CO 2 and O 2/N 2 variations in and just below the bubble–clathrate transformation zone of Antarctic ice cores

Abstract

CO 2 measurements on the EPICA (European Project for Ice Coring in Antarctica) DML ice core in depth levels just below the bubble ice–clathrate ice transformation zone (1230–2240 m depth) were performed. In the youngest part (1230–1600 m), they reveal variations of up to 25 ppmv around the mean atmospheric concentration within centimetres, corresponding to a snow deposition interval of a few years. Similar results are found at corresponding depth regions of the Dome C and the Talos Dome ice cores. Since we can exclude all hitherto known processes altering the concentration of CO 2 in ice cores, we present a hypothesis about spatial fractionation of air components related to episodically increasing clathrate formation followed by diffusion processes from bubbles to clathrates. This hypothesis is supported by optical line-scan observations and by O 2/N 2 measurements at the same depth where strong CO 2 variations are detected. Below the clathrate formation zone, this small-scale fractionation process is slowly smoothed out, most likely by diffusion, regaining the initial mean atmospheric concentration. Although this process compromises the representativeness of a single CO 2 measurement on small ice samples in the clathrate formation zone of an ice core, it does not affect the mean atmospheric CO 2 concentration if CO 2 values are averaged over a sufficiently long depth scale (> 10 cm in case of the EPICA DML ice core).