Pb and Sr isotopic compositions of snowpack from québec, canada: inferences on the sources and deposition budgets of atmospheric heavy metals (original) (raw)

2000, Geochimica et Cosmochimica Acta

Elemental 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 along two main transects from the province of Quebec (Canada); one north-south (between 47°N and 55°N; 1994) and the other within the St. Lawrence Valley (1997). Median enrichment factors (relative to upper crustal abundances) for Cd, Cu, Mn, Pb and Zn for all samples range from Ϸ300 to Ϸ42,000 and are indicative of an anthropogenic origin. Pb isotope ratios for snow samples retrieved in 1994 are highly variable ( 206 Pb/ 207 Pb ϭ 1.148 to 1.193) and are characterized by the most radiogenic Sr isotope values ( 87 Sr/ 86 Sr Ն 0.710). In contrast, the Pb and Sr isotope results for 1997 snow samples collected along the St. Lawrence Valley (below latitude 47°N), yield the most radiogenic Pb isotope ratios ( 206 Pb/ 207 Pb ϭ 1.180 to 1.190) and 87 Sr/ 86 Sr ratios between 0.708 and 0.710. The former indicate that the atmospheric pollution in this region of Quebec is dominated by a mixture of anthropogenic emissions from U.S. ( 206 Pb/ 207 Pb Ϸ 1.20) and Canadian ( 206 Pb/ 207 Pb Ϸ 1.15) sources. Pb isotope ratios ( 206 Pb/ 207 Pb ϭ 1.160 to 1.180) for 1997 samples collected north of latitude 47°N indicate input of an additional anthropogenic component, possibly that of Eurasian pollution being transported over the high Arctic during the winter season.