PALAIOS, 2007, v. 22, p. 528–538 Research Article DOI: 10.2110/palo.2005.p05-138r
FERNS AND FIRES: EXPERIMENTAL CHARRING OF FERNS COMPARED TO WOOD AND IMPLICATIONS FOR PALEOBIOLOGY, PALEOECOLOGY, COAL PETROLOGY, AND ISOTOPE GEOCHEMISTRY LAURA C. McPARLAND, MARGARET E. COLLINSON,* ANDREW C. SCOTT, DAVID C. STEART, NATHALIE V. GRASSINEAU, and SHARON J. GIBBONS Geology Department, Royal Holloway University of London, Egham, Surrey, TW20 0EX, UK e-mail:
[email protected]
ABSTRACT We report the effects of charring on the ferns Osmunda, Pteridium, and Matteucia with coniferous wood (Sequoia) for comparison. Like charred wood, charred ferns shrink, become black and brittle with a silky sheen, and retain three-dimensional cellular structure. Ferns yield recognizable charcoal (up to 800ⴗC) that could potentially survive in the fossil record enabling reconstruction of ancient fire-prone vegetation containing ferns. Charred fossils of herbaceous ferns would indicate surface fires. Like charred wood, cell-wall layers of charred ferns homogenize, and their reflectance values increase with rising temperature. Charcoalified fragments of thick-walled cells from conifer wood or fern tissues are indistinguishable and so cannot be used to infer the nature of source vegetation. Charred conifer wood and charred fern tissues show a relationship between mean random reflectance and temperature of formation and can be used to determine minimum ancient fire temperatures. Both charred conifer wood and charred fern tissues show some tendency toward increasingly lighter ␦13C values up to charring temperatures of 600ⴗC, which should be taken into account in analyses of ␦13C in charcoals. Charred fern tissues consistently have significantly more depleted ␦13C values (ⱕ4‰) than charred wood. Therefore, if an analysis of ␦13C through time included fern charcoal among a succession of wood charcoals, any related shifts in ␦13C could be misinterpreted as atmospheric changes or misused as isotope stratigraphic markers. Thus, charcoals of comparable botanical origin and temperatures of formation should be used in order to avoid misinterpretations of shifts in ␦13C values. INTRODUCTION The aim of this paper is to determine the effects of charring on ferns and apply this knowledge in paleobiology, paleoecology, coal petrology, and isotope geochemistry. Ferns have an extensive fossil record from the early Carboniferous onward (Collinson and van Konijnenberg-van Cittert, 2002), and charred ferns have been reported throughout this time range (Scott, 2000; Collinson and van Konijnenberg-van Cittert, 2002). The earliest fossil charcoal assemblages from the Silurian and Devonian include charcoals derived from extinct plants, some of which are related to ferns (Cressler, 2001; Edwards and Axe, 2004; Fairon-Demaret and Hartkopf-Fro¨der, 2004; Glasspool et al., 2004), and charcoal of true ferns occurs in the Lower Carboniferous (Mississippian; Scott and Galtier, 1985; Scott et al., 1985), Upper Carboniferous (Pennsylvanian; DiMichele and Phillips, 2002), and Jurassic (Collinson and van Konijnenburg-van Cittert, 2002). Cretaceous fern charcoals are abundant (Alvin, 1974; Harris, 1981; Gandolfo et al., 1997; Herendeen and Skog, 1998; Herendeen et al., 1999; Collinson et al., 2000; Falcon-Lang et al., 2001; Collinson and van Konijnenburg-van Cittert, 2002; Scott and Stea, 2002), and, although
rarer after flowering plants became dominant, fern charcoals are also reported from the Cenozoic (Collinson, 2002). Sediments from the Paleocene-Eocene (P-E) boundary (⬃55 Ma) at Shorne Wood, Kent, have been found to contain high numbers of ferns spores and exceptionally high amounts of charcoal, much of which is derived from ferns (Collinson et al., 2003, 2007). A major, rapid, but short-lived global warming event occurred at the P-E boundary (Bains et al., 1999; Harrington, 2001; Harrington et al., 2005), so an interpretation of associated fire regimes has potential value for understanding the link between fire regime and climate change. Investigations into charring temperature and its impact upon plant tissue preservation and quantified cell-wall reflectance values have been used to interpret the minimum charring temperature (Jones et al., 1991; Scott and Glasspool, 2005, 2007) and the behavior of ancient fires from fossil charcoals (Scott, 2000, 2002; Scott and Glasspool, 2007). Previous experimental calibrations were based only on wood (Correia et al., 1974; Scott, 1989, 2000, 2002; Jones et al., 1991; Scott and Jones, 1991a, 1991b; Guo and Bustin, 1998; Bustin and Guo, 1999; Scott and Glasspool, 2005). This paper uses an experimental approach to determine the effects of charring on ferns—including the effects of charring temperature on organ, tissue, cell, and anatomical preservation—on carbon isotopic composition and on mean random reflectance under oil. These results are directly compared with those for wood samples charred in the same experiments. The morphological characteristics of charred ferns, as seen in reflected light under oil immersion, are important for interpreting the origin of inertinite macerals in coal (Scott, 2002; Scott and Glasspool, 2007). The experimental calibrations for fern charcoals developed in this study will enable both fern and wood charcoal to be used to determine the role of fires in influencing vegetation and environments throughout Earth history. MATERIAL AND METHODS Materials
* Corresponding author.
