X-ray microtomographic imaging of charcoal
Introduction
Charcoal is produced by the pyrolysis of biomass under conditions of limited oxygen availability (Chaloner, 1989, Scott, 2000). This material, known as fusain in the fossil record (Scott, 1989) and as inertinite by coal petrographers (Scott and Glasspool, 2007), typically comprises 60–90% carbon, a proportion of which exists in condensed aromatic molecular configurations (Forbes et al., 2006, Eckmeier et al., 2007). Charcoal often retains much of the original plant macrostructure, enabling identification of the species or genus that was pyrolysed (Scott, 2000, Scott, 2001, Emery-Barbier and Thiebault, 2005, Marguerie and Hunot, 2007).
Charcoal represents one of the most widely used materials for radiocarbon dating (Bird, 2006), and a key factor limiting the ability to obtain robust radiocarbon dates from charcoal is the potential for post-depositional contamination by extraneous carbon. At least some components of charcoal appear to be largely ‘inert’ (Scott and Glasspool, 2007), based on a high degree of resistance to a range of oxidants (Bird and Gröcke, 1997, Wolbach and Anders, 1989, Skjemstad et al., 1996) and the persistence of some charcoals for long periods of time in the geologic record (Cope and Chaloner, 1980, Scott, 2000, Scott, 2008). However, mass balance studies of charcoal in both tropical and boreal soils have indicated that charcoal can be progressively lost from the soil (e.g. Bird et al., 1999a, Bird et al., 1999b). The mechanisms postulated for this include photo-oxidation, inorganic chemical or biologically mediated oxidation possibly coupled with physical commutation to finer particle sizes and translocation (Skjemstad et al., 1996, Bird et al., 1999a, Bird et al., 1999b, Czimczik et al., 2005).
It has been demonstrated that charcoal can interact in several ways with its local depositional environment, having a beneficial effect upon soil microbial communities through the provision of microhabitats (Wardle et al., 1998, Zackrisson et al., 1996, Pietikäinen et al., 2000, Warnock et al., 2007), improving soil nutrient retention capacity and sorbing a range of chemical contaminants (Cornelissen et al., 2004, James et al., 2005, Keech et al., 2005). These observations suggest that charcoal is unlikely to be a fully closed system in most depositional environments. Potential sources of post-depositional extraneous carbon therefore include: the reaction of organic compounds in soil solutions with the charcoal surface, the passive microbial or fungal colonization of pore spaces and possibly the active use of some charcoal components by these organisms as a metabolic substrate.
One key to evaluating the potential for interaction between a charcoal fragment and its local depositional environment is the internal physical structure of the material. Porosity, pore size and the connectivity of available pore space are likely to exert significant control on the ability of soil solutions to diffuse through the charcoal and of soil microbiota to colonize the charcoal. While the primary control on charcoal structure is the nature of the material pyrolysed, it is likely that pyrolysis conditions will also exert significant control on charcoal structure (Pietikäinen et al., 2000, Cornelissen et al., 2004). Optical examination of polished sections of charcoal and scanning electron microscopy of charcoal fragments can provide some information on charcoal structure and these techniques are routinely used in charcoal taxonomy (e.g. Collinson et al., 2007). Nitrogen adsorption/desorption (László et al., 2005) and mercury porosimetry (Klose and Schinkel, 2002) techniques can also provide information on porosity and pore size distribution. However, none of these techniques provide three-dimensional information on charcoal structure and particularly information on the degree to which pore space is connected throughout a charcoal fragment.
Computerized reconstructions of serial sections of anatomically preserved plants have proved successful in the study and identification of plant fossils (Smith and Stockey, 2007), however, this technique is inherently destructive. Phase contrast synchrotron X-ray computerized microtomography has been shown to yield detailed internal anatomy of charcoal, but currently this technique is applicable only to specimens 1–2 mm in size (Friis et al., 2007).
X-ray microtomography employs the same principles as medical CAT-scanning to generate a three-dimensional virtual image of opaque materials, and has previously found application in the study of soil micromorphology (O'Donnell et al., 2007) and sedimentology (Appoloni et al., 2007). This study explores the potential of this technique as a new tool that has not previously been utilized in the study of charcoal. While microtomography can be readily used as a non-destructive technique for charcoal taxonomy, we focus here on its use as a tool for examining the effect of pyrolysis conditions and chemical oxidation on the development of porosity and pore space connectivity in laboratory-produced charcoal. We also assess the potential of the technique for determining the distribution of mineral contaminants in archaeological and geological charcoal.
Section snippets
Samples and methodology
Approximately 0.7 cm3 cubes were cut from the outer 15 rings a sample of Scots Pine (Pinus sylvestris), obtained from Tentsmuir Forest, Fife, in November 2005. Charcoal was produced from this material in a Carbolite controlled-atmosphere rotary furnace, with the exact pyrolysis temperature monitored via a thermocouple inserted into one of the wood cubes in each run. The furnace was set to a heating rate of 10 °C min−1, and then held at either 300 or 600 °C for 60 min (hereafter referred to as PC300
Results and discussion
Fig. 1 shows the estimates of porosity and pore size distribution for a range of threshold greyscale values for two of the end-member laboratory-produced pine charcoals. It can be seen that both porosity and pore size are critically dependant on the threshold chosen, with porosity, for example, varying by a factor of four for thresholds varying from 20 to 80. While visual comparison of thresholded images with the raw image clearly demonstrated that thresholds of 20 and 80 do not yield realistic
Conclusions
X-ray microtomography has been able to elucidate the internal structure and location of mineral phases in a range of ancient charcoals enabling a better understanding of the processes that can lead to alteration and degradation of charcoal in the natural environment. With each new generation of μCT systems available, sample spatial resolution will improve; a nano-tomography system is presently available, however the analytical capability of this system is limited to small sample sizes (//www.skyscan.be
Acknowledgements
Funding for this research was provided partly by NERC standard grant NE/C004531/1 ‘Charcoal Degradation in Natural Environments’ to Bird. Scott thanks the Royal Holloway research strategy fund and a personal charitable donation for his research on modern and ancient charcoal. This research represents a contribution from SAGES (The Scottish Alliance for Geosciences, Environment and Society).
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Present address: School of Geography & Geosciences, University of St Andrews, St Andrews, Fife, KY16 9AL, Scotland, UK.