Assessing Modern Calluna Heathland Fire Temperatures Using Raman Spectroscopy: Implications for Past Regimes and Geothermometry

Abstract

Charcoal geothermometry continues to offer considerable potential in the study of palaeowildfires over decadal, centennial, millennial, and deep time scales—with substantial implications for the understanding of modern wildfire intensification. Recent developments in the application of Raman spectroscopy to carbonaceous organic material have indicated its capability to potentially reconstruct the palaeocharcoal formation temperature, and equivalent palaeowildfire pyrolysis intensity. Charcoal reflectance geothermometry (which also relies upon microstructural change with thermal maturation) has also been the subject of extensive modern evaluation, with multiple studies highlighting the key influence of energy flux on the resultant charcoal microstructure. The ability to accurately quantify modern wildfire temperatures based upon novel Raman-charcoal analyses has not yet been attempted. Using Raman band width-ratios (i.e., FWHMRa) and accompanying geothermometric trends to natural wildfire charcoals, our results identify differences between microstructurally-derived fire temperatures compared to those recorded during the fire event itself. Subsequent assessments of wildfire energy flux over time indicate no dominant influence for the observed differences, due to the inherent complexity of natural fire systems. Further analysis within this study, regarding the influence of reference pyrolysis methodology on microstructural change, also highlights the difficulty of creating accurate post-fire temperature reconstructions. The application of Raman spectroscopy, however, to the quantification of relative changes in fire temperature continues to prove effective and insightful.

Timing and causes of forest fire at the K–Pg boundary

“…All spectra were deconvolved within Renishaw WiRE 3.4 software, applying smoothing and a cubic spline interpolative baseline, and bands D and G fit solely. For geothermometric purposes, parameter FWHMRa (D-and G-band width ratio) was utilised within the following equation 74 See also Supplementary Information, Table S4, Fig. S3.…”

Towards a kerogen-to-graphite kinetic model by means of Raman spectroscopy

“…Raman spectroscopy on wood-derived charcoals has been used to assess charring temperatures with implications in paleo-wildfires for paleoecological and volcanological studies (Ascough et al, 2010;Mauquoy et al, 2020;Theurer et al, 2021Theurer et al, , 2022Schito et al, 2022), or to evaluate coal and wood reactivity for metallurgical application (Urban et al, 2003;Paris et al, 2005;Zickler et al, 2006;Chabalala et al, 2011;Morga, 2011;Surup et al, 2019) (Table 4). Studies dealing with other fuel types (i.e., biomass char) or on the effect of catalysts on char reactivity at very high temperatures (from 1500 to 2700 • C) are not considered.…”

“…The application to charcoal geothermometry is probably the most recent advance, and has shown attractive potential application in archaeological, paleo-environmental and geological studies (Deldicque and Rouzaud, 2020;Mauquoy et al, 2020;Theurer et al, 2021Theurer et al, , 2022Schito et al, 2022) as well as for industrial application (Surup et al, 2019). Such large spectrum of applications results in a wide range of temperatures used in calibration experiments, up to almost 2000 • C. Most of the results shown in Fig.…”

“…1. Nevertheless, the application to diagenetic and metamorphic studies (Beyssac et al, 2002;Guedes et al, 2010;Lahfid et al, 2010;Liu et al, 2013;Lünsdorf et al, 2017;Wilkins et al, 2014;Schito et al, 2017;Henry et al, 2018;Corrado et al, 2020;Muirhead et al, 2020) still outnumbers alternative applications like maturation around shallow intrusions (Muirhead et al, 2012;Chen et al, 2017;Muirhead et al, 2017;Henry et al, 2019aHenry et al, , 2019bLi et al, 2020), contact metamorphism at deeper levels (Aoya et al, 2010;Beyssac et al, 2019;Mori et al, 2017;Li et al, 2019), frictional heating on fault planes (Ito et al, 2017;Nakamura et al, 2015;Kedar et al, 2020;Muirhead et al, 2021) or charring temperatures of charcoal in natural wildfires (Theurer et al, 2021(Theurer et al, , 2022. In other words, the research has been mainly focused on the evolution of Raman data under low heating rates and is not clear how different heating (and pressure) conditions can affect it.…”

Charcoal analysis for temperature reconstruction with infrared spectroscopy

“…As a result of significant preheating and the absence of smoldering combustion, muffle furnace charcoal exhibits higher optical reflectance compared to charcoal from vegetation fires at the same peak heat intensity (kWm −2 ) (Belcher and Hudspith, 2016) and therefore, the molecular structure may also be different. This was further tested by Theurer et al (2022) by generating charcoal using a laboratory tube furnace with pyrolysis under a constant nitrogen flow and comparing it to charcoals obtained from a prescribed heathland fire. They found inconsistencies based on thermocouplemeasured temperatures of heathland charcoal compared to microstructure changes of tube furnace charcoal at the same pyrolysis temperature (Theurer et al, 2022).…”