We used three fern species, Matteucia struthiopteris (L.) Tod., Osmunda regalis L., and Pteridium aquilinum L., and one coniferous species, Sequoia sempervirens (D. Don) Endler, in this study. We selected Osmunda regalis petioles (⫽ leaf stalks) as the main experimental fern material because the simplicity of the C-shaped leaf trace (vascular tissue) meant that comparable pieces could be obtained along the length of the petiole and that the response to charring would be easier to record because of their anatomical consistency. Osmunda has a good fossil record at least back to the Permian (Yatabe et al., 1999; Matsumoto and Nishida, 2003) and has a wide modern distribution, both in the wild and in cultivation. Therefore, not only is material widely available for experimental use, but osmundaceous ferns (as well as others, including those with similar simple leaf traces) are also potential components of charred residues from both modern and ancient wildfires. In order to compare the charring response of fern tissues with that of wood, we included naturally dry branch wood of the conifer Sequoia sempervirens (branches ⬃3–5 cm thick),
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(⫽ leaf stalk) of the frond (Fig. 1B): the stipe and lower main rachis below and immediately above the first primary pinna, the middle main rachis below the uppermost primary pinna that was fully expanded at the time of collection (July), and the longest primary pinna rachis. After recording their weight when fresh, three pieces of each segment length were placed into steel tubes 15 mm in diameter and 75 mm long, with lids designed to exclude oxygen while allowing volatiles to escape during heating. A control sample of Sequoia wood (which was naturally air dried, having been stored in the laboratory for over a year), milled to fit the steel tubes exactly (Scott and Glasspool 2005, 2007), was placed in another steel tube for each charring experiment. Typically ⬃30–40 min elapsed between collection of the fresh fern fronds and the placement of the samples into the oven. The charring experiments used a preheated Carbolite temperature-controlled oven or furnace set to one of 11 experimental temperatures (200⬚C, 250⬚C, 300⬚C, 350⬚C, 400⬚C, 450⬚C, 500⬚C, 600⬚C, 800⬚C, 900⬚C, and 1000⬚C). Samples were charred at each temperature for 1 h to simulate one example of charring duration in natural wildfires; the charring time was measured from the time the sample reached the experimental temperature. The oven and sample temperatures were measured using two thermocouples linked to Pico Log娂 for Windows (Pico Technology, 2000). One thermocouple was placed within the fern sample while the other was placed in the oven to determine the accuracy of the oven’s internal thermostat. Based on the thermocouple in the sample, it took ⬃40 min for the samples to reach the experimental temperature. We also ran an experiment at 350⬚C using air-dried Osmunda litter consisting of stipe and lower main rachis. Subsequently, we ran experiments at 200⬚, 350⬚, 600⬚, 800⬚, 900⬚, and 1000⬚C using stipes of fresh Pteridium and at 350⬚C using stipes of fresh Matteucia. After 1 h, we removed the steel containers from the oven, left them to cool, and weighed the samples to quantify the loss of moisture and other volatiles. Before each experiment, we placed the steel containers in a furnace at 600⬚C without their lids for 1 h to volatilize and remove any residues. Analytical Methodology
FIGURE 1—Features of Osmunda fronds. A–B) A single frond with an indication of the portions used in the experiments. The term petiole (⫽ leaf stalk) is used to refer collectively to all portions of the frond studied. C) Diagrammatic sketch of a transverse section of Osmunda petiole to show the location of tissues and regions mentioned in the text and shown in detail in other figures. OCS ⫽ outer cortical sclerenchyma; MR ⫽ middle main rachis; PR ⫽ largest primary pinna rachis; S&LR ⫽ stipe and lower main rachis.
