A sublimation technique for high-precision measurements of δ¹³CO₂ and mixing ratios of CO₂ and N₂O from air trapped in ice cores

Abstract. In order to provide high precision stable carbon isotope ratios (δ¹³CO₂ or δ¹³C of CO₂) from small bubbly, partially and fully clathrated ice core samples we developed a new method based on sublimation coupled to gas chromatography-isotope ratio mass spectrometry (GC-IRMS). In a first step the trapped air is quantitatively released from ~30 g of ice and CO₂ together with N₂O are separated from the bulk air components and stored in a miniature glass tube. In an off-line step, the extracted sample is introduced into a helium carrier flow using a minimised tube cracker device. Prior to measurement, N₂O and organic sample contaminants are gas chromatographically separated from CO₂. Pulses of a CO₂/N₂O mixture are admitted to the tube cracker and follow the path of the sample through the system. This allows an identical treatment and comparison of sample and standard peaks. The ability of the method to reproduce δ¹³C from bubble and clathrate ice is verified on different ice cores. We achieve reproducibilities for bubble ice between 0.05 ‰ and 0.07 ‰ and for clathrate ice between 0.05 ‰ and 0.09 ‰ (dependent on the ice core used). A comparison of our data with measurements on bubble ice from the same ice core but using a mechanical extraction device shows no significant systematic offset. In addition to δ¹³C, the CO₂ and N₂O mixing ratios can be volumetrically derived with a precision of 2 ppmv and 8 ppbv, respectively.