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Re: Extinction

At 08:40 PM 3/25/00 +0100, Tommy Tyrberg wrote:

There is one going on in Iceland right now (latest flood basalt eruption
there was Laki in 1783).

What is happening in Iceland is small potatoes compared to the Deccan eruptions. That emplaced an average of over 4 cubic km of lava *per* *year*. At that rate, few villages in Iceland would remain above the lava flows!

 Hey! Maybe we can blame the ongoing mass
extinction on that!

Maybe it has contributed. Humans, oceanic regression, ice ages, and increased volcanic activity is a fairly significant combination.

Jokes aside the Laki eruption certainly demonstrated that flood basalt
eruptions have very nasty effects over large areas, and I am inclined to
blame the Siberian Traps for the Permo/Triassic extinctions myself. However
that was as exceptional in size as the Chicxulub crater (each the largest
of its kind during the Phanerozoic). I'm much more doubtful about any
word-wide effect of the Deccan eruptions since the intertrappan beds show
that dinosaurs survived even on the (isolated) indian continent for about a
million year after the eruptions started.

The Deccans were the second or third largest set. That puts them securely in the "exceptional" range in *my* book.

Again, in a *multi*causal model it is not necessary or expected that every cause have an *immediate* major effect. The presence of dinosaurs in the middle of the Traps is *not* counter-evidence to their contribution, only to their *sufficiency* by themselves. But I never suggested they were the *sole* cause of the extinctions. (Also, I am not sure it was 1 million years after the start, since the main eruptions may have only *lasted* one million years).

 By the way which extinction
episodes do You associate with e. g. the Paraná, Columbia plateau and
Ethiopian flood basalts?

Actually, I should not have said "every", as my main source only covers the Mesozoic. However, I seem to remember there was a minor mass extinction mid-Tertiary, about the right time for the Columbia basalts, though I lack exact dates on either.

>???The current ongoing mass extinction doesn't count?
>
>Or do you simply attribute the whole of the Late Pliestocene/Holocene
>extinctions to human action?
>
Yes, though it is possible that climatic stress may have rendered some
animal population more vulnerable to human predation.

Actually, I suspect the combination was a major factor. Also some of the Late Pleistocene extinction in North America appear to have *preceded* human arrival here, at least based on the dating of finds in the La Brea Pits here in LA. (Even though the peak extinctions occurred shortly after our arrival). Certainly both the giant sequoia and the giant condor were well on the way out even before we arrived. Indeed we may actually have *delayed* the mainly natural extinction of the California Condor (I doubt it would have survived to the next glaciation even without human presence).

 The difficulty with
the climatic explanation though is that the extinctions were diachronic and
very well correlated with the spread of Homo sapiens. Also there is nothing
in this last glacial cycle to make it any different from the previous ones
which caused no megafaunal extinctions.

This fits equally with a multicausal model in which humans are the *final* cause, but not the sufficient cause.

Actually it was much lower just 10,000 years ago (and rising very fast).
About the only extinctions I can think about that can definitely be
assigned to Pleistocene sea-level changes are some extinctions on Aldabra
atoll which was completely drowned during the previous (Eemian) interglacial.

However, I suspect you will find a higher than normal turnover in shallow-water species over the last 2 million years compared to the previous 5 million.

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