from the same tree as used by Scott and Glasspool (2005, 2007) in each experiment. We obtained Osmunda, Sequoia, and the fern Pteridium from the campus at Royal Holloway University of London, Surrey, UK, and the fern Matteucia from a local garden. Experimental Methodology We collected living fronds of Osmunda (Figs. 1A–B) and cut them carefully to avoid damage, using new single-edged razor blades, into segments 20, 15, 10, or 4 mm long. Cutting prior to charring provided comparable pieces for study and reduced the risk of fragmenting the brittle charred residues. We took segments from three parts of the petiole
Visual Analysis.—We assessed charcoalification visually according to the criteria outlined by Scott (1989, 2000, 2001) and Scott and Jones (1991a, 1991b). These included preservation of three-dimensional morphology and anatomy of the sample, a black color accompanied by the presence of a silky sheen, brittle behavior, and a black streak left on hands and paper. Reflectance Microscopy.—The 10 mm segments of fern and fragments of wood were embedded in polyester resin, cut, and polished so that a transverse section (Fig. 1C) could be studied. Reflectance was measured under a Nikon Microphot microscope using Leica QWin image analysis software (Leica Image Systems Ltd., 1997). We measured specimens under Cargill immersion oil (refractive index of 1.518 at 23⬚C) using a 40⫻ objective lens. We used four standards: cubic zirconium (R0 3.188), GGG (R0 1.7486), YAG (R0 0.929), and Spinel (R0 0.393). The light source was a Nikon fiber optic LS-101 with a 546 nm filter. We determined the reflectance of the charcoals produced by each experimental treatment by measuring reflectance values at 33 random points on three replicates from each temperature treatment (with an extra point on one replicate). We then combined these points and obtained the mean random reflectance for each experimental treatment, repeating the procedure on all three portions of the fern petioles and on Sequoia wood pieces, at all temperatures ⬎300⬚C (uncharred samples and those charred at lower temperatures gave no measurable reflectance). In the Osmunda sections, we studied four tissue and cell types (Fig. 1C): xylem (lignified, water-conducting vascular tissue), thin-walled inner cortical parenchyma, thick-walled outer cortical sclerenchyma, and cuticle. Scanning Electron Microscopy (SEM).—The 4 mm fern segments and fragments of Sequoia wood (5–15 mm in diameter) were mounted on stubs using disks of double-sided tape for viewing in transverse section
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TABLE 1—Visual analysis of selected criteria for charcoalification in fern petiole tissues (Osmunda) and conifer wood (Sequoia).
Temperature 200⬚C 250⬚C 300⬚C 350⬚C (and higher)
Species
Silky sheen
Brittleness
Color of specimen
Color of streak on hands or paper
Osmunda Sequoia Osmunda Sequoia Osmunda Sequoia Osmunda Sequoia
No No No No Yes No Yes Yes
Not brittle Not brittle Slightly brittle Slightly brittle Quite brittle Quite brittle Very brittle Very brittle
Light brown Light brown Dark brown Dark brown Dark grey-black Dark grey Black Black
None None Faint dark brown-grey None Black Black Black Black
(Fig. 1C). For study of cell-wall layers, we made fresh fractures. We fixed uncharred fresh fern samples using 3% glutaraldehyde in 0.1M phosphate buffer (pH 7.2). These were then rinsed through 0.1M phosphate buffer (pH 7.2), run through a series of ethanol baths of increasing concentration from 30% to 100%, and critical-point dried. Using a Polaron Coating unit E5100, all the samples were gold coated to ⬃50–150 nm thickness and examined using a Hitachi S-3000 scanning electron microscope. Carbon Isotopic Analysis.—Charcoal samples from each experiment were stored in thermally decontaminated foil, cleaned in methanol in an ultrasonic bath for 10 min, then dried and weighed. Analytical procedures followed Grassineau (2006). An average of 85 g of charred sample (ⱖ300⬚C) and 130 g of samples from temperatures ⱕ250⬚C were analyzed by helium continuous flow EA-IRMS (elemental analyzer–isotopic ratio mass spectrometer; 1500 Fisons EA coupled with an OPTIMA GVMicromass MS), with two or three replicates for each sample. A blank was run every 10 samples. Results were calibrated using international standard NBS 21 graphite ([National Bureau of Standards]; ⫺28.16‰) and IAEA-CO9 (International Atomic Energy Agency) barium carbonate (⫺47.10‰), and two working standards (RHBNC calcite [Royal Holloway Bedford New College] at ⫹3.25‰ and GF graphite [Good Fellow] at ⫺23.95‰). The reproducibility on the standards was better than ⫾0.09‰. The carbon isotopic composition was expressed in conventional ␦13C notation as per mil (‰) variations relative to Vienna-Peedee Belemnite (V-PDB) carbonate standard.
