Usefulness of Fallout 241Am as a Geochronology Tool in Salt Marsh Sediments: Insights from Oregon on the U.S. West Coast

Abstract Studying coastal ecogeomorphic change relies on reliably and accurately dating recent sediment deposits. While a handful of short-lived radioisotopes are proven suitable geochronometers, all have limitations. One particularly useful anthropogenic radionuclide, 137 Cs, is rapidly approaching extinction due to its half-life. We evaluate whether the bomb-produced radionuclide 241 Am is a reliable alternative to 137 Cs. In 75 cores from eight Oregon intertidal zones, 241 Am was readily detectable in higher-elevation marsh sediments and, when present, was less mobile post-deposition than 137 Cs. Accretion derived from the depth of the 241 Am and 137 Cs peaks associated with the height of nuclear proliferation were statistically similar to rates determined from excess 210 Pb. Although 241 Am activities are relatively low in these environments, their detectability is expected to increase as 241 Pu continues to decay. 241 Am may therefore be an accurate event-horizon chronometer in high marsh sediments, and will likely become an increasingly preferable dating technique compared to 137 Cs geochronology.