Hi,

> I'm an archeologist trying to identify/interpret the nature 
> and formation 
> of an unusual indurated carbonate layer 

Regarding the different structures of various types of calcretes you may refer to:

Freytet, P., and Plaziat, J.-C., 1982, Continental carbonate sedimentation and pedogenesis-Late Cretaceous and early Tertiary of southern France, in Purser, B.H., ed., Contributions to Sedimentology 12: Stuttgart, E. Schweizerbart'che Verlags-buchhandlung (Nägele u. Obermiller), 213 p.

Retallack, G.J. 1997. A colour guide to paleosols. John Wiley and Sons, Chichester, 175 p. (ISBN 0-471-96711-4).

A good place to start for stable isotopes may be:
Ekart, D.D., Cerling, T.E., Montañez, I.P. & Tabor, N.J. (1999): A 400 million year carbon isotope record of pedogenic carbonate: implications for paleoatmospheric carbon dioxide. - American Journal of Science, 299, 805-827.

regards
Robert

-- 
Dr. Robert van Geldern
Leibniz Institute for Applied Geosciences (GGA)
S3: Geochronology and Isotopehydrology
Stilleweg 2, 30655 Hannover, Germany
phone: +49-(0)511-643-2313 (office), -2539 (lab)
fax: +49-(0)511-643-3665
mailto:[log in to unmask]

> -----Original Message-----
> From: Stable Isotope Geochemistry 
> [mailto:[log in to unmask]] On Behalf Of Margo Schwadron
> Sent: Monday, February 27, 2006 10:37 PM
> To: [log in to unmask]
> Subject: [ISOGEOCHEM] 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
>