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

Detailed measurements of crystal outlines and fabrics have been performed on 35 000 crystals in fifteen 10 × 20 cm2 vertical thin sections from the North Greenland Icecore Project (NorthGRIP) ice core, evenly distributed in the depth interval 115–880m. 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 becomes increasingly irregular, and the area distribution of crystals develops from a single log-normal distribution into a bimodal lognormal distribution. The c-axis fabric of the ice shows a smooth development of an increasingly stronger vertical fabric with depth, and the 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.