History of the atmospheric deposition of major and trace elements in the industrialized St. Lawrence Valley, Quebec, Canada

Abstract

The history of the atmospheric deposition of major and trace elements over southwestern Quebec, Canada, was reconstructed using multielemental analysis of lacustrine sediments sampled in a small and undisturbed lake located on top of a mountain in the heart of the industrialized St. Lawrence Valley. Acid leachable and residual elements were extracted from a 37-cm long core (1-cm resolution) using clean techniques and analyzed by inductively coupled plasma mass spectrometry. Organic matter and sulfur concentrations were high and played a major role in the low postdepositional diagenetic remobilization of many trace elements. Sulfur, manganese, iron, arsenic, molybdenum and barium displayed a high mobility making it exceedingly difficult to infer unambiguously time-dependent changes in atmospheric deposition for these elements. Atmospheric deposition rates for the less mobile elements (e.g., potassium, vanadium, chromium, cobalt, nickel, copper, zinc, rubidium, cadmium, tin, antimony, mercury, thallium, lead, and bismuth) increased regularly between 1942 and 1960–1975 in the Lake Hertel area and then stabilized for most of these elements, with the exception of nickel, copper, zinc and tin. Lead deposition rate was reduced by about 25% between 1982 and 1995, and a slight decreasing trend was also found for cobalt, mercury, and thallium during the same period. Present-day atmospheric deposition of metals directly on the lake surface represents a small percentage of the sedimentary deposition rates at this location. Deposition followed by surface runoff and outwash of terrestrial organic and inorganic matter most likely is the driving mechanism leading to the non-diagenetic enrichment of metals in Lake Hertel sediments.