The isotopic signature of the gases

The air trapped in bubbles of ice cores represents a unique archive of the past atmospheric composition. By extracting the gas in the bubbles, researchers can determine the concentration of e.g. greenhouse gases hundreds of thousand years into the past.

But not only can the concentration of these gases be measured. A deeper insight into the processes leading to changes in the atmospheric concentrations can be gained from the studies of the isotopic composition of these gases, for example the 13C/12C ratio measured in CO2 or CH4. This is due to the fact that the isotopic signature of a gas contains information about which process the gas was formed under, and thus researchers can identify the most important sources of e.g. greenhouse gases in the ancient atmosphere.

In order to determine concentration changes along with changes in the isotopic signature over time for a specific gas, the 'total air' (i.e. the mixture containing all the gases) trapped in the ice bubbles has to be released and the gases have to be carefully separated from each other prior to measurement.


The isotopic composition of methane CH4 from
different sources. δ13C indicates the deviation in per mill of the 13C/12C ratio to a defined standard (PDB). Data from Quay et al. (1999) and Whiticar (1999).

Due to the fact that the amount of available ice is restricted, typical sample sizes are rather small, varying between 10-1000 g of ice depending on the gas to be analyzed and the technique applied. That results in 1-100 ml of air only (roughly 10 % of the volume). Accordingly, 1 ml of air results in 0.00035 ml (= 0.35 μl) of CO2 with 1.1 % of the carbon isotopes contained being 13C. With such small amounts, even small contaminations or small changes in the laboratory setup during extraction, separation, and final measurement for the individual samples will have large effects on the final results. Achievement of low and stable procedure blanks and a frequent calibration against standards with known concentrations and isotopic composition of the entire set-up is therefore essential.

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