Detection of a hot 137Cs particle in marine sediments from Norway: potential implication for 137Cs dating

An anomalous value in the 137Cs record from a marine sediment core in Veafjord, Norway, appears to be due to the presence of a micron-size hot radioactive particle. The 137Cs concentration in a subsample from the surficial 0–1 cm sediment slice was over 3 times higher than in the adjoining slice and double that in a concentration peak significantly deeper in the core dating from the period of Chernobyl fallout. To determine the cause of the anomaly, the sample was divided into two equal parts each of which were then reanalysed. Whereas one had a normal concentration similar to that of the adjoining slice, the concentration in the other was now over 5 times higher. Analyses of three further splits of the high activity subsamples followed a similar pattern with one part having a normal concentration, and the other a concentration that nearly doubled each time. The results are consistent with the presence of a hot 137Cs particle embedded in the sediment matrix. Its activity was estimated to be 15 ± 2 mBq. Possible sources are fallout from the 1986 Chernobyl accident or marine discharges from UK nuclear installations in the 1970s. The delayed input of such a particle into the sedimentary record highlights the need for care in using 137Cs records for dating sediment accumulations in areas open to contamination from these or similar sources.