Textures and fabrics in the GRIP ice core

Abstract

Abstract. A comprehensive study of textures and fabrics has been carried out on the Greenland Ice Core Project (GRIP) ice core. Crystal sizes and c axis orientations have been measured on thin sections with conventional techniques, yielding new information on the growth, rotation and recrystallization of ice crystals in the Greenland Ice Sheet. Normal grain growth is found to persist to a depth of 700 m in the core, where the onset of polygonization due to increasing strain prevents a further increase in grain size in the Holocene ice. Smaller crystals are observed in the Wisconsin ice, larger crystals are found in the Eemian ice, and the crystal size is found to vary with climatic parameters in these periods. This dependence, which probably results from variable impurity content in the ice, persists to a depth of 2930 m. Coarse-grained ice, probably resulting from rapid growth of crystals at comparatively high temperatures, is found in the lowest 100 m of the core. The data on c axis orientations reveal a steady evolution of the fabric from random near the surface to a strong single maximum in the lower part of the ice sheet. A significant strengthening is not observed at the Holocene-Wisconsin transition. The fabric development indicates that vertical compression at the ice divide is the main mode of deformation down to a depth of 2850 m. The evolution toward a single maximum fabric hardens the ice against vertical compression but softens it against simple shear. Evidence of simple shear deformation is clearly observed between 2850 m and 2950 m depth. Stretched fabrics in coarse-grained ice in the lowest 100 m could be due to tensional stresses; this ice is unlikely to be undergoing any significant horizontal deformation at the present time.

Fabric development with nearest‐neighbor interaction and dynamic recrystallization

“…For comparison, the fabric at 1293 m depth in the GRIP borehole is also shown [Thorsteinsson et al, 1997], where the strain is $0.5 [Castelnau et al, 1996b]. There is active polygonization at this Figure 11.…”

“…The preferred orientation of mantle minerals, mainly olivine crystals, is known to cause seismic shear wave splitting in the crust and the upper mantle [Savage, 1999]. In ice sheets, fabric evolution has been well documented from extensive thin section measurements on ice cores [Alley et al, 1995;Gow et al, 1997;Thorsteinsson et al, 1997] and from sonic logging in boreholes and on the ice cores themselves [Kohnen and Gow, 1979;Taylor, 1982;Anandakrishnan et al, 1994;Thorsteinsson et al, 1999]. A consequence of fabric development is that bulk physical properties become anisotropic, as shown by experiments and theory [Steinemann, 1958;RussellHead and Budd, 1979;Duval, 1981;Duval and LeGac, 1982;Budd and Jacka, 1989;van der Veen and Whillans, 1990;Wenk and Christie, 1991;Alley, 1992;Anandakrishnan et al, 1994;Azuma, 1994Azuma, , 1995Azuma and Goto-Azuma, 1996;Castelnau et al, 1996a].…”

“…Ice I h has only two independent easy slip systems, the basal plane (0001) [Ashby and Duval, 1985]. Good information about fabric and texture exists from recent ice core studies [Thorsteinsson et al, 1997;Gow et al, 1997]. Resistance to shear on other slip systems (prism 10 " 10 ð Þ 11 " 20 h iand pyramidal 11 " 22 ð Þ 1123 is 20 times, or higher, than on the basal plane slip system [Castelnau et al, 1997].…”

“…In the Greenland Ice Core Project (GRIP) ice core from the Greenland ice sheet, poly gonization (rotation recrystallization) is active below 650 m depth, where the vertical strain is $0.25 [Thorsteinsson et al, 1997]. De La Chapelle et al…”

“…[40] Keeping in mind the complications mentioned above, Figure 8 shows a comparison of the model results, using mild NNI with polygonization, with fabric data from the GRIP borehole, Greenland [Thorsteinsson et al, 1997] The normalized strain rate versus axial strain, the strain density of single crystals, and the final fabric. (bottom) The polygonization events after each strain step as a percentage of the total number of crystals.…”