EXPERIMENTAL-DETERMINATION OF CARBON-ISOTOPE EQUILIBRIUM FRACTIONATION BETWEEN DISSOLVED CARBONATE AND CARBON-DIOXIDE

By mixing aliquots of pure Na2CO3 and CO2 solutions with precisely kno wn molal fractions of CO3=, HCO3-, and dissolved CO2 were prepared. Th e apparent fractionation factor between gaseous CO2 and total carbon i n the solution was determined mass spectrometrically from which the CO 3= - CO2 fractionation factor was calculated taking into account the k nown fraction of HCO3- and respective isotope fractionation. Although the measurements have been made for a rather narrow temperature range, from 4 to 80 degrees C, a theoretical curve was fitted through the ex perimental points, and thereby, the isotopic partition function ratio of (CO3=)-C-13 and (CO3=)-C-12 molecules has been obtained for an exte nded temperature range. The results obtained for CO3=-CO2 isotope frac tionation are significantly lower than those for HCO3--CO2 exchange (w ith 10(3)(alpha - 1) = 5.0 +/- 0.2 at 25 degrees C and cross-over poin t at about 63 degrees C). Copyright (C) 1997 Elsevier Science Ltd.