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 150 years. Sediment cores were taken from the four prevalent aquatic environments found on Block Island: saltwater pond, saltwater marsh, seepage pond, and freshwater wetland. Total mercury analysis was performed using a Milestone DMA-80 Direct Mercury Analyzer. Cores were dated using Lead-210 and Cesium-137 geochronology. An elemental analysis of the cores was performed using X-Ray Fluorescence (XRF) spectroscopy. Total mercury data was converted to mercury accumulation rates and compared with atmospheric records. Total mercury data showed background levels of 30-40 ppb prior to the onset of anthropogenic emissions. Mercury concentrations peaked in 1983 (322 ppb), at 7 times pre-anthropogenic levels. Most sites showed a decline in mercury concentrations over the past 20 years. Mercury concentrations varied with depositional environment. Mercury was highest in freshwater wetland cores (peak at 322 ppb), and lowest in Great Salt Pond (peak at 190 ppb). Differences in grainsize 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.
Neurath, Rachel, "Atmospheric mercury deposition in an isolated environment : a 150-year record at Block Island, Rhode Island" (2009). Honors Project, Smith College, Northampton, MA.
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