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

摘要

An important constraint on mechanisms of past carbon cycle variability isprovided by the stable isotopic composition of carbon in atmospheric carbondioxide (δC-CO) trapped in polar ice cores, butobtaining very precise measurements has proven to be a significantanalytical challenge. Here we describe a new technique to determine theδC of CO at very high precision, as well as measuringthe CO and NO 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 CO and NO 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 δC-CO and 2 ppm and 4 ppbfor the CO and NO 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 NO history of theatmosphere. The oxygen isotopic composition of CO is alsoinvestigated, confirming previous observations of oxygen exchange betweengaseous CO and solid HO within the ice archive. These dataoffer a possible constraint on oxygen isotopic fractionation during HOand CO exchange below the HO bulk melting temperature.