A new set-up for simultaneous high-precision measurements of CO₂, δ¹³C-CO₂ and δ¹⁸O-CO₂ on small ice core samples

Abstract. Palaeoatmospheric records of carbon dioxide and its stable carbon isotope composition (δ¹³C) obtained from polar ice cores provide important constraints on the natural variability of the carbon cycle. However, the measurements are both analytically challenging and time-consuming; thus only data exist from a limited number of sampling sites and time periods. Additional analytical resources with high analytical precision and throughput are thus desirable to extend the existing datasets. Moreover, consistent measurements derived by independent laboratories and a variety of analytical systems help to further increase confidence in the global CO₂ palaeo-reconstructions. Here, we describe our new set-up for simultaneous measurements of atmospheric CO₂ mixing ratios and atmospheric δ¹³C and δ¹⁸O-CO₂ in air extracted from ice core samples. The centrepiece of the system is a newly designed needle cracker for the mechanical release of air entrapped in ice core samples of 8–13 g operated at −45 °C. The small sample size allows for high resolution and replicate sampling schemes. In our method, CO₂ is cryogenically and chromatographically separated from the bulk air and its isotopic composition subsequently determined by continuous flow isotope ratio mass spectrometry (IRMS). In combination with thermal conductivity measurement of the bulk air, the CO₂ mixing ratio is calculated. The analytical precision determined from standard air sample measurements over ice is ±1.9 ppm for CO₂ and ±0.09 ‰ for δ¹³C. In a laboratory intercomparison study with CSIRO (Aspendale, Australia), good agreement between CO₂ and δ¹³C results is found for Law Dome ice core samples. Replicate analysis of these samples resulted in a pooled standard deviation of 2.0 ppm for CO₂ and 0.11 ‰ for δ¹³C. These numbers are good, though they are rather conservative estimates of the overall analytical precision achieved for single ice sample measurements. Facilitated by the small sample requirement, replicate measurements are feasible, allowing the method precision to be improved potentially. Further, new analytical approaches are introduced for the accurate correction of the procedural blank and for a consistent detection of measurement outliers, which is based on δ¹⁸O-CO₂ and the exchange of oxygen between CO₂ and the surrounding ice (H₂O).