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

Bauska, T. K.; Brook, E. J.; Mix, A. C.; Ross, A.

An important constraint on mechanisms of past carbon cycle variability is provided by the stable isotopic composition of carbon in atmospheric carbon dioxide (δ 13C-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 13C of CO 2 at very high precision, as well as measuring the CO 2 and N 2O 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 2O 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 13C-CO 2 and 2 ppm and 4 ppb for the CO 2 and N 2O 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 2O 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 2O within the ice archive. These data offer a possible constraint on oxygen isotopic fractionation during H 2O and CO 2 exchange below the H 2O bulk melting temperature.

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Bauska, T. K. / Brook, E. J. / Mix, A. C. / et al: High-precision dual-inlet IRMS measurements of the stable isotopes of CO2 and the N2O / CO2 ratio from polar ice core samples. 2014. Copernicus Publications.

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