GSA Connects 2021 in Portland, Oregon

Paper No. 118-6
Presentation Time: 2:30 PM-6:30 PM


VOLANTE, Paul, Department of Geology, William & Mary, Williamsburg, VA 23187 and KASTE, J.M., Department of Geology, College of William & Mary, Williamsburg, VA 23187

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.