A meter of snow on top of the Mount Logan Icefield today may represent ~ 1 year of snowfall, but a meter of ice at the bottom of the Icefield could represent thousands of years of snowfall compressed and stretched by ice compaction and thinning.
Caption for figure: The relationship between depth, age, and layer thickness is shown for a 188-m long ice core that was drilled through the Mount Logan plateau in 2001-02. The blue line of the graph show how the thickness of an ice layer diminishes rapidly with depth, while the red line show how the age of the ice increases gradually at deeper levels. The lowest few metres of core contain most of the very old ice.
There are several ways that scientists use to determine the age of ancient ice layers in ice caps:
Using a mathematical model, they can predict the approximate rate at which snow layers are being buried and compressed in the glacier. The relationship between depth and age of the ice is not linear but gives an approximate guide.
They can count ice layers. This is only possible when there is abundant snowfall and no summer melting. Seasonal or yearly variations in the density, dirt content or the chemistry of the ice can be used to resolve individual layers back in time. However beyond a certain depth, the ice gets too flattened by compaction for these variations to be resolved.
They can use "time markers" – distinctive layers that correlate with an historical or prehistoric event. For example, volcanic ash (‘tephra’) from eruptions whose date is known, are detected as dark horizons of microscopic particles, or as sharp spikes in the chemistry (acidity) of the ice core. In more recent times, trace amounts of radioactive dust from nuclear surface explosions can also be detected in the ice.
Abrupt climatic changes, such as the end of the last ice age ~11,500 years ago, also leave a very distinct "signature" that can be recognized in the chemistry of the ice.
The older and deeper the ice core, the greater the uncertainty of its true age. Typically in Canadian Arctic and alpine cores it is difficult to establish a reliable time scale for ice older than about 15,000 years.
Presently it is estimated that the oldest core drilled through the icy plateau of Mount Logan in 2001-02 contains a record of:
the current interglacial period (the last ~11,500 years, called the Holocene).
at the bottom: several meters of older ice from the last Ice Age or before. This older ice could be as old as 80,000 or 100,000 years, but the age or length of time it represents is uncertain.