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A classification for macroscopic charcoal morphologies found in Holocene lacustrine sediments Colin J. Courtney Mustaphi and Michael F.J. Pisaric Progress in Physical Geography published online 8 October 2014 DOI: 10.1177/0309133314548886 The online version of this article can be found at: http://ppg.sagepub.com/content/early/2014/10/07/0309133314548886

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A classification for macroscopic charcoal morphologies found in Holocene lacustrine sediments

Progress in Physical Geography 1–21 ª The Author(s) 2014 Reprints and permission: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0309133314548886 ppg.sagepub.com

Colin J. Courtney Mustaphi Carleton University, Canada, and University of York, UK

Michael F.J. Pisaric Brock University, Canada

Abstract Macroscopic charcoal analysis of lake sediment stratigraphies is a widely used approach to reconstruct past biomass burning patterns of ecosystems. The development of fire records often relies on a single quantification method of charcoal in a sediment subsample; however, recent studies have shown that additional paleoecological information can be obtained by classifying charcoal morphologies. The morphologies and diagnostic features of charcoal yields information about fuel sources, fire type, and charcoal taphonomy, and can aid in calibrating sediment records to known historical fires. This additional information enhances paleoecological inferences by providing more paleoenvironmental information than studies of total charcoal as the only metric. Here we present a classification of 27 macroscopic charcoal morphologies observed in Holocene sediments of lakes located in the mixed-conifer forests of southeastern British Columbia, Canada. This classification system builds on other morphological classifications that have been previously utilized, but is more inclusive of the morphological variability observed and is flexible to modification for use when applied to other study settings. The morphological classification presented here was developed following the observation of >100,000 macroscopic charcoal fragments >150 mm. This paper focuses on the observed morphological classes, their identification, potential fuel sources, and the morphotype assemblage stratigraphy from one site as an example. The charcoal assemblages varied throughout the mid-to-late Holocene contemporaneously with known regional scale hydroclimatic changes in British Columbia. Major changes in fire frequency were also concomitant with morphotype assemblage changes. Future work focusing on linking fuel types with charcoal morphotypes, post-fire observations of charcoal taphonomy, and the analysis of multiple attribute charcoal data sets from a variety of ecosystems will improve our understanding of biomass burning and long-term fire ecology. Keywords anthracology, biomass burning, disturbance, fire, lake sediments, morphology, morphotypes, paleobotany, paleoecology, Quaternary, wildfire

I Introduction Sedimentary charcoal analysis is a wellestablished paleoenvironmental research method that is used to investigate past fire activity across

Corresponding author: Colin J. Courtney Mustaphi, York Institute for Tropical Ecosystems, Environment Department, University of York, York YO10 5DD, UK. Email: [email protected]


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Progress in Physical Geography

multiple spatiotemporal scales (Conedera et al., 2009; Scott, 2010; Scott and Damblon, 2010). These studies are critical for understanding past climate-fire-vegetation interactions, landscapescale ecosystem evolution, human-environment interactions, and future fire risk that informs land-management policy and decision-making (Gavin et al., 2007; Hessl, 2011). Proxy evidence for fire occurrence, such as charcoal remains and polycyclic aromatic hydrocarbons, are preserved in depositional environments and can be used to reconstruct past biomass burning (Conedera et al., 2009). Charcoal is produced during wildfires through the incomplete burning of biomass fuels that is then transported and accumulates in sediments, soils, and ice (Whitlock and Larsen, 2001). Deposited macroscopic charcoal (>125 mm) in lake sediments tends to have a source area of 24 hours, and wet sieved using a 150 mm mesh (Carcaillet et al., 2001; Courtney Mustaphi and Pisaric, 2013; Lynch et al., 2004). The remaining material was examined in a Petri dish using a Nikon SMZ800 stereomicroscope at 6–40 magnification. Observations of charcoal morphologies were made at each site as total charcoal was tallied (Courtney Mustaphi and Pisaric, 2013). Morphology and morphological features were observed from >100,000 charcoal fragments and common and rare morphologies were photographed. The fragments were classified into seven broad categories, which included a total of 27 subclasses of charcoal morphologies. These classifications were based on the conspicuous overall shape of the fragments and were further discriminated by the dominant surface texture and/or major features present. Qualitative observations of the degree of luster on the fragments were also noted as it is an indicator of the fire conditions (Belcher et al., 2005). Morphotype classification was then applied during macroscopic charcoal counting of each contiguous subsample of Pyatts Lake sediments and is presented here as an example study. Morphotype zones were delineated through constrained incremental sums of squares cluster analysis (CONISS) of the relative abundance of all morphotype classes and the number of significant clusters was prescribed by a broken stick model.

