Re: extinction

["Dewey M. McLean" <dmclean@vt.edu>]

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