The stable carbon isotope ratio of atmospheric CO2 (d13Catm) is a key parameter in deciphering past carbon cycle changes. Here we present d13Catm data for the past 24,000 years derived from three independent records from two Antarctic ice cores. We conclude that a pronounced 0.3 per mil decrease in d13Catm during the early deglaciation can be best explained by upwelling of old, carbon-enriched waters in the Southern Ocean. Later in the deglaciation, regrowth of the terrestrial biosphere, changes in sea surface temperature, and ocean circulation governed the d13Catm evolution. During the Last Glacial Maximum, d13Catm and atmospheric CO2 concentration were essentially constant, which suggests that the carbon cycle was in dynamic equilibrium and that the net transfer of carbon to the deep ocean had occurred before then.

The ice cores were analyzed in three different laboratories which each lab uses a dedicated, independent analytical system referred to as Bern sublimation, Bern cracker and Grenoble mill data. The names of the different methods are composed of the University (Grenoble and two groups at Bern) and an identifier of the special extraction system ('ball mill', 'cracker', 'sublimation'). In short, the three methods involve the following analytical steps:First, enclosed atmospheric air from the ice sample is released using a dedicated extraction device (sublimation, needle cracker and a ball mill for the respective data sets: Bern sublimation, Bern cracker and Grenoble mill). In a second step, the CO2 from the released ice core air is separated from the bulk air (N2, O2 and Ar) using cryogenic trapping. Third, a gas chromatographic column purifies the CO2 sample from other trace gases such as N2O. Finally, the stable carbon isotopic ratio of the CO2 sample is measured against a bracketing standard using an isotope ratio mass spectrometer. The typical measurement reproducibilities of the three methods are 0.05 permil - 0.07 permil for the Bern sublimation, 0.07 permil for the Bern cracker, and 0.10 permil for the Grenoble mill data set.

Supplement to: Schmitt, Jochen; Schneider, Robert; Elsig, Joachim; Leuenberger, Daiana; Lourantou, Anna; Chappellaz, Jérôme A; Köhler, Peter; Joos, Fortunat; Stocker, Thomas F; Leuenberger, Markus Christian; Fischer, Hubertus (2012): Carbon isotope constraints on the deglacial CO2 rise from ice cores. Science, 336(6082), 711-714