A simple but improved protocol for measuring carbon and oxygen isotope ratios of calcite in calcite-dolomite mixtures

The extraction of stable carbon and oxygen isotope ratios (delta C-13 and delta O-18) of primarily precipitated calcite from partially dolomitized limestone is of prime interest because the delta C-13 and delta O-18 values of primary calcite are widely used in paleoclimate research. The differential acid extraction method (DAE) has been widely used to separate CO2 evolved during the calcite-orthophosphoric acid reaction from CO2 evolved from the dolomite- orthophosphoric acid reaction. However, the various laborious offline methods with inevitable uncertainties make DAE very challenging. We proposed a fully automatic-online DAE method in the present study, using an autosampler-GasBench II-Isotope Ratio Mass Spectrometer (IRMS). The results show that 6-values of calcite can be obtained from the mixture (grain size between 180 and 250 mu m) having >50% calcite after similar to 15-100 min of reaction with orthophosphoric acid at 30 degrees C. Further, both the precision and the accuracy of the measurement using the autosampler-GasBench II-IRMS can be improved by attaching micro-vibrators to the sample tray.