Sources and Evolution of Anthropogenic Lead in Dated Sediments from Lake Clair, Québec, Canada

Two sediments cores were collected from the deepest part of Lake Clair (Québec, Canada) to assess the historical sources of Pb additions to the lake. The cores were collected by divers by carefully inserting a Plexiglas tube into the sediments. To determine the stratigraphic ages of the sediments, ²¹⁰Pb and ¹³⁷Cs activities were counted by γ-ray spectroscopy. Lead concentrations and isotopic ratios were performed by inductively coupled plasma–mass spectrometry (ICP–MS), following digestion of the samples with a mixture of HF, HNO₃, and HClO₄ acids and Pb separation by anion-exchange chromatography. Starting at the middle of the 19th century, Pb content of the sediments increased until 1975. The maximum Pb enrichment factor of 35 times (relative to the natural background) was found in sediments deposited in 1975. At this time, excess Pb flux was estimated to be about 0.03 g m⁻² yr⁻¹ Before 1872, the Pb isotopic ratios were relatively stable (mean ²⁰⁶Pb/²⁰⁷Pb = 1.20 ± 0.01), reflecting the natural Pb background. Between 1872 and 1894, the source of anthropogenic Pb was highly radiogenic as shown by the Pb isotopic signatures of the sediments (mean ²⁰⁶Pb/²⁰⁷Pb = 1.22 ± 0.01), possibly reflecting deforestation and agricultural developments in the St.-Lawrence Valley. Between 1894 and 1937, widespread use of industrial and domestic charcoals may explain the isotopic composition of Pb accumulated in the sediments (mean ²⁰⁶Pb/²⁰⁷Pb = 1.19 ± 0.01). From 1937 to 1975, Pb isotopic compositions became less radiogenic (²⁰⁶Pb/²⁰⁷Pb from 1.18 to 1.17) even though elemental Pb abundance reached extremely high values (623 mg kg⁻¹). This isotopic shift reflects increased use of alkyl-lead in gasoline. For sediments accumulated between 1967 and 1996, the U.S. contribution to anthropogenic Pb accumulated in Lake Clair sediments amounted to between 30 and 63%.