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 241 Am records in peat cores from acidic, ombrotrophic bogs. The source of 241 Am (half-life 432.7 years) in these natural archives is radioactive decay of the short-lived plutonium isotope 241 Pu (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 241 Am records with 210 Pb dates, examining the alignment of the 241 Am activity peak with the depth corresponding to the year AD 1963, as determined using the 210 Pb 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 210 Pb dating. Among the 35 cores, 10 were excluded because of significant uncertainty in 241 Am peak quantification. Among the remaining 25 cores, 22 displayed a single 241 Am peak and 3 exhibited two peaks with similar 241 Am 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, 14 C 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 210 Pb CRS model AD 1963. Factors contributing to age discrepancies included (i) analytical limitations of 241 Am measurements in peat using gamma spectrometry, (ii) the impact of peat accumulation rates on 241 Am activities, i.e., dilution effects, (iii) post-depositional migration of 241 Pu and (or) 241 Am, (iv) uncertainties introduced during 210 Pb age-modelling, and (v) the limited number of samples dated using 14 C. The usefulness of 241 Am as an Anthropocene marker (assuming AD 1963) hinges on the accepted uncertainty level in defining this epoch. Detection of 241 Am 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 137 Cs 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 241 Am.