1). Brittleness increased with temperature, the largest changes being between 250⬚C and 300⬚C for fern tissues and 300⬚C and 350⬚C for conifer wood (Table 1). A silky sheen and a colored streak developed between these temperatures (Table 1). The smaller Osmunda primary pinna rachis segments were grayish black and very brittle at 250⬚C. At 300⬚C, the fern material would be described as fusain if found fossilized (Scott, 1989, 2000, 2001), while Sequoia wood did not exhibit this degree of alteration until 350⬚C. The fern specimens underwent increasing shrinkage with rising temperatures. Above 500⬚C, fern samples became increasingly fragile and frequently fragmented if touched, resulting in elongate pieces of outer cortical or vascular tissues. At temperatures ⱖ600⬚C, some of the primary pinna rachis and middle main rachis segments did not survive as charcoal. At 900⬚ and 1000⬚C, only fragments of fern samples survived, and disintegration of tissues was extensive, but wood was still intact at 1000⬚C. A white residue was left on the steel containers and on the samples at temperatures ⱖ500⬚C. Weight Loss
Visual Analysis
The percentage weight lost from Osmunda was high (⬎80%) even at the lowest temperature, with only a slight subsequent increase in weight loss with rising charring temperature (Fig. 2). In contrast, Sequoia wood showed only small (10%–20%) weight loss at the lower temperatures with a sharp rise in weight loss between 250⬚ and 350⬚C (to 60%), followed by a slight continued increase at higher temperatures (Fig. 2). Even at 1000⬚C, charred Sequoia wood exhibited less weight loss than the lowest temperature value for Osmunda. Pteridium showed the same weight loss response as Osmunda.
All samples remained intact at temperatures from 200⬚ to 500⬚C but changed color from light to dark brown and then to black by 350⬚C (Table
Reflectance
RESULTS
All sections heated to ⱖ300⬚C showed visible reflectance (Fig. 3). Uncharred samples and those exposed to temperatures ⬍300⬚C showed no measurable reflectance. All samples with measurable reflectance followed a similar trend of increased reflectance with rising charring temperature (Figs. 3–6). This trend was seen in both wood and fern tissue (Fig. 4), in all fern tissue and cell types studied (Fig. 5), and in all portions of fern petiole (Fig. 6). At temperatures ⱖ600⬚C, reflectance values for fern outer sclerenchyma and xylem no longer overlap (Fig. 4), but the significance of this is unclear as insufficient intact charcoal was produced at 900⬚ or 1000⬚C for reflectance measurements of these tissue types. The Sequoia wood reflectance temperature curve shows a shift to a steeper gradient at ⱖ600⬚C. This may be related to differential breakdown of cellulose and lignin during charring. Scanning Electron Microscopy
FIGURE 2—Percentage of weight loss at different charring temperatures for conifer wood and fern petiole tissues. The value for Osmunda was averaged from a data set of all segment lengths from the middle main rachis, stipe, and lower main rachis (see Fig. 1).
Overall Condition of the Ferns as Seen in Transverse Section.—In the uncharred fern petioles and those heated to 200⬚C, the inner cortex and leaf trace boundary remained intact (Fig. 7A) or almost intact (Fig. 7B). With exposure to increasing temperature, however, these regions separated (Figs. 7C–D). We observed that this is a zone of structural weakness, as it
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FIGURE 3—Reflectance images, from polished blocks under oil, of plants after charring for 1 h at varying temperatures, showing comparable trend of increasing reflectance in thick-walled outer cortex of fern petioles (Osmunda) at A) 300⬚C, C) 350⬚C, E) 500⬚C, and G) 800⬚C, and in conifer wood (Sequoia) at B) 300⬚C, D) 350⬚C, F) 500⬚C, and H) 800⬚C.
