Significance Burn markers are observed in many records of the Cretaceous–Paleogene asteroid impact and mass extinction event. These materials could be derived from wildfires on land or from sedimentary rocks hit by the asteroid. We present a detailed record of molecular burn markers (polycyclic aromatic hydrocarbons [PAHs]) from the Chicxulub crater and in ocean sediments distant from the impact site. PAH features indicate rapid heating and a fossil carbon source and are consistent with sedimentary carbon ejected from the impact crater and dispersed by the atmosphere. Target rock-derived soot immediately contributed to global cooling and darkening that curtailed photosynthesis and caused widespread extinction. PAH evidence indicates wildfires were present but less influential on global climate and extinction.