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

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

Mar 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000e%26psl.176..413r&link_type=abstract

Earth and Planetary Science Letters, Volume 176, Issue 3-4, p. 413-424.

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A 140 m snow/ice core drilled at Mont Blanc, France, has been analysed for Pb isotopes, Pb and Ba concentrations. The 206Pb/207Pb ratio, which was measured by thermal ionisation mass spectrometry, decreased steadily from ~1.18 about two centuries ago to ~1.17 in 1960, then fell rapidly to ~1.15 by 1968. Evidence of the Italian (Turin) isotopic lead experiment (IILE) was found in samples dated ~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. Summer samples were associated with smooth changes in the 206Pb/207Pb ratio while larger fluctuations were encountered in winter which is consistent with a low altitude inversion near Mont Blanc in the winter and free transfer of pollutants from lower to higher altitudes at other times. A plot of 208Pb/207Pb versus 206Pb/207Pb 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 dominate. 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 Mont Blanc and Summit (central Greenland) records shows they contain similar 206Pb/207Pb ratios between 1960 and 1968, although small differences in isotopic composition can be detected by also considering the 208Pb/207Pb ratio. However, after 1969 the two records diverge markedly, with the Greenland ratios being dominated by the highly radiogenic Mississippi valley-type Pb from the USA and with the Mont Blanc ratios moving to lower values particularly about the time of the IILE.

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