When an ice core has been drilled there is still information to be gained by studying the borehole itself. After the drilling is finished, the casing of the borehole is extended above the surface so that it is possible to gain access to the borehole in the following years after the camp is closed.
When the borehole is revisited, an instrument measuring the geometry of the borehole and the temperature of the surrounding ice is lowered into the borehole. The changes in the shape of the borehole since the ice core was drilled give information on the flow of the ice, and the measured temperatures reveal the past temperatures at the site.
The temperature of the ice at the time of deposition is given by the mean temperature at the site. The temperature at the site changes with time due to changing climate. However, ice is a poor heat conductor and "remembers" its original temperature for a long time as it moves down through the ice sheet. In other words: the ice deposited during the glacial period is still cold, and the coldest temperatures in the ice sheet are found in the middle of the ice sheet. The warmest temperatures are found at the bottom because the bedrock releases geothermal heat. Heat is released from the bedrock everywhere on Earth, but the ice sheets act like blankets and prevent the heat from getting away. Thus the ice sheet is heated from below.
In southern Greenland the temperature at the base is below freezing, but in northern Greenland the underground consists of materials that release more heat and the ice is at the melting point at the base. This was experienced first hand by the crew drilling the NorthGRIP ice core, when they reached the bottom in 2003 and 40 m of liquid water was pushed up in the lowest part of the borehole.
G. D. Clow, E. D. Waddington and N. Gundestrup
High-precision temperature measurements in the GISP2 and GRIP boreholes
N. S. Gundestrup, H. B. Clausen and B. L. Hansen
The UCPH borehole logger
D. Dahl-Jensen, K. Mosegaard, N. Gundestrup, G.D. Clow, S. J. Johnsen, A. W. Hansen and N. Balling
Past Temperatures Directly from the Greenland Ice Sheet
Ice Core Drilling Projects
For more information on the topic please contact Dorthe Dahl-Jensen