MOBILITY AND DIAGENESIS OF PB AND PB-210 IN PEAT

Peatlands long have been considered to preserve the record of atmospheric deposition of anthropogenic contaminants such as Pb. In the past two decades, Pb-210 hs been widely used to date recent strata of peat and to calculate accumulation rates. The assumption that Pb and Pb-210 are immobile and not subject to diagenesis in peat has been questioned but not rigorously tested. We attempted to determine if Pb is mobile in peatlands and Pb profiles are altered by diagenic processes by constructing a mass balance for Pb about a small peatland, by comparing inventories, concentrations, and accumulation rates of Pb and Pb-210 in peatlands across northeastern North America, and by examining the relationship between concentrations of Pb in bog waters and peat in numerous sites. Our results clearly demonstrate that Pb and Pb-210 are mobilized by the organic-rich waters of peatlands. Profiles of Pb and Pb-210 at depths below the water table do not preserve the record of atmospheric deposition, and inventories fo Pb and Pb-210 are depleted in peatland hollows. Concentrations of Pb in bog waters are regulated by the concentration of Pb in the peat and the concentration of dissolved organic carbon. The mass balance for one bog indicated that over the specific three-year period of study more than 30% of inputs of Pb were not retained within the peat. As a result of this mobility, dates based on Pb-210 can be biased and inaccurate by as much as 30 years. Dates based on Pb-210 should be verified by other techniques, especially when the inventory of Pb-210 is less than that expected from local rates of deposition.