The applicability of Raman spectroscopy in the assessment of palaeowildfire intensity

Abstract

Evidence of wildfires in deep time is preserved as fossilised charcoal fragments in the rock record and inertinite macerals in coal. Historically, charcoal reflectance has been utilised to assess the formation temperature of these charcoals, and thus burning intensities of prehistoric fires. This is achieved by quantifying reflectance variability as a function of changes in charcoal microstructure with temperature. Raman spectroscopy been shown to similarly assess microstructure in carbonaceous organic matter with thermal maturation. However, there have been few applications of Raman spectroscopy to wildfire-derived charcoals, modern or prehistoric. Little consideration has also been paid to the nature and applicability of derived parameters in assessing intensity. This study presents a novel assessment of Raman spectroscopy as a method for interpreting palaeowildfire burning intensity. Spectra were obtained from experimentally pyrolysed Calluna vulgaris material, generated across a range of natural wildfire temperatures, and subsequent derived parameters were compared with established principles. For assessing changes in palaeowildfire intensity, this study has found the best correlations between thermal maturity and; D-band full-width at half-maximum (D-FWHM) and the D−/G-band full-width at half-maximum ratio (D-FWHM/G-FWHM). Additional parameters, commonly applied to Raman studies of charcoal, are otherwise influenced by non-linearity. The influence of precursor material on charcoal microstructure has also been derived, indicating further complexity when assessing heterogenous samples. Our results indicate that, whilst Raman spectroscopy offers extraordinary potential for understanding prehistoric and modern wildfire intensity, parameters and further analyses used require measured consideration.