137Cs-ajoitusmenetelmän tarkkuus ja luotettavuus

137Cs is an artificial isotope generated by fission reactions, whose deposition times are well known both globally and locally. The 137Cs dating method measures the 137Cs activity in sediment layers from different depths, and by utilizing knowledge of the deposition moment dating accuracy can be up to one year. The prerequisites of the dating method are not only the knowledge of precise moments of deposition, but also calm sedimentation and understanding of the sediment disturbances. The best destinations for using the 137Cs dating method are deep basin sediments, where bioturbation is very low and sedimentation is calm. This study looked at the accuracy and reliability of the 137Cs dating of the bottom sediment from five different lakes. There were clear annual varves in four out of five lakes, so the sediment was not mixed and mechanical movements of 137Cs had not occurred. The most significant fall-off affecting Finland in 1986 was the Chernobyl nuclear accident and when analyzing sediment samples in 1986, it could even be traced back to a single varve. Another significant 137Cs fall-off has been the nuclear tests of the 1950s to 1960s. The 137Cs peak of these events was almost invisible compared to the 137Cs activity of 1986. Regardless of the limnological features of the lake, 1986 had also migrated downward in the analysis results. The more porous sediment was, the stronger the spread had occurred. According to the results of this study the 137Cs dating method is still usable, especially in Finland at the time of the Chernobyl nuclear accident of 1986. From 1950 to 1960, the time of nuclear testing, this method is far more uncertain in Finland, although the marker horizon is more globally distinguishable. However, downward migration of 137Cs was clearly evident even in a calm sedimentation environment, which is a major hindrance to the method. Chemical follow-up studies would be useful to clarify the causes of migration.