Margo,

A good place to start is with the work of Thure Cerling at the U of
Utah. His papers provide insight into how one might distinguish between
C3, C4 and atmospheric CO2 input to pedogenic isotope carbonate
chemistry.

Dave Cole
Oak Ridge National lab

-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On
Behalf Of Margo Schwadron
Sent: Monday, February 27, 2006 4:37 PM
To: [log in to unmask]
Subject: Help Interpret Carbonate/Calcrete Isotope Values

Hi all,

I'm an archeologist trying to identify/interpret the nature and
formation 
of an unusual indurated carbonate layer (Calcrete? Caliche? Duricrust? 
Marl?) found between two distinct prehistoric Indian occupations on 
several freshwater tree island in Everglades National Park, Florida.
This 
was found at many sites, and there is a temporal correlation, that based

on c14 dates, suggests the layer formed sometime between 3800 BP and
2700 
BP. There seems to be potential for data suggesting paleoenvironmental 
change, and abandonment of the site during a possible wetter
hydrological 
regime.

Nevertheless, I'm having extreme difficulty in trying to interpret this 
layer. I do not have the background in geochemistry, and most geologists
I 
consult have never seen anything like it and don't know what to call it,

or how it formed. I had isotopic analysis done for inorganic carbon and 
oxygen:

   d13C-cal.   d18O-cal.
MS-EU-1   -1.6   -3.4
MS-EU-2   -1.7   -1.2
MS-EU-3   -5.9   -2.1
MS-EU-4   -0.9   -1.0

The geologist who ran them said that "the data show that they were not 
formed in a soil dominated by trees. If they were pedogenic carbonates, 
the soil would have supported more grasses than trees during the period 
when these carbonates were formed. It's also possible they are
groundwater 
carbonate or pond/lake carbonate.  We could do some calculation to 
estimate the temperatures of their formation. "

I was hoping to find a "database" of known isotopic values that would
help 
us interpret this data. Does anyone on this listserve have any
suggestions 
for what these isoptopic values indicate? Any additional research
avenues 
I should take, and any references I should look at? Any help is GREATLY 
appreciated!

Sincererly,

Margo Schwadron, M.A., RPA
Archeologist, National Park Service