Working on carbonates in pond waters having pH <7 I am aware that
calculation of the expected ('equilibrium') d13C of calcite, from measured
values of the d13C of TDIC, is particularly sensitive to pH.

The spreadsheet I have made which corrects for this pH effect
calculates the concentrations of the different carbon species (from
dissociation constants) assuming that the activity coefficients are unity.

Does anyone: 1) have a spreadsheet/programme method which
calculates the concentrations from 'proper' estimation of the activity
coefficients; and/or 2) know if assuming the coefficients are unity makes
much difference anyway (for fairly low salinity water)?

Tim Heaton

NERC Isotope Geosciences Laboratory
British Geological Survey
Keyworth, Nottingham NG12 5GG

Tel. +44(0)115 936 3401
Fax +44(0)115 936 3302
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