NEW CONSTRAINTS ON THE THERMAL ENERGY RELEASED FROM THE K-T IMPACTOR: EVIDENCE FROM MULTI-METHOD CHARCOAL ANALYSIS

It has been suggested by various authors 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 the K-T rocks. Probably the most distinctive product from combustion of biomass is charcoal. The abundance of charcoal across the K-T boundary at 8 terrestrial K-T boundary sites, stretching from Colorado in the south, to Saskatchewan in the north, is recorded using 3 separate methods that allow quantitative analyses of microscopic through to macroscopic charcoal particles. This study not only provides the first extensive study of charcoals across the K-T boundary but uses the presence/absence of charred material to predict the extent and severity of the thermal pulse released by the K-T impact. The K-T sedimentary rocks contain on average 9 times less charcoal than the Cretaceous rock record and non charred plant remains are abundant in the K-T rock layers. The below-background charcoal abundance and the high proportion of noncharred material in the K-T and lowermost Tertiary sedimentary rocks across the western interior of North America suggest that there were no significant wildfires in this area associated with the K-T event. For spontaneous ignition of vegetation temperatures >545 °C are necessary whilst smouldering will begin at 325 °C. The below background levels of charcoal in the K-T rocks allows 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, the thermal power delivered from the impact to North America did not have the destructive potential previously predicted.