Petrographic characteristics of young sea ice, Point Barrow, Alaska

A pilot study is made of interrelations between structural features readily observed in horizontal thin sections of sea ice under low magnification. The core used was 31.4 cm in length and was collected from Elson Lagoon, Point Barrow, Alaska, on Oct. 26, 1960. The growth of the ice sheet from its formation on Oct. 1 to the time of sampling can be described by Z = 1.20 (S + Sigma theta ) (super 0.60) where Z is the thickness of the ice sheet in cm, Sigma theta is the accumulated degree days, and S is a fictitious number of degree days, in this case, 38, assigned to the initial conglomeration of slush ice. Important intercrystalline structural features are a systematic increase in crystal size with depth and a fairly constant 2:1 ratio between the length and width of the ice crystals. Comparisons are made with recent observations on crystal growth in metals. The frequency distributions of plate widths, a, the distance between the centers of adjacent subgrains measured parallel to the c-axes of the ice crystals, commonly show significant positive skews. The average value of a shows a slight but significant increase with increasing depth in the ice sheet. Within the limited range of observations a strong linear correlation (r = -0.95) exists between a and v, the growth velocity in mm/hr: a = 0.70 - 0.10 v. This relation however will not explain observations at either low or high growth rates. Plots of sub-plate widths vs sub-plate lengths show an extreme scatter and a slight increase in the modal value of the plate length with increasing plate width. The packings of the sub-plates in sea ice are compared with packings observed in Zn and are found to be similar.