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Re: extinction



Dear Colleagues,

Much of my work on the K-T extinctions has involved searching for mass extinction killing mechanisms whose evidences are preserved in the geobiological record. Much of that work involved searching for evidences of a K-T impact winter, and global impact-induced wildfires that theoretically consumed most of earth's forests.

My post of Sat. 10 Jan 2004 10:07:57 cited my paper, "Impact Winter in the Global K-T Extinctions: No Definitive Evidences" (in Levine, J. S., ed., _Global Biomass Burning: Atmospheric, Climatic, and Biospheric Implications_, Cambridge, MIT Press, 1991, p. 493-503). That paper was the culmination of a nearly 10 year search for evidences of an impact triggered dust-induced "impact winter" (global dust-induced blackout and refrigeration) that began with the 1980 publication of the Alvarez impact theory.

Recall that the theoretical global blackout purportedly triggered extinctions among plants that served as food for herbivorous dinosaurs, thus causing extinctions among the herbivores, and then among predatory dinosaurs that used the herbivores as a food source.

That paper also included a search for evidences of theoretical K-T global wildfires that burned most of earth's forests via sparks and radiation from a K-T impact as proposed by Wolbach and others.

I found no definitive evidences for either a K-T impact winter or global wildfires. Now, two recent papers seem to support my work.

The first is Kevin Pope's paper, "Impact dust not the cause of the Cretaceous-Tertiary mass extinction" (Geology, 2002, v. 30, pp. 99-102). Following are some quotations from Kevin's paper:

"These findings indicate that the original K-T impact extinction hypothesis--the shutdown of photosynthesis by submicrometer-size dust--is not valid, because it requires more than two orders of magnitude more fine dust that is estimated here."

"These results shed doubt on the importance of impact dust in the mass extinction that marks the K-T boundary. A global atmospheric loading of...submicrometer-sized dust would not cause the catastrophic impact winter often proposed...."

Kevin did leave open other impact hazards such as global cooling caused by impact production of sulfate aerosols, and soot from global wildfires.

The second is the Belcher et al. paper, "Fireball passes and nothing burns--The role of thermal radiation in the Cretaceous-Tertiary event: Evidence from the charcoal record of North America (Geology, 2003, v. 31, pp. 1060-1064). Following are some quotations from the paper:

"The K-T and lowermost Tertiary sedimentary rocks of six nonmarine sequences (Colorado to Saskatchewan) contain no charcoal or below-background levels of charcoal and a significant quantity of noncharred organic materials, revealing that there was no distinctive wildfire across the North American continent related to the K-T event."

"Therefore, the thermal power delivered from the impact to North America did not have the destructive potential previously predicted. High amounts of thermal radiation were not responsible for the environmental perturbations or extinctions associated with the K-T event."

With the seemingly virtual elimination of a K-T impact winter, and global wildfires, not many potential mass extinction killing mechanisms remain. A couple contenders are climatic cooling via impact-induced injection of sulfate aerosols into the atmosphere, and greenhouse warming via a combination of several factors.

Cordially,
Dewey McLean