Pb isotope ratios of lake sediments in West Greenland: inferences on pollution sources

Lead (Pb), like many other pollutants, is carried into the Arctic by long-range atmospheric transport from industrial centers at lower latitudes. Unlike other pollutants, Pb can be used to assess emission source regions through the use of stable Pb isotope analyses. Using sediment cores from 17 lakes (three profiles and 14 top/bottom sample pairs) in the Søndre Strømfjord (Kangerlussuaq) region, West Greenland (67°N), this study assesses the extent and origin of Pb pollution along a 150 km transect between the Inland Ice and Davis Strait. Like ice core analyses from the interior of Greenland, the isotope analyses suggest pre-industrial contamination, although significant concentration changes in the lake sediments do not occur until the 18th/19th centuries, with the maximum concentrations occurring about 1970. Compared to the background, the Pb concentrations in recent sediments have increased about 2.5-fold, with slightly higher enrichments towards the coast, where annual precipitation is highest. For all of the lakes, there is a major decline in the ²⁰⁶Pb/²⁰⁷Pb ratio in the recent sediments (mean 1.218±0.030) as compared to deeper sediments (mean 1.365±0.084). Using a Pb isotope mixing model, we calculated an excess Pb isotope ratio, i.e. the isotope ratio necessary to produce the observed declines in recent sediments. While studies of atmospheric aerosols in the high Arctic (²⁰⁶Pb/²⁰⁷Pb ratio ～1.16) have indicated that Russian emissions (²⁰⁶Pb/²⁰⁷Pb ratio ～1.15–1.16) are a dominant source of arctic pollution, the excess Pb ratios of the lake sediments in the Søndre Strømfjord region (²⁰⁶Pb/²⁰⁷Pb ratio ～1.14–1.15), in the low Arctic, suggest that W Europe (²⁰⁶Pb/²⁰⁷Pb ratio ～1.14) is also a major emission source for this region.