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The densification of layered polar firn

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Hörhold, M. , Kipfstuhl, S. , Wilhelms, F. , Freitag, J. and Frenzel, A. (2011): The densification of layered polar firn , JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, F01001PP. , 15 . doi: 10.1029/2009JF001630
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Abstract:

High-resolution density profiles of 16 firn cores from Greenland and Antarctica are investigated in order to improve the understanding of densification of layered polar firn.A resolution of 1 to 5 mm enables us to study the detailed densification processes and the evolution of the layering and the resulting variability in density with increasing depth. The densification of layered firn is important for the process of air enclosure in the ice and is connected with the observed formation of a non-diffusive zone.We find that (1) mean density profiles, obtained from high-resolution measurements, do only partly or not show clear transitions in densification rate at densities of 550, 730 or 820-840 kg/m^3, as they are commonly proposed in literature. (2) The density variability, induced by the layering, shows a similar pattern at all sites: High variabilities at the surface, a rapid drop to a relative minimum at mean density of 600 - 650 kg/m^3, followed by a second relative maximum. (3) This leads to increased variabilities at densities of the firn-ice transition for most of the sites. (4) The variability at the surface does decrease with increasing annual mean temperature and accumulation rate, whereas the variability at the firn-ice transition increases.The firn cores in this study cover a broad range in annual mean temperature, accumulation rate and age. We therefore can exclude a change in local climate conditions as explanation. Overall, high-resolution density profiles deliver a more complex picture of compaction of polar firn as layered granular medium, than has been concluded from mean density profiles in the past.

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