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FIGURE 4—Mean random reflectance under oil of charcoals produced at different temperatures for Osmunda petiole tissues and conifer wood (see Fig. 1 for correspondence). Error bars are within size of symbols. OCS ⫽ outer cortical sclerenchyma; S&LR ⫽ stipe and lower main rachis.
is the first point of breakage when fern petioles are left to dry naturally in the lab or when pressure is applied to a fresh petiole. At charring temperatures of ⱖ250⬚C, fractures occurred between or across the cells of the outer sclerenchyma, causing radial cracking around the edges of the stipe and rachis (Fig. 8A). The pinna rachis (Fig. 8B) showed the same charring response as the stipe and main rachis with detachment along the cortex-leaf trace junction and with radial cracking. At temperatures ⬎500⬚C, the separation and cracking led to fragmentation (see the Visual Analysis section above), which became more severe with increasing temperature. Condition of the Thinner-Walled Fern Tissues.—The xylem tissue showed little change across the range of charring temperatures. The inner cortical parenchyma cells (Fig. 1C) showed little change at lower temperatures (Fig. 9A), but at 400⬚C the cell walls began to break down (Fig. 9B). What began as isolated ruptures in the cell walls at 400⬚C (Fig. 9B) became more extensive breakages at higher temperatures (Fig. 9C). Above 500⬚C, the cell wall damage became sufficient to cause disintegration of the parenchyma tissue. The Condition of the Cell Walls in Sequoia Wood and Osmunda Cortical Sclerenchyma.—Cell-wall layers are very hard to distinguish in the relatively thin-walled primary xylem (Fig. 7) of Osmunda, therefore the thick-walled outer sclerenchyma (Fig. 10) was used to study the charring effects on cell-wall layering in the fern. This tissue is physically comparable, in terms of wall thickness, to secondary xylem of wood. Furthermore, it is lignified (⫽ sclerenchyma tissue), as demonstrated by a red color, identical to that of the fern xylem, when stained with phloroglucinol and hydrochloric acid. Cell-wall layers were visible in fern thickwalled outer sclerenchyma in the uncharred sample and in charred samples ⱕ300⬚C. At temperatures ⱖ350⬚C, the cell-wall layers have homogenized (Fig. 11), though faint hints of a middle lamella may remain in places. The same temperature response was seen in the Sequoia wood (Fig. 12).
FIGURE 5—Mean random reflectance under oil of charcoals produced at different temperatures for four tissues and cells from Osmunda (see Fig. 1 for correspondence). Error bars are within size of symbols. S&LR ⫽ stipe and lower main rachis.
4‰ more 13C enriched than ferns, with the exception of two (of six) values for Pteridium, which are only 1‰–2‰ lighter than the wood. As there is no overlap in the error of the data sets, it can be seen that there is a significant difference between the isotopic compositions of Sequoia wood and fern tissue. We have therefore demonstrated a difference in three different ferns from different habitats, that is, Osmunda (a lakeside wetland fern with upright stem), Pteridium (a woodland-open landscape fern with creeping rhizome), and Matteucia (from garden cultivation, also with creeping rhizome). DISCUSSION Weight Loss, Shrinkage and Cell-Wall Changes As the fern and wood samples are exposed to increasing temperatures, moisture and other volatiles contained within them are released. In woods, bound water is lost first (ⱕ270⬚C), followed by breakdown of cellulose and then lignin, from which CO, CH4, and other volatiles are released (Pyne et al., 1996). These stages of wood charring may account for the steps in the pattern of the weight loss with increasing temperature seen in the Sequoia wood (Fig. 2). In contrast, the percentage weight loss from fern tissues was high at all temperatures. This suggests that moisture loss, rather than tissue breakdown, may account for most of the weight loss from fern tissues. This is consistent with the fact that the Sequoia wood was not collected fresh prior to charring (and so would have had a low initial moisture content) and because wood consists mostly of lignified
Carbon Isotope Analysis There was a slight trend to lighter ␦13C values with increasing temperature in both Sequoia wood and all studied parts of Osmunda up to 600⬚C (Fig. 13). Values became heavier again, however, at temperatures ⱖ800⬚C . This may be due to the differential breakdown of cellulose and lignin during charring. One value for Pteridium at 350⬚C is anomalously heavy, but the single analyses of fresh Matteucia and Osmunda litter charred at 350⬚C both yielded values very close to that of the fresh Osmunda samples. Although the trends are the same, Sequoia wood is typically 3.5‰–
FIGURE 6—Mean random reflectance under oil of charcoals produced at different temperatures from different portions of fern petioles (Osmunda; see Fig. 1 for correspondence). Error bars are within size of symbols. MR ⫽ middle main rachis; S&LR ⫽ stipe and lower main rachis.
