Reconstructing past fire temperatures from ancient charcoal material

Charcoal abundance measurements are commonly used to estimate fire activity in palaeoecological studies; however, fire temperature is not directly measured. Reconstructing fire temperature is desirable because the ecological response to fire is, in part, a function of the temperature of the fire, e.g. crown fires >500 °C, slash and burn agriculture <400 °C. Here, we determine whether charcoal chemistry, as inferred from Fourier Transformed Infrared Spectroscopy (FTIR), is a reliable proxy for fire (combustion) temperature. We generated reference charcoal material from a grass species (Panicum capillare) and a woody species (Alnus glutinosa), prepared with three different laboratory treatments (untreated, water, and hydrogen peroxide), and heated to six temperatures (200–700 °C). We picked individual charcoal fragments from lake sediments deposited between ca. 1400 and 450 years ago to compare with the reference charcoal material. FTIR spectra were used to infer the chemical composition of both modern reference and ancient charcoal. The FTIR spectra of the datasets were analysed with model-based clustering.

The inferred chemistry of the reference charcoal from the FTIR spectra was in broad agreement with previous studies, and the model-based clustering algorithms were able to distinguish clusters based on the temperature to which the material was heated. The FTIR spectra from the ancient charcoal fragments fell within the range of variability of the modern reference charcoal, allowing for successful classification of the fragments created by fires in the long distant past. We used a probability density function of each statistically significant cluster to infer combustion temperatures for the ancient charcoal fragments. Our results suggest that the use of FTIR analysis of charcoal can differentiate low (200 °C–300 °C), medium (400 °C–600 °C) and high (600 °C–700 °C) temperature fires. Our findings pave the way for generating a better understanding of the role of fire in Earth's system through time.