Reconstructing Fire Temperature Using Infrared Spectroscopy of Charcoal Materials

Charcoal is the result of incomplete combustion of organic materials during fire and is ubiquitous in the natural environment. Sedimentary charcoal is used to infer long-term fire regimes and vegetation dynamics that extend the historic record, providing valuable information for improving forest management. Abundance counts and morphology of sedimentary charcoal are often used to quantify aspects of past fire regimes such as fire frequency and types of fuels burned in past fires. While these previous sedimentary charcoal methods have increased our understanding of past fire dynamics, our understanding of metrics including fire intensity and temperature has been limited by existing charcoal analysis methods. To address this knowledge gap, charcoal was produced in the laboratory from species present in the mixed-conifer forests of the western United States, an area that has been experiencing increasing frequency of catastrophic wildfires. The charcoal was then analyzed using infrared spectroscopy. Using a muffle furnace, vegetation samples were combusted at temperatures ranging from 200-800ºC (in 100ºC increments) and remaining material (i.e., charcoal) was ground to a size fraction <53 micrometers, separated by species and temperature. This laboratory produced charcoal was analyzed with a Fourier Transform Infrared (FTIR) spectroscopy microscope and the resulting spectra were compared by temperature and species using standardized methodologies. For comparison, vegetation samples were also combusted in an open burn chamber, instrumented with thermocouples, to simulate more realistic wildfire conditions. Burn chamber samples were also analyzed using the FTIR microscope and compared to muffle furnace generated sample spectra. The spectra generated from laboratory produced charcoal will serve as an important reference library, potentially applicable to future sedimentary charcoal studies to inform past fire behavior and provide context into burn severity and associated ecological effects.