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Re: astroblemes and extinctions



>>>       - No vertebrates larger than a breadbox proliferated from the
>>>         late Cretaceous through to the Tertiary. In other words, 
>>>         "the meek inherited the earth". 
>>Not entirely true, probably for the reasons you describe below.  Among the
>>"larger than a breadbox" sized animals which persisted were several genera
>>of crocodillians, river turtles, and champsosaurs (crocodile-like diapsids
>>of uncertain affinity).
>I stand corrected. I should have said "few" vertebrates larger than
>a breadbox...
>Any reactions to the point I was trying to make, i.e. that big animals 
>have a tough time surviving large impacts? I think that has important
>implications for models as to what actually happens when a big one hits.

Actually, this is probably true, but doesn't require an impact.  Large
animals have, of course, larger requirements for food, water, and space,
whereas smaller animals can get by with less.  So, the pattern of large
animal extinction is wholly consistant with an impact model, but does not
require it.

Another interesting example are the Quaternary extinctions.  Almost every
single invertebrate, small vertebrate, and plant which existed before the
Ice Ages are still around, while many large mammals, birds, and reptiles
have gone extinct.  Of course, the main culprit in this case is probably
humans, but environmental changes are suspected for some cases.

>
>>Actually, there has been no supported evidence of an astrobleme at the
>>Permian/Triassic extinction, which is by far the most severe in the
>>post-Cambrian world.
>
>Are you familiar with Rampino's P/T impact hypothesis? It does not have
>anywhere near the supporting evidence that the K/T impact does, but 
>when you add the Chinese P/T stratigraphic evidence, it becomes a pretty 
>strong case for a massive impact(actually 2 huge impacts) at that boundary.
>If you haven't seen the data, there are two boundary clays very reminiscent
>of the Gubbio, Italy K/T section, _with_ large Ir/siderophile spikes.
>

The problem is, there is problems both with the stratigraphy and the lack
(as of the present) of finding Ir spikes elsewhere (Russia, South Africa,
etc.).  So, the evidence for a Permian/Tr (Tr is the official symbol for the
Triassic) impact is far more tenuous at present.

>>It is important to note that the Deccan Traps begin millions of years prior
>>to any iridium spike, so anyone who wants them to be generated by the K/T
>>impact has to believe in time-travel as well...
> 
>I assume you refer to another Rampino hypothesis concerning antipodal
>volcanism. I don't know what to make of the K/T impact-Deccan traps
>correlation or the P/T impact-Siberian traps correlation, but there
>is evidence on planet Mercury for astroblemes having antipodal crustal
>effects. Perhaps a very large impact can have antipodal or other associated
>volcanic effects if the underlying mantle/crust are favorable towards it. 
>For example, a medium flood basalt province(e.g. Columbia River) becomes 
>a Siberian trap level event, or a mantle plume volcano such as Hawaii
>gets elevated to a Deccan trap level eruption sequence.

However, all the evidence points to no major change in size or intensity
before, during, or after the impact.  The null hypothesis is that there was
no significant effect.  I agree that antipodal volcanism is strong for
Mercury, and in theory should apply to VERY big impacts on the Earth, but
the evidence is not present as of now.

>It is important to
>note that the Deccan traps span the K/T interval, the lava poured out
>some before, some during and some after the Iridium spike. It might be
>erroneous to insist on volcanic activity as solely caused by the impact,
>or conversely that the impact had no effect on the volcanism.

Actually, it is erroneous to "insist" that there was no effect, but it is
entirely correct to say that there is no evidence as of now that the impact
had no effect.  It's not an unreasonable idea, but some one has to go out
and find the proper data to support the idea.

Thomas R. HOLTZ
Vertebrate Paleontologist, Dept. of Geology
Email:Thomas_R_HOLTZ@umail.umd.edu (th81)
Phone:301-405-4084