Recent declines in atmospheric mercury deposition in the upper Midwest

Historic increases in atmospheric mercury deposition caused by anthropogenic emissions have been well documented from sediment cores from lakes and peatlands in North America and Europe. Few previous studies have addressed the question of whether mercury deposition has increased continuously to the present or whether it has declined in recent decades. We present stratigraphic data from a suite of Minnesota lakes that indicate mercury deposition peaked in the 1960s and 1970s, at least for the upper Midwest. Recent declines, which appear in both rural and urban lakes, are not evident in sediment cores from remote coastal lakes in southeastern Alaska. Because the Alaskan sites provide an integrated sample of mercury pollution in the Northern Hemisphere, these results imply that global mercury emissions have not abated and that decreased inputs to Midwestern lakes are caused by reduced emissions from regional sources. U.S. inventory data suggest that decreased emissions likely resulted from reduced industrial use of mercury, use of pollution-control technologies that incidentally capture mercury, a shift from coal to natural gas for commercial and residential heating, and a decrease in uncontrolled waste incineration. Increased stack height and other factors that favor long-distance transport could be partially responsible for the trend.