The discovery of large amounts of soot in clays deposited at the Cretaceous-Paleogene (K-Pg) boundary and linked to the ~65 Ma Chicxulub impact crater led to the hypothesis that major wildfires were a consequence of the asteroid impact. Subsequently, several lines of evidence, including the lack of charcoal in North American sites, were used to argue against global wildfires. Close to the impact site fires are likely to be directly ignited by the impact fireball, whereas globally they could be ignited by radiation from the re-entry of hypervelocity ejecta. To-date, models of the latter have yet to take into account that ejection – and thus the emission of thermal radiation – is asymmetric and dependent on impact angle. Here, we model: 1) the impact and ejection of material; 2) the ballistic continuation of ejecta around a spherical Earth; and 3) the thermal pulse delivered to the Earth's surface when ejecta re-enters the atmosphere. We find that thermal pulses in the downrange direction are sufficient to ignite flora several thousand kilometers from Chicxulub, whereas pulses at most sites in the uprange direction are too low to ignite even the most susceptible plant matter. Our analyses and models suggest some fires were ignited by the impact fireball and ejecta re-entry, but that the non-uniform distribution of thermal radiation across the surface of the Earth is inconsistent with the ignition of fires globally as a direct and immediate result of the Chicxulub impact. Instead we propose that the desiccation of flora by ejecta re-entry, as well as the effects of post-impact global cooling/darkness, left much of the terrestrial flora prone to fires, and that the volume of soot in the global K-Pg layer is explained by a combination of syn- and post-impact wildfires.
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