Pb and 206Pb/207Pb isotopic analysis of a tree bark pocket near Sheffield, UK recording historical change in airborne pollution during the 20th Century

A section of tree trunk (beech, Fagus sylvatica) containing a bark pocket progressively enclosed at the junction of two branches was collected from a semi-rural location near Sheffield, UK. According to the annual growth rings, the bark pocket formed between ca. 1919 and 1998 (the date of felling). The bark pocket was divided into consecutive samples of differing radial depth (and thus age), that were analysed by ICP mass spectrometry. The Pb concentration varied from 7 to 78 mg kg−1 and the 206Pb/207Pb isotope ratio from 1.11 to 1.15. In contrast, the current surface bark contained 46 mg kg−1 Pb and recorded a 206Pb/207Pb ratio of 1.11. The changing elemental and isotopic composition of the bark pocket recorded historical change in the level and sources of airborne Pb pollution. An overall increase in Pb concentration with time was accompanied by a progressive reduction in 206Pb/207Pb from ca. 1935 to 1943. Mass balance calculations indicated that Pb additives in petrol contributed significantly to the rise in concentration, accounting for a maximum of 50% of the total Pb for ca. 1986–1998, but that other sources were generally dominant. The highest Pb concentrations were recorded from ca. 1951 to 1973, suggesting a high level of industrial pollution. A reduction in Pb concentration and reversal of the trend in 206Pb/207Pb was observed in the current bark.