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

Abstract

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 ≈300 to ≈42,000 and are indicative of an anthropogenic origin. Pb isotope ratios for snow samples retrieved in 1994 are highly variable ( 206Pb/ 207Pb = 1.148 to 1.193) and are characterized by the most radiogenic Sr isotope values ( 87Sr/ 86Sr ≥ 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 ( 206Pb/ 207Pb = 1.180 to 1.190) and 87Sr/ 86Sr 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. ( 206Pb/ 207Pb ≈ 1.20) and Canadian ( 206Pb/ 207Pb ≈ 1.15) sources. Pb isotope ratios ( 206Pb/ 207Pb = 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. A comparison of the Pb isotope results between lichens and snow samples from identical sample locations indicate that these either overlap (along St. Lawrence Valley), or are significantly different (north–south transect). The latter discrepancy may be attributed to either: (1) different time scales for the integration of the atmospheric signal (months for snow vs. years for lichens); (2) recording of the atmospheric signal at substantially different altitudes; or (3) the presence of an important, local point source of atmospheric pollution. Annual depositional budgets have been estimated for Pb, Cd, Zn, Cu, and Mn, and average values (g km −2 yr −1) are 1500, 130, 196,000, 1900, and 6400, respectively. Compared to previous depositional fluxes (1993–1994) estimated from adjacent regions in North America, those reported here are slightly lower with the exception of Mn and Zn. The nondecrease in depositional fluxes of Mn may be attributed to combustion of Mn-bearing fossil fuels by automotive vehicles. The exact cause for the elevated annual depositional values for Zn, however, remains enigmatic.