Dear Mark,

        The VSMOW to VPDB conversion equation you list was derived using an
acid fractionation factor for calcite of 1.01025 (Sharma & Clayton, 1965)
and a CO2-H20 fractionation factor of 1.04115.  The Advisory Group on
Stable Isotope Reference Samples for Geochemical and Hydrological
Investigations recommends using the same acid fractionation factor but also
the newer CO2-H20 fractionation factor of 1.04120 (Gonfiantini, 1984).
Using these values and the VSMOW CO2-VPDB CO2 fractionation factor of
1.00027 (Hut, 1987) yields the conversion equation:  d18OVSMOW =
1.03091*d18OVPDB + 30.91 (see equation 11 and discussion of Coplen, 1988,
Normalization of oxygen and hydrogen isotope data, Chemical Geology, 72,
pp. 293-297).

        As an aside, Kim and O'Neil (1997, Geochimica et Cosmochimica Acta)
recently determined a new acid fractionation factor for calcite of
1.01050.  Perhaps Tyler Coplen and others could comment on possibility of
favoring this value in the future?

Cheers,

Bryan


>I submitted CO2 samples for stable isotopic analyses that were produced
>by reacting the CaCO3 in biohydroxyapatite with H3PO4 at 90C.  Results
>for d13C and d18O values were returned relative to PDB.  If I wanted to
>convert the d18O values to SMOW, would it be safe to use the equation
>provided by Friedman and O'Neil (1977, pg KK4), where
>dSMOW=1.03086*dPDB+30.86?  This equation is based on CO2 liberated in
>H3PO4 from calcite at 25C.  Dr. Yang Wang provided a fractionation
>factor for CO2 liberated from CaCO3 with H3PO4 at 25C of 1.01025, which
>is the same as for calcite under similiar conditions.  Is there a more
>applicable conversion equation?  Thanks in advance for any insights...
>-Mark
>
>
>
>************************************************************************
>*****
>R. Mark Larson
>Department of Rangeland Ecology and Watershed Management
>University of Wyoming
>Laramie, WY 82071-3354
>
>Phone: (307) 766-5470
>Fax: (307) 766-6403
>Email: [log in to unmask]
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______________________________________
Bryan Bemis
Department of Geology
University of California
Davis, CA 95616

Lab (530) 752-3311
Fax (530) 752-0951
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