Caesium-137 profiles in the sediments of a partial-meromictic lake on Great Sandy Island (Fraser Island), Queensland, Australia

Hidden Lake is a perched, brown-water lake located in the centre of Great Sandy Island (Fraser Is.), S.E. Queensland. It is highly acid (pH 4.0), oligotrophic and is thermally and chemically stratified for most of the year. The sediments revealed a super(137)Cs profile which departed from the temporal pattern of super(137)Cs fallout in Brisbane and was represented by an exponential increase of super(137)Cs towards the surface sediments from ca. 32 cm depth. The possible causes of the divergent profile are discussed, including physical and biological mixing, lag in the transport of catchment material to the sedimentary basin, diffusion, recycling and biological concentration. It is hypothesised that a combination of the last four processes, with diffusion facilitated by the highly acid conditions, are the major causes of the observed super(137)Cs profile. Possible recycling and bioconcentration of super(137)Cs raises questions as to the validity of this method of dating in similar environmental conditions, and as to the interpretation of other palaeochemical data.