Sources and chronology of atmospheric lead deposition to a Canadian Shield lake: Inferences from Pb isotopes and PAH profiles

摘要

The depth-distribution of lead and its stable isotope ratios were determined in a dated sediment core from a Canadian Shield lake receiving anthropogenic Pb inputs exclusively from atmospheric deposition. The results demonstrate that anthropogenic Pb deposited to the sediments of this lake since the preindustrial period can be modeled successfully using as little as two isotopically distinct Pb types. The first, whose flux was not detectable before 1850, reached a maximum value around 1950, and then decreased significantly thereafter; it was characterized by Pb-206/Pb-207 and Pb-206/Pb-208 ratios of 1.222 and 0.495, respectively, and was derived mainly from coal combustion. The second, whose flux was not detectable before 1880, increased sharply to exceed that of the Pb type derived from coal combustion around 1930, and reached a maximum in the mid 1970s; it is characterized by Pb-206/Pb-207 and Pb-206/Pb-208 ratios of 1.179 and 0.482, respectively, and was derived mainly from leaded gasoline combustion and industrial sources. The chronology of deposition of these two anthropogenic lead types agrees well with the historical records of fossil fuel uses in Canada and the USA, and also with the history of sediment-deposited polycyclic aromatic hydrocarbons (PAHs) originating from coal combustion. The inventory of Pb derived from coal combustion (0.09 mu mol cm(-2)) is similar to 30% of that derived mainly, but not exclusively, from leaded gasoline (0.31 mu mol cm-2). Apportionment among source regions of lead deposited to the sediments during the period when leaded gasoline dominated Pb atmospheric emissions indicates that similar to 50% of this lead originated in the USA. Copyright (C) 2005 Elsevier Ltd.