Dating by annual layer counting

An ice sheet consists of layers (strata) of snow and ice, almost like a giant sandwich. In the inner part of the ice sheet where more snow accumulates than melts and evaporates (the so-called accumulation zone), a new layer of snow is deposited on top of the previous layer of snow every year.  

The buried snow layers get compressed as more snow falls on top (read more about the transformation of snow to ice here). The layers are slowly transformed into ice and they are becoming thinner and older as they move downwards through the ice sheet and eventually melt or break off as icebergs at the ice sheet's margins. The mapping and study of the layering in the ice is called stratigraphy.

ice flow

A vertical profile of the ice sheet. Note that in reality, the ice sheet is about 1000 km wide and 3 km thick. The green, horizontal lines represent annual layers that are becoming thinner and older at increasing depth. The arrows show the typical flow pattern of the ice: downwards and outwards from the interior to the margins. This flow pattern arises from the fact that there is surplus of snow in the cold, interior parts (the accumulation zone) and melting and/or calving of icebergs at the margins (the ablation zones).

The stratification of the ice can be observed in many ways. The most direct way is to dig a pit in the snow on top of the ice sheet.

A double pit at a location close to the NEEM deep drill site. Two pits are dug closely together separated by a thin layer of snow. The observer sits in one pit which is closed with a sheet of plywood. Light shines through the snow layer from the other pit and makes the layers, the strata, clearly visible.

Each depositional event (e.g. a snow storm) is clearly seen as a distinct layer. Summer and winter snow can often be distinguished by a hard surface, or even sometimes a melt layer, at the top of each summer layer. This is very handy, as the layers in this way can be used for dating purposes, counting from the top how many summer and winter layers there are above a given depth. However, more sophisticated methods are needed in order to get accurate dating, especially because the visible stratification is less clear when the snow has been compressed into glacial ice. To distinguish the layers, analysis of the core is needed.

Measurements of the isotopic composition of the ice and the small amounts of impurities in the ice all show stratification that can be used for annual layer counting. The most well-known method is to use the stable isotope composition of the ice, but the best dating is achieved when many different records with annual layering are used in parallel to identify the annual layers.

Read more about
- dating using stable isotope data
- how diffusion limits in old ice can be dated using stable isotope data
- dating using multiple parallel series of impurity measurements
- the GICC05 time scale that is based on both approaches