The radioactive lead isotopes come from decay of uranium and thorium, so lead from different mines will have different isotope ratios depending on how much U and Th happened to be in that ore.

Not all leaded gasoline was the same either:

> 206Pb/207Pb ratios commonly found in Pb ores throughout the world range between 16.0–18.5 and 1.19–1.25, respectively (Hansmann and Köppel, 2000). Exception to this rule is the commonly used Pb ore from the Broken Hill deposit, Australia, which is characterised by extremely low 206Pb/207Pb ratios (1.03–1.10). On the other hand, Pb originating from the Mississippi Valley ore deposit, USA, exhibits significantly more radiogenic Pb isotopic composition (206Pb/204Pb N20.0; 206Pb/207Pb= 1.31–1.35) (Doe and Delevaux, 1972). American leaded gasoline reflected therefore significantly higher 206Pb/207Pb ratios compared to European gasoline (Fig. 1). The introduction of the European leaded gasoline around 1945 resulted in a steep decrease of the 206Pb/207Pb ratio of atmospheric Pb (Weiss et al., 1999; data from peat deposits). The isotopic composition of leaded gasoline was to some extent dependent on economical factors, such as the availability and price of Pb ores and has evolved due to the different Pb ores used. For example, Pb used for French leaded gasoline originated from Australian, Moroccan and Swedish ores and the contribution of the separate ores changed during time (Véron et al., 1999). It is therefore indispensable to gather data concerning the origin of gasoline used in studied regions.

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