A sublimation technique for high-precision δ13C on CO2 and CO2 mixing ratio from air trapped in deep ice cores
Glacier ice from polar ice sheets represents the only direct archive to retrieve information about the composition of the paleoatmosphere. From deep Antarctic ice cores like Vostok it is known for more than a decade that the concentration of the greenhouse gas CO2 periodically varied between glacials and interglacials and its variations are strikingly correlated with temperature. During the glacials the CO2 concentration was about 90 ppmv lower than during the warm interglacials
With the Dome C ice core drilled within the European Project for Ice Coring in Antarctica (EPICA) the CO2 concentration record now covers the last 650,000 years. The task to quantitatively understand the processes behind these observed CO2 changes is of outstanding importance not only for the paleo climate community, but also to predict the CO2 concentration in the future. One crucial key to unravel the open questions about the coupling of the atmospheric CO2 concentration with the dynamics of the global carbon cycle is the stable carbon isotope ratio of CO2 (δ13C). Methodological constraints have, so far, restricted the δ13C analysis from ice cores to the uppermost core section. Moreover, the available δ13C data sets are difficult to interpret as accuracy and temporal resolution are insufficient and a large centimeter scale variability of δ13C was recently measured on the EPICA ice cores, which cannot be explained by atmospheric fluctuations.
Therefore, we developed a new sublimation technique (Figure 2) allowing a quantitative extraction of air from small ice core samples coupled to a high precision gas chromatography-isotope ratio mass spectrometry system (Figure 3) to determine δ13C on the released CO2. Additionally, this technique precisely determines the CO2 concentration on the same ice sample. This new technique makes it possible to surmount major analytical limitations and shortcomings encountered during previous studies. Using a unique sublimation technique a nearly complete gas extraction has been achieved which represents a prerequisite for high-precision δ13C analysis in deep clathrate ice. As the sublimation technique is equally suitable to analyze bubble and clathrate ice, this new method allows for the first time to measure δ13C values and the CO2 concentration on the entire length of an ice core without adapting the extraction to changing ice conditions.