The origin of lead in the organic horizon of tundra soils: Atmospheric deposition, plant translocation from the mineral soil or soil mineral mixing?

Abstract

Knowledge of the anthropogenic contribution to lead (Pb) concentrations in surface soils in high latitude ecosystems is central to our understanding of the extent of atmospheric Pb contamination. In this study, we reconstructed fallout of Pb at a remote sub-arctic region by using two ombrotrophic peat cores and assessed the extent to which this airborne Pb is able to explain the isotopic composition ( 206Pb/ 207Pb ratio) in the O-horizon of tundra soils. In the peat cores, long-range atmospheric fallout appeared to be the main source of Pb as indicated by temporal trends that followed the known European pollution history, i.e. accelerated fallout at the onset of industrialization and peak fallout around the 1960s–70s. The Pb isotopic composition of the O-horizon of podzolic tundra soil ( 206Pb/ 207Pb = 1.170 ± 0.002; mean ± SD) overlapped with that of the peat ( 206Pb/ 207Pb = 1.16 ± 0.01) representing a proxy for atmospheric aerosols, but was clearly different from that of the parent soil material ( 206Pb/ 207Pb = 1.22–1.30). This finding indicated that long-range fallout of atmospheric Pb is the main driver of Pb accumulation in podzolic tundra soil. In O-horizons of tundra soil weakly affected by cryoturbation (cryosols) however, the input of Pb from the underlying mineral soil increased as indicated by 206Pb/ 207Pb ratios of up to 1.20, a value closer to that of local soil minerals. Nevertheless, atmospheric Pb appeared to be the dominant source in this soil compartment. We conclude that Pb concentrations in the O-horizon of studied tundra soils – despite being much lower than in boreal soils and representative for one of the least exposed sites to atmospheric Pb contaminants in Europe – are mainly controlled by atmospheric inputs from distant anthropogenic sources.