Wet sieved charcoal analysis
By: Colin J. Courtney Mustaphi
January 22, 2014; November 3, 2020
Sieved charcoal analysis can be performed at various depths down core to examine broad changes in macroscopic charcoal deposition or contiguously subsampled down core at a high resolution to reconstruct fire episodes and to estimate fire return intervals over long (millennial) time scales (Higuera et al. 2009). Charcoal should be gently separated from the sediment matrix of a known volume of sediment e.g. 1 cm3 of sediment. Larger volumes may be necessary within the top sediments that have very high water contents (>80% water weight) and for systems with rapid accumulation of sediments or peat that may dilute the charcoal accumulation signal (Aleman et al. 2013; Carcaillet et al. 2001). The sediment subsample should be soaked in a Calgon or sodium metaphosphate solution to help disaggregate the sample and remove clay particles from the organic material (Bamber 1982). Hydrogen peroxide may also be added to help bleach and distinguish dark black organic matter from true charcoal pieces (Schlacter and Horn 2010; and see Tsakiridou et al., 2020 for a comparison of pretreatment methods). The samples may need to sit from a few hours to a few days depending on how easily the samples disaggregate and sieve – a few test iterations may be needed. Once softened, the subsamples can be washed through a sediment sieve. The mesh size may vary between studies but should be consistent with the literature or between sites within your study for comparability. Multiple size fractions can also be used to gain more insight into biomass burning and charcoal taphonomy. Commonly used size fractions are 100, 125 (Colombaroli and Gavin, 2010), 150 (Courtney Mustaphi and Pisaric 2013; 2014), or 250 μm meshes (see also Vachula, 2019). The retained material can be examined and counted under a stereomicroscope at a magnification of 10-50× (Whitlock and Larsen, 2001; Hawthorne et al., 2018). Ensure to record the depth interval (top and bottom of sample interval), the volume (or mass) of wet or dry sediment extracted, and the number of charcoal pieces counted. Concentrations (pieces cm-3) will be converted to charcoal accumulation rates (CHAR, pieces cm−2 y−1) if there is a continuous age-depth model for your study site and you can use tools like CharAnalysis software. It may also be useful to collect the water weight values of each sample to convert the charcoal concentration to charcoal per mass of dry sediment (Courtney Mustaphi et al., 2015).
Photographs taken at the University of York by Colin J. Courtney Mustaphi.
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