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High-precision dual-inlet IRMS measurements of the stable isotopes of CO₂ and the N₂O/CO₂ ratio from polar ice core samples Public Deposited

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  • High-precision dual-inlet IRMS measurements of the stable isotopes of CO2 and the N2O/CO2 ratio from polar ice core samples
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  • An important constraint on mechanisms of past carbon cycle variability is provided by the stable isotopic composition of carbon in atmospheric carbon dioxide (δ¹³C-CO₂) 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 δ¹³C of CO₂ at very high precision, as well as measuring the CO₂ and N₂O 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 CO₂ and N₂O 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 δ¹³C-CO₂ and 2 ppm and 4 ppb for the CO₂ and N₂O 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 N₂O history of the atmosphere. The oxygen isotopic composition of CO₂ is also investigated, confirming previous observations of oxygen exchange between gaseous CO₂ and solid H₂O within the ice archive. These data offer a possible constraint on oxygen isotopic fractionation during H₂O and CO₂ exchange below the H₂O bulk melting temperature.
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  • Bauska, T. K., Brook, E. J., Mix, A. C., & Ross, A. (2014). High-precision dual-inlet IRMS measurements of the stable isotopes of CO₂ and the N₂O/CO₂ ratio from polar ice core samples. Atmospheric Measurement Techniques, 7(11), 3825-3837. doi:10.5194/amt-7-3825-2014
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  • This work was supported by NSFgrant 0839078 (E. J. Brook and A. C. Mix). Oregon StateUniversity provided additional support for mass spectrometerpurchase and management of the OSU/CEOAS stable isotopelaboratory. We thank the WAIS Divide Science Coordination Officefor the collection and distribution of the WAIS Divide ice core(Kendrick Taylor (Desert Research Institute of Reno Nevada), NSFgrants 0230396, 0440817, and 0944348; and 0944266 – Universityof New Hampshire) and the Taylor Glacier 2010-2011 Field Team(Daniel Baggenstos, James Lee, Hinrich Schaefer, Tanner Kuhl,Robb Kulin, Jeff Severinghaus, Vas Petrenko, and Paul Rose) forcollection of Taylor Glacier samples with support through NSFgrant 0838936 (E. J. Brook).
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