C13: C12 Ratios...

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

Hey,
Bear with me, as this may be a bit off topic, but it is part of a much larger 
picture.  

Before the terrorist attacks of two weeks ago I was beginning an on-list 
conversation on a possible Upper Pennsylvanian extinction event in Illinois.  I 
had a question of whether there was evidence for this event at other sites 
around the world, or if it was localized.  

As far as I can now tell, the event probably was localized.  However, that is 
assuming that there WAS an extinction event.  The rocks certainly preserve what 
appears to be a mass-death layer of brachiopods.  But, this could simply be an 
artifact of preservation.  

In order to test whether or not this mass-death event may be real, I would like 
to employ a little C13: C12 ratio test.  Basically speaking, as far as I can 
tell from my research, the ratio of C13 to C12 (sorry about the scripts) in 
ideal carbon dioxide emitted from volcanoes is about 1:99 (of course, this can 
vary, but it is the ideal ratio).  Correct me if I'm wrong, please.  

However, this ratio changes as this carbon dioxide is used by photosynthesizing 
plants and algae.  Organisms that photosynthesize prefer C12.  Therefore, in 
carbonate rock samples that were formed in an area of high life diversity, the 
ratio of C13: C12 is higher.  

I have two basic questions:

1) Would testing this ratio in a few samples of Upper Pennsylvanian limestone 
be a good indicator of diversity (low ratio=less life, high ratio=more life)?

2) What would be the ratio of C13: C12 in an ideal sample of Paleozoic 
limestone?  By this, I mean a limestone that was formed when there was a 
relatively high diversity of life.

Thanks for bearing with this.  Have a nice weekend!

Steve

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Steve Brusatte-DINO LAND PALEONTOLOGY
SITE: http://www.geocities.com/stegob
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