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An important constraint on mechanisms of past carbon cycle variability isprovided by the stable isotopic composition of carbon in atmospheric carbondioxide (δ13C-CO2) trapped in polar ice cores, butobtaining very precise measurements has proven to be a significantanalytical challenge. Here we describe a new technique to determine theδ13C of CO2 at very high precision, as well as measuringthe CO2 and N2O mixing ratios. In this method, ancient air isextracted from relatively large ice samples (~400 g) witha dry-extraction "ice grater" device. The liberated air is cryogenicallypurified to a CO2 and N2O mixture and analyzed with a microvolume-equipped dual-inlet IRMS (Thermo MAT 253). The reproducibility of themethod, based on replicate analysis of ice core samples, is0.02‰ for δ13C-CO2 and 2 ppm and 4 ppbfor the CO2 and N2O mixing ratios, respectively (1σ pooledstandard deviation). Our experiments show that minimizing water vaporpressure in the extraction vessel by housing the grating apparatus in aultralow-temperature freezer (−60 °C) improves the precision anddecreases the experimental blank of the method to −0.07 ± 0.04‰. We describe techniques for accurate calibrationof small samples and the application of a mass-spectrometric method based onsource fragmentation for reconstructing the N2O history of theatmosphere. The oxygen isotopic composition of CO2 is alsoinvestigated, confirming previous observations of oxygen exchange betweengaseous CO2 and solid H2O within the ice archive. These dataoffer a possible constraint on oxygen isotopic fractionation during H2Oand CO2 exchange below the H2O bulk melting temperature

atmospheric measurement techniques

High-precision dual-inlet IRMS measurements of the stable isotopes of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and the N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O / CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; ratio from polar ice core samples