Fossil charcoal: techniques and applications

Review of Palaeobotany and Palynology, 63 (1990): 269-279

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Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Fossil charcoal" techniques and applications P. M a r t i n S a n d e r 1'3 a n d C a r o l e T. G e e 2'3 Paleontological Institute and Museum of the University of Zurich, Kiinstlergasse 16, CH-8006 Ziirich (Switzerland) aSwiss Federal Institute of Technology, Geological Institute. ETH-Zentrum. CH-8092 Ziirich (Switzerhmd)

(Received October 4, 1989; revised and accepted March 7, 1990)

ABSTRACT Sander, P.M. and Gee, C.T., 1990. Fossil charcoal: techniques and applications. Rev. Palaeobot. Palynol., 63: 269-279. Although its origin was formerly in dispute, it is now clear that fusain is the result of wildfire and does provide an additional source of paleobotanical information. Through charring, wood is rendered into an excellent preservation state which is best studied with the SEM. In comparison to permineralized wood, fossil charcoal is easily and quickly prepared for microscopic work. It most commonly occurs in organic rich, gray or yellow, terrestrial mudstones as a minor but frequent and regular component, from the Silurian to the present. Charcoal has distinctive macroscopic characteristics that are clearly recognized in the field as well as microscopic characteristics readily observed in the lab. Although both permineralized wood and fossil charcoal can be used for anatomical and taxonomic studies, only charcoal indicates the past occurrence of fire which may be instrumental to solving paleoecological or taphonomic problems.

Introduction Burnt plant matter, especially charcoal, is a subordinate c o m p o n e n t o f m a n y post-Silurian terrestrial sediments and is the only fossil plant matter in m a n y cases (Scott and Collinson, 1978; Cope and Chaloner, 1985). Its excellent preservation and the c o m m o n occurrence make charred plant remains a source o f paleobotanical information that has been c o m m o n l y neglected in the past. This should change with the ever-increasing availability o f scanning electron microscopes (SEM's). The aim o f this paper is to discuss the value o f fossil charcoal in paleobotanical studies and to offer some practical hints a b o u t collecting, preparation and study. The paper is also intended to update and extend the discussion o f fossil charcoal in Cope and Chaloner's (1985) g r o u n d - b r e a k i n g paper. Occurrences o f burnt plant matter not previously recorded by Scott and Collinson (1978) nor by C o p e and C h a l o n e r (1985) and published 3present address: Institute of Paleontology, University of Bonn, Nussallee 8, D-5300 Bonn 1 (West Germany) 0034-6667/90/$03.50

@, 1990 Elsevier Science Publishers B.V.

since 1985 are mentioned by Francis and Coffin (in press), H e a t o n (1980), K a m p m a n n (1983), H61der and N o r m a n (1986), Sander (1987, 1989), Wells and Stewart (1987) and Scott (1989). In most cases, the remains o f charred w o o d are found, but burnt leaf remains (Remy, 1954; Harris, 1958, 1981; Alvin, 1974; Scott, 1974; Alvin et al, 1981; Scott and Chaloner, 1983) and flower parts have also been reported (Friis and Skarby, 1981, Friis et al., 1986). Possibly the earliest occurrence o f charred plant remains are specimens o f Cooksonia reported by Edwards et al. (1986) from the lowermost D e v o n i a n (Gedinnian) o f England. Interestingly enough, the analysis o f charcoal and other charred plant remains have played a m u c h greater role in the archaeological sciences (e.g. Schweingruber, 1976a, 1976b; Wille, 1978; Bar-Yosef and Kislev, 1986; Schoch, 1986) and this resource seems to be well exploited. Existence of fossil charcoal Acceptance o f fossil charcoal as a paleobotanical resource has been hampered by conceptual

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factors. In particular, American workers doubted the existence of fossil charcoal for a long time and a special mode of preservation was invoked to explain its genesis (Schopf, 1975; Beck et al., 1982) consisting of anaerobic microbial degradation in organic sediments. Coal petrographers (Stach, 1982), on the other hand, identify fossil charcoal as fusain and state that most, if not all, fusain is created by pyrolysis (burning). In their 1985 review paper, Cope and Chaloner could find no evidence for non-pyrolitic mechanisms of fusain formation and argue convincingly that all fusain is fossil charcoal. In the most recent discussion of the issue, Scott (1989) again strongly supports the pyrolitic origin of fusain which is based on his experimental work in addition to a detailed review of the evidence. An additional argument presented here for a charcoal origin of fusain is the nature of occurrence of many specimens. Fusain occurs not only in coal seams but macroscopic specimens are also found isolated in terrestrial mudstones which were certainly unaffected by anaerobic microbial action of the kind envisaged by Beck et al. (1982). The enrichment of fusain in layers at the bottom of ponds in coal swamps (Edwards, 1953) and on top

of coal seams (Gee and Sander, in prep.) is best explained by transport through water or burning in situ (see Teichmfiller, 1982). Examples from our study collection

