The interior of the Greenland ice sheet never melts. Every year, new snow simply falls on top of the previous year’s. As the glacier flows, these snow layers slowly sink to the bottom where they are stretched and compressed, but the sequence of annual layers remains undisturbed. Drilling ice cores through the ice sheet reveals layers of ice dating back thousands of years.

With a five year grant from The Carlsberg Foundation, researchers from the Niels Bohr Institute have been able to undertake detailed studies of Greenland ice cores. Analysing the isotopic composition and the impurity concentrations in the ice allows them to identify the separate layers of summer and winter snow, even in very old ice. By counting the layers, like tree rings, the ice core sections can be dated with high precision.

A time template
The Greenland Ice Core Counted Chronology 2005 (GICC05) now stretches back 62,000 years. For geologists, archaeologists and biologists, it has created a time template on climate variations and events like volcanic eruptions from the middle of the last ice age to the present day. It also provides an important, independent accuracy check for researchers working on radiometric dating methods.

To create the GICC05 chronology, the researchers studied 5,700 metres of ice from three Greenland cores. The annual layer counting involved 175,000 isotope measurements and more than a million measurements of chemical impurities in layers of ice less than 1cm deep.

The grant also funded the development of new techniques which will enable the layer counting to go back even further back in time in the future.