Dating of glaciers by lead-210

A method is described of dating neve and ice, based on the radioactive decay of 210Pb or RaD (T1/2 = 22 yr). Two techniques have been developed to measure the very low activities of 210Pb (0.1 to 5 dpm). One uses a semi-conductor detector to take spectral measurements of the α-activity of 210Po labelled with 208Po; the other is based on measurement of the β-radiation of 210Bi by a proportional counter. The method has been applied to several sections of neve taken in the Antarctic, in Greenland and on an alpine glacier, each covering a period of more than 100 yr. The results show that in the Polar regions the neve behaves as a closed system from the moment of precipitation, the 210Pb dating this moment. The accumulation rates deduced from the 210Pb distribution as a function of depth are in excellent agreement with those of other methods. In temperate glaciers, because of the homogenization of the neve caused by percolation of the melting water, the event dated by the 210Pb is the transformation of the neve into compact ice. The 210Pb ages obtained with ice samples from the ablation area of the Kesselwandferner (Austrian Tyrol) are in good agreement with current ideas on glacier dynamics. Comparison of the 210Pb and fission product concentrations has revealed no evidence of the artificial production of 210Pb by the explosion of nuclear devices. With samples of 2 kg, this method can be used to date ice with an age of 100 yr in the Antarctic and more than 120 yr in Greenland and in temperate zones. (author)