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Re: extinction

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.

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