VARIABILITY OF BACKGROUND MERCURY LEVELS IN ADIRONDACK LAKE SEDIMENT CORES

Establishing correct background mercury concentrations in sediment cores is essential for accurately determining recent atmospheric fluxes. The most common practice has been to assign lake-specific values based on mercury levels reached in the deepest core sections thereby representing pre-industrial deposition. Using this method, background mercury concentrations reported in the literature for lake sediments range from 10 to over 200 ppb. The several lakes in the Adirondack mountains that comprise this study also showed lake-specific mercury background concentrations with a somewhat smaller range of 110 to 273 ppb. These values were based on direct mercury analysis (Milestone DMA-80) of core sections that were dated on the basis of Cs-137 and Pb-210 profiles determined by gamma ray spectrometry. Measured differences in sediment composition, specifically the relative percentages of organic matter, amorphous silica derived from diatom shells, and residual mineral matter, can explain much of the variability in lake-specific background mercury levels. Measurements on Adirondack soils were used to constrain the organic plus mineral components of background mercury.