Photomicrograph in cross-polarized light of ice sample from 333 m depth in the GISP2 core. Mid-sized crystals show homogeneous ice that has not been sheared by ice flow. Small bubbles appear as round inclusion, for example in the orange crystal near center. Record of electrical conductivity from the GISP2 ice core, Summit, Greenland. The record shows annual variations for a portion of the mid-1600s.
Electrical conductivity is especially sensitive to ice acidity.
The major peaks represent volcanic eruptions known from the historical record. Since 1972, Russian scientists have been drilling into the ice sheet. They were joined by French and American scientists in collecting 3.7 km of core samples representing 420,000 years of glacier history. The Vostok ice record covers four glacial-interglacial cycles and represents perhaps the most detailed and complete evidence for climatic change for this entire period anywhere. The location of Vostok Station and other research stations in Antarctica.