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

Low-temperature scanning electron microscopy (SEM) was used to observe meta morphosed snow, glacial firn, and glacial ice obtained from South Cascade G lacier in Washington State, USA. Biotic samples consisting of algae (Chlamy domonas nivalis) and ice worms (a species of oligochaetes) were also collec ted and imaged. In the field, the snow and biological samples were mounted on copper plates, cooled in liquid nitrogen, and stored in dry shipping con tainers which maintain a temperature of -196 degrees C. The firn and glacie r ice samples were obtained by extracting horizontal ice cores, 8 mm in dia meter, at different levels from larger standard glaciological (vertical) ic e cores 7.5 cm in diameter. These samples were cooled in liquid nitrogen an d placed in cryotubes, were stored in the same dry shipping container, and sent to the SEM facility. In the laboratory, the samples were sputter coate d 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 sno w and ice are difficult to interpret because of internal reflection and ref raction, the SEM images provide a clear and unique view of the surface of t he samples because they are generated from electrons emitted or reflected o nly 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 bondin g 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 sma ller than the expected crystal size. Microscopic air bubbles, less than 15 mu m in diameter, clearly marked the boundaries between these crystal-like features. The life forms associated with the glacier were easily imaged and studied. The low-temperature SEM sample collecting and handling methods pr oved to be operable in the field; the SEM analysis is applicable to glaciol ogical studies and reveals details unattainable by conventional light micro scopic methods.