The basal ice, which can be found close to the bedrock under the ice sheet, is characterized by containing impurities like silt, rocks and biological remains which have accumulated in the bottommost part of the ice sheet as the ice has gradually moved across the rock. 

The basal ice can also be folded and thoroughly mixed with younger ice from the overlying clean ice and older ice from previous glaciers that were later incorporated into the large ice sheet. The age can therefore not be determined using the same stratigraphic methods that are used on the clean overlying ice, which is described under the section on ”stratigraphy and dating”.

New methods of dating the ice are combined to estimate the age from both physical and biological angles. This combination allows for an overlap between the ages of the different components of the basal ice:

a.) Precipitation
b.) The time since the sediment was incorporated into the ice
c.) How much time has passed since the cells of the organisms broke down and their DNA was bound to soil particles, which are stored in the ice.

Together, these methods can provide an estimate of which time period we should assign to the formation of the basal ice and with that, the build-up of the ice sheet in the area where the ice core was drilled.

Dating of the basal ice using physical and biological methods