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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
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.
January 16, 1990
Dr. Thomas J. Ahrens
Seismological Laboratory 252-21
California Institute of Technology
Pasadena, CA 91125
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
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.
Dewey M. McLean
Professor of Geology