Smoke without Fire - Geochemical Evidence for Combustion of Hydrocarbons at Chicxulub

Models have proposed that the Cretaceous-Paleogene (K-P) impact event was associated with extensive wildfires. An abundance of soot and polycyclic aromatic hydrocarbons (PAHs) in marine K-P rocks have been considered to support for the wildfire hypothesis. However, non-marine K-P rocks, from across North America, contain limited amounts of charcoal and abundant non-charred plant remains, indicating that wildfires did not play a role in end Cretaceous events. PAHs and soot can be formed from a variety of sources, including combustion of vegetation and hydrocarbons whereby modern pyrolitic PAH signatures are traceable to their source. Here we present the results of a study of PAHs from 6 non-marine K-P sections from North America and reveal that the K-P boundary rocks yield pyrolitic PAH abundances that are typically lower than that of the background record. This pattern is similar across all 6 sites that stretch 1183km in distance and follow a similar trend to charcoal distribution, further confirming that extensive wildfires are unlikely to have played a role in the K-P events. A comparison of the signature of the PAHs with those from the combustion of coal, lignite, oil, diesel, gymnosperms and angiosperms indicates that the K-P PAHs show evidence of being sourced from hydrocarbon combustion. The global distribution of PAHs need not invoke global wildfires and in fact provides compelling evidence that a significant volume of hydrocarbons or organic fossil carbon were combusted during the K-P impact event. The K-P charcoal and PAH record requires that extinction and recovery mechanisms be reassessed in order to include mechanisms that do not involve high amounts of thermal radiation.