Abstract
Lead isotopic data are reported for epiphytic lichens, vegetation samples, and lacustrine sediments collected in the boreal forest of Québec between 47° and 55°N, and along the St. Lawrence Valley between 45° to 48°N. Lichens located up to 500 km north of Noranda (48°N) record a significant input of anthropogenic Pb emitted to the atmosphere from smelting activities. This input is not apparent beyond 53°N where only the isotopic signal typical of Canadian aerosols is recorded. Lichens along the St. Lawrence Valley show evidences for a dominant input from U.S. sources. The lead isotopic composition of lichens allow quantitative monitoring of the sources of atmospheric Pb. However, their slow metabolism and their unknown age detract from recording the Pb signal on short and precise timescales. Spruce needles have isotopic compositions undistinguishable from that of lichens; this reflects integration of the atmospheric Pb signal over a comparable time span, a result confirmed by the lead isotopic record in lacustrine sediments. Vegetation samples such as spruce bark, spruce wood, and deciduous tree leaves are more radiogenic than lichens from the same site. This may reflect mixing of radiogenic Pb metabolized from soil solutions through the root system with atmospheric Pb.
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