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Re: astroblemes and evolution



Now that I have my references handy, I can post my more definitive
(and longer) rebuttal of the 'meteorites cause most extinctions'
position.

First, I will discuss some new material I have gotten on the
P-Tr extinctions. This is largely from "The Book of Life"
edited by Stephen Jay Gould.

Starting about 5 million years before the boundary, oceanic
isotope curves start to swing wildly. Several show a sharp peak
in one direction, and then a rapid shift to the opposite extreme
persisting across the P-Tr boundary.  (This is obvious in both the
Oxygen isotope and Carbon isotope ratios).  The Strontium isotope
ratio shows a rapid, but steady, decline over this interval.

Another factor, the sea-level was at a near all-time low (perhaps
even lower than it was during the last glaciation).  This low was
caused by a combination of uplifted continents (due to mountain
building incident upon the formation of Pangea), and to the terminal
Permian glaciations (which cover about the same time span as the
isotope fluctuation).

The final 10 to 15 million years of the Permian were characterized
by the formation of the Siberian Flood Basalts, and by extensive
volcanism in China (the probably cause of the Chinese siderophile
spike - since that spike is restricted to China).

Finally, at this time there was a major "Ocean Anoxic Event".
This is where the oxygen compensation depth reduces to a point
such that most continental shelf areas are under anoxic waters.
Some workers have even speculated, based on the OAE and the
rapidly changing oxygen ratios, that the *atmospheric* oxygen
concentration may have dropped by as much as 50%.

So, with *three* major sources of stress (oceanic anoxia,
glaciation, and major volcanism) ocurring at this time,
what need is there for an additional factor to explain the
extinction?

Certainly, the known pattern of extinction seems to be a gradual
one across the last few million years of the Permian rather than
an abrupt termination.


Now, I will discuss *patterns* of extinction and associated
events.  This material is from an article by Leary and Rampino
in "Extinction Events in Earth History", edited by Kauffman
and Walliser.

The key factor is the table on page 48, which I reproduce in
modified form below.  (I have elided some highly questionable
associations to make the pattern clearer).

Extinction      Ocean           Flood           Probable
                Anoxic          Basalts         Impacts
                Event
---------------------------------------------------------
K-T             no              Deccan          YES

Cenom./Turon.   YES             no              YES     [mid late
Cret.]

Aptian/Albian   YES             Raj-Mahal       NO      [mid Cret.]

early Cretac.   YES             no              NO
[very minor]

Kimmer./Tith.   YES             Serra Geral     NO      [late Jurassic]
                                [SW Africa]

Callovian       YES             ?possible       Maybe   [mid Jurassic]
                                [Antarctic]

Pliensbachian   YES             Karroo (Africa) NO      [early
Jurassic]

Norian          YES             ?East N. Amer.  NO      [late Triassic]

P-Tr            YES             Siberian Traps  Maybe
----------------------------------------------------------------------

Note the pattern here, the two factors with the *highest* correlation
with extinctions are flood basalts and ocean anoxic events.  Most
substantial extinctions are associated with *both* factors.
[The Antarctic Traps are not dated close enough to the mid
Jurassic extinction to be a likely contributor, and the
timing of the East Northa American flood basalts relative
to the Norian/Rhaetian extinction is questionable, but
within possibility].


For a discussion of the issue of the K-T extinctions and the
contribution of the impact, see the article: "Catastrophic
versus non-catastrophic extinction of the dinosaurs ..."
in Journal of Paleontology, vol 68, nbr 2.

In summary, Dr. Williams notes that there is an interval
of about 2-3 meters immediately below the iridium spike
in which dinosaur bones, though present, are extremely
rare compared to lower levels.  This depauperate zone
has persisted despite recent intensive searching in that
zone (Sheehan et alii, and unpublished work by W.A. Clemens).

Dr. Williams also points out that the fact that those few bones
found are *not* partially dissolved rules out the 'acid rain'
hypothesis for the lack of bones in this interval.  [High
acidity *can* dissolve bones, but there is no way it would
completely dissolve some and leave the rest completely untouched].

Dr. Williams also points out that this pattern is *not* what
would be expected from a catastrophic extinction caused by a
meteor impact (where the fossils should continue in full
abundance right up to the iridium spike, and even perhaps
show a peak in abundance at the spike due to mass death).

However, it is *also* not what would be expected from the
more naive gradualistic extenction models - 2-3 meters
only represents about 100,000 to 200,000 years.
[This is also compatible with Sheehan's report of no
diversity drop over the Late Maastrichtian, since his
method only split the epoch into three intervals, each
more than a million years long, and it would thus fail
to detect changes on the order of 250,000 years or less].

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

The peace of God be with you.