Our first-hand experience is based on a collection of fossil conifer wood from North America and Europe ranging in age from Early Permian to Miocene (Table I). Some specimens came from coal seams, for example, from the Maastrichtian and Paleocene of the Red Deer River valley (a famous dinosaur locality in western Canada) and from the Miocene mammal locality of K/ipfnach (Switzerland). The majority of specimens came from mudstone facies, that is, from the Lower Permian and Upper Cretaceous of Texas (U.S.A.). Specimens from this study collection underscore the frequent occurrence as well as the paleobotanical potential of fossil charcoal. Various anatomical features of wood important for identification are clearly evident in the specimens. They include rays (Plate I, 1), bordered pits in face view (Plate I, 2, 3), bordered pit pairs in side view (Plate I, 4), crossfield pitting (Plate I, 5) and resin canals (Plate I, 6). Identification on the

TABLE I Collection of fossil charcoal used in this study No.

Locality

Formation

Age

Lithology

Identification

C1

Geraldine Bonebed Archer County, Texas Rattlesnake Canyon 2 Archer County, Texas Rattlesnake Mountain Big Bend National Park, Texas Horsethief Canyon Red Deer River, Alberta, Canada Knudsons Farm Red Deer River, Alberta, Canada Knudsons Farm Red Deer River, Alberta, Canada K/ipfnach, Kanton Zfirich, Switzerland

Nocona

Wolfcampian (Early Permian) Wolfcampian (Early Permian) Maastrichtian (Late Cretaceous)

gray mudstone

Dadoxylon sp.

Lower Horseshoe Canyon

Maastrichtian (Late Cretaceous)

top of coal seam

unident. (collapsed internal structure)

Whitemud

Maastrichtian (Late Cretaceous)

grayish-white mudstone

Scollard

Paleocene

coal

Taxodioxylon sp. (Gee and Sander, in prep.) unident. (Collapsed internal structure)

Obere Siisswassermolasse

Miocene

coal

C2 C3

C4

C5

C6

C7

Nocona Aguja

gray mudstone yellowish mudstone

(Sander 1987) Dado~:ylon sp. (Sander 1989) Pinuxylonsp.

Pinuxylon sp.

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FOSSIL CHARCOAL: TECHNIQUES AND APPLICATIONS

generic level are based on criteria discussed by Core et al. (1979); Barefoot and Hankins (1982), and Wheeler et al. (1986).

Techniques of investigation Stereomicroscope The low magnification of most stereomicroscopes does not permit the recognition of much anatomical detail but does allow study of the growth rings. Additionally, the stereomicroscope offers enough detail to distinguish the three important planes of wood study (transverse, radial, tangential) so that the specimen can be prepared for SEM investigations. The usefulness of a specimen for SEM study is easily assessed with the stereomicroscope. For instance, collapse of cellular structure or the infilling of the cell lumina with minerals is detectable under low magnification.

SEM All fossil charcoal exhibits three-dimensional preservation which is studied with the greatest success using an SEM. Charcoal shows almost the same amount of detail as recent wood samples and identification to at least the generic level is possible. Preparation for SEM investigation is very simple and commonly only involves fracturing the specimen to expose fresh faces of the three planes of study (Plate II, 7). It should be noted that cutting with a blade does not yield satisfactory results, as previously noted by Schweingruber (1976a). Nor does grinding (Brown, 1945; Schlfipfer and Brown, 1948). Both of these processes disrupt the microstructure of the charcoal. In some samples of fossil charcoal, a weak acid bath is advisable to clean the calcite out of the cell lumina. Shortly before sputter coating, surface cleaning of the specimen with pressurized air serves to get rid of charcoal dust produced during the fracturing process. The sample is then glued onto the stub with a somewhat viscous glue; thin glues will be sucked into the wood air spaces through capillary action and will obscure the

microstructure. Despite the very high carbon content of the samples, sputter coating is advisable. Our specimens were successfully coated with a gold palladium alloy. Recent charcoal, however, does not necessarily require sputter coating (McGinnes et al., 1974), especially with specimens charred under high temperatures. After mounting, the three planes can then be easily viewed with the SEM within a single specimen (Plate II, 8, 9, 10) and at various magnifications (Plate I, 1 6).

