ISOTOPIC EVIDENCE TO ACCOUNT FOR CHANGES IN THE CONCENTRATION OF LEAD IN GREENLAND SNOW BETWEEN 1960 AND 1988

Preserved layers of snow deposited between 1960 and 1988, taken from 10.7 m and 70 m snow cores drilled at Summit in central Greenland in 1989, were analyzed for lead isotopes (Pb-206/Pb-207, Pb-208/Pb-207 and Pb-206/Pb-204.). The lead has an isotopic composition consistent with a mixture of USA and Eurasian origins. Although the Pb-206/Pb-207 ratio fluctuates, presumably due to seasonal changes in the weather patterns, there is a clear trend in values from about 1.16 in the early 1960s to 1.18 by 1976, then back to the initial value by the mid 1980s. The initial trend upwards reflects the progressive introduction of relatively radiogenic Mississippi Valley-type lead into USA gasoline during the 1960s, while the return to lower values indicates a decreased contribution from this source due to the switch to unleaded gasoline for motor vehicles. Eurasian lead, assumed to be the second component of the mixture, appears to have a relatively constant Pb-206/Pb-207 ratio over this period.