A two century record of lead isotopes in high altitude Alpine snow and ice

A 140 m snow/ice core drilled at Mont Blanc, France: has been analysed for Pb isotopes, Pb and Ba concentrations. The Pb-206/Pb-207 ratio, which was m easured by thermal ionisation mass spectrometry, decreased steadily from si milar to 1.18 about two centuries ago to similar to 1.17 in 1960, then fell rapidly to similar to 1.15 by 1968. Evidence of the Italian (Turin) isotop ic lead experiment (IILE) was found in samples dated similar to 1977 where the ratio dipped to 1.117. By the early 1990s it had returned to mid 1960s values. Large seasonal variations were found in Pb and Ba concentrations. S ummer samples were associated with smooth changes in the Pb-206/Pb-207 rati o while larger fluctuations were encountered in winter which is consistent with a low altitude inversion near Mont Blanc in the winter and free transf er of pollutants from lower to higher altitudes at other times. A plot of P b-208/Pb-207 versus Pb-206/Pb-207 ratios reveals three isotopic groupings, associated with the periods pre-1923, 1923-1968 and 1969-1991. In the first group, the isotopic composition is consistent with local mining, smelting and coal burning, while in the second, motor vehicle exhaust emissions domi nate. In the third group, motor vehicle emissions also dominate but the Pb is even less radiogenic. During this period the IILE occurred and there was a reduction in the use of leaded gasoline in Europe. A comparison of the M ont Blanc and Summit (central Greenland) records shows they contain similar Pb-206/Pb-207 ratios between 1960 and 1968, although small differences in isotopic composition can be detected by also considering the Pb-208/Pb-207 ratio. However, after 1969 the two records diverge markedly, with the Green land ratios being dominated by the highly radiogenic Mississippi valley-typ e Pb from the USA and with the Mont Blanc ratios moving to lower values par ticularly about the time of the IILE. (C) 2000 Published by Elsevier Scienc e B.V. All rights reserved.