Pb and Sr isotopic evidence for sources of atmospheric heavy metals and their deposition budgets in northeastern North America

Abstract

Concentrations of Al, Ba, Cd, Cu, Mg, Mn, Pb, Rb, Sr, and Zn, as well as Pb and Sr isotopic compositions were determined in samples of snowpack obtained during the 1998 winter season from northeastern North America. Median enrichment factors, relative to upper crustal abundances, for Cd, Cu, Mn, Pb, and Zn for all samples range from ca. 50 to 36,000 and are indicative of an anthropogenic origin. The Pb isotope ratios correlate with geographic location because snowpack from eastern Ontario and the northeastern USA are characterized by the most radiogenic 206Pb/ 207Pb ratios (ca. 1.18–1.19), and these decrease systematically in an easterly direction towards the Atlantic coast (ca. 1.16). The Pb isotope data for the 1998 samples of snowpack indicate that atmospheric pollution in this region of North America is dominated solely by a mixture of anthropogenic emissions from US ( 206Pb/ 207Pb ca. 1.20) and Canadian ( 206Pb/ 207Pb ca. 1.15) sources. This result contrasts with that obtained for 1997 snowpack from similar geographic regions, because the Pb isotope data for the latter suggest the involvement of an additional anthropogenic component, possibly that of Eurasian pollution transported over the high Arctic. This difference in the distribution pattern of atmospheric pollution over northeastern North America between the 1997 and 1998 winter seasons may be related to the El Niño phenomenon. Sr isotope data for all the 1998 snowpack samples define a large range in 87Sr/ 86Sr values but most are between 0.709 to 0.710, similar to the value of present-day seawater (ca. 0.7092). However, the majority of snowpack samples from eastern Ontario are characterized by lower 87Sr/ 86Sr ratios (<0.709), and these correlate negatively with their enrichment factors of Pb. Such trends suggest the presence of a relatively ‘unradiogenic’ source of anthropogenic Sr, possibly related to emissions from coal-fired power plants located in the western and midwestern regions of the USA. Annual depositional budgets estimated from 1998 snowpack indicate that those for Pb, Cd, and Cu are highest in eastern Ontario, and significantly lower in the remaining regions. The former reflect a “meteorological corridor” in northeastern North America, which coincides with heavily industrialized and densely populated areas. Depositional budgets of Zn are extremely high in all areas investigated and their exact source(s) remains enigmatic. These may reflect either an increase in atmospheric industrial emissions (since the 1980s), a lower residence time in the atmosphere, or particular chemical speciation that affects the deposition of Zn from the lower troposphere. Depositional budgets for Mn are also high and may be related to the combustion of Mn-bearing fossil fuels by automotive vehicles.