Long-term stability of records of fallout radionuclides in the sediments of Brotherswater, Cumbria (UK)

Results from a recent multi-core study of 137Cs and 210Pb in Brotherswater, Cumbria, are compared with those from two similar multi-core studies carried out at the same lake in 1976/1977 and 1988/1989. The purpose of this new study was to assess of the long-term stability of fallout records of these radionuclides in the sediments of Brotherswater and their reliability as tools for dating the sediments. Six cores were taken from four different areas of the lake, similar to those used in the earlier studies. Dried sediment samples from each core were analysed by gamma spectrometry for 210Pb, 226Ra, 137Cs and 241Am following a similar protocol to that used in the 1988/1989 study. The 137Cs results graphically illustrate the progressive burial of a peak in concentrations recording the 1963 fallout maximum from the atmospheric testing of nuclear weapons, and its subsequent reduction due to radioactive decay. The post-1986 cores are characterised by the appearance and burial of a second peak recording fallout from the 1986 Chernobyl accident. Identification of the 1963 137Cs peak in post-1986 cores was confirmed by the co-presence of traces of 241Am, also a product of nuclear weapons test fallout. In both the 1988/1989 study and the present study, 210Pb dates calculated using the CRS model for the most part placed 1963 and 1986 at depths very similar to those determined from the 137Cs records. The maintenance of this agreement over a period of more than two decades provides evidence of the reliability of sediment records in this lake and the validity of models used to interpret them. The 137Cs records were too indistinct and 1963 too recent to make similar comparisons in the case of the 1976/1977 cores. Agreement between 210Pb and 137Cs dates was best at sites where the net rate of supply of 210Pb to the sediment record was comparable to the atmospheric flux. Small but significant discrepancies were observed at sites where the 210Pb supply rate greatly exceeded the atmospheric flux, most probably due to additional time-dependent inputs from the catchment, or post-depositional sediment redistribution within the lake.