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A new set-up for simultaneous high-precision measurements of CO2, delta C-13-CO2 and delta O-18-CO2 on small ice core samples

A new set-up for simultaneous high-precision measurements of CO2, delta C-13-CO2 and delta O-18-CO2 on small ice core samples Thumbnail


Abstract

Palaeoatmospheric records of carbon dioxide and its stable carbon isotope composition (d13C) 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 d13C and d18O-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 8–13g 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 ±1.9ppm for CO2 and ±0.09‰ for d13C. In a laboratory intercomparison study with CSIRO (Aspendale, Australia), good agreement between CO2 and d13C results is found for Law Dome ice core samples. Replicate analysis of these samples resulted in a pooled standard deviation of 2.0ppm for CO2 and 0.11‰ for d13C. 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 d18O-CO2 and the exchange of oxygen between CO2 and the surrounding ice (H2O).

Citation

(2016). A new set-up for simultaneous high-precision measurements of CO2, delta C-13-CO2 and delta O-18-CO2 on small ice core samples. Atmospheric Measurement Techniques, 3687 - 3706. https://doi.org/10.5194/amt-9-3687-2016

Acceptance Date Jun 21, 2016
Publication Date Aug 10, 2016
Journal ATMOSPHERIC MEASUREMENT TECHNIQUES
Print ISSN 1867-1381
Publisher European Geosciences Union (EGU)
Pages 3687 - 3706
DOI https://doi.org/10.5194/amt-9-3687-2016
Publisher URL https://www.atmos-meas-tech.net/9/3687/2016/

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