Benzo[ghi]fluoranthene and benz[a]anthracene as potentially useful wildfire temperature indicators

Abstract

Polycyclic aromatic hydrocarbons (PAHs), along with charcoal, are key indicators of wildfires in the geological record. This study explores the distribution of PAHs in charcoal and fusinite-rich sedimentary rocks from the Late Triassic and Early Jurassic periods. During the Upper Triassic, conifers were the main source of biomass for forest fires. In contrast, in the Lower Jurassic, almost all main plant groups could provide fuel for wildfires including lycopsids, horsetails, ferns, cycads, bennettitaleans, ginkgoaleans, and conifers. For the Upper Triassic, fire temperatures ranged widely from 272 °C to 742 °C, which includes ground, surface, and crown fires. Regarding the Lower Jurassic fires, the range of measured temperatures was smaller, from 251 °C to 451 °C, corresponding to ground and surface fires. As expected, the reflectance of fusinite correlates well with total organic carbon for charcoals (R² = 0.52) apart from a few outliers that lower the correlation. By analysing the correlation between the benzo[ghi]fluoranthene (B[ghi]Fl) to benz[a]anthracene (B[a]A) ratio and fusinite reflectance, a new method for estimating wildfire temperatures was developed. The B[ghi]Fl/(B[ghi]Fl + B[a]A) ratio exhibits a robust correlation (R² = 0.62) with fire temperatures calculated from fusinite reflectance. A novel formula for calculating wildfire temperatures was proposed, offering a valuable proxy complementary to fusinite reflectance. The results obtained are related to the higher thermal stability of B[ghi]Fl compared to B[a]A, making this ratio a reliable indicator of combustion temperature. The use of PAHs to determine the temperature of wildfires should be limited to unweathered rocks, as B[a]A is rapidly oxidised. Further research is needed to verify the application of this indicator in other sedimentary basins.