The suggestion that globally extensive wildfires were ignited as part of the Cretaceous-Tertiary (K-T) boundary events (e.g., Robertson et al., 2004; Melosh et al., 1990; Kring and Durda, 2002) ignores the following: (1) absence of above background levels of charcoal, (2) absence of charred peats, (3) absence of geomorphological and sedimentological evidence, and (4) presence of significant quantities of noncharred material.

Belcher et al. (2003) and Scott et al. (2000) have shown that the K-T sedimentary rocks contain below-average background levels of charcoal (but not lower than the minimum background level recorded) when compared to the charcoal record of the Late Cretaceous. Robertson et al. claim high temperatures would destroy charcoal produced in a K-T firestorm. They favor the model of Melosh et al. (1990), which predicted atmospheric temperatures of ~827 °C following the K-T impact. Such temperatures do not explain the lack of charcoal, as high temperature does not destroy charcoal. We have produced charcoals in ovens at temperatures over 900 °C for one week. Furthermore, the K-T charcoal does not show very high reflectance. Scott (200 0) demonstrated that charcoal reflectance increases with temperature. High temperatures would have generated very highly reflecting charcoals. The key in charcoal destruction is not temperature but oxygen availability. Robertson et al. (2004) suggest local oxygen deficiencies occurred under the K-T fires, as in firestorms over burning cities during World War II. Plant material may be totally consumed by hot flames fed with oxygen (Scott, 2000), whereas charcoal formation occurs where there is heat and a lack of oxygen. Therefore, the suggestion of Robertson et al. (2004) would provide the ideal situation for creating beautifully preserved, highly reflecting charcoal particles.

A long standing problem with K-T ejecta reentry models (e.g., Melosh et al., 1990; Toon et al., 1997; Kring and Durda, 2002) is the assumption that the upper K-T layer consists of ballistically distributed ejecta. This may be true for the lower claystone (found only across the United States) that decreases in thickness with distance from Chicxulub following a power law relation with exponent of −3 (Hildebrand and Stansberry, 1992), a characteristic of ballistically distributed ejecta, but this is not the dispersal mechanism for the upper layer, which is uniform in thickness (~3 mm) across the world. Its presence was a puzzle finally explained by dispersal of impact fireballs seen from remote observations of the SL9 impacts on Jupiter. These showed that impact fireballs collapse hydrodynamically (e.g., Hammel et al., 1995), indicating the assumption that the upper layer is ballistically distributed is invalid. Therefore, ballistic dispersal does not produce a layer of uniform thickness across the globe; hydrodynamic collapse of an impact fireball would globally disperse impact products and release an order of magnitude less energy, which would be insufficient to ignite vegetation.

There are several reports of the K-T boundary in coal (peat deposits) (Sweet et al., 1999; Scott at al., 2000; Belcher et al., 2003). Peat is an excellent fuel for wildfires, especially if the temperatures and duration envisioned by Robertson et al. had been achieved. Charred peats occur elsewhere in the fossil record (Petersen, 1998) but we see no evidence of such material in our K-T peat sequences.

Robertson et al. argue a few centimeters of soil would provide sufficient insulation to prevent charring. While thermal penetration of soil is not high, it has been shown that at 2.5 cm depth fires of 700 °C (Rundel, 1981) increase the temperature to 200 °C. For the temperatures and durations indicated by Robertson et al., much of the litter layers would have produced charcoal (beginning at >300 °C). By suggesting that unburned organic matter in the soil could be the source of noncharred material in the K-T layers, Robertson et al. ignore stratigraphy. The K-T stratigraphy consists of two distinctive layers: a lower cream-colored claystone overlain by a thinner brown to orange claystone. What soil there was before the impact is below the K-T layers and has no part in the noncharred material story. Moreover, the pollen and spores immediately under/in/above the K-T layers are impeccably preserved (i.e., no evidence of heating during deposition). Palynomorphs in the K-T layers are not derived from the latest Cretaceous sediments or from the overlying coals as leakage, as the assemblages are so different (see Sweet et al., 1999). The noncharred plant fragments (which are not root material) cannot be sourced from soil.

If Robertson et al. argue for complete removal of organic material then we ought to see major erosion and deposition events (as is common following wildfires; e.g., Meyer et al., 1992). If there was a major global fire we would see evidence in the sedimentological record. We see no evidence of this in the areas we studied, confirming our view that extensive fires did not occur as part of the K-T events.