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FIGURE 7—SEM images showing shrinkage and breakage at the junction between the leaf trace and cortex of Osmunda fern petioles after charring for 1 h at varying temperatures: A) uncharred, B) 200⬚C, C) 350⬚C, and D) 450⬚C.
cell walls of dead cells. In contrast, the fern samples were fresh material with many thin-walled cells containing moisture and cytoplasm, with only a small amount of thick-walled tissue (Figs. 7, 10). The high weight loss that occurs in fern tissues, even at very low temperatures, raises the question of whether all the moisture loss and associated shrinkage, with breaks along the cortex-leaf trace boundary and radial cracking, occur during the hour of the experimentation or as the sample dries out while the oven returns to the experimental temper-
ature after the material has been inserted. Furthermore, the speed with which plant material would reach various charring temperatures during a natural wildfire would probably vary according to the speed of approach of the fire (among other factors, such as water content). To address these issues, it would be necessary to find a way to avoid oven-temperature change when specimens are inserted into the oven and to obtain data on speed of heating for charcoals produced in natural wildfires. In wood, many chemical reactions involved in the breakdown of cel-
FIGURE 8—SEM images of transverse sections from fern petioles (Osmunda) charred at 400⬚C. A) Characteristic radial cracking of the outer cortex in a main rachis. B) Pinna rachis showing radial cracking and separation at the cortex-leaf trace junction.
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FIGURE 9—SEM images to show the condition of the inner parenchyma cells of fern petioles (Osmunda) after charring for 1 h at varying temperatures. Damage to cell walls increases with rising temperature. A) 250⬚C, B) 400⬚C, C) 500⬚C.
lulose and lignin take place between temperatures of 450⬚ and 500⬚C; in particular, cellulose breaks down to volatiles (Guo and Bustin, 1998; Bustin and Guo, 1999). The partial disintegration of fern parenchyma cells (Fig. 9), commencing at 400⬚C and intensifying at temperatures ⱖ450⬚C, may also reflect similar chemical breakdown. Paleobiological Implications The results clearly demonstrate that ferns yield charcoal that could be recognized readily in the fossil record. The process of charcoalification may aid the preservation of ancient fern material owing to the chemical
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inertness of charcoal, which is composed of nearly pure carbon (e.g., Scott and Jones, 1991a, 1991b). DiMichele and Phillips (2002) noted that small fern debris in Carboniferous coal balls is commonly preserved as charcoal and argued that this might reflect a taphonomic bias (favoring inert charcoal) in the litter input into the fossil record rather than indicating that these small ferns only grew in fire-prone habitats. Our results show that, when charcoalification occurs, the original anatomical structure of the fern is damaged. Thinner-walled parenchyma cells begin to show disintegration of their walls, and shrinkage-related breaks occur in the form of radial cracks and separation of the leaf trace from the cortex. This damage becomes more severe with higher charring temperatures. The rising levels of damage in fern tissues associated with higher charring temperatures would probably cause fragmentation during transport to depositional settings, resulting in a bias in the fossil record in favor of material charred at lower temperatures. Our experiments show that at charring temperatures ⬎ 300⬚C, fern petioles typically shrink and separate in the zone between the cortex and leaf trace. This phenomenon is very evident in Cretaceous charred Gleicheniaceae fern petioles (Gandolfo et al., 1997). Such disruption to the original anatomy does not imply that the ferns were senescent or dried in the litter before they were burned. Where anatomical preservation is otherwise excellent, it can be inferred that fresh living fern biomass was burned. Charring temperatures of ⱖ350⬚C result in homogenization of cellwall layers in thick-walled fern cells; similar cells are evident in fossil fern charcoal, such as the gleicheniaceous fern described by Gandolfo et al. (1997). Therefore, a minimum charring temperature of 350⬚C can be inferred for those fossils. Mean random reflectance of all tissue and cell types in all three portions of the Osmunda petiole studied increases with rising temperature. Therefore, reflectance values of charcoalified fern tissues can be used to determine minimum ancient fire temperatures in a comparable manner to charcoalified woods. This brings additional benefit because fragmentary charcoals, which may be used in petrographic studies on coals, often cannot be identified to their source tissue or cell type. Our results imply that a range of plant tissues and their constituent cells (including thickwalled sclerenchyma, thin-walled cortical parenchyma, and primary xylem) can be used as temperature proxies, not only the secondary xylem of wood. Experiments charring