High-precision dual-inlet IRMS measurements of the stable isotopes of CO 2 and the N 2 O / CO 2 ratio from polar ice core samples

• Published in: Atmospheric Measurement Techniques
• Main Authors: T. K. Bauska, E. J. Brook, A. C. Mix, A. Ross
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2014
• Subjects: Environmental engineering; TA170-171; Earthwork. Foundations; TA715-787; ice core
• Online Access: https://doi.org/10.5194/amt-7-3825-2014; https://doaj.org/article/4eadbf24b5e24c59b1f4067457212d0f

An important constraint on mechanisms of past carbon cycle variability is provided by the stable isotopic composition of carbon in atmospheric carbon dioxide (δ 13 C-CO 2 ) trapped in polar ice cores, but obtaining very precise measurements has proven to be a significant analytical challenge. Here we describe a new technique to determine the δ 13 C of CO 2 at very high precision, as well as measuring the CO 2 and N 2 O mixing ratios. In this method, ancient air is extracted from relatively large ice samples (~400 g) with a dry-extraction "ice grater" device. The liberated air is cryogenically purified to a CO 2 and N 2 O mixture and analyzed with a microvolume-equipped dual-inlet IRMS (Thermo MAT 253). The reproducibility of the method, based on replicate analysis of ice core samples, is 0.02‰ for δ 13 C-CO 2 and 2 ppm and 4 ppb for the CO 2 and N 2 O mixing ratios, respectively (1σ pooled standard deviation). Our experiments show that minimizing water vapor pressure in the extraction vessel by housing the grating apparatus in a ultralow-temperature freezer (−60 °C) improves the precision and decreases the experimental blank of the method to −0.07 ± 0.04‰. We describe techniques for accurate calibration of small samples and the application of a mass-spectrometric method based on source fragmentation for reconstructing the N 2 O history of the atmosphere. The oxygen isotopic composition of CO 2 is also investigated, confirming previous observations of oxygen exchange between gaseous CO 2 and solid H 2 O within the ice archive. These data offer a possible constraint on oxygen isotopic fractionation during H 2 O and CO 2 exchange below the H 2 O bulk melting temperature.