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

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 obtain ed during the 1998 winter season from northeastern North America. Median en richment 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 a n anthropogenic origin. The Pb isotope ratios correlate with geographic loc ation because snowpack from eastern Ontario and the northeastern USA are ch aracterized by the most radiogenic Pb-206/Pb-207 ratios (ca. 1.18-1.19), an d these decrease systematically in an easterly direction towards the Atlant ic coast(ca. 1.16). The ph isotope data for the 1998 samples of snowpack in dicate that atmospheric pollution in this region of North America is domina ted solely by a mixture of anthropogenic emissions from US (Pb-206/Pb-207 C a. 1.20) and Canadian (Pb-206/Pb-207 ca. 1.15) sources. This result contras ts with that obtained for 1997 snowpack from similar geographic regions, be cause the Ph isotope data for the latter suggest the involvement of an addi tional anthropogenic component, possibly that of Eurasian pollution transpo rted over the high Arctic. This difference in the distribution pattern of a tmospheric pollution over northeastern North America between the 1997 and 1 998 winter seasons may be related to the El Nino phenomenon. Sr isotope dat a for ail the 1998 snowpack samples define a large range in Sr-87/Sr-86 Val ues but most are between 0.709 to 0.710, similar to the value of present-da y seawater (ca. 0.7092). However, the majority of snowpack samples from eas tern Ontario are characterized by lower Sr-87/Sr-86 ratios (<0.709), and th ese correlate negatively with their enrichment factors of Pb. Such trends s uggest the presence of a relatively 'unradiogenic' source of anthropogenic Sr, possibly related to emissions from coal-fired power plants located in t he western and midwestern regions of the USA. Annual depositional budgets e stimated from 1998 snowpack indicate that those for Pb, Cd, and Cu are high est in eastern Ontario, and significantly lower in the remaining regions. T he 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 incre ase in atmospheric industrial emissions (since the 1980s), a lower residenc e time in the atmosphere, or particular chemical speciation that affects th e 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 fue ls by automotive vehicles. Copyright (C) 2000 Elsevier Science Ltd.