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3.3.2.3. Dating ice cores

One of the biggest problems in any ice core study is determining the age-depth relationship. Many different approaches have been used and it is now clear that fairly accurate time scales can be developed for the last 10,000 years. Prior to that, there is increasing uncertainty about ice age. The problem lies with the fact that the age-depth is highly exponential, and ice flow models (e.g. Dansgaard & Johnson, 1969) are often needed to determine the ages of the deepest sections of ice cores. For example, the upper 1000m of a core may represent 50,000 years, whilst the next 50m may span another 100,000 year time period, due to the severe compaction, deformation and flow of the ice sheet in question.

Radio isotope datingIsotopes of certain elements, rather than being stable (like H and O) are radioactive and unstable, and decay by emitting nuclear particles (a or b particles). The rate at which they do this is invariable so that a given quantity of a radioactive isotope will decay in a known interval of time; this is the basis of radio-isotope dating methods., using 210Pb (lead) (Crozaz & Langway, 1966), 32Si (silicon), 39Ar (argon) (Oeschger et al., 1977) and 14C (carbon) (Paterson et al., 1977) have all been used with varying degrees of success, over different time scales, to determine the age of ice cores.

Certain components of ice cores may reveal quite distinct seasonal variations which enable annual layers to be identified, providing accurate time scales for the last few thousand years. Such seasonal variations may be found in 18O values, trace elements and microparticles (Hammer et al., 1978).

Where characteristic layers of known ages can be detected, these provide valuable chronostratigraphic markers against which other dating methods can be verified. So-called reference horizons have resulted from major explosive volcanic eruptions. These inject large quantities of dust and gases (principally sulphur dioxide) into the atmosphere, where they are globally dispersed. The gases are converted into aerosols (principally of sulphuric acid) before being washed out in precipitation. Hence, after major eruptions, the acidity of snowfall increases significantly above background levels (Hammer, 1977). By identifying highly acidic layers (using electrical conductivity) resulting from eruptions of known age, an excellent means of checking seasonally based chronologies is available.