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

Abstract

The extraction of stable carbon and oxygen isotope ratios (δ13C and δ18O) of primarily precipitated calcite from partially dolomitized limestone is of prime interest because the δ13C and δ18O 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 δ-values of calcite can be obtained from the mixture (grain size between 180 and 250 μm) having >50% calcite after ∼15–100 min of reaction with orthophosphoric acid at 30 °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.