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Re: extinction



Dewey McLean (dmclean@vt.edu) wrote:

<Impactors claim that the Chicxulub impact occurred 65 million years ago.
The initial phase of Deccan Traps volcanism occurred somewhat _earlier_
than K-T boundary time.>

  This argument of dating was one of the reasons why I commented upon
accuracy of dating. This works for both events. The dates are off, even if
it seems one may have been triggered by the other. The Boltysh (Kelley &
Gurov, 2002) and Silverpit (Stewart & Allen, 2002) craters appear to be
similarly, if partially offset, correlated to the end Cretaceous, either
into the Maastrichtian before the transition by a hundred thousand years,
or after by the same into the Danian. This argues for the multi-impact
scenario (I advocate no theory). Pre-K/T major level anomolies are
reported elsewhere, though, and equatorially (as in within Oman). The
various layers suggest that precipitation or gradual fall, as David noted,
is inconsitent with such outpourings, is false, but that large-scale PGE
and IR-deposits were emplaced in shorter-term events, rather than over
steady periods for a few thousand years (also in which case erosion and
tectonics would rework the deposits and likely skew the finer precipitate
metals, instead of forming distinct and absolute layers.

  I think the reference David may be thinking about is this:

  Luck, J.M. and Turekian, K.K. 1983. Osmium-187/Osmium-186 in Manganese
    Nodules and the Cretaceous-Tertiary Boundary. _Science_ 222: 613.

  otherwise:

  Kelley, S.P. and Gurov, E. 2002. Boltysh, another end-Cretaceous impact.
    _Meteoritics and Planetary Science_ 37: 1031-1043.
  Stewart, S.A. and Allen, P.J. 2002. A 20-km-diameter multi-ringed impact
    structure in the North Sea. _Nature_ 418: 520-523.

  I think perhaps that a single, or even serial event, should not account
for the extensive deposits, but that multiple impacts may also potentially
lead to the varying ages and thicknesses/concentrations of the metals, as
it would likely appear that a truly expulsive volcanic activity can only
produce locally, and an effusive one that is being implicated cannot
(based on its own local sampling). Perhaps the various world-wise traces
should be allowed as multiple-origin impactors, rather than singular
culprits.

  And not to remark on the attitude of one over another, as Dr. McLean has
been VERY polite even to what can be seen as an obvious detractor such as
myself (though I am not), but there is a decidedly Deccan-biased argument
in favor where, even given the generalized conclusionary work he has
offered in his detailed studies on the subject. The volcanics give another
possible source to the K/T event, but the presence argues that the
inter-Trappean vert fauna would have to have bene rendered moot and
imaginary for the extinction-level effects applied to it can be worked
out, and this does not seem to be the case. If anything, the
inter-Trappean beds show an extensive diversity of very large (several
40ft+ sauropods and ~30ft abelisaurs, smaller abelisaurs, mammals, birds,
turtles, crocs) that should be environmentally sensitive, as in Rahioli
and Pisdura localities. Such sensitivity shows these forms pervading
inter-Trappean beds, contra the extinction-level events applied. Even
Hawaii's Big Island supports trhiving life (including offshore) despite
these calamitous events -- and these are observable.

  No ill-respect is meant.

=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

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