Source: Adapted and updated from Huggett (1997a, 293-4).

Source: Adapted and updated from Huggett (1997a, 293-4).

lethal radius that would release soot into the atmosphere (e.g. Wolbach et al. 1985, 1988; Kring and Durda 2002; Robertson et al. 2004). For 10-km bolides, the lethal radius could include areas of continental size. The mechanisms that might ignite wildfires are somewhat debatable. It is possible that the thermal radiation produced by the ballistic re-entry of ejecta condensed from the vapour plume of a 1015-1016 kg bolide would increase the global radiation flux by up to 150 times the input from solar energy for periods of one to several hours (Melosh et al. 1990). Jay Melosh and his collegues (1990) estimated temperatures in the upper atmosphere - at about 70 km - to rise to up to 827°C for several hours. Thermal radiation inputs of such a magnitude would probably spark off wildfires as well as directly damaging animals and plants. Alternatively, an infrared thermal pulse from the global rain of hot spherules splashed from the impact site might have been the primary killing agent and the lighter of wildfires (Robertson et al. 2004). However, not all researchers agree that wildfires burned, citing as evidence the presence of 'normal' levels of charcoal, the presence of significant quantities of uncharred material, the absence of charred peats, the absence of geomorphological and sedimentological evidence for soil erosion (Belcher et al. 2003).

The high temperatures near the impact would lead to the formation of large quantities of the oxides of nitrogen. A very large impact could produce up to 3 x 1018 t of nitric oxide that, in less than a year, would spread through the atmosphere. This huge injection of gas would give world-wide atmospheric nitrogen dioxide concentrations of 100 parts per mil-

Table 7.2 Proposed causes of Phanerozoic mass extinction events.








Anoxia (with

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