No. 43: Jan-Feb 1986
The worldwide deposit of iridium at the end of the Cretaceous implies, to many at least, that the great biological extinctions of this period were the consequence of a meteorite impact. It has now been discovered that clay samples from the Cretaceous-Tertiary boundary also contain 0.36-0.58% graphitic carbon. It is fluffy stuff and suggests that the planet was once covered by a thick layer of soot. Quantitatively, the soot layer is equivalent to the carbon in 10% of the earth's present biomass. The authors speculate that this soot was created by huge wildfires that consumed much of the earth's vegetation and perhaps fossil fuel as well. Terrestrial life was, of course, devastated -- just as it is in the currently popular "nuclear winter" scenarios. The end-of-the-Cretaceous soot is in fact, thicker and more widely spread than nuclear winter theories predict.
(Wolbach, Wendy S., et al; "Cretaceous Extinctions: Evidence for Wildfires and Search for Meteoric Material," Science, 230:167, 1985.)
Comment. Questions arise, though: How could a single meteorite impact ignite worldwide wildfires? Why haven't other meteorite impacts, recorded abundantly by large craters and astroblemes, also set fire to the planet and left iridium layers?