Americium-241 in peat bogs as a global marker of the beginning of the Anthropocene: examples from Europe and North America

Fallout radionuclides are being considered as potential global indicators of the beginning of the Anthropocene, yet questions persist about their preservation in bogs. This study assessed the reliability of Am-241 records in peat cores from acidic, ombrotrophic bogs. The source of Am-241 (half-life 432.7 years) in these natural archives is radioactive decay of the short-lived plutonium isotope Pu-241 (half-life 14.329 years) present in fallout from the atmospheric testing of nuclear weapons. We leveraged previously gathered data to gauge the congruence of Am-241 records with Pb-210 dates, examining the alignment of the Am-241 activity peak with the depth corresponding to the year AD 1963, as determined using the Pb-210 constant rate of supply (CRS) model. Our analysis employed 35 peat cores from 30 sites across Europe and North America, collected over the last three decades for trace metal deposition studies. Each core underwent independent Pb-210 dating. Among the 35 cores, 10 were excluded because of significant uncertainty in Am-241 peak quantification. Among the remaining 25 cores, 22 displayed a single Am-241 peak and 3 exhibited two peaks with similar Am-241 activities. Of the 22 cores with a single peak, 18 were within 10 years of AD 1963, with 10 of those being within 5 years. For 10 of the 22 cores, C-14 dates obtained from the atmospheric bomb pulse curve enabled comparison, with 9 showing <= 10 years of difference and 7 of those showing <= 5 years from the Pb-210 CRS model AD 1963. Factors contributing to age discrepancies included (i) analytical limitations of Am-241 measurements in peat using gamma spectrometry, (ii) the impact of peat accumulation rates on 241Am activities, i.e., dilution effects, (iii) post-depositional migration of Pu-241 and (or) Am-241, (iv) uncertainties introduced during 210Pb age-modelling, and (v) the limited number of samples dated using C-14. The usefulness of 241Am as an Anthropocene marker (assuming AD 1963) hinges on the accepted uncertainty level in defining this epoch. Detection of 241Am through gamma spectrometry was statistically validated as a real peak in 22 out of a total of 35 cores: this fraction (63%) is within the range reported in other studies. Given that the cores were gathered from 16 European and 14 North American sites, these peaks can be taken to denote the hemispheric onset of the Anthropocene. Lastly, common post-depositional migration of Cs-137 in ombrotrophic peatlands contrasts with its immobility in minerotrophic peatlands, where the presence of mineral matter and neutral pH inhibit its movement. This observation suggests that fens and swamps, characterized by minerotrophic conditions, may also serve as repositories of nuclear weapons test fallout, including Am-241.