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Re: Extinction

At 10:44 PM 3/24/00 +0100, Tommy Tyrberg wrote: 
>
This is just quibbling. An impact that is large enough form a 150 km crater
and to emplace a centimeter-thick deposit world-wide


I have not heard that the ejecta deposits were that thick *everywhere*!

Indeed, I can think of places where there is no detectable change in deposition across the boundary.

>
>But these "alternatives" have solid evidenciary basis. Volcanism,
>sea-level regression, mountain building, climate change, novel
>speciation--these things really did happen!!
>
Yes, they happen all the time, and usually with no drastic effect on the
biosphere.

Flood basalt volcanism does NOT occur "all the time". In fact every single well-dated flood basalt episode is correlated with a detectable peak in extinction rate!

There has been about 8 regressions of approximately the same
order of magnitude as the Maastrichtian (but probably a great deal faster)
just in the last 800,000 years with no discernable effect on either marine
or land biota.


???The current ongoing mass extinction doesn't count?

Or do you simply attribute the whole of the Late Pliestocene/Holocene extinctions to human action?

Just because the regression doesn't have an *immediate* effect of increasing extinction rates doesn't mean it cannot be contributory to elevated extinctions. For instance it is easily possible that the marine fauna is particularly sensitive to human activity now because the ocean level is so very low.

Alternatively, it may be that *short* regressions do not have the sustained effect necessary to drive significant numbers of species to extinction.

 Climate changes have also been extreme in the last several
million years, also without causing any large-scale extinctions.

Really? There are certainly many specialists who would argue with this! The cause of the Late Pliestocene extinctions is still hotly debated, and climate is one of the leading contenders (along with Homo sapiens)!

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May the peace of God be with you.         sarima@ix.netcom.com