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Re: The Deccan Traps



 From: bk090@freenet.carleton.ca (David Brez Carlisle)
 > ... The very existence of dinosaur
 > fossil remains in the interstadial layers between the basal
 > Deccan lava flows indicates that periodic vulcanism on this scale
 > did not produce mass extinctions, or there would have been none
 > to come back to colonize the land after the fi rst ph ase of the
 > eruption died down.

There is an invalid assumption hidden in this reasoning.
The assumption is that the cause of a mass extinction must
produce its effect quickly, and immediately.  This need not
be so.

Given a *gradual* stressing of the ecosystem to the breaking point,
one does not expect to see significant extinctions early in the
flood vulcanism. Indeed all one might see early is a slight decrease
in alpha diversity. (Hmm, by some accounts this is exactly what
we *do* see).  And then the main extinction would tend to occur
over a prolonged span of time, perhaps in bursts as different
ecological guilds reach the limit of their tolerance. (Again,
there are some indications that this is exactly what is seen).

 >      The total energy released in the eruption of the Deccan
 > Traps is of the same order of magnitude as the amount released
 > at Ch icxolub. 

Now you seem to be assuming that the mechanism of extinction involved
is driven mainly be the primary eruptive effects!  This is unlikely,
as trap vulcanism is one of the least violent sorts known.

Another factor: how do you explain the fact that flood basalts are
known to be associated with all but one of the mass extinctions
through the Mesozoic?  Such a high level of co-occurance certainly
suggests a causative relationship.

 >  The difference is that the energy from Deccan
 > was spread over a million years, while that from the impact was
 > released in a matter of seconds (if we i nclude the atmospheric 
 > passage) or less than a second (if we count o nly the time of
 > the impact itself). 

Note, I do not say the impact was not significant. I suspect it made
the difference between a "minor" mass extinction (like the Turonian
extinctions) and a the major blowout that actually happened.

However, I also suspect that such an impact occuring in the *absence*
of prior stress would not even cause a minor mass extinction. (A
rather large crater dated to a point in the Cenozoic without any
detectable mass extinction tends to support this conclusion).


In short - I believe all major mass extinctions are *multicausal*.
One effect, short of a planet buster, cannot by itself cause that
much of a crisis.

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

The peace of God be with you.