Direct observations of pore close-off in stratified firn by means of large scale X-ray computer tomography

Katharina.Klein [ at ]


Very recently it has been observed, that in contradiction to former beliefs polar firn exhibits almost everywhere a significant layering at the firn-ice-transition (FIT). Even at low accumulation sites like Dome C (Antarctica) the porosity shows variations in adjacent layers of around 30% at the FIT. Because of this stratified structure impermeable layers occur within the firn and act like a sealing, so that the air is completely isolated from the overlying firn layers. It is out of question that this layering will alter the predicted age distribution of enclosed air as they are simulated by modelling the close-off in homogeneous firn columns. In our contribution we present first direct observations of the pore structure at the FIT of stratified firn of B36/37 drilled at Kohnen station (Antarctica) by means of the large scale X-ray computer tomograph (AWI-ICE-CT). Three slabs of 30-40cm length covering the layered structure of the firn column in the depth around the FIT are measured with a spatial resolution of 15μm. Thus we are able to dissolve the inner pore connectivity and investigate the effect of layering at pore close-off at once in a unique way as it was impossible up to now. The fraction of the closed pores and the developement of the geometrical properties including size distributions and inner connectivity are studied and compared with a percolation model for stratified firn. By combining these findings with the new firn densification model that describes the layering at the FIT as an effect of different impurity load, a first prediction of the expected age distribution for B36/37 is made.

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Open Science Conference IPICS 2012, 01 Oct 2012 - 05 Oct 2012, Giens, France.
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Klein, K. and Freitag, J. (2012): Direct observations of pore close-off in stratified firn by means of large scale X-ray computer tomography , Open Science Conference IPICS 2012, Giens, France, 1 October 2012 - 5 October 2012 .

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