gilder G200TH Cu/Pd grids and examined with a Hitachi H600 Transmission Electron Microscope. Micrographs were taken with Ilford "EM" black and white film.
Stable carbon isotopes The experimental charcoal samples, taken directly from the muffle furnace and without any
43
FOSSIL C H A R C O A L , ITS R E C O G N I T I O N A N D P A L A E O A T M O S P H E R I C S I G N I F I C A N C E
PLATE 1 SEM MICROGRAPHS OF EXPERIMENTAL CHARS.
DESCRIPTION.
CELL WALL STRUCTURE SEEN.
SEM MICROGRAPHS OF FOSSIL MATERIAL.
E. HOMOGENISED CELL WALLS. CARBONIFEROUS CHARCOAL FROM E KIRKTON, SCOTLAND. i
A. 210C, 1 hr.
10UM
220C
10UM
i
230C
SUDDEN APPARENT HOMOGENISATION OF CELL WALLS.
i
340C
FIRST APPEARANCE OF CRACKING ALONG SITE OF MIDDLE LAMELLA.
10UM
CRACKING BECOMES MORE PRONOUNCED AT EDGES OF SAMPLE. INDIVIDUAL CELLS PULL APART.
D. 560C, 1 hr.
10UM
F. CARBONIFEROUS CHARCOAL FROM E KIRKTON, SCOTLAND.
COMPLETE COMBUSTION
Scanning electron micrographs of experimental chars of P ~vh,estris, and comparison with mierographs of Carboniferous fossil material. Cracking ahmg the site of the middle lamella occurs in both early and late wood.
T.P. JONES AND W.G. CHALONER
44 PLATE I (continued)
i .................
!~
G. 200C, l hr.
] 10UM
[ H. 200C, 1 hr.
I'0UM
J. 260C, 1 hr.
10'1'1'1'1'1'1'1'1"~
i i ii Transmitted electron micrographs showing the homogenization of cell walls. Note the disappearance of the pit membrane in micrograph J.
FOSSIL CHARCOAL, ITS RECOGNITION AND PALAEOATMOSPHERIC SIGNIFICANCE
chemical treatment, were either first ground to a fine powder using a ceramic pestle and mortar, or small intact fragments were used. Between 200 #g and 350/zg were weighted out in quartz glass buckets and the buckets topped up with analar Cu(II)O. These were then placed inside 6 mm OD quartz glass tubes, the tubes were evacuated and sealed with a oxyhydrogen torch. The tubes were placed in a furnace for 10 h at 1000°C, then for 10 h at 600°C, and finally left overnight at 450°C to reabsorb any surplus oxygen. Once the combustion gases were released in the mass spectrometer preparation line by means of a flexible cracking joint, the CO~ was purified by cryo-distillation using liquid nitrogen and methanol slush. Measurements were taken using a PRISM mass spectrometer. This is an adaption of the method of Northfelt et al. (1981).
Results
Under the experimental conditions outlined, the apparent homogenization of the cell walls is seen to occur between 220°C and 230°C. SEM micrographs A and B of Plate I show detail of the cell wall in transverse section, before and after homogenization. The face for micrograph A was prepared by cutting the wood block with a razor blade, while the face for micrograph B is a fracture plane through the charcoal. TEM micrographs G to J of Plate II also illustrate the transition from structured to homogenized cell walls. Micrographs G and I show the change at the junction of 3 cells, as seen in transverse section. Micrographs H and J show the homogenization, as seen in sections through bordered pits; note that in micrograph J, while the walls are homogenized and the 3-dimensional shape of the bordered pit is retained, the pit membrane has disappeared. Sections through pit-pairs often miss one or both pit apertures, giving the false impression that the pit chambers are partially or fully enclosed. Both micrographs H and J give the impression of a partially closed chamber. With TEM, after the comparative ease of obtaining sections from non-homogenized material (micrographs G and H), extreme difficulties were expe-
45
rienced once the cell walls were homogenized. The excessive 'chattering' of the microtome knife, thickness of the sections and marks from imperfections in the glass knife (such as the vertical, darker band in micrograph I), can obscure fine structural detail. Accepting these limitations, cell wall structure is not apparent in micrographs I and J. In all cases where the cell wall appeared homogenized when viewed under SEM, the TEM micrographs of the same specimen confirmed the lack of internal wall structure. It is important to note here that this comparison was made of the same specimens, that is, after SEM, the fragments were washed off the SEM stub, embedded in resin, sectioned and then viewed under TEM. The technical difficulties