The History of Atmospheric Deposition of Cd, Hg, and Pb in North America: Evidence from Lake and Peat Bog Sediments

Abstract

Lake and ombrotrophic peat bog sediments record increases in the concentrations and accumulation rates of Cd, Hg, and Pb for most of temperate North America for the last 100 years. These increases are largely related to the burning of coal, smelting of nonferrous metals, the transportation industry, and the industrial production of chlorine. Modern atmospheric fluxes of Cd in central North America are about 1,000 × background fluxes; accumulation rates for Cd in sediments have increased two to three times above background, beginning about 100 years ago. The anthropogenic component of the total Cd flux in recent sediment is about 0.1 to 0.2 mg/m 2 /yr. Global-scale Hg pollution of the atmosphere is suggested by concentrations of Hg in northern hemisphere air that are double the Hg content of southern hemisphere air. The accumulation rates of Hg in sediment have approximately doubled over the last 100 years. However, these accumulation rates are approximately an order of magnitude less than those for Cd. Modern increases in Pb concentrations are ubiquitous for all lakes examined thus far in North America. Input is from multiple sources and thus the timing of increased accumulation rates in sediment varies across the continent. Typical modern accumulation rates reach maxima of 20 to 30 mg/m 2 /yr, or 100 × that of Cd and 1,000 × that of Hg. Recent decreases in atmospheric Pb are reflected in decreases in the accumulation rate of Pb in both lake and peat bog sediment in eastern North America.