ADVANTAGES OF 241AM AS A DATING TOOL IN SEDIMENTARY SYSTEMS

¹³⁷Cs is the most common tracer of direct fission products from the Cold War Era nuclear testing period of 1953-1963, and thus widely used as a sediment dating tool. However, in certain sedimentary environments, Cs can be geochemically mobile and prone to biological uptake, which can limit its accuracy in sedimentation studies. Americium-241 (²⁴¹Am) is also derived from Cold War atomic detonations following ²⁴¹Pu decay and has a substantially longer half-life of 432.2 years compared to the half-life of 30.2 years of ¹³⁷Cs. Americium has strong gamma emission at 59.5 keV and can be measured concurrently with ²¹⁰Pb and ²²⁶Ra analysis. We hypothesize that Pu and Am form stronger associations with solid phases and are less prone to biological uptake, and are thus more accurate sediment chronometers. Here we compare ²⁴¹Am and ¹³⁷Cs profiles in sediments and vegetation from the eastern U.S. and find that the ²⁴¹Am concentration maxima is sharper than the ¹³⁷Cs profile, and we show preferential ¹³⁷Cs uptake by native vegetation compared with ²⁴¹Am. The ¹³⁷Cs profile shows an enrichment in the surface compared with ²⁴¹Am, and we find up to 1070 Bq ¹³⁷Cs/kg in live native vegetation compared with ²⁴¹Am, which is rarely detected in foliage. Further demonstrating the prevalence of biological uptake of ¹³⁷Cs, in a separate analysis of honey samples produced in the US east coast, 68 of 122 honey samples contain detectable levels of ¹³⁷Cs ( ≥0.03 Bq ¹³⁷Cs kg⁻¹ ), while virtually no samples possess detectable levels of ²⁴¹Am. We expect that ¹³⁷Cs based dating models will be unreliable in low potassium regions, while ²⁴¹Am models will be more accurate. With continued biological uptake and radioactive decay of ¹³⁷Cs, ²⁴¹Am may become a preferable alternative in future sediment dating studies.