[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
At 18:48 2000-03-24 -0800, Stanley Friesen wrote:
>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
>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
There is one going on in Iceland right now (latest flood basalt eruption
there was Laki in 1783). Hey! Maybe we can blame the ongoing mass
extinction on that!
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. By the way which extinction
episodes do You associate with e. g. the Paraná, Columbia plateau and
Ethiopian flood basalts?
>>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?
Yes, though it is possible that climatic stress may have rendered some
animal population more vulnerable to human predation. 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.
>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.
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.
>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
>May the peace of God be with you. email@example.com