experienced when attempting to cut ultra-thin sections of embedded charcoal, and the fragility of the sections obtained, with the electron beam tending to disintegrate the sections, may be considered as another physical characteristic of the charcoal. With temperatures rising from 230 to 340°C, above the level at which homogenization occurs, no further change is detectable in the cell walls under the SEM. Fracture faces are clean, often with concoidal fracture, and no internal cell wall structure is seen. Attempts to cut TEM sections were eventually abandoned as the charcoal became increasingly brittle. From 340°C onwards, cracking starts to appear along the site of the middle lamella, as can be seen in micrograph C of Plate I. This was observed in all specimens up to 600°C; above this range, complete combustion of the material occurred. Typically, the centre of the specimens in the range 340-600 °C still had uncracked, homogenized cell walls. Towards the edges, where the material was less insulated from heat and oxygen, cracking appears. This cracking progressively worsens, until the cells are completely isolated by cracks along the site of the middle lamella. In the final stages, the cells are seen as separate units. The extent of this breakdown was progressively more advanced with increase in temperature over the range 340-600°C. Measurement of the stable carbon isotopes ranging from unheated wood through to charcoal prepared at 600°C, showed that charcoalification does not "freeze" stable carbon isotopic ratios,
4(3
FP. JONES AND W.G. CHALONER
can be considered as fully charcoalified. In this phase the cell wall is apparently homogenized, structure is anatomically preserved, pit membranes have usually disappeared, the tissue is more chemically inert, the reflectance (R r) is moderate to high and the charcoal has undergone gl3C isotopic fractionation. In the third phase the material is still charcoal, but is breaking-up. In this phase the cell wall is cracking along the site of the middle lamella, structural preservation is becoming blurred, tissue is chemically inert, reflectance (R r) is high and the charcoal is 6~3C isotopically fractionated. This information is summarised in Table 3. All three of these phases are responses to pyrolysis, and the fossilization potential and likely diagenesis of each of these phases can be reliably predicted. The first phase, a response to 'scorching' or 'charring' certainly changes the nature of the wood. This information is not new; primitive man hardened wooden arrow points in open fires and modern man expects to hit a baseball a few more yards if his wooden baseball bat is 'flame tempered'. The vitrinite-fusinite 'transitional' material found in bituminous coals (Cope and Chaloner, 1985), has an intermediate layer of semifusinite with remnant traces of a middle lamella. This material had sufficient preservation-
but rather results in an isotopic fractionation, favouring the lighter isotopes, Fig. 2. Untreated whole tissue gave a value averaging 613C = - 24.5, charred wood gave an average value of 613C = -24.3, charcoal prepared at temperatures of 300-500°C gave values averaging 6~3C ~ -24.9, and charcoal prepared at 600°C gave an average result of gl3c = -25.3. The spread of data for each temperature range is believed to be due to the inherently heterogeneous nature of the wood. Results at each temperature level were much closer for the finely powdered charcoal, than corrcsponding results obtained from whole fragments. Discussion
The experimental material is seen to have three distinctive phases of response to pyrolysis, with the three important controlling parameters being temperature, time and availability of oxygen. The first phase results from 'scorching' or 'charring' of the tissue. In this phase the cell wall has internal structure, structure is anatomically preserved, pit membranes are intact, tissue is not chemically inert, reflectance (R,) is low and the material retains the ~13C isotopic ratio of the unheated wood. In the second phase the material
-24.0
I
I
I
1
[
I
-24.5
O
D ?
Z -25.0 0
m i
-25.5
-26.0
I OC
I 200C
I 3OOC
I 400C
I 500C
I 600C
TEMPERATURE OF FORMATION Fig. 2. Plot of stable carbon isotopes 8C against temperature of formation °C for experimental chars of Psyh'estris.
