Crystalline Texture of the 2083 m Ice Core at Vostok Station, Antarctica

Abstract

Crystalline texture and c-axis orientation of the 2083 m ice core at Vostok Station, covering more than 150kyear, reveal the existence of strong anisotropics. Changes in crystal size with depth are compatible with the growth of grains driven by the free energy of grain boundaries. A smaller growth rate appears to be associated with cold periods. A gradual increase in the horizontal elongation of grains was observed between 350 and 680 m. But, the mean value of the coefficient of the linear dimensional orientation of grains does not change below 700 m.The c-axis orientation of ice grains tends to orientate perpendicular to the direction of the elongation of grains, forming a vertical girdle pattern. This characteristic fabric has been interpreted as resulting from the gradual rotation of grains by basal glide under uniaxial longitudinal tension. The rotation of grains was calculated with respect to the total strain, simulating the formation of the girdle fabric pattern. The fabric-enhancement factor was calculated at various depths. It appears that Vostok ice hardens gradually with depth when considering the transverse convergent flow. No significant variation of the enhancement factor was observed with changes in climate and impurity content.

Comment on “Superplastic deformation of ice: Experimental observations” by D. L. Goldsby and D. L. Kohlstedt

“…As indicated by Duval et al [2000], GBS as a dominant deformation mode is clearly not compatible with the development of fabrics in ice sheets. This conclusion is supported by results on the relationship between grain flattening and strain along the Vostok [Lipenkov et al, 1989] and Dome Concordia ice cores. It has been shown that the strain rate deduced from the flattening of crystals roughly corresponds to the estimated strain rate near the surface of these two sites in Antarctica.…”

Modelling firn thickness evolution during the last deglaciation: constraints on sensitivity to temperature and impurities

“…At Vostok, a high densification rate is observed well above the critical density of about 550 kg m −3 . One possible reason is the very different flow regimes of the two sites, one being at a Dome summit, and the other on a flow line and subject to a horizontal tension (Lipenkov et al, 1989). This is not taken into account in our simplified 1-D model.…”

How old is the ice beneath Dome A, Antarctica?

“…This anisotropic ice may be an order of magnitude "softer" against deformation in certain directions than ice with random fabric, and has an important influence on the age of ice in the lower 1/3 of the ice thickness (Martín and Gudmundsson, 2012;Seddik et al, 2011). Deep ice cores from sites such as Vostok and NorthGRIP in Greenland, which are not located on a true dome geometry, exhibit a girdle-type fabric pattern (Lipenkov et al, 1989;Montagnat et al, 2014), whereas deep ice in Dome C and the Greenland summit GRIP ice core exhibit single maximum; that is, the c axes are concentrated along the vertical direction (Thorsteinsson et al, 1997;Wang et al, 2003). At Dome F, which is perhaps the closest analogue to the Dome A region, a single maximum fabric dominated the bottom 1/3 of the ice core (Seddik et al, 2011).…”