Re: extinction

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

Dear Colleagues,

My computer lit up this morning to display some worthy challenges to 
my post of Wed, 14 Jan 2004 19:31:30. I will respond to them (as best 
as I am able) but first, I want to make sure that everyone 
understands my position as regards the K-T impact versus volcanism 
debate.

As I noted in a recent post, Powell's book "Night comes to the 
Cretaceous..." categorized me as an "anti-impactor," an incorrect 
statement that causes me much distress. In fact, nothing could be 
further from the truth.

The K-T seems a unique event in earth history in which both impact, 
and volcanism, were involved in triggering a mass extinction. Both 
were undeniably involved in the K-T mass extinction. My work is 
simply to try to sort out the relative involvements of impact and 
volcanism in the vast, complex, and extremely messy K-T geobiological 
record. This involves integration of a vast complex of 
multidisciplinary data in which a major problem involves conflicting 
interpretations of single data sets. It seems that nearly every 
aspect of the K-T is controversial.

That I am not an "anti-impactor" is demonstrated by my 1990 letter to 
Tom Ahrens (below the dashed line) in which I proposed a combination 
impact-volcanism model that might unify the K-T transition 
geobiological record via both impact and volcanism.

I sent my Ahrens letter to Walter Alvarez. In his 3 April 1990 letter 
to me, Walter stated: "Won't it be something if it turns out all the 
K-T tracks are converging?!"

For the purpose of this list, I am attempting to provide a window so 
that you can look into the inner workings of a scientific revolution. 
I am most appreciative of the kindly tone of our exchanges--a tone 
that has not characterized the actual inner political workings of the 
K-T debate that most people know little, to nothing, about.

Cordially,
Dewey McLean

-------------------------------------------

January 16, 1990

Dr. Thomas J. Ahrens
Seismological Laboratory 252-21
California Institute of Technology
Pasadena, CA 91125

Dear Tom:

It's good to see you and John O'Keefe advocating K-T greenhouse 
conditions. Since the 1970s (Science, 1978, v. 201, pp. 401-406), a 
greenhouse has been my primary killing mechanism in the K-T 
extinctions. In fact, a major K-T carbon cycle perturbation has been 
confirmed, and a record of K-T transition warming has emerged. Put 
the pieces together, and a major long-duration K-T greenhouse seems 
apparent. Hitherto, my problem with a K-T impact event was that I 
could not find definitive evidences of an "impact winter"--a 
prediction of the impact theory, and the impact-related killing 
mechanism--in the K-T geobiological record.

For a K-T greenhouse killing mechanism, since the 1970s, I have 
worked to isolate out principles linking climatic warming to 
reproductive fertility, by which a greenhouse can trigger global 
extinctions (I have the Ph.D. in geology from Stanford, and all 
course work for the Ph.D. in biology, which provide a basis for my 
work). I believe that I now have a heat-embryogenesis dysfunction 
killing mechanism which has universal applicability to mammals, 
reptiles, and birds that would have been operative in a K-T 
greenhouse (I enclosed some reprints for you).

My primary interest in the K-T has always been to couple greenhouse 
conditions to global extinctions. I evoked the Deccan Traps volcanism 
(1981) as a means for initiating a greenhouse, believing that a 
long-term mantle C02 release could explain the long-term aspect of 
the K-T transition geobiological record. I believe that I have made a 
case for a K-T transition greenhouse coeval with Deccan Traps 
volcanism and bioevolutionary perturbation.

On the other hand, the K-T boundary shocked minerals remain as 
possible evidence of an impact--even though some volcano advocates 
think they are volcanic. This impasse could go on for years, and 
never provide definitive direction on cause of the K-T extinctions. 
The K-T is not so much about impact versus volcano, as about 
extinctions, and the killing mechanism that is recorded in the 
geobiological record. Your impact-induced greenhouse might cut the 
Gordian Knot, and allow unification of our current state of knowledge 
of the K-T. Perhaps, like the Blind Men of Hindustan, we have all 
been feeling different parts of the same K-T killing mechanism: a 
greenhouse.

Is it possible that a short-duration K-T boundary impact-induced 
greenhouse, superimposed upon and intensifying a long-duration 
volcano-induced K-T greenhouse, might unify our current state of 
knowledge of the K-T geobiological record? Such unification would 
accommodate the K-T boundary shocked minerals and intensification of 
ecological stresses within the long-duration K-T transition carbon 
cycle and bioevolutionary perturbations that are preserved in the 
record. This unification--which accords with the K-T record--offers a 
step forward in isolating the cause of the extinctions while impact 
versus volcano details are sorted out down through the years.

Whereas a K-T "impact winter" analogue of a modem "nuclear winter" 
remains elusive in the record, a K-T transition greenhouse seems 
essentially confirmed. I have long argued that the K-T is the classic 
example in earth history of an analogue for a modern greenhouse. A 
civilization facing a potential modern greenhouse can gain insight on 
the dangers of a greenhouse to life by examining the K-T, regardless 
of the greenhouse triggering mechanism.

Cordially yours,

Dewey M. McLean
Professor of Geology