Paleo Slide Set: Polar Ice Cores
Thin wafers of ice for core viewed between crossed polarizers.

Researchers also study the physical properties of the GISP2 ice. Crystal size and orientation, 
temperature of the borehole at the time of drilling, and ice density all hint at how the ice formed 
and the conditions of its stay in the ice sheet.

By examining thin wafers of ice between crossed polarizers, scientists determine the orientation, 
shape, and size of the ice crystals. A crystal's orientation is indicated by its color under polarized 
light, and is used to determine whether a given ice segment has undergone deformation significant enough 
to distort the ice record (for example, researchers originally located a period of rapid and intense climate 
change in the last interglacial; ice crystal research, however, revealed that sections of the ice segments upon 
which these findings were based may have been disturbed or removed due to shearing). In the upper part of the 
GISP2 core, crystal size increases with depth; this segment from 333 m is made up of mid-sized crystals. Minute 
air bubbles appear in the photo as small round inclusions, the most visible being in the large orange crystal 
near the center of the photo. These samples of fossil air provide scientists and policy-makers with direct evidence 
of past atmospheric composition for the last 200,000 years, documenting the effects that modern man has had on 
Earth's atmosphere and providing information that is crucial to informed scientific and political debates about 
greenhouse warming. 

Photo Credits:
Anthony Gow 
United States Army Corps of Engineers, Cold Regions Research and Engineering Laboratory.