Compound microscope In the study of charcoal, transmitted light microscopy is mainly used in archaeological work. Specimens must be impregnated with some type of plastic and then cut with a wood microtome (e.g. Schweingruber, 1976a; Longo Marziani and Iannone, 1986). Impregnation can be difficult, especially with geologically older specimens. The main problems are dehydration of the specimens and the removal of air from the pores of the wood. The standard technique of coal petrography (Mackowsky, 1982) can be used fruitfully with fossil charcoal as well (Scott, 1989). The specimens are embedded and impregnated with polyester resin, polished and studied under reflected light using a coal petrographic microscope. Because fusain has the highest reflectance of all the macerals of coal (Stach, 1982), reflectance microscopy produces an image of good contrast. There is no depth of field, however, and only the polished surface of the specimen can be studied. Another disadvantage of this method is that coal petrographic microscopes are only found in institutions specializing in coal geology. Thus, for the study of non-Quaternary specimens, especially those with mineral infillings of the cell lumina, SEM work is far superior to any other method of study.

Occurrence, recognition, collecting hints There seems to be no lower size limit for charred plant remains but only specimens larger than 200 ~tm are large enough for paleobotanical investigation. The maximum size of charred plant

t~

FOSSIL C H A R C O A L : T E C H N I Q U E S A N D A P P L I C A T I O N S

remains and charcoal rarely exceeds a few centimeters in area. Except for in coal seams, macroscopically visible charcoal specimens are mainly found in organic rich, gray or yellow mudstones. These mudstones usually represent deposits of small bodies of water with a reducing bottom environment. Apart from charcoal, they commonly yield other plant remains and vertebrate fossils (e.g. Sander, 1987, 1989). Fossil charcoal or fusain can be recognized with the naked eye and in the field by its silky luster (resulting from the high reflectivity of individual cell wall cross sections), the cuboidal shape of the specimens and the brittleness that readily results in a gray dust stain on the hands. The color is either a deep black, grayish-black, or dark gray, in contrast to m a n y other coal colors which have brownish hues. Many charcoals almost always show a distinct "cleavage" along the radial plane of the wood and fracture well along tangential and transverse planes (Plate II, 11). Vitrinitic coal, on the other hand, shows conchoidal fractures and a resinous to vitreous luster (Plate II, 12). Charcoal has a low density (usually between 0.45 and 0.75). Some recent specimens will float in water for several days. This depends mostly on the charring conditions but also on internal wood structure, that is, the degree of air-filled spaces open to the atmosphere (Schweingruber, 1976a). In many cases, fossil charcoal will float, whereas vitrinitic coal with a density greater than 1.0 will sink. The floating ability of fossil charcoal results from open cell lumina which remain open during charring. During coalification (resulting in vitrinite), cellular structure is compressed and the cell lumina are closed if any microstructure is preserved at all. The floating ability of charcoal may

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be lost by some fossil specimens if the cell lumina are diagenetically filled with minerals such as calcite or pyrite. Such examples come from the Lower Permian of Texas (Plate III, 13, 14). Floating ability may also be influenced by the collapse of cellular structure. Collapse commonly occurs in alternating bands (Plate III, 15), usually affecting only the weakest zones, that is, the earlywood. It should be noted that this collapse is a brittle fracture involving the shattering of the cell walls and is not plastic deformation. This brittle shattering is microscopically reflected in the characteristic "bogen structures" (Plate III, 16: originally called Bogenstrukturen in Teichmfiller, 1982) which are also indicative of fusain. Specimens affected by the collapse of cellular structure can be recognized with the naked eye by their flattened appearance and the well developed cleavage along the plane of flattening. Such specimens are obviously not desirable for wood anatomical studies (Plate III, 17). Flattened specimens are commonly compressed to one-fifth of their original thickness by the overlying sediment. This can be deduced by assuming that the gross morphology of the flattened specimens was originally isometric (cube shaped). Uncompressed charcoal, usually found as discrete occurrences in noncoal lithologies, are characteristically cube shaped. Other macroscopic features of fossil charcoal, listed by Harris (1958, 1981) and by Cope and Chaloner (1985), include its resistance to maceration and its low flammability. Fossil charcoal merely glows upon combustion whereas vitrinized plant remains burn with a smoky flame. Features of fossil charcoal using the SEM, listed by Cope and Chaloner (1985) and Scott (1989), are the undeformed structure and fabric of the tissue