3 Sediment analyses of the Pyatts Lake stratigraphy Several analyses were performed to examine the influence of vegetation, fire frequency, and taphonomy on the charcoal morphology assemblages. Magnetic susceptibility was measured with Bartington Systems MS2B and MS2E sensors at


Mustaphi and Pisaric

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contiguous 0.5 cm intervals. Loss-on-ignition analysis (Dean, 1974; Heiri et al., 2001) used 1 cm3 subsamples taken at contiguous 0.5–5 cm intervals down core that were dried at 105 C for 24 hours and then burned at 550 C for four hours to estimate sediment organic content. The sample was then reheated to 950 C for two hours to estimate carbonate content. At 10 cm intervals, particle size distributions were measured from 1 cm3 subsamples that were digested with 30% hydrogen peroxide (H2O2) in a water bath (~75 C) until reactions stopped. Following this, 5 mL of 50 g L-1 sodium hexametaphosphate (Na6P6O18) was added to disperse the sample before triplicated measurement runs of 60 s, at an obscuration of 10+2%, using a Beckman Coulter LS 13 320 laser diffraction particle size analyzer. Pollen preparations, with the addition of exotic Lycopodium tablets (Stockmarr, 1971), used 1 cm3 subsamples taken at 10–20 cm resolution using standard digestion techniques (Fægri and Iversen, 1989). Pollen was counted under 400–1000 magnification and included terrestrial pollen sums of 539–1095 grains. Charcoal concentration data was analyzed using CharAnalysis version 1.1 software (Higuera et al., 2009) and parameterized using the same methods as Courtney Mustaphi and Pisaric (2014). Sediment macroscopic charcoal concentrations (pieces cm-3) were converted to charcoal accumulation rates (CHAR; pieces cm-2 yr-1) and resampled to the global median sampling interval of nine years. A LOWESS smoother robust to outliers over a 500-year window was used to calculate the varying background charcoal accumulation rate (bCHAR). The identification of locally defined peaks within a 500-year window was performed by subtraction of the bCHAR component series from the CHAR and identified through a 99% noise cut-off probability established by a 0- or 1-mean Gaussian mixture model (Gavin et al., 2006; Higuera et al., 2010). The minimum charcoal concentration, within a 75-year period prior to a peak, required a probability 1 side or structures on either side 2D, very thin and Thin filaments with embranched branching structures of similar thickness 3D, thick and Conspicuously embranched thicker, obvious cross-section faces Long with short Can be thin or ramifications, thick. filaments, spurs, Ramifications spines can be alternate or opposite.

of textures that ranged from dark and opaque with simple to complex structuring. Surfaces exhibited plain, striated, or latticed structures, and could also be foliated, lignified, or mottled with rounded void spaces. Occasionally, the polygons had a few tangential linear or dendritic spurs. Observed lusters ranged from gray to brilliant.

Luster

Fuels

Related types

M-B Conifer Opaque needles

C2, B5

M-B Needle Opaque fragments

C1

D-B Twigs, Thuja Opaque needles D-B twigs, woody Opaque and herbaceous material M-B Leaf veins C6, D1 Opaque (Enache and Cumming, 2006) M-B Leaf stems, C5, F1 Opaque herbaceous roots, stolons M Rootlets, fine F1 Opaque stems

4 Type B: ortho-angular polygons and polyhedra (blocky shaped; Table 2) These morphotypes were blocky or rectilinear with most corner angles near 90 . The pieces were either conspicuously prismatic (threedimensional, quadrilaterally faced polyhedra)



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Table 4. Type D, E, and F morphotypes: long and simple, spheroidal, and irregular. Type G not pictured. Lusters: G ¼ gray; D ¼ dull; M ¼ moderate; B ¼ brilliant; Gy ¼ glassy; O ¼ oily. Morphotype Figure

Morphology Surface features

D

D1 Linear

Elongated with few features Very thin and long

D2 Flat linear

Thin, long, and flat

D3 Flat linear with voids

Thin, long, and flat, with oval voids

Long (simple)

Luster

Fuels

Related types

Plain or includes voids

Easily broken

Can break or fray into multiple pieces of D1 charcoal. May have parallel venation Voids may be burned out stomatal complexes

or planar (two-dimensional, quadrilaterals), and often with one longer axis, but not necessarily extreme (i.e. rectangular). Polyhedral pieces tended to be larger (>1 mm). Surface textures ranged from plain to striated and lignified, or contained void spaces. The edges were smooth, serrated, or denticulated, which reflected the leaf edge morphologies of the fuels. Observed lusters ranged from gray to highly brilliant.

