PB-210 AGE DATING OF 3 PEAT CORES IN THE JURA MOUNTAINS, SWITZERLAND

Peat cores collected from three sites in the Jura region of Switzerlan d, La Tourbiere des Genevez, Etang de la Gruere and Praz Rodet, were a nalysed for the fallout radionuclides Pb-210, Cs-137, Cs-134 and Am-24 1, and the minerogenic radionuclide Ra-226. Unsupported Pb-210 invento ries of the cores were all in the range 3900-4784 Bq m(-2) and are con sistent with their having largely retained the atmospheric flux (ca.13 0 Bq m(-2)y(-1)). In contrast, comparisons with earlier (1986) cores f rom these sites suggest that there have been significant losses of fal lout Cs-137. Samples from all three sites had low but significant leve ls of supported Pb-210 (Ra-226) activity, the origin of which is presu mably wind blown soil dust. Pb-210 dates calculated using the CRS mode l were independently validated by Am-241 and pollen stratigraphy. The core from La Tourbiere des Genevez had a fairly conventional Pb-210 ac tivity versus depth profile indicating more or less constant net accum ulation during the past 100 years. At the other two sites however the Pb-210 profiles contained significant deviations from simple exponenti al decline that may record episodes in the bog during which there were major variations in the net dry mass accumulation rate. These episode s are dated 1930-50 (Etang de la Gruere) and 1960-80 (Praz Rodet). Alt hough the depths at which total Pb-210 reaches equilibrium with the su pporting Ra-226 range widely, from ca.37 cm in Etang de la Gruere to c a.60 cm in Praz Rodet, mean net dry mass accumulation rates for the pa st 100 years are remarkably similar, the values for all three sites ar e in the range 0.023-0.027 g cm(-2)y(-1).