PLATE 1 Wood anatomical features preserved in fossil charcoal. 1. A radial view with tracheids and rays. The ray parenchyma cells occur in two shapes, procumbent and upright. Scale bar equals 50 jam. Spec. no. C7. 2. Bordered pit of Dadoxylon viewed from inside a tracheid. Scale bar equals 2 jam. Spec. no. C2. 3. Circular bordered pits in pinaceous wood. Scale bar equals 5 jam. Spec. no. C3. 4. Bordered pit pair. Scale bar equals 2 jam. Spec. no. C4. 5. Crossfieldpitting of Taxodioxylonviewed from inside the tracheids. Note the small size of the simple circular crossfield pits. Scale bar equals 20 jam. Spec. no. C5. 6. Normal resin duct in pinaceous wood. Such ducts are restricted to the Pinaceae. Scale bar equals 20 jam. Spec. no. C3.

FOSSIL CHARCOAL:TECHNIQUES AND APPLICATIONS

with open cell lumina and pits in contrast to vitrinite where the cells are deformed and the cell lumina are closed. However, charring will result in a certain amount of shrinkage and complete homogenization of the cell wall (Schlfipfer and Brown, 1948; McGinnes et al., 1971, 1974). Homogenization of the cell walls is a very important feature of charcoal. As a result, the different layers of the cell wall and adjacent cell walls cannot be distinguished from one another (Plate III, 18). Due to this feature, charcoal specimens are inferior to wood permineralizations because it may be difficult to differentiate between parenchyma cells and tracheids. Our own observations and a survey of the literature indicate that rocks most likely to yield fossil charcoal and other charred plant remains are all grayish or yellowish terrestrial mudstones and siltstones (Alvin, 1974; Alvin et al., 1981; Harris, 1981~ Scott and Chaloner, 1983; Friis et al., 1986). Sandstones only occasionally contain charred plant remains (Scott, 1989) because the high energy levels necessary for sand deposition destroy the brittle charcoal. Limestones with enough terrestrial biotic input appear to be potential sources for charcoal as well (e.g. Heaton, 1980). Many coals contain macroscopically visible fusain specimens (Scott, 1979; Teichmiiller, 1982; Gee and Sander, in prep.), but structural collapse can be a problem because of the high compaction rates typical of coal. A very interesting source of fusain is the gray mudstones of karst fissure fills which also contain vertebrates (Harris, 1958; Kampmann, 1983; H61der and Norman, 1986). It is easiest to recover charcoal specimens by collecting on the sediment surface, but screenwash-

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ing (e.g. Bar-Yosef and Kislev, 1986; Friis et al., 1986) and bulk maceration (e.g. Scott and Collinson, 1978) are also applicable for small fragments which are commonly encountered in microvertebrate concentrates. In natural outcrops of mudstones, the charcoal is enriched on the surface by weathering and erosion and may form an erosional lag.

Applications Paleobotanical applications of fossil charcoal are manifold and are compared here with those of permineralized wood. Of major importance are potential wood anatomical studies using fossil charcoal which have been made possible with the general availability of SEM's. Permineralized wood cannot be studied profitably with the SEM but must be viewed with transmitted light using thin sections or acetate peels. Thin sectioning is particularly time consuming and acetate peels do not offer the same depth of field. Exceptionally well preserved permineralized wood shows the same amount of detail as sections of recent wood, but this type of preservation is not common. On the other hand, charcoal can never contain the same amount of information as uncharred wood because some anatomical information is lost during the charring process through homogenization of the cell walls, shrinkage and the loss of translucency. Another fact to keep in mind is that almost all fossil wood is described from silicified specimens and some difficulties in translating these wood characters into SEM structures will certainly be encountered. Similarly, recent wood samples look

P L A T E 11

Dadoxylon sp.

(C2) from the Lower Permian of Texas. A cube of fossil charcoal with the three planes of study exposed by breaking. Scale bar equals 500 jam. Spec. no. C2. Transverse view. Note the lack of growth rings, the regular size of the cells, and the presence of rays. Scale bar equals 50 p.m. Spec. no. C2. 9. Radial view. Note the abundance of rays. Scale bar equals 200 jam. Spec. no. C2. 10. Tangential plane. The height and width of the rays are now clearly visible. Scale bar equals 50 p.m. Spec. no. C2. 7. 8.

