11,000 YEARS OF MERCURY DEPOSITION IN ELK LAKE, MINNESOTA--EFFECTS OF CLIMATE AND VEGETATION CHANGE ON THE MERCURY CYCLE

Elk Lake, in west central Minnesota, contains a continuous sequence of annually laminated (varved) sediments extending nearly 11,000 years into the past. Prior studies of sediment cores, presented mostly in GSA Special Paper 276, document a sequence of climatic and consequent biotic changes in the lake and its surroundings. Profiles of mercury concentration and accumulation rate reported here show the widely detected anthropogenic pulse of mercury deposition (up to about 200 ppb total Hg and about 23 ugHg/m2/yr) coincident with the onset and growth of industrialization in the past century. The profiles also show numerous intervals during the past 8,000 years when mercury concentration and/or accumulation rate exceeded modern values. Many of these intervals correlate with changes in climate and vegetation in the region. The greatest recorded concentration of mercury (about 360 ppb) occurred at about 8 cal. ka, a time when a conifer forest was rapidly replaced by prairie grass and sage vegetation. This vegetation change was a result of changes in atmospheric circulation, which brought warm, dry, windy conditions to the Midwestern U.S. Breakdown of a forest soil may have liberated mercury that had gradually accumulated in forest-floor organic material during the 4,000-year interval from glacial retreat to the invasion of prairie vegetation. Other spikes in mercury deposition (up to about 200 ppb total Hg and 134 ugHg/m2/yr) occurred in the mid-Holocene (4-8 Ka) during periods of dry climate and high input rates of eolian dust. Windborne dust apparently contained higher mercury content than autocthonous lake sediment and resulted in both higher concentrations and faster accumulation rates of mercury during dry and windy periods. Our results indicate that climate change can significantly impact the geochemical cycling of mercury and that eolian processes are significant in mercury transport on at least a regional scale.