ATMOSPHERIC MERCURY DEPOSITION IN AN ISOLATED ENVIRONMENT: A 300-YEAR RECORD AT BLOCK ISLAND, RHODE ISLAND

Block Island, located 20 km off the Rhode Island coast, is isolated from local point sources of mercury, providing a record of atmospheric mercury deposition in southern New England over the last 300 years. Sediment cores were taken from the four prevalent aquatic environments found on Block Island: salt marsh, freshwater bog, salt pond, and freshwater pond. Total mercury analysis was performed using a Milestone DMA-80 Direct Mercury Analyzer. Cores were dated using Lead-210 and Cesium-137, as well as pollen data. An elemental analysis of the cores was performed using X-ray Fluorescence (XRF). Total mercury data was converted to mercury accumulation rates and compared with precipitation records for Block Island, as well as present-day wet deposition of mercury at West Point, NY, and Barnstable County, MA, and dry deposition in Amherst, MA. Total mercury data showed background levels of 20-40 ppb prior to the onset of an anthropogenic signal that reached a maximum of 320 ppb. Most sites showed a decline in mercury over the past 40 years. Concentration patterns show some similarities with cores from Long Island Sound and marshes along the coast of Connecticut. Mercury concentrations vary with depositional environment. Mercury concentrations were highest in freshwater cores (peak at 320 ppb), and lowest in the Great Salt Pond of Block Island (peak at 190 ppb). Differences in grain size and sediment focusing in ponds with larger watersheds could account for these variations, however the ponds studied on Block Island appear to be seepage lakes, which would minimize sediment focusing. Peaks in mercury concentrations are thought to correspond to times of maximum mercury input into the atmosphere, and may also be associated with periods of unusually high precipitation.