Constraints on the thermal energy released from the Chicxulub impactor: new evidence from multi-method charcoal analysis

It has been suggested by various workers that an extraterrestrial impact at the K–T boundary delivered sufficient thermal power to ignite globally extensive wildfires. Numerous models have sought to predict the amount of thermal power released by the impact, but none have considered the distribution of wildfire indicators in K–T rocks. Probably the most distinctive product from combustion of biomass is charcoal. The abundance of charcoal across the K–T boundary at eight non-marine sites in North America, stretching from Colorado in the south to Saskatchewan in the north, is recorded using three separate methods that allow quantitative analyses of microscopic to macroscopic charcoal particles. This study not only provides the first extensive study of charcoals across the K–T boundary but also uses the presence or absence of charred material to predict the extent and severity of the thermal pulse released by the K–T impact across the area predicted to have suffered the most extreme environmental effects. The K–T rocks contain on average between four and eight times (according to the method used) less charcoal than the Cretaceous rock record and non-charred plant remains are abundant in the K–T rocks. The below-background charcoal abundance and the high proportion of noncharred material in the K–T and lowermost Tertiary rocks across the Western Interior of North America suggest that there were no significant wildfires in this area associated with the K–T event. Although soot and polyaromatic hydrocarbons (PAHs) have been reported in the K–T rocks we suggest that the soot morphology and PAH types are more consistent with a source from the vaporization of hydrocarbons rather than biomass. For spontaneous ignition of vegetation temperatures >545 °C are necessary, whereas smouldering will begin at 325 °C. The below-background levels of charcoal in the K–T rocks allow the ground temperatures following the K–T impact to be constrained to between no more than 545 °C at any point and not above 325 °C for any significant period. This implies a maximum irradiance of <19 kW m−2 at the ground surface and that no more than 6 kW m−2 of thermal power was delivered to the ground for more than a few hours. Therefore our results show that the fossil record indicates that the impact at Chixculub did not generate sufficient thermal power to ignite extensive wildfires.