Depending on the amount of extracted air, three to five successive analyses are done.
Some volcanic events which were sufficiently powerful to have distributed material around the globe leave a signature in the ice which can be detected in many different cores, allowing synchronization of the time scales between two different locations.
Glaciologists drill and extract cylindrical cores from high-latitude ice sheets and use different techniques to estimate the cores’ age at certain depths.
Greenland ice cores contain layers of wind-blown dust that correlate with cold, dry periods in the past, when cold deserts were scoured by wind.
Radioactive elements, either of natural origin or created by nuclear testing, can be used to date the layers of ice in the cores.
In part, they measure the age of rocks and other natural materials by dating techniques.
They can date rocks by gauging the amount of decay of radioactive elements.
Research teams from the United States, the Soviet Union, Denmark, and France have bored holes over a mile deep into the ice near the poles and removed samples for analysis in their laboratories.
Based on flow models, the variation of oxygen isotopes, the concentration of carbon dioxide in trapped air bubbles, the presence of oxygen isotopes, acid concentrations, and particulates, they believe the lowest layers of the ice sheets were laid down over 160,000 years ago.
Since heat flow in a large ice sheet is very slow, the borehole temperature is another indicator of temperature in the past, and these sources of information can be combined to find the climate model that best fits all the available data.
Impurities in ice cores may depend on location; for example, coastal areas are more likely to include material of marine origin, such as sea salt ions.
The Greenland Society of Atlanta has recently attempted to excavate a 10-foot diameter shaft in the Greenland ice pack to remove two B-17 Flying Fortresses and six P-38 Lightning fighters trapped under an estimated 250 feet of ice for almost 50 years (Bloomberg, 1989).