Recent Pb, Cs and Am accumulation in an ombrotrophic peatland from Amsterdam Island (Southern Indian Ocean).

Over the past 50 years, Pb, Cs and Am have been abundantly used in reconstructing recent sediment and peat chronologies. The study of global aerosol-climate interaction is also partially depending on our understanding of Rn-Pb cycling, as radionuclides are useful aerosol tracers. However, in comparison with the Northern Hemisphere, few data are available for these radionuclides in the Southern Hemisphere, especially in the South Indian Ocean. A peat core was collected in an ombrotrophic peatland from the remote Amsterdam Island (AMS) and was analyzed for Pb, Cs and Am radionuclides using an underground ultra-low background gamma spectrometer. The Pb Constant Rate of Supply (CRS) model of peat accumulations is validated by peaks of artificial radionuclides (Cs and Am) that are related to nuclear weapon tests. We compared the AMS Pb data with an updated Pb deposition database. The Pb flux of 98 ± 6 Bq·m·y derived from the AMS core agrees with data from Madagascar and South Africa. The elevated flux observed at such a remote location may result from the enhanced Rn activity and frequent rainfall in AMS. This enhanced Rn activity itself may be explained by continental air masses passing over southern Africa and/or Madagascar. The Pb flux at AMS is higher than those derived from cores collected in coastal areas in Argentina and Chile, which are areas dominated by marine westerly winds with low Rn activities. We report a Cs inventory at AMS of 144 ± 13 Bq·m (corrected to 1969). Our data thus contribute to the under-represented data coverage in the mid-latitudes of the Southern Hemisphere.