This proportion of carbon is normal: 0.5% of carbon occurs in modern marine sediments, which accumulate very slowly and are continually churned and mixed by crustaceans, worms, sea urchins, rays, and other digging and burrowing animals.
But the Chicago group chose to assume that all the KT clay layer was formed from impact glass and was laid down in a year. For geophysical and geochemical reasons that are laid out in the group's research papers, this wildfire scenario can be true only if the following scenario is also true: that the impact dust roiling through the atmosphere generated enough electrostatic charges to cause continent-wide lightning storms that started fires on all important land surfaces on Earth. The dust stayed suspended in the atmosphere while wildfires burned up most of the vegetation on Earth. The dust then mixed uniformly with the soot in the air, so that the soot stuck to the dust particles; then the dust settled through the air and the ocean to settle on the seafloor within a year, absorbing as it sank much of the antimony, arsenic, and zinc in the ocean. For chemical reasons, this scenario in turn works only if there was five times as much zinc in the Cretaceous ocean than there is now (and there's no evidence at all for that).
If (and only if) the KT boundary clay is formed largely from impact material, and if it was laid down in a year, then the amount of carbon in it is extraordinarily large, forcing the conclusion about wildfires. We have to assume that wildfires burned unchecked from one end of a continent to the other, and that the rate of soot production was 10 times greater than we see in fires today.
Even the Black Dragon forest fire in 1987 in northern China and Siberia, the largest ever recorded, was a regional, not a continental event. The Kuwait oil fires of 1991 and the Indonesian and Brazilian fires of 19971998 were similarly regional, not global, in their effects.
Only temperate forests and savanna woodlands burn with frightening intensity. Rainforest hardly ever burns naturally: it has to be chopped down first. That's why the 1997 and 1998 fires in Southeast Asia and Brazil were so bad: they burned rain forest that had been cleared for palm plantations or peasant agriculture.
There's practically no underbrush in the rainforest, only shallow, damp leaf litter, and the canopy is high above the ground. Fires usually peter out at the forest edge‹that's why so many cultures can operate small-scale slash-and-burn agriculture in rainforests without fires getting out of control.
Altogether, the firestorm scenario for the KT boundary requires such special conditions that it must have been an event different from anything we have available as a comparison. We have to rely on calculations for the physics and chemistry of its effects, and we have no effective baseline data.
It's certainly unlikely that the impact material fell out of the atmosphere in one year. Ash from the eruption of Tambora in April 1815 was so thick in the atmosphere that colored snow fell in Europe the following January, and dust and aerosols dropped temperatures so much in August 1816 that harvests failed all over the Northern Hemisphere. Even ash and aerosols from the smaller eruption of Mount Pinatubo in 1991 took several years to fall out of the atmosphere. Therefore, a major assumption is suspect.
At this point, one must make some sort of personal judgment: I conclude that the firestorm scenario is highly unlikely. Nevertheless, it is annoying to see that the speculation of the firestorm group quickly turned to assertion as they published their series of papers.
I had concluded that the firestorm scanario did not work by the 3rd edition, but recent research by Claire Belcher and others has shown conclusively that there was no firestorm.
I dealt with this before, when it was suggested in the 1980s by Jay Melosh. But briefly, what about birds? They were neither underground nor underwater, no matter what these folks say. There's a disease that geophysicists are prone to. When you do a calculation, you tend to believe the result, and think of it as evidence, instead of seeing it as only a hypothesis to be tested against REAL evidence. So these folks find that there are REAL pieces of evidence that are potentially dangerous to their conclusions, so they pile cockamamie idea on top of cockamamie idea to explain away the contrary and REAL evidence. In the end the whole construct is a shoddy pile of cardboard (in my opinion).
Thankfully, new calculations by Goldin and Melosh have led them to retract the microwave summer scenario.
Goldin, T. J. and H. J. Melosh. 2009. Self-shielding of thermal radiation by Chicxulub impact ejecta: Firestorm or fizzle? Geology 37, 1135-1138. Only the abstract is available on the Web
Page last updated March 23, 2013.
Links last checked March 23, 2013.
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