Isotopic Character of Lead Deposited from the Atmosphere at a Grassland Site in the United Kingdom Since 1860

Abstract

Lead isotope analysis, using high-precision thermal ionization mass spectrometry, of herbage samples collected annually since the 1860s has revealed a steady reduction in the 206Pb/207Pb ratio from about 1.170 in 1880 to about 1.098 in the period 1980−1985. The value of 1.170 is very close to that (1.171) found previously in Scottish lake sediments for anthropogenic lead deposited prior to the introduction of leaded petrol. Analysis of the data suggests two dominant components in the lead deposited from the atmosphere over the period of 130 years studied. Prior to the turn of the century, lead of industrial or coal origin had a typical 206Pb/207Pb ratio of 1.170 following which a nonradiogenic component with a lower 206Pb/207Pb ratio (1.09 or less) made an increasingly more significant contribution. The decline in ratio commenced well before the introduction of tetraethyllead in petrol, indicating either changes in the source of lead-containing ores or coal or changes in the relative contributions of different sources. The generally smooth change in the isotopic character of the deposited lead suggests a slow change in the relative proportions of the dominant sources. The trend appears however to have been reversed following the declining use of lead additives in petrol. The deposition, in particular that with the low ratios associated in Britain with petrol additives, has had relatively little effect on the bulk soil. The 206Pb/207Pb ratio in bulk soil samples (to 23 cm) from the site, which had been undisturbed for at least 200 years, decreased from 1.187 in 1876 to 1.180 in 1984 but at no time reached the ratio found in the deposition on herbage. Analysis of sections taken down a soil core showed a steady increase in the 206Pb/207Pb ratio from 1.159 at the surface to 1.181 at 15 cm depth. Recent deposition with low 206Pb/207Pb ratios had a significant but not dominant contribution to the lead in the surface soil, which also contained a substantial proportion of older deposition with higher ratios. The ratios in the sections did not reach however that measured in bulk soil samples collected in 1876, indicating that lower ratio lead had reached a depth of at least 16 cm in the soil. Up to 50% or more of the lead at a depth of 16 cm could be anthropogenic in origin.