Seasonal variability in ice crystal properties at NorthGRIP. A case study around 301 m depth

The aim of this case study is to quantify the seasonal variability in crystal properties, and to discuss the reason for the variability. A continuous 1.10 m long vertical thin section profile covering approximately five annual cycles has been obtained from the NorthGRIP ice core at around 301 m depth. The crystal outline and the c-axis orientation of more than 13k crystals in the profile have been measured on a new Australian automated ice crystal analyzer. In 2.5 cm resolution we observe a strong seasonal variability in crystal areas of more than 30 % deviation from the average value of 6.7 mm2. Each year, a band of smaller crystals is observed in ice deposited during springtime. The area distribution function is found to be close to a log normal distribution. The crystal areas are compared to the concentration of chemical impurities in the ice; at a 5 cm resolution, the best correlation is found with the concentration of Ca2+. Our results show no seasonal variability of the average c-axis orientation of ice crystals