ferns for up to 24 h would be needed to determine if reflectance values stabilize like those of woods; these experiments would potentially extend the use of fern charcoals as proxies for emplacement temperature of pyroclastic flows (Scott and Glasspool, 2005). Time as well as temperature is important in causing an increase in reflectance (Guo and Bustin, 1998; Bustin and Guo, 1999; Scott and Glasspool, 2005), and it has been established that reflectance data can provide minimum temperature of formation of charcoals (Scott and Glasspool, 2005). Our results indicate that fern charcoals with reflectance values ⬎1.75% would not have been formed at ⬍450⬚C. Fern charcoal formed at ⬎350⬚C becomes increasingly more fragile, with fragmentation notable at temperatures ⱖ500⬚C, and becomes progressively more severe with rising charring temperature. Therefore, most large recognizable fossil fern charcoal probably formed at temperatures ⬍500⬚C and would be considered semifusinite when encountered in polished blocks (Scott, 2002). Much of this charcoal would probably have been formed during surface fires (such as in modern heathlands) where temperatures are around 400–500⬚C (Scott et al., 2000). For example, much early Carboniferous fern charcoal is derived from scrambling zygopterid ferns (e.g., Metaclepsydropsis, Diplolabis; see Phillips and Galtier, 2005). An interesting variant on this would be if fires spread through the crowns of tree ferns, which, in the Carboniferous, could range from ⬍1 m to ⬃3 m and might even form a low-stature canopy. An analysis of charcoal from peats of the late Pennsylvanian coals that are dominated by tree ferns (DiMichele and Phillips, 2002) would be a valuable future study. Charcoal from epiphytic or climbing ferns should be distinguishable, as it would probably be accompanied by other charred canopy elements. As
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FIGURE 10—SEM images of the outer portion of Osmunda petioles in transverse section before (A) and after (B) charring for 1 h at 500⬚C, showing the thick-walled outer sclerenchyma tissue that was used for studying homogenization of cell-wall layers in response to charring (see Fig. 11).
canopy fires are often hotter, such ferns might not survive at all as charcoal, being completely consumed by fire. In fires with temperatures ⱖ900⬚C, the presence of ferns in the original vegetation would not be detected in the charcoal fossil record. In contrast, wood would have survived, as indicated by the intact Sequoia wood at 1000⬚C and as expected based on Belcher et al. (2005). If thick-walled fern cells are charred and subsequently fragment, they would be indistinguishable from wood cell fragments (unless fragments
of wood cell-wall pitting could be identified to confirm the identification). The same would be true for charred thick-walled cells of the moss Polytrichum figured by Edwards and Axe (2004). Thus, subcellular fragmentary charcoal must be interpreted with care and can never be used to infer the presence of a particular vegetation or plant type (e.g., conifer trees vs. ferns or forest vs. herbaceous vegetation; see Scott and Glasspool, 2007, for further discussion on the application of inertinite group macerals).
FIGURE 11—SEM images of thick-walled outer sclerenchyma cells in Osmunda petioles after charring for 1 h at varying temperatures, showing homogenization of wall layers at temperatures ⱖ350⬚C. A) uncharred, B) 300⬚C, C) 350⬚C, and D) 400⬚C.
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FIGURE 12—SEM images of conifer wood cells after charring for 1 h at varying temperatures, showing homogenization of wall layers at temperatures ⱖ350⬚C. A) uncharred, B) 300⬚C, C) 350⬚C, and D) 400⬚C.
Paleoatmospheric and Carbon Isotope Implications If both fern tissues and wood are present in bulk charcoal samples (or bulk organic carbon samples) used for carbon isotopic analysis, up to 4‰ variation could be encountered in results that would merely reflect changes in the proportions of the two constituents. Such variation would have
no implications for paleoatmospheric compositions, isotopic excursions, or isotopic stratigraphy. A single fern-rich sample within a suite of wood samples could give a spurious, apparently negative, carbon isotopic excursion. Thus it is important to determine the botanical nature (tissue, organ, and plant type) of charcoals and organic carbon so that comparable samples can be used for carbon isotopic analysis. Charcoals from the Wealden of England (Collinson et al., 2000) include both fern and conifer-dominated assemblages; therefore, in the absence of botanical data, carbon isotopic shifts through this sequence could be easily misinterpreted. Interpretation of Charcoal Formation
FIGURE 13—Average ␦13CV-PDB (‰) of charcoals produced at different temperatures for conifer wood and petiole tissues of different fern genera. There is a consistently significant difference between ␦13C values for fern petiole tissues and conifer wood. Error bars are within size of symbols. RT ⫽ room temperature; MR ⫽ middle main rachis; PR ⫽ largest primary pinna rachis; S&LR ⫽ stipe and lower main rachis.
