The influence of pyrolysis time, moisture, and plant species on carbon bridgehead fraction of charcoal

Paleofire reconstructions are a challenging endeavor primarily due to the numerous factors involved in wildfire frequency, behavior, and regimes. These factors include, but are not limited to fuel composition, moisture, soil types, climate/weather conditions, and topographical features. Therefore, development of robust wildfire proxies requires vigorous experimental testing for multiple variables. Here, we explore the influence of pyrolysis time, moisture, and plant species on a novel proxy for fire intensity—carbon bridgehead fraction of charcoal. Experimentally, 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 (300-700°C), moisture (0-100% moisture capacity), and time (0-1 hr) conditions in a tube furnace. Samples were analyzed using solid-state C-13 nuclear magnetic resonance (NMR) spectroscopy with two experiments to calculate carbon bridgehead fraction: cross polarization – magic angle spinning (CP-MAS) to quantify total aromatic carbon and dipolar dephasing (DD) to quantify aromatic bridgehead carbon. Results reveal significant differences between vegetation types, with moisture delaying or slowing the rate of carbon bridgehead formation. Relationship between carbon bridgehead fraction and time are less clear and may be influenced by the formation of pyrolysis byproducts (such as pyroligneous acids and free radicals) and/or signal losses in the cross-polarization spectra. To assess the influence of these factors on carbon bridgehead fraction we plan to conduct additional analyses on our experimental charcoals, including electron paramagnetic resonance (EPR) spectroscopy to quantify the free radicals in samples and C elemental analysis to assess carbon observability by NMR. Future work involves ground truthing the proxy to modern wildfires and subsequently applying it to paleorecords.