MERCURY ACCUMULATION IN DEVILS-LAKE, NORTH-DAKOTA - EFFECTS OF ENVIRONMENTAL VARIATION IN CLOSED-BASIN LAKES ON MERCURY CHRONOLOGIES

Sediment cores were collected from lakes in the Devils Lake Basin in N orth Dakota to determine if mercury (Hg) accumulation chronologies fro m sediment-core data are good indicators of variations in Hg accumulat ion rates in saline lakes. Sediment cores from Creel Bay and Main Bay, Devils Lake were selected for detailed analysis and interpretation. T he maximum Hg concentration in the Creel Bay core was 0.15 micrograms per gram at 8 to 9 centimeters. The maximum Hg concentration in the Ma in Bay core was 0.07 micrograms per gram at 5 to 7 centimeters. The ge neral decreases in Hg concentrations with depth are attributed to hist oric variations in atmospheric Hg deposition rate. Hg stratigraphies c ombined with Pb-210 and (CS)-C-137 dating analyses yield Hg chronologi es that indicate a general increase in Hg accumulation rates in Devils Lake since the middle of the 19th century. Mean modern Hg accumulatio n rates in Creel Bay were 4.9 nanograms per square centimeter per year , and rates in Main Bay were 1.8 nanograms per square centimeter per y ear. Mean preindustrial Hg accumulation rates in Creel Bay were 1.2 na nograms per square centimeter per year, acid rates in Main Bay were 1. 6 nanograms per square centimeter per year. Relatively low Hg concentr ations in recent sediments in the Devils Lake Basin, along with simila rities in Hg accumulation rates between lakes in the Devils Lake Basin and other lakes in the northern interior of North America, indicate t hat local sources of Hg are not important sources of Hg. Results of th e study indicate that accurate Hg chronologies are discernible in sedi ment cores collected from saline lakes. However, spatial and temporal variations in lake level and water chemistry common to saline lakes ma ke interpretation of radioisotopic and geochemical chronologies diffic ult. Hg geochemistry in Devils Lake, and presumably in other saline la kes, is dynamic. The results of this study indicate that the absolute amount of sediment transported to Devils Lake, along with the associat ed Hg and total organic carbon, and the distribution of sedimentation patterns in Devils Lake may be affected by changing lake levels.