A new setup for simultaneous high precision measurements of COsub2/sub, δsup13/supC-COsub2/sub and δsup18/supO-COsub2/sub on small ice core samples

摘要

Paleo–atmospheric 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 data exist only from a limited number of sampling sites and time periods. Additional analytical resources with high analytical precision and throughput are thus desirable to extend and confirm the existing datasets. Also, consistent measurements derived by independent laboratories and a variety of analytical systems helps to further increase confidence in the global CO₂ paleo reconstructions. Here, we describe our new setup for simultaneous measurements of atmospheric CO₂ mixing ratios, atmospheric δ13C and δ18O-CO₂ in air extracted from ice core samples. The core 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 the 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, although rather conservative estimates of the overall analytical uncertainty for a single measurement. Facilitated by the small sample requirement, replicate measurements are feasible and the achievable method precision accordingly higher. Further in this study, new analytical approaches are introduced for the accurate correction of the procedural blank and for the reliable and consistent detection of measurement outliers which is based on δ18O-CO₂ and the exchange of oxygen between CO₂ and H₂O of surrounding ice.