Comparison of macroscopic features of fossil charcoal and vitrinitic coal. 11. A lump of Permian fusinite. Note the silky luster and preservation of cellular structure. Scale bar equals 1.5 cm. Spec. no. C13. 12. A piece of Miocene vitrinite. Note the conchoidal fracturing and vitreous luster. Scale bar equals 1.23 cm. Spec. no. C14.

-]

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FOSSIL C H A R C O A L : T E C H N I Q U E S A N D APPLICATIONS

very different in the SEM than in a microtome section. SEM descriptions of at least some recent woods (Meylan, 1978) are available, however, which should facilitate comparison with Tertiary charcoal. The taphonomic and paleoecologic information potential of fossil charcoal is considerably greater than that of permineralized wood because fossil charcoal is the result of an event of great environmental impact - - wildfire. W o o d permineralizations merely indicate the presence of rafting in of trees in the area. Wildfires do have profound short term as well as long term effects on ecosystems and such effects should be visible in the fossil record (see Cope and Chaloner, 1985; Scott, 1989 and references therein). The short term effect is that of a catastrophe for the local or regional biota, especially for the fauna, and may serve to explain the origin of fossil deposits that show catastrophic mortality patterns (e.g. Sander, 1987). On the paleoecological side, a past fire regime can be inferred from patterns of charcoal occurrence in stratified sediments like lake varves or coals. Examples of this line of work were pioneered by Harris (1958) and later summarized by Cope and Chaloner (1985). Given the indiscriminating nature of a forest fire and the great transportability of charcoal, a greater diversity of plants is to be expected in a charcoal sample than in a fossil wood sample. This assumption, however, has not been tested so far. In comparison with permineralized wood, fossil charcoal is only of linlited value for paleoclimatological studies based on growth rings. Dendrological techniques have been successfully applied in

several instances, one of the most recent being the work by Parrish and Spicer (1988) on Cretaceous wood from the north slope of Alaska. A good discussion of the principles is found in Creber (1977) and studies of modern and Quaternary climates are available in Hughes et al. (1982). The main problem in using fossil charcoal for this kind of analysis is the generally small size of the charcoal fragments resulting in the preservation of only a few growth rings. It is, however, generally possible to ascertain the presence or absence of growth rings, allowing some very general conclusions to be made in comparison with modern environments. Creber (1977) cites some growth ring presence/absence ratios of tropical rain forest communities from various parts of the world. In neither the Indian nor the Amazon rain forests are there tree species with growth rings totally lacking, nor are forest fires unknown (Sanford et al., 1985), the two prerequisites for successful paleoclimatological studies using presence/absence ratios of growth rings.

Summary Fossil charcoal is ubiquitous in fine-grained clastic terrestrial sediments and is easily distinguished in the field from vitrinitic coal. We believe that the paleobotanical potential of these fossils has only been insufficiently explored and thus, with this description of techniques and applications, would like to stimulate interest in this unique kind of fossil wood. Advantages and disadvantages of fossil charcoal as compared to traditionally studied wood permineralizations are listed in "Fable II.

PLATE Ili Various preservational features in fossil charcoal. I3. Mineral infilling of the open cell lumina. Scale bar equals 10 gm. Spec. no. CI. 14. Mineral infilling of the open cell lumina. Note the calcite columns still protruding from the cells. Scale bar equals 100 gm. Spec. no. (72. 15. Bogen structures found only in the earlywood, alternating with bands of the unaffected latewood. Scale bar equals 50 I.tm. Spec. no. (74. 16. An enlargement of bogen structures which are composed of curved fragments of the cell walls. Scale bar equals 2 p_m.Spec. no. C4. 17. Tangential view of a wood composed only of bogen structures and obviously of little paleobotanical value. Scale bar equals 50 gm. Spec. no. C6. 18. Cell walls homogenized by the effect of charring. Scale bar equals 2 lam. Spec. no. C2.

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P.M SANDER AND (.f. GEE

TABLE 11 Comparison of fossil charcoal and permineralized wood Charcoal

Permineralized wood

Properties: occurrence preservation SEM investigation light microscope specimen size

very common very good easy good small fragments

common bad to excellent not advisable very good fragments to entire trunks

Applications: wood anatomy

very good

poor to very good very good moderate moderate

paleoclimatology paleoecology taphonomy

moderate good good

Acknowledgments W e w o u l d like t o t h a n k M r . U r s J a u c h f o r his technical assistance with the SEM work which was u n d e r t a k e n at the Institute o f Plant Biology o f the University o f Ziirich.

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