A simple but improved protocol for measuring carbon and oxygen isotope ratios of calcite in calcite-dolomite mixtures. (March 2023)

• Record Type: Journal Article
• Authors: Sreenivasan, Sruthi P.; Bera, Melinda Kumar; Samanta, Arpita
• Is Part Of: Applied geochemistry. Volume 150(2023)
• Publication Date: 2023-03
• Subjects: Differential acid extraction -- Calcite-dolomite mixture -- Rapid online measurement -- GasBench II-Isotope ratio mass spectrometer; Environmental geochemistry -- Periodicals; Water chemistry -- Periodicals; Geochemistry -- Social aspects -- Periodicals; Geochemistry -- Periodicals; 551.9
• DOI: 10.1016/j.apgeochem.2023.105600
• Languages: English
• ISSNs: 0883-2927

Abstract

Abstract: The extraction of stable carbon and oxygen isotope ratios ( δ 13 C and δ 18 O) of primarily precipitated calcite from partially dolomitized limestone is of prime interest because the δ 13 C and δ 18 O 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. Highlights: We present a differential acid extraction method for measuring calcite stable isotopic ratio from dolomitized limestone. Proposed online method is fully automatic but gives comparable isotopic composition with the other laborious offline methods. The micro-vibrator attachment improves the precision and accuracy and can be adapted for online gas preparation devices.