Abstract
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.
Highlights
CO2 concentration measurements on polar ice cores provide direct atmospheric information of past carbon dioxide concentrations over up to the last 800 000 years (Fischer et al, 1999; Petit et al, 1999; Monnin et al, 2001; Ahn and Brook, 2008; Luthi et al, 2008)
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 (GCIRMS)
We have presented an analysis system capable of high precision measurements of δ13C on CO2 on ice core samples
Summary
CO2 concentration measurements on polar ice cores provide direct atmospheric information of past carbon dioxide concentrations over up to the last 800 000 years (Fischer et al, 1999; Petit et al, 1999; Monnin et al, 2001; Ahn and Brook, 2008; Luthi et al, 2008). Extraction efficiency becomes important for ice from the bubble/clathrate transition zone (BCTZ), where fractionation between different gas species during the bubble clathrate phase transition has been observed (Ikeda et al, 1999; Luthi et al, 2010) It becomes crucial for high precision measurements of δ13C from partly or fully clathrated ice. Here, dry extraction methods are only able to extract the CO2 from opened bubbles and decomposing clathrates, where fractionation processes may come into play during the clathrate relaxation process. This allows to measure many extracted gas samples within a short time span and, to take advantage of identical measurement conditions in the IRMS for a large set of samples This is crucial as changes in the performance of the IRMS measurement, like source tuning, variations in the H2O background are a common problem. We compare our data with previous results from other methods to evaluate the absolute accuracy of the measurements
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