Re: Pennsylvanian Extinction Query

["Steve Brusatte" <dinoland@lycos.com>]

On Sun, 09 Sep 2001 05:13:02  
 Ken Kinman wrote:
>
>
>Steve,
>     As for any local big extinction events in Illinois at the end of the 
>Carboniferous, I would guess it most likely was caused by deltaic flooding, 
>with silt smothering the brachiopods (and other groups) in the offshore 
>area.  I suppose this could have be intense during this particular time due 
>to a combination of glaciations and volcanic activity causing rapid melting.
>     As for worldwide extinctions of various taxa, it wasn't much compared 
>to the mass extinctions that would follow later.  But if there was a fairly 
>rapid decrease in oxygen levels, some of the larger brachiopods could have 
>been hit hard by that.
>      In any case, I suspect that any mini-extinction at that time would 
>have been caused by mainly geological and environmental stresses, not by any 
>kind of major impact event.  Well past my bedtime, but if I think of 
>anything pertinent tomorrow, I'll post more information.
>                     ------Ken

Ken et al.,
I agree that if there was indeed a period of mass death, then the event was 
likely something very localized.  Let's face it, during this time (Upper 
Pennsylvanian), Illinois was situated near the Equator and was home to a 
friggin' huge delta.  Of course, this delta is better understood by examining 
earlier Pennsylvanian (i.e. Mazon Creek and Francis Creek Shale) sediments, but 
it was still present during the Upper Pennsylvanian.  

This delta was formed by a very large river as it emptied into a nice 
continental sea.  I really don't know exactly how far away the river was from 
my site, but I would guess that it was less than 40 miles away.  Judging by the 
fossils and rocks preserved at the main site I am working on, the shore of this 
large continental sea was, at the most, 10 miles away when the mass death layer 
is preserved.  

With such a big river emptying into such a large sea there are several 
localized events that can explain such an extinction.  For some reason the 
rivers may have poured in a plethora of sediment, rapidly covering the 
brachiopods and other fossils.  This could have been a result of either intense 
volcanism (possibly from as far away as New York State, since the river system 
likely extended that far!) or a large flood.  Or, possibly, the river altered 
the chemistry of the shelf of this sea, causing low oxygen conditions.  This 
may be (I stress: may be) supported by a pyrite layer found at the site.  This 
layer is very small, and may have formed well after the sediment was deposited, 
though.  

I am fully aware that any number of localized events relating to the large 
river may have caused a one-time burying of the brachiopods and other fossils, 
making it appear as if there was a large extinction.  But, the reason that I 
brought my dilemma to the attention of the list is that it may possibly fit 
into a bigger picture: the Raup-Sepkoski extinction cycles.  

As I wrote last night, their data only cover the past 250 million years.  
However, if their 26-million year cycles are indeed correct, then some sort of 
extinction is predicted about the same time that my site was deposited.  So 
far, though, other than a nice suggestion by Steven Coombs, I have been unable 
to find any bona fide proof in the literature.  

Suggestions still welcome!!  

Steve

---
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