PRECIPITATION OF DISSOLVED CARBONATE SPECIES FROM NATURAL-WATERS FOR DELTA-C-13 ANALYSIS - A CRITICAL-APPRAISAL

Methods involving the chemical precipitation of carbonate from natural waters by the addition of either SrCl2NH4OH or BaCl2-NaOH solutions are frequently used for δ13C analysis. Precipitates are generally filtered and dried in air. It is shown that certain stages of the overall preparation procedure are subject to change the δ13C of the precipitate due to contamination and isotope exchange with atmospheric CO2 and associated isotope fractionation effects. The stages normally most affected are filtration and drying of the precipitate in air, but contaminated alkaline-reagent solutions may also introduce error unless precautions are taken. In ideal circumstances, filtering and drying precipitates in inert media yields carbonate with °13C-values accurate to the original solution. © 1990.