M-B Monocot leaves, C6, D2 Opaque wood? (Enache and Cumming, 2006) M-B Leaves D1 Opaque

M-B Poaceae leaves Opaque (Jensen et al., 2007)

B4

6 Type D: long and simple (Table 4) These morphotypes were highly elongated, simple, thin filaments, and were either featureless or contained oval voids similar to morphotype B4. The thin filaments were nearly hair-like in thickness and very fragile (D1), yet the slightly thicker charcoal (D2) was manipulated with little breakage. Observed lusters varied from moderate to brilliant.

7 Type E: spheroidal (Table 5) 5 Type C: long and complex (Table 3) These morphotypes were elongated pieces with one dominant long axis and had conspicuous structures that branched outward. Embranchments were long or short, regularly patterned (opposite or alternate), and were straight or irregular. These charcoal fragments strongly reflected the structures of the fuels; such as veined leaves, twigs, rootlets, and thin stem parts. Observed lusters varied from dull to brilliant.

This class contained charcoal that was threedimensional and highly rounded, bulbous, broadly spheroidal, ellipsoidal, or ovoid, and with faceted or smooth surfaces. Some pieces had simple geometries, such as spherules and fusiforms. Other complex morphologies were agglomerations of multiple spheroids, in asymmetric chains or raspberry-shaped, similar to microscopic charcoal and tar geometries identified by Franze´n and Malmgren (2012). Common to these morphotypes, surfaces appeared lustrous or exhibited a coal- or tar-like luster with coloration. Pieces


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Table 5. Type E, F, and G morphotypes: long and simple, spheroidal, and irregular. Type G not pictured. Lusters: G ¼ gray; D ¼ dull; M ¼ moderate; B ¼ brilliant; Gy ¼ glassy; O ¼ oily. Morphotype

Figure

Morphology

E

Spheroidal

3D, rounded, spheroidal, ellipsoidal, ovoid

E1 Spheroid

E2 Vesiculate spheroid

E3 Agglomerative

F F1 Bulky

Irregular

G

Glassy

G1 Glassy

Surface features Luster

Irregular rounded surface; although may have some faceting 3D, rounded E1 charcoal fits the broad description of E 3D, rounded, with Smooth with vesicles voids and vesicles. Fractures are sharp. Complex Smooth agglomerations, or rough raspberry-like undulated shape, or surface chained Complex shapes Bulky with tangled Rough, may be tubes, encrusted with clastic material 2D or 3D, Dark, dense, vitreous, complex shapes, sharp or resinous, rounded reflective corners Occasional voids Smooth. and sometimes Material is dendritic dense and brittle, and fragments energetically.

Fuels

Related types

M-B, Gy, O Fossil fuels, accretion of volatiles, sap?

M-B, Gy, O Smooth forms E2, E3 Opaque can be burned seeds M-B, Gy, O Sap, resin, and Opaque waxes?

E1

B, Gy, O Opaque

E1

D Opaque

Root masses

B, Gy, O Opaque

Resins, seeds, phytoliths (see photographs in Jensen et al., 2007)


C6


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Figure 2. Classification key for sedimentary macroscopic charcoal morphology analysis. Note that the blocky (B1) pathway follows the gray lines and the D3 morphotype follows from the elongated-simple to the matrix supported curved features pathway.

were often hard and brittle when manipulated or broken. Observed lusters varied from moderate to brilliant with glassy or oily textures.

et al., 2007). Type G charcoal pieces were dense and brittle and fragmented differently than wood charcoal. Lusters were brilliant, glassy, and occasionally oily.

8 Type F: irregular (Table 5) Type F morphotypes were characterized by complicated shapes with tubes, lobes, and more massive nodes. These were occasionally encrusted with mineral soil grains and often had a dull luster with a wide range of hardness from powdery to rigid fragmentation.

