Properties of ice crystals in NorthGRIP late- to middle-Holocene ice 

Annals of Glaciology, Vol. 37, p. 113-118, 2003 

A. Svensson, K.G. Schmidt, D. Dahl-Jensen and S.J. Johnsen
Geofysisk Afdeling, Niels Bohr Instituttet for Astronomi, Fysik og Geofysik, Københavns Universitet
Y. Wang and S. Kipfstuhl
Alfred Wegener Institute for Polar and Marine Research, P.O. Box 120161, D-27515 Bremerhaven, Germany.
T. Thorsteinsson
Science Institute, University of Iceland, Dunhaga 3, IS-107 Reykjavik, Iceland.

Detailed measurements of crystal outlines and fabrics have been performed on 35k crystals in fifteen 10 x 20 cm2 vertical thin sections from the NorthGRIP ice core evenly distributed in the depth interval 115-880 m. The crystals exhibit important changes over this period. As the ice gets older the mean crystal area increases towards a constant value, the shape of the crystals is getting increasingly irregular, and the area distribution of crystals develops from a single log normal distribution into a bi-modal log normal distribution. The c-axis fabric of the ice shows a smooth development of an increasingly stronger vertical fabric with depth and a formation of a weak vertical girdle. Already in the younger samples the fabric is rather strongly oriented towards vertical. The fabric and the area of individual crystals are found not to correlate. A simple model, which takes into account the vertical strain of the ice, is applied in an attempt to determine the crystal growth rate at NorthGRIP.