When a fire starts, an ignition source, usually lightning, causes plant materials such as cellulose and lignin to begin breaking down. Volatile material is driven off, and the remaining materials char in the absence of air. The volatile matter mixes with oxygen in the air, and combustion takes place, providing heat for the reaction to continue. Visible reflectance is generated by charring at 300⬚C, but not at 250⬚C, for both fern tissues and woods. Decomposition of cellulose does not begin below 270⬚C (Pyne et al., 1996), therefore little change in reflectance would be expected below this temperature. The lack of visible or measurable reflectance at charring temperatures ⬍300⬚C means that reflectance values cannot be used to understand the early phase of the charring response; visual analysis of hand specimens and SEM observations must be used instead. Based on our visual criteria, it appears that full charcoalification did not occur ⬍300⬚C for Osmunda petiole tissues and ⬍350⬚C for Sequoia wood. Cell-wall layers are homogenized when charred at 350⬚C, but not at 300⬚C, in both fern tissues and woods. These observations
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indicate that a definition of full charcoalification is hard to provide, as it would differ slightly for different plant types and tissues and would depend on the criteria used. Fern tissue and wood charcoals display all the criteria—three-dimensional preservation; anatomical preservation; black, brittle, shiny, increased reflectance seen in polished blocks under oil; and homogenization of cell-wall layers—at charring temperatures ⱖ350⬚C.
to ACS from the Royal Society. DCS is funded by a grant from the Leverhulme Trust (Grant ref. F/07/537/0) to MEC and ACS. We would like to thank Patricia Goggin and Kevin d’Souza for technical support and Neil Holloway for producing the polished blocks. We are grateful to the editors and referees whose comments greatly improved this paper. REFERENCES
CONCLUSIONS 1. The average ␦13C for Sequoia wood was consistently more 13C enriched than that of fern petiole tissue by typically 3.5‰–4‰, and the presence of ferns within charred wood samples could produce a depleted carbon isotopic excursion. Such a shift could be misinterpreted as an atmospheric change or misused as an isotopic stratigraphic marker. To avoid this problem, the botanical nature of charcoals needs to be determined so that comparable samples can be analyzed. 2. Both charred fern tissue and Sequoia wood showed some tendency to lighter ␦13C values with increasing temperature up to 600⬚C, reversing at higher temperatures. Thus, small shifts in charcoal ␦13C values do not necessarily equate to shifts in atmospheric composition and may instead be related to temperatures of formation. To take account of this, charcoals produced at comparable temperatures should be analyzed. 3. Charring of fresh petioles of the fern Osmunda resulted in a change in color from green through brown to finally a brittle, silky black, implying that charcoalification had taken place at temperatures ⱖ300⬚C. 4. Even at low charring temperatures, fern weight loss was considerable (⬎80%), and the original anatomical structure was preserved only at temperatures ⬍250⬚C. The loss of anatomical integrity above 250⬚C can be used as an indicator of fire intensity and would probably result in fragmentation during subsequent transport, biasing the fossil record in favor of low-temperature fern charcoals. The presence of homogenized cell-wall layers occurs at temperatures ⱖ350⬚C, thus giving a minimum wildfire temperature of 350⬚C. 5. Fern samples of all tissue and cell types with visible reflectance values—that is, those charred at temperatures ⱖ300⬚C—showed that rising temperature of formation led to increasing mean random reflectance under oil. Therefore minimum fire temperatures (across a wide range of numerical values) can be inferred from fern charcoal as well as from wood charcoal. 6. Sequoia wood exhibited most of the same responses to charring as fern tissues. An exception was weight loss, which was significantly less at lower temperatures than that seen in Osmunda and increased sharply between 250–350⬚C. Based on visual criteria, the Sequoia wood did not appear to be completely charcoalified until 350⬚C, in contrast to 300⬚C for Osmunda. Both fern tissues and wood displayed all criteria of charcoalification at charring temperatures ⱖ350⬚C. 7. Results showed that ferns produce recognizable charcoal at charring temperatures up to at least 800⬚C. Therefore, ancient fire-prone vegetation containing ferns can be potentially reconstructed but will be biased in favor of lower temperature charcoals (⬍500⬚C) because of fragmentation with rising temperature. At temperatures ⱖ900⬚C, fern charcoal is very unlikely to be represented in the fossil record. Fossil charcoal derived from herbaceous ferns would indicate production by surface fires because charcoal from epiphytes or climbing ferns should be accompanied by other charred canopy elements. 8. Fragments of thick-walled charred cells derived from ferns or woods are likely to be indistinguishable. Therefore, subcellular fragmentary charcoals, which may be encountered in coal petrological studies, should not be used to infer source vegetation. ACKNOWLEDGMENTS This study was funded by an Undergraduate Research Bursary (to LCM supervised by MEC and ACS) from the Nuffield Foundation (Grant ref URB/02092/A 30103), which is gratefully acknowledged. The experimental charcoalification was undertaken in ovens purchased by a grant
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