A comparative study of long-term Hg and Pb sediment archives

Environmental contextLead and mercury are toxic atmospheric pollutants emitted in large quantities since 1850. Accumulating lake and peat sediments capture the pollutants from the atmosphere and indirectly record changes in deposition through time. This study of four long-term sediment records addresses the questions, What proportion of this atmospheric deposition is natural background?' and Does the archive faithfully represent true rates of atmospheric deposition?' AbstractAtmospheric phenomena have a large influence on the flux of mercury (Hg) and lead (Pb) to terrestrial and aquatic ecosystems. Some direct phenomena involve high-frequency variations in air movement; other indirect processes involve longer-term changes in climate and associated vegetation and hydrology. Here, we use evidence from sediment cores to explore how these atmospheric and landscape processes produced large natural variations in Hg and Pb deposition over thousands of years before industrial pollution. Cores from Sargent Mountain Pond, coastal Maine, USA (16600 years), Plene Lake, south-western Czech Republic (15000 years), Lake Tulane, central Florida, USA (45000 years), and Caribou Bog, Orono, Maine, USA (10000 years) each illustrate how long-term local environmental changes influence the deposition and net retention of Hg and Pb. Important natural factors that emerge from comparisons among these four sites include forestation, changing groundwater hydrology, evolution of the watershed and lake system and (watershed area)/(lake area) ratio, all overlain by late-Holocene anthropogenic atmospheric pollution.