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

Preserved layers of snow deposited between 1960 and 1988, taken from 1 0.7 m and 70 m snow cores drilled at Summit in central Greenland in 19 89, 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 rati o fluctuates, presumably due to seasonal changes in the weather patter ns, there is a clear trend in values from about 1.16 in the early 1960 s to 1.18 by 1976, then back to the initial value by the mid 1980s. Th e initial trend upwards reflects the progressive introduction of relat ively radiogenic Mississippi Valley-type lead into USA gasoline during the 1960s, while the return to lower values indicates a decreased con tribution from this source due to the switch to unleaded gasoline for motor vehicles. Eurasian lead, assumed to be the second component of t he mixture, appears to have a relatively constant Pb-206/Pb-207 ratio over this period.