Ice Core. An ice core is a section of ice drilled from a glacier or ice sheet. Ice deposits contain samples of the atmosphere at the time the ice formed; they also record seasonal fluctuations of temperature and dust. That’s why ice cores are extremely valuable sources of paleoclimate data—data on the earth’s climate in the distant past. Researchers drill sections of ice cores hundreds of meters long and then match sections at different depths with particular eras in the earth’s past. These sections can then be analyzed for clues about atmospheric gases and temperatures from hundreds of thousands of years ago.