Re: extinction

[Stan Friesen <swf@tdat.ElSegundoCA.NCR.COM>]

HORTON@bcrssu.agr.ca writes:
 >   In my humble opinion, the evidence for dinosaur decline prior to
 > the KT boundary layer is due to sampling effects.

Then you haven't read the latest article on that subject in
the literature.  (I will try to bring the cite tomorrow).

In background, it has long been known that dinosaur bone
drops out about 3 meters below the K-T boundary in the
Lance/Hell Creek formations.

A recent intensive search of this 'barren interval' has only
yielded a few isolated dinosaur bones.  Enough well preserved
one to rule out the idea that acid rain from the impact dissolved
them (since even surviving bones would be damaged by this),
but no where near as many bones as below the barren zone.  Thus,
even after a very specific, very intense search of the beds
immediately preceding the impact layer, the barren zone remains
barren (even if no longer entirely empty).

The abundance of dinosaur teeth in channel sands also drops off
at about the same level, and then peters out *across* the boundary
layer (or at least up to it - the stratigraphic position of channels
is often hard to determine, since they erode into surrounding
sediment).

 > The other theories, eg. disease,
 > don't make much sense.

You seem to be unaware of the Deccan Trap volcanism which spanned
the boundary.   This would have many of the same effects as an
impact, but spread out over a longer period.

Combine this with the cooling climate, the lowering shelf seas
(for marine organisms), and as a final blow, the Chicxulub impact,
and you get a fairly good multi-causal model for the extinction.

Almost all major extinctions are associated with several stress
factors, not just one.  The Permo-Triassic extinction had lowering
sea levels, increased aridity, lowering temperatures, and flood
basalt volcanism similar to the Deccan series (in this case, I
believe it was the Siberian series) - but NO evidence of any impact.
And this was the greatest extinction in the Phanerozoic, and indeed
probably the greatest extinction since the oxygen crisis.

Most of the lesser extinction events of the Mesozoic are associated
with both flood basalts and with Ocean Anoxic Events - periods
in which the oxygen compensation depth decreased enough to put
most shelf areas under anoxic waters. (There may have been one of
these associated with the P-T extinction as well).

 >. It has to explain
 > not only this extinction event, but other mass exinction events.

Why?  Why do all extintion events have to have the same cause,
or set of causes?

Actually, if they do, you have ruled out the impact, as impacts
are *not* associated with most extinction events.

In fact if you insist on one cause, then it has to be flood
basalt volcanism, which is the one thing that has the *highest*
correlation with extinction events.  [The only extinction event
I know of that violates it for certain is the Pliestocene extinction]

 >   For an excellent discussion of the impact theory vs. other
 theories, read
 > "Nemesis: the Death Star" by Rich ...(I've forgotten his last name:
 Mueller?).
 > He was a friend of Louis and Walter Alvarez, two of the men who
 proposed the
 > impact theory. The author of this book takes the next step, to
 propose a
 > mechanism for periodic impacts.

This stuff is seriously out of date.  Further studies have:
        - cast doubt on the periodicity
        - failed to find evidence of impacts in association with
          most other extinction events.
        - failed to find any evidence of any other planetary
          body in the Solar System.

swf@elsegundoca.ncr.com         sarima@netcom.com

The peace of God be with you.