Permalink : http://hdl.handle.net/2115/45437

KEYWORDS : ice core;ice crystal;texture;fabric;Greenland

In this study we discuss new and published profiles of c-axis fabric and size distributions of a large number of crystals in the upper 1000 m of the Greenland GRIP and NorthGRIP ice cores as obtained with an automatic c-axis fabric analyzer. We show that in both cores the fabric is strongly anisotropic already just below the firn (about 110 m depth) with a degree of orientation around 30%. We determine the mean crystal area profiles which are somewhat contrasting to earlier findings based on manual measurements. The crystal size distributions are compared to a simple ID model which takes into account crystal growth and polygonization (rotation recrystallization). The model suggests that the crystal size distribution develops into a Bessel type distribution that differs from the commonly applied log normal distribution. Finally. we combine the fabric and microstructure results to determine the sub-grain boundaries in the GRIP samples. Both the size distribution model and the sub-grain study suggest that polygonization is significant already below the firn and that the physical processes governing the conventionally adapted 'normal grain growth' and 'polygonization' regimes are identical. The new measurements provide a dataset for ice sheet models that take into account the anisotropy of the Greenland ice sheet.