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
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.