The surface of the Earth is constantly being hit with high-energy radiation, or cosmic rays. These react with, and are absorbed by, the Earth surface. As the rays hit they produce different radioactive isotopes called cosmogenic nuclides in the top meter or so of the surface rock. Some of these radionuclides are produced and decay at an established rates, and therefore can be used to determine how long the rock surface has been exposed to the elements, making them an extremely useful tools in dating glacial advance and retreat.
For the Snow on Ice Project a recently developed technique of pairing two radioactive isotopes, 14C/10Be, will be used to determine surface exposure. Both 14C/10Be are produced at a known rate in mineral quartz, 14C at 5,730 yrs. and 10Be at 1.39 million years, and if a rock surface has been continuously exposed to the atmosphere the production ratio of these two isotopes is ~3.0. If the ratio falls below the continuous exposure line rate of 3.0 it can be determined that the surface must have had a period of ice cover after its original exposure. The lower the ratio between these two isotopes the longer the area must have been covered over again by ice. This technique offers an extremely valuable tool in determining ice sheet retraction and re-expansion in an area.
The surface exposure dating is being correlated with the ages from the lake sediment cores, applying a second independent method of dating Greenland’s ice sheet extent. Additionally, the dating expands the overall area of dating coverage as it provides a method that can be used to connect between the areas where lake sampling is possible.