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Re: patterns in extinction, etc.



From: "King, Norm" <nking.ucs@smtp.usi.edu>

 > I don't think we can demonstrate that those groups snuffed it at the same 
 > time--i.e., that's not a _fact_.  It's an _interpretation_ that their 
 > demise was synchronous.  ...

Indeed - this point is one of the themes Archibald's book.
[He casts considerable doubt on full synchrony].

 > I think that most commentators on the terminal Cretaceous extinction
 > allow that its cause has not been certainly determined--even those who
 > favor the impact scenario.  There's no dogma.  But I'm still sitting
 > here waiting for an equally competent alternative.

My alternative, also suggested by Archibald, is the "no one cause"
alternative.  That is no one event or process *by* *itself* was
sufficient to cause the full range of extinctions seen.

Many possible factors were present that may have been contributory.

- A marine regression not matched until recent times.  This reduces the
continental shelf area, fragments coastal and oceanic habitats, and
increases the seasonality of the climate (a switch from a "maritime"
to a "continental" climate).  It is intetesting to not the the even
more extensive Permo-Triassic extinctions took place in a time of even
more widesread "continentality".

- Massive volcanism in India (the formation of Deccan Traps). This
could have altered the climate and caused acidic rain. In this regard
it is interesting to note that there are some K-T depositional sequences
that subtly suggest a stepwise extinction pattern, which *may* relate
to the fact that the Deccan volcanism was intermittant, or at least
highly variable.  Again, an even more massive flood basalt formation
is associated with the Permo-Triassic extinctions.

- Climatic cooling was occuring.  This may be related to the two
previous items.  Near the time of the Permo-Triassic extinctions there
was an actual *ice* *age*.

- The Yucatan meteorite impact. [I hardly need explain how this would
induce stress].

So, the idea is that *all* of the above combined together to produce
the extensive extinctions found at tha K-T boundary.
[Archibald calls this "three strikes and you're out"].

swf@elsegundoca.attgis.com              sarima@netcom.com

The peace of God be with you.