Properties of GRIP ice crystals from around Greenland interstadial 3

Abstract

Ice-crystal textures (sizes and shapes) and fabrics (c-axis orientations) have been determined in 3 m of vertical thin sections from the Greenland Icecore Project (GRIP) ice core. The samples cover ice from before, during and after Greenland interstadial 3 (IS3) that occurred about 25 kyr BP. The texture of 60 000 crystals has been obtained from stacked digital images by semi-automated methods, and the fabric of 5000 selected crystals has been measured on the Automatic Ice Fabric Analyzer at the Alfred Wegener Institute, Bremerhaven. The area distribution function of the crystals is close to a lognormal distribution, and the mean area is found to be 3.36 mm2 in ice from IS3, and 3.04 mm2 in ice from colder periods before and after IS3. The overall c-axis orientations show a strong single maximum with vertical orientation. These results agree well with earlier GRIP fabric studies obtained by manual methods. From comparisons of crystal areas with the concentrations of dust, Ca2+, SO42-, Cl- and NO3- in the ice we determine that NO3 correlates best with crystal areas. Crystals within clearly visible cloudy bands are significantly smaller than the surrounding crystals, and their c-axis orientations are less well confined than the average.