Charcoal Bridgehead Carbon Fraction: A Proxy for Fire Temperature in Sediment Records

Wildfire reconstructions are critical to understanding past changes in landscapes and ecology across the planet. However, fire proxy records are often plagued by a lack of essential information, i.e., fire temperature, heat, etc., that are fundamental to interpreting the intricate interplay between fire and environments. To aid in solving this issue, we present a unique proxy for fire temperature, charcoal bridgehead carbon fraction, and explore the extent of variability recorded in charcoal chemistry. To do so, we have produced charcoals from three native Texas plants: live oak (Quercus sp.), Ashe juniper (Juniperus ashei), and broomsedge bluestem (Andropogon virginicus) under a range of temperature, moisture, and time conditions in a tube furnace. Samples are being analyzed using 13C nuclear magnetic resonance (NMR) spectroscopy, revealing a direct relationship between bridgehead carbon fraction (a function of charcoal aromaticity) and furnace temperature, with slight variations between plants. Based on preliminary experiments conducted on applewood (Malus domestica), we hypothesize a near-logarithmic relationship between bridgehead carbon fraction and pyrolysis time in Texas live oak and broomsedge bluestem. Yet, experiments conducted on Ashe juniper reveal a deviation from this relationship, potentially due to the presence of free radicals produced during the pyrolysis reaction. We also find that moisture appears to have a delaying effect on the production of charcoal, likely reducing bridgehead carbon fraction at identical pyrolysis temperature and time of dry plant experiments. Accordingly, our initial findings suggest that bridgehead carbon fraction has utility as a proxy for the severity of paleo-wildfire regimes. Future work involves continuing NMR experiments, assessing the variability of the proxy spatially across wildfire transects, and applying it to paleorecords.