FOSSIL CHARCOAL, ITS RECOGNITIONAND PALAEOATMOSPHERICSIGNIFICANCE
potential to resist being converted to vitrinite, but is not fully fossil charcoal, but rather is fossil charred wood. The charring of the plant tissue has clearly made it more resistant to biodegradation and diagenetic alteration, and it has survived as a fossil with relatively little alteration. However, regardless of whether we consider the charcoal or the charred wood, both are the product of wildfire, and if there was a palaeowildfire, the oxygen level constraints on the palaeoatmosphere must stand. This argument does not include charcoal or charred wood found in association with volcanic rocks or other signs of active volcanism, which clearly could have undergone pyrolysis regardless of palaeoatmospheric oxygen levels. This is an important proviso in any general statements about global oxygen levels based on charcoal occurrences. Cope and Chaloner (1985) responded to Beck et al.'s suggestion (1982) that Callixylon fusain could have had a non-pyrolysis origin, by suggesting that the Callixylon shows features consistent with charring produced by wildfire conditions. The experiments described in this paper strongly support this suggestion, and they place limits on the degree of pyrolysis that the Callixylon must have undergone. It is proposed that the material
47
lies across the boundary between phases 1 and 2 of the experimental material; that is, that the Callixylon material is transitional between charred wood and charcoal. This suggestion is supported by the presence of both homogenized and structured cell walls, and the preservation of pit membranes as seen under TEM. As already quoted, Beck et al. (1982) states that under SEM the cells lack any evidence of discrete walls of contiguous cells, but importantly adds "with few exceptions". Schmid (1967, p. 723), while understandably concentrating on the more interesting structured walls, does record "Other micrographs show little or no layering in the walls". With the experimental chars, without exception, where the wall had internal structure, this was seen under both SEM and TEM; where the walls appeared homogenized, this again was seen under both SEM and TEM. It should be remembered that the same fragment was used for both SEM and TEM. It is suggested that the "few exceptions" of Beck et al. are the structured cell wails of Schmid, and the homogenized walls of Beck are the "no layering in the walls" of Schmid. Research is currently underway to clarify this. The argument for charring is further supported by the presence of preserved pit membranes,
TABLE 3 Physical and chemical parameters of phases 1, 2 and 3 of pyrolysis of xylem tissue Phase 1
Phase 2
Phase 3
Charred wood.
Charcoal
Charcoal breaking-up.
Experimental chars "180 ° C-220 o C".
Experimental chars "230 ° C-340 ° C".
Experimental chars "3411 o C-6110 ° C".
Internal cell wall structure seen.
Cell walls apparently homogenised.
Cracking along site of middle lamella.
Preservation potential moderate, usually compressed.
Preservation potential excellent. Anatomic preservation of high fidelity.
Preservation potential poor. Very fragile, typically smashed to minute pieces.
Pit membranes intact.
Pit membranes usually disappeared.
Pit membranes disappeared.
Attacked and oxidised by Schultze's solution within a few hours.
Highly resistant to oxidation by Schultze's solution.
Highly resistant to oxidation by Schultze's solution.
Reflectance Rr _<"0.4%".
Reflectance R r ="0.4%-1.2%".
Reflectance Rr >-"1.2~".
Stable carbon isotopes same as parent plant whole tissue.
Stable carbon isotopes fractionated. exp' chars ---"0.5" lighter.
In
Stable carbon isotopes fractionated. In exp' chars ~"1.0" lighter.
Seen in fossil record as semifusinite.
Seen in fossil record as fusain, fusinite and semifusinite.
Seen in fossil record as fusain, inertodetrinite.
48
beautifully seen in Schmid's TEM sections of Callixylon. It is hard to envisage a biotic process which homogenizes cell walls; indeed, a modern (biotic) equivalent has yet to be found. It is even harder to imagine anaerobic microorganisms which not only homogenize cell walls, but preserve pit membranes as well. If we allow that the CalFtxylon was charred, then this dilemma is removed. Not only is the pit membrane retained in the proposed phase l, but the charring enhances the preservation potential of the membrane as well. A final consideration is 'selectivity' of preservation. A biotic process is hard to reconcile with the observed gradient of charcoal to uncharted material. This problem is again removed if we invoke a palaeowildfire origin, with some of the tissue pyrolysed to charcoal and the rest sufficiently insulated to remain only charred. The experimental charring also roughly support the temperature of charring of the Call~,lon ~ 250°C suggested by Cope and Chaloner (1985). The changeover from phase 1-2 was seen to occur between 220°C and 230°C, under the experimental conditions outlined. That is, heating from cold, one hour at H.T.T., then allowed to cool overnight. With slightly higher temperatures the same effect would have been reached more quickly, and conversely, slightly lower temperatures would