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TitleDensification of layered firn of the ice sheet at NEEM, Greenland
AuthorFujita, S; Hirabayashi, M; Goto-Azuma, K; Dallmayr, R; Satow, K; Zheng, J; Dahl-Jensen, D
SourceJournal of Glaciology vol. 60, no. 223, 2014 p. 905-921,
Alt SeriesEarth Sciences Sector, Contribution Series 20140033
PublisherInternational Glaciology Society
Mediapaper; on-line; digital
File formatpdf
Subjectssurficial geology/geomorphology; geophysics; ice; ice sheets; icefields; isotopes; oxygen isotopes
Illustrationstables; plots
Programenvironmental impacts and adaptation in the northern environment, Environmental Geoscience
AbstractDensification of firn at the North Greenland Eemian Ice Drilling (NEEM) camp is investigated using density surrogates: dielectric permittivities Ev and Eh at microwave frequencies with electrical fields in the vertical and horizontal planes, respectively. Dielectric anisotropy ?E (=Ev -Eh) is then examined as a surrogate for the anisotropic geometry of firn. Its size, fluctuations and mutual correlations are investigated in samples taken at depths from the surface to ?90 m. The initial ?E of ?0.06 appears within the uppermost 0.2 m. After that, ?E decreases rapidly until 21 - 26m depth. Below this, ?E decreases slowly. Layers with more ions of fluorine, chlorine and some cations deposited between the autumn and the subsequent summer deform preferentially during all these stages. This layered deformation is explained partly by the textural effects initially formed by the seasonal variation of metamorphism, and partly by ions such as fluorine, chlorine and ammonium, which are known to modulate dislocation movement in the ice crystal lattice. Insolation-sensitive microstructure appears to be preserved all the way to the pore close-off, within layers of the summer-to-autumn metamorphism. Like previous authors, we hypothesize that calcium is not the active agent in the reported deformation - calcium correlations.
Summary(Plain Language Summary, not published)
In this study, research on densification from firn (a transition state between snow and ice) into ice was carried out. Samples used in the experiment, 9 sections and 1.65 meter each, were taken from NEEM (The North Greenland Eemian Ice Drilling) site, Greenland. In order to understand how firn gradually converts into ice, the experiment was carried out by measuring vertical and horizontal dielectric permittivities using electrical fields. Dielectric anisotropy, a proxy of anisotropic geometry of ice and pore spaces, can be calculated from measurements of the experiment. Results from this experiment show that dielectric anisotropy was about 0.06 in the top 20 centimeters and then decreased rapidly to a depth slightly lower than 20 meters. After that depth, dielectric anisotropy decreased slowly, representing a stage of dominant dislocation creep and pressure sintering et al. with shrinkage of pore spaces. Ions in firn play a role in deformation and dislocation of ice crystal lattice.