Richard M. Larson wrote the following about PDB conversion to SMOW:

> 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?

Yes, it's always safe to use this equation on d18O-PDB data to convert
to d18O-SMOW.  SMOW is 30.86 (or 30.91 according to some sources.
Don't ask me which :þ) per mill lighter than PDB.  It has nothing to
do with the source of the number, in other words.  As long as you know
the PDB delta, you can calculate the SMOW delta.

I'd be interested to know if 30.86 is more 'valid' than 30.91.  Any
takers?

Totty  <8^)