10 Additional types Other burned and charcoalized plant parts maintained their morphology after burning, transport, and deposition, and can be used for paleoecological interpretations. Additional morphotype classes and subclasses can be added by analysts as needed in future studies.

9 Type G: glassy (Table 5) Glassy pieces were similar in appearance to amorphous obsidian and were harder than other morphotypes. These charcoal types had complex shapes, with sharp corners, smooth and reflective edges, and occasional voids, and occasionally had a dendritic shape (see photographs in Jensen

11 Fuel sources Figure 3 (a–n) presents some morphotypes produced by burning known fuel sources and below we present descriptions of the charcoal when observed under stereomicroscopy. Some morphotypes were produced from the burning of


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Figure 3. Photographs of representative charcoal morphotypes under a stereomicroscope produced by open flame burning of known fuels. White scale bar represents 500 mm. Note that some morphotypes could be produced by multiple fuel sources and the samples presented here are only those produced from the fuel types described.

specific fuels, such as morphotypes A5, C1, C2, C3, C6, and F1 (Figure 3). Morphotypes A3 (h) and B3 (i) could be produced from the burning of a wide range of materials including wood, punky (decomposing) wood, leaves, and herbaceous parts. The burning of conifer needles

produced charcoal of morphotypes B3 (Table 2), C1 (k,l), C2 (Table 3), and C3 (m). A three-dimensional lattice similar to B5 charcoal could be created through the breaking of the charred internal structure of conifer needles in the laboratory; yet this morphotype was not



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observed in any of the sediment records. Open flame burning of plant parts common to the forest understory of the region, such as monocot leaves, produced abundant A3 (h), B3 (i), D1 and D2 morphotypes (h). The burning of grasses produced A3 (Table 1), A4 (a), B3 (Table 2), B4 (b,c), B5 (d), B6 (Table 2), and D1 morphotypes (also see Jensen et al., 2007). Grass roots produced C6 (e), C7 (f), and F1 morphotypes (g). Wood produced B1 (n), B2, B3, and D1 (Table 4) morphotypes. Deciduous leaves produced abundant and fragile A4, A5 (j), C5, and C6 (Table 3) morphotypes.

V Pyatts Lake case study results 1 Charcoal morphotype assemblages

Figure 4. Photographs of charcoal morphotypes observed during macroscopic charcoal analysis of Holocene-aged lake sediments (>150 mm wide) collected from southeastern British Columbia Canada (Figure 1). White scale bar represents 500 mm and black bars represent 250 mm. (a) A small fragment of A1 charcoal that is striated and possibly from wood. (b) A2 charcoal. It has fine circular voids and is likely attributable to herbaceous sources. (c) A small fragment of A3 charcoal. (d and e) Morphotype A6 that had not been described in other studies and could not be reproduced in the laboratory. (f) B4 charcoal, from a grass source, with void spaces where the stomata had burned differentially from the remainder of the plant parts and were similarly reproduced in the laboratory (Figure 3, b and c; Walsh et al., 2010a). (g) A delicate, gray, charcoal

Examples of subfossil morphotypes observed in the British Columbian lake sediment stratigraphies are shown in Figure 4. The age-depth model developed using iterative random walks through the calibrated radiocarbon dates and parameterization are shown in Figure 5 and Table 6. Charcoal morphotypes were counted from contiguous subsamples (n ¼ 703) at Pyatts Lake and 24 of the morphotypes presented here were observed (Figure 6). Charcoal concentrations averaged 15.1 pieces cm-3 with a range of 0–175.5 pieces cm-3 with an extreme peak of 620 pieces cm-3. Within the entire 6890year record, morphotype B2 was dominant and comprised 29.0% of all charcoal observed. Also abundant were D1 (22.6%), A3 (21.3%), and B3 (12.2%). Morphotypes A1 and D2 each Figure 4 continued. piece that was much thinner than B5 charcoal. (h and i) Distinct charred plant parts; (h) was readily identifiable as a Pinuscontorta needle fragment with stomata still visible. (j) Morphotypes D1, D2 and D3, which were all observed in a single sample and probably represent grassy fuels. (k) E1 charcoal that had a smooth surface and was hollow that may have resulted from the burning of a seed. (l) E3 charcoal that had an agglomerated, raspberry-like shape, and was very dense with an oily luster.


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Figure 5. Age-depth model produced using random iterative walks through the calibrated radiocarbon ages.

comprised