Detection of fallout 241Am in US Atlantic salt marsh soils

We report the presence of the fallout radionuclide Am-241 (t(1/2) = 433 years) in salt marsh soils from two U.S. Atlantic estuaries and discuss its utility as a particle tracer and geochronometer. This work is motivated by the knowledge that Cs-137, the most widely used geochronometer in environmental studies, will decay to extinction during the next century. At the same time, levels of Am-241, produced by radioactive decay of fallout Pu-241, will continue to increase on Earth's surface as they have since the height of atmospheric nuclear weapons testing in the 1960s. Measurements of Am-241 in soils at eighteen salt marsh locations were made by non-destructive gamma spectrometry and compared to activities of Cs-137 in the same samples. Results indicate that decay of fallout Pu-241 can explain the presence of Am-241 in the soils, and that the activities are sufficiently high to provide meaningful chronological information with acceptable confidence limits. We achieved a detection limit of 0.28-1.47 Bq kg(-1) using low-energy, planar germanium detectors and 11-55 g powderized samples. Activities of Am-241 (0.08-6.44 Bq kg(-1)) were similar in mineral- and organic-rich marsh soils indicating that soil composition does not appear to influence the initial capture of Pu-241 and retention of its Am-241 progeny. Given its high affinity for fine particles, long half-life, and ease of measurement by non-destructive gamma spectrometry, Am-241 has potential to serve as an alternative to Cs-137 geochronometry in salt marshes and perhaps other estuarine and coastal environments. (C) 2017 Elsevier Ltd. All rights reserved.