Impacts and wildfires - An analysis of the K-T event

Models of the Cretaceous-Tertiary (K-T) impact at Chicxulub have suggested that thermal radiation would have been sufficient to have ignited extensive or near global wildfires. The discovery of abundant soot, increased levels of polyaromatic hydrocarbons (PAHs), and the possible occurrences of the fullerenes C60 and C70 have been considered to support the wildfire hypothesis. However, the charcoal record from K-T sites stretching the length of the Western Interior of the USA reveals amounts of charcoal below background levels and an abundance of non-charred material in the K-T and earliest Tertiary rocks. Explanations to account for this disagreement between the charcoal record and the other potential wildfire indicators include: charcoal formation but subsequent oxidation (by acid rains or during diagenesis), and intense thermal radiation converting charcoal directly to CO2, In both scenarios significant quantities of non-charred material would not be expected to survive. There is no satisfactory hypothesis to explain how charcoal could be transported away from the entire Western Interior, and it is unlikely that conditions prevailed in the Western Interior that prevented the fires. Following re-analysis of the proposed wildfire evidence, the abundance of soot, PAHs, and fullerenes can be explained without invoking the global wildfire hypothesis. It has been concluded that fullerenes are not a suitable indicator for impact-related palaeo-wildfires. The morphology of the K-T soot is characteristic of soots produced during combustion of hydrocarbons. The uniformity of its carbon isotope signature between sites across the globe is better explained by the vaporisation of one pool of hydrocarbons. The PAH record of the K-T rocks includes compounds that are never formed from the burning of biomass but that are released during the combustion of hydrocarbons. Recent Chicxulub drill cores reveal that the target rocks contain hydrocarbons, the vaporisation of which could produce the soot and PAHs found at the K-T boundary.