Snow crystal imaging using scanning electron microscopy: III. Glacier ice, snow and biota

Low-temperature scanning electron microscopy (SEM) was used to observe metamorphosed snow, glacial firn, and glacial ice obtained from South Cascade Glacier in Washington State, USA. Biotic samples consisting of algae (Chlamydomonas nivalis) and ice worms (a species of oligochaetes) were also collected and imaged. In the field, the snow and biological samples were mounted on copper plates, cooled in liquid nitrogen, and stored in dry shipping containers which maintain a temperature of-196°C. The firn and glacier ice samples were obtained by extracting horizontal ice cores, 8 mm in diameter, at different levels from larger standard glaciological (vertical) ice cores 7.5 cm in diameter. These samples were cooled in liquid nitrogen and placed in cryotubes, were stored in the same dry shipping container, and sent to the SEM facility. In the laboratory, the samples were sputter coated with platinum and imaged by a low-temperature SEM. To image the firn and glacier ice samples, the cores were fractured in liquid nitrogen, attached to a specimen holder, and then imaged. While light microscope images of snow and ice are difficult to interpret because of internal reflection and refraction, the SEM images provide a clear and unique view of the surface of the samples because they are generated from electrons emitted or reflected only from the surface of the sample. In addition, the SEM has a great depth of field with a wide range of magnifying capabilities. The resulting images clearly show the individual grains of the seasonal snowpack and the bonding between the snow grains. Images of firn show individual ice crystals, the bonding between the crystals, and connected air spaces. Images of glacier ice show a crystal structure on a scale of 1-2 mm which is considerably smaller than the expected crystal size. Microscopic air bubbles, less than 15 firn in diameter, clearly marked the boundaries between these crystal-like features. The life forms associated with the glacier were easily imaged and studied. The lowtemperature SEM sample collecting and handling methods proved to be operable in the field; the SEM analysis is applicable to glaciological studies and reveals details unattainable by conventional light microscopic methods. Imagerie des cristaux de glace par microscopie électronique à balayage: III. Glace de glacier, neige, matière vivante Résumé La microscopie électronique à balayage à basse température a été utilisée pour observer de la neige transformée, de la glace de névé et de la glace de glacier du glacier "South Cascade" (Washington, Etats-Unis). Des algues (Chlamydomonas nivalis) et des vers de glace (une espèce d'oligochètes) ont également été recueillis et observés. Sur le terrain, les échantillons biologiques et de neige ont été montés sur des plaques de cuivre, refroidis dans l'azote liquide et stockés dans des conteneurs de transport secs maintenant la température à -196°C. Les échantillons de glace de névé et de glacier ont été obtenus par extraction de carottes de glace horizontales de 8 mm de diamètre à différents niveaux de carottes de glace verticales de 7.5 cm de diamètre. Ces échantillons ont été refroidis dans l'azote liquide et placés dans des cryotubes eux Open for discussion until 1 December 2000