Ice-lattice distortion along the deepest section of the Vostok core from X-ray diffraction measurements Akira HORI, 1 Takeo HONDOH, 1 Mitsugu OGURO, 2 Vladimir Ya. LIPENKOV 3 1 Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan E-mail: hori@hhp2.lowtem.hokudai.ac.jp 2 Physics Laboratory, Asahikawa Campus, Hokkaido 070, Japan 3 Arctic and Antarctic Research Institute, 38 Beringa Street, 199397 St Petersburg, Russia ABSTRACT. We performed X-ray diffraction measurements on eight ice samples taken between 3200 and 3611m depth of the Vostok (Antarctica) ice core to observe lattice distortions of ice crystals. Selected samples represent three distinct sections of the core: (i) glacier (meteoric) ice with well- preserved climatic record (down to 3310 m), (ii) ‘shear zone’ at the base of the glacier ice (3450– 3537 m) within which the climatic record is disturbed by ice deformation, and (iii) accretion ice formed by freezing of subglacial Vostok lake waters at the base of the ice sheet (from about 3537m depth to the bottom of the core). The dislocation density decreases from 10 12 to 10 8 m –2 with increasing depth. In the accretion ice, lattice distortion tends to decrease with depth. However, the dislocation density does not reach a level typical for laboratory-grown columnar ice even at 3610 m. This reflects plastic deformation which accretion ice has undergone after its formation. INTRODUCTION The deepest section of the Vostok (Antarctica) ice core (Fig. 1) is known to have characteristic features and to have been intensively investigated (Jouzel and others, 1999). Its three distinct layers are as follows: (i) glacier (meteoric) ice with well-preserved climatic record (down to 3310m), (ii) ‘shear zone’ (Lipenkov and Barkov, 1998; Simo ˜es and others, 2002) at the base of the glacier ice (3450–3537 m) within which the climatic record is likely disturbed by ice deformation, and (iii) accreted ice formed by freezing of subglacial Vostok lake waters at the base of the ice sheet (from about 3538 m depth to the bottom of the core). To elucidate the deformation and recrystalliza- tion processes of the Vostok ice core, both theoretical and experimental studies have been carried out (Lipenkov and others, 1989; De La Chapelle and others, 1998; Duval and others, 1998). The accreted ice has attracted growing attention, and extensive studies have been carried out on the lake ice (Souchez and others, 2000; Simo ˜es and others, 2002). The grain-size of accretion ice is >10 cm (Jouzel and others, 1999). Therefore, conventional fabric analyses are not helpful although the orientations of large crystals of accretion ice have been reported (Souchez and others, 2000). Instead, X-ray studies on the samples from a few depth levels were carried out to investigate the crystal quality of this ice (Montagnat and others, 2001; Cullen and Baker, 2002). High crystalline quality, i.e. low dislocation density, of these samples has been reported, but the depth dependence of dislocation density in this ice was not studied. In the present work, lattice distortion of eight ice-core samples between 3200 and 3611 m was investigated by X- ray diffraction, and the dislocation densities were estimated for these samples. From the depth profile of the dislocation densities, information on the plastic deformation which the accreted ice experienced after its formation was obtained for the first time. EXPERIMENTAL All ice samples studied here were transported in cooler boxes kept at a low temperature using dry ice and stored in a cold room at –508C after drilling. The samples were cut into thin sections of about 5 mm thickness using a bandsaw and were smoothed using a microtome until the scratches disappeared to eliminate the effect of dislocations which can be introduced during the sample cutting. To prevent sublimation, silicone oil was put on each sample and then its surface was covered with a Mylar film 1.5 mm thick. The rocking-curve measurements (Warren, 1990; Bowen and Tanner, 1998) were carried out using an X-ray diffract- ometer (Rigaku, SLX2000) set in a cold room at –208C, Annals of Glaciology 39 2004 Fig. 1. Sketch of the Vostok ice core and the subglacial lake at the bottom. 501