Belcher et al. (2003) reported that at six locations across North America quantities of charcoal in the Cretaceous-Tertiary (K-T) boundary clay are about an order of magnitude below background levels. They interpreted this absence of charcoal as evidence for the absence of a global conflagration at the K-T boundary. The case for their argument would be much stronger had they found no change in charcoal levels at the boundary layer rather than a marked decrease from background levels. The general background levels of charcoal presumably were generated by ordinary wildfires ignited largely by lightning. Therefore, in order to explain the absence of background charcoal levels in terms of an absence of unusual fire at the K-T boundary, Belcher et al. must also assume either an absence of ordinary lightning-induced fires (which does not seem likely) or the existence of a mechanism other than fire that would destroy the background charcoal only at the K-T boundary.
We think, instead, that fire is the only mechanism that is likely to have destroyed charcoal so thoroughly at the K-T boundary. Obviously, fire can destroy charcoal as well as create it. Low intensity fires will create charcoal, but high-intensity (i.e., high-temperature) fires will tend to destroy it. The observed unusual absence of charcoal therefore constitutes strong prima facie evidence for a fire of unusually high intensity at the K-T boundary.
There is good reason to think that the conflagration at the end of the Cretaceous attained unusual intensity. Any fire is essentially in thermodynamic balance between heat production (by combustion) and heat loss (largely through convection and radiation). A large fire will generally have a lower surface-to-volume ratio and will therefore have lower heat losses and higher temperatures than a smaller fire. Fires at the K-T boundary, which were ignited essentially simultaneously across entire continents by the infrared radiation from reentrant ejecta, were therefore likely to have been of unprecedented intensity. Also, heat from this infrared radiation continued to be added to the fires for several hours even as they burned. The resulting extraordinarily high temperatures very plausibly destroyed vegetative fuel to a level much lower than that required to explain normal preservation of charcoal at background levels. No other simple explanation easily accounts for an order-of-magnitude decrease in preserved charcoal at the K-T boundary. Certainly the idea of an absence of unusual fire fails to do so.
Today we have no first-hand experience with the intensity of continental-scale wildfires. Perhaps the only similar phenomena in recent history are the firestorms that were caused by incendiary bombing in World War II. In July 1943, for example, the firestorm in Hamburg, Germany, burned approximately ten square kilometers in a few hours; it attained temperatures of perhaps 800 °C in the streets and wind speeds of hurricane force (Middlebrook, 1980), clearly intense enough to destroy charcoal. The fires that were ignited simultaneously across millions of square kilometers at the K-T boundary can hardly have been any less intense than these much smaller-scale firestorms.
Belcher et al. (2003, p. 1063) stated that their conclusion implies that Melosh et al. (1990) “overestimated the mass of the high-velocity ejecta by at least two orders of magnitude.” Because the Melosh et al. mass estimate is based on direct observation of the spherules in the boundary clay (remnants of the reentrant ejecta), we find the suggestion of a two-order-of-magnitude error to be, at best, implausible. It would imply that the spherule layer has an average accumulated thickness of less than a tenth of a millimeter. Smit (1999) estimated the ejecta layer to have had an accumulated thickness of 2–3 mm at points more than 7000 km from the impact site.
Belcher et al. noted that significant quantities of uncharred organic remains are present in the K-T boundary layers. They claimed, incorrectly, that presence of such material is inconsistent with the idea of an intense continental firestorm. However, a substantial amount of organic matter would be expected to be present in the soil at any given time prior to a conflagration, and a few centimeters of soil would provide sufficient insulation to prevent charring from even the most intense fires. Perhaps still more important, many plants would have started to regenerate from buried roots, seeds, and spores on time scales of days after a global conflagration. The roots of these new-grown plants might well have proliferated in a nutrient-rich ash layer, thereby providing the observed record of uncharred plant fossils.
Therefore, precisely opposite to the interpretation by Belcher et al., the important new evidence that they report is consistent with the occurrence of continental-scale firestorms at the end of the Mesozoic era. For a more complete discussion of the biological effects of this thermal stress following the Chicxulub impact see Robertson et al. (2004).