have required longer. Allowing for a small degree of variation, the pyrolysis temperatures would still have been significantly above average diagenetic temperatures of 10°C/kin (Winkler, 1965; Rolfe and Brett, 1969). Material physicochemically identical to the phase 2 experimental charcoal is abundant in the fossil record from the Lower Carboniferous onwards. Fossil charcoal is so abundant, that in many sedimentary sequences it is the dominant fossil, and occasionally the only fossil recovered. It is found in coal seams and sedimentary sequences of many different lithologies. Little imagination is required to visualise fires sweeping through the Carboniferous coal-swamps during phases of drought, or Jurassic conifer forests. Such a process is mirrored today in the wildfires of the North American Everglades (Cohen et al., 1987), or conifer forests of Yellowstone National Park, USA (Wuerthrer, 1988; Despain et al.,
T.P. JONES A N D W.G, CHALONER
1989). We can test and confirm that this fossil material meets all the requirements of phase 2 experimental charcoal, with the exception of the fractionation of the 8~3C isotopes. Certainly we can measure the ~13C ratios of conifer fossil charcoal, and the readings are approximately as expected; however, since we do not know the original isotopic ratio, the fractionation can only be assumed. Modern conifer xylem cell walls are composed of two main components, cellulose and lignin, on average 2/3 cellulose and 1/3 lignin. As a result of the plant's photosynthetic and ensuing biochemical pathways lignin is isotopically lighter than cellulose (Degens, 1967; Schidlowski, 1983, 1984; DeNiro and Hastorf, 1984; Benner et al., 1987). Unfortunately, the two distinct end products of fossil charcoal and vitrinite (compression fossil) both tend to be isotopically fractionated towards the lighter isotope, even though the fractionation processes are quite different. In the formation of fossil charcoal, cellulose is removed before the [ignin by pyrolysis (Beall et al., 1974), while in vitrinite formation degradation has preferentially removed the cellulose. Some confirmation of the fractionation may be found in sampling across the fusinite, semifusinite to vitrinite, 'transition' material found in bituminous coals. The third phase recognised in the experimental charring, with the charcoal breaking-up, is also seen in the fossil record, micrograph F, Plate I. However, such very fragile material is not commonplace, especially in charcoal which has undergone transportation. The 'fibrous' nature of charcoal has been recognised for a long time (Stopes, 1919). However there are three separate reasons why charcoal can appear fibrous when examined macroscopically. Firstly the wood, anatomically preserved, could appear fibrous as a result of the essentially elongate nature of its xylem cellular structure. Secondly, a very common phenomenon seen in fossil charcoal is 'bogen-struktur' (Stach, 1927a, b), where as a result of compression after burial cell walls are fractured, usually perpendicularly across the weakest or thinnest part of the wall and lengthwise down the cells. Once the containing matrix or any diagenetic cementation is removed, such
FOSSIL CHAR('OAL, ITS RECOGNITION AND PALAEOATMOSPHERIC SIGNIFICANCE
charcoal tends to disaggregate into separate tiny spikes or "fibres". Thirdly, the fossil charcoal may have cracks along the site of the middle lamella, as seen in phase three of the experimental chars. The maceral inertodetrinite, composed of masses of tiny, granular, high reflecting particles, could be partially composed of smashed and broken minute charcoal fragments, corresponding to the break-up material of phase 3 of the experimental chars. Conclusion Xylem tissue has three distinctive phases of response to pyrolysis. These phases can be physically and chemically defined, and approximately equated to formation temperatures. Fossil equivalents of these phases are seen from the late Devonian onwards. The presence of this fossil charcoal record indicates a history of palaeowildfire, and this places upper and lower constraints on oxygen levels in palaeoatmospheres during this period. Material which is categorically fossil charcoal, corresponding to phases 2 and 3 is abundant in the fossil record from the early Carboniferous onwards. Fossil material equivalent to phases 1-2 can be found from the late Devonian onwards. Although the pyrolysis origin of some of the latter material is less certain, evidence is increasingly pointing towards a 'charred wood' origin. Charring enhances the preservationpotential of plant tissue, and can be seen in the fossil record as the maceral semifusinite. Further experimental work is required to increase the certainty of this pyrolysis origin. Doubts remain about the pyrolysis origin of some of the more problematic semifusinites, but the origins of 'typical' fusain are evidently pyrolytic. Acknowledgements We would like to thank the staff of the Electron Microscope Unit, R.H.B.N.C. and Dr D. Mattey, Stable Isotope Laboratory, Geology Dept., R.H.B.N.C.T.J gratefully acknowledges the receipt of a NERC studentship.
49
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