ABSTRACT. This work presents a method of mapping deformation-related sublimation patterns, formed on the surface of ice specimens, at microscopic resolution (3-4 mm pixel -1 ). The method is based on the systematic sublimation of a microtomed piece of ice, prepared either as a thick or a thin section. The mapping system consists of an optical microscope, a CCD video camera and a computer-controlled xy-stage. About 1500 images are needed to build a high-resolution mosaic map of a 4.5 Â 9 cm section. Mosaics and single images are used to derive a variety of statistical data about air inclusions (air bubbles and air clathrate hydrates), texture (grain size, shape and orientation) and deformation-related features (subgrain boundaries, slip bands, subgrain islands and loops, pinned and bulged grain boundaries). The most common sublimation patterns are described, and their relevance for the deformation of polar ice is briefly discussed.
Abstract. The origin of a strong continuous radar reflector observed with airborne radio-echo sounding (RES) at the EPICA deep-drilling site in Dronning Maud Land, Antarctica, is identified as a transition in crystal fabric orientation from a vertical girdle to an increased single-pole orientation seen along the ice core. The reflector is observed with a 60 ns and 600 ns long pulse at a frequency of 150 MHz, spans one pulse length, is continuous over 5 km, and occurs at a depth of about 2025-2045 m at the drill site. Changes in conductivity as reflector origin are excluded by investigating the icecore profile, synthetic RES data, and a RES profile with different electromagnetic polarisation azimuths. The reflector's magnitude shows maximum values for polarisation parallel to the nearby ice divide and disappears for polarisation perpendicular to it, identifying the orientation of the girdle to lie in the vertical plane parallel to the ice divide. Observations allow us to extrapolate the crystal orientation feature along the reflector in space, with implications for ice-sheet dynamics and modeling.
[1] Microstructural analyses have been performed on polar firn from the European Project for Ice Coring in Antarctica drilling site in Dronning Maud Land, Antarctica. The results derived from images of the firn structure in microscopic resolution indicate that dynamic recrystallization is active in firn at all depths, and it dominates the evolution of the microstructure when the firn density exceeds a critical value of 730 kg/m 3 (overburden snow load $0.2 MPa). At the firn-ice transition (density $820 kg/m 3 ) the microstructure is characterized by many small grains and bulged or irregularly shaped grain boundaries. More than half of all grains show subgrain boundaries. Thus, strain-induced boundary migration is an essential feature to describe the irregular grain structure. In agreement with previous studies, significant grain growth has been observed with depth for the largest grains in the samples. However, our microscopic analysis reveals that the grain growth with depth in fact vanishes if all grains larger than 65 mm in diameter are taken into account. This result reflects the fact that the growth of the largest grains is counteracted by grain size reduction by shrinking and subdivision of old grains, as well as production of new grains. Consequently, previous conclusions that grain growth in polar firn is essentially analogous to normal grain growth in metallic and ceramic sinters and that the stored strain energy is small in comparison with grain boundary energy can no longer be supported. Additionally, our observations show that the incipience of dynamic recrystallization in polar ice sheets is not as sensitive to temperature as supposed so far. A discussion of the change of the mean grain size due to the measuring technique is imperative.
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