Lead-210 age dating of three peat cores in the Jura Mountains, Switzerland

Peat cores collected from three sites in the Jura region of Switzerland, La Tourbiere des Genevez, Etang de la Gruere and Praz Rodet, were analysed for the fallout radionuclides Pb-210, Cs-137, Cs-134 and Am-241, and the minerogenic radionuclide Ra-226. Unsupported Pb-210 inventories of the cores were all in the range 3900-4784 Bq m(-2) and are consistent with their having largely retained the atmospheric flux (ca.130 Bq m(-2)y(-1)). In contrast, comparisons with earlier (1986) cores from these sites suggest that there have been significant losses of fallout Cs-137. Samples from all three sites had low but significant levels of supported Pb-210 (Ra-226) activity, the origin of which is presumably wind blown soil dust. Pb-210 dates calculated using the CRS model were independently validated by Am-241 and pollen stratigraphy. The core from La Tourbiere des Genevez had a fairly conventional Pb-210 activity versus depth profile indicating more or less constant net accumulation during the past 100 years. At the other two sites however the Pb-210 profiles contained significant deviations from simple exponential decline that may record episodes in the bog during which there were major variations in the net dry mass accumulation rate. These episodes are dated 1930-50 (Etang de la Gruere) and 1960-80 (Praz Rodet). Although the depths at which total Pb-210 reaches equilibrium with the supporting Ra-226 range widely, from ca.37 cm in Etang de la Gruere to ca.60 cm in Praz Rodet, mean net dry mass accumulation rates for the past 100 years are remarkably similar, the values for all three sites are in the range 0.023-0.027 g cm(-2)y(-1).