ISOTOPIC COMPOSITION OF EPIPHYTIC LICHENS AS A TRACER OF THE SOURCES OF ATMOSPHERIC LEAD EMISSIONS IN SOUTHERN QUEBEC, CANADA

Lead isotopic data are reported for epiphytic lichens, vegetation samples, and lacustrine sediments collected in the boreal forest of Quebec between 47 degrees and 55 degrees N, and along the St. Lawrence Valley between 45 degrees to 48 degrees N. Lichens located up to 500 km north of Noranda (48 degrees N) record a significant input of anthropogenic Pb emitted to the atmosphere from smelting activities. This input is not apparent beyond 53 degrees 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.