Radiolead (²¹⁰Pb) and stable lead in the lichen Cladonia alpestris

Lichens of the genus Cladonia, mainly C. alpestris, collected during 1882-1972 were analyzed for stable lead and ²¹⁰Pb. The specific activity of ²¹⁰Pb (²¹⁰Pb/Pb ratio) indicates a constant value of about 2 mCi/kg up to 1940, and then decreased to 0.6 mCi/kg during 1950-1960. As the average annual ²¹⁰Pb deposition is relatively constant, the decrease in the ²¹⁰Pb/Pb ratio after 1940 indicates an increased deposition of stable lead after 1940. A single exponential variation of the ²¹⁰Pb/Pb ratio, in respect to latitude, was found in lichens collected in 1964. Values for ²¹⁰Pb and stable lead in C. alpestris, collected in central Sweden during 1961-1972, appear to be relatively constant and the average area content of ²¹⁰Pb and stable lead in lichen carpets was 15 ± 2 nCi/m² and 25 ± 5 mg/m² respectively. The pattern of vertical distribution was converted to a model for the translocation of lead in lichens, which included the diffusion principle. The diffusion coefficient for lead in the top layer of the lichen carpet was estimated to be about (0.2 ± 0.1) x 10-⁶ cm² x s-¹.