A variety of methods are used to date an ice core. The most direct method is to count annual layers in much the same way that tree-rings can be counted to determine the age of the tree.
However, the layers in ice cores are not generally visible in the ice. They only become apparent when the core is analysed for a chemical signal that varies with the seasons, which most signals do, to some extent. In fact the clearest dating is obtained when several seasonal signals are examined and compared.
Many cores however come from regions where the yearly snowfall accumulation is too small for the annual layers to be distinguished, and other methods of dating must be used.
Even in cores where the yearly snowfall produces thick layers, the nature of glacier flow stretches and thins layers as they get buried deeper. This flow-thinning means that annual layer counting eventually becomes impossible in all deep cores.
Where layer-counting is not possible, dating generally relies upon mathematical models of ice flow.
Another useful technique is to identify events which are seen in other types of climate records, such as historical, tree ring and sedimentary records. These all have independent methods of dating, and so the timing of a major climate shift or volcanic eruption can be used to synchronise the age scales.
Most Australian ice core research has been conducted at Law Dome, a small icecap some 200km in diameter on the coast of Antarctica near Casey station.
The summit of Law Dome is approximately 1400m above sea level, and several cores have been drilled. The longest core extends all the way to bedrock some 1200m below. This deep core, the “Dome Summit South”, or DSS, core provides a record that extends back over 80 000 years. The record is particularly detailed for an ice core, especially at the time of the emergence from the last glacial period to present. This is a result of the high snowfall rate on Law Dome, which leaves a thick layer of snow every year.
The disadvantage is that, especially with an icecap only 1200m thick, the record does not extend as far back in time as some other ice cores.