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

Andrii Oleksandrenko,Michael John Maclachlan Duke,Tommy Noernberg,Peter G Appleby,William Shotyk

doi:10.1139/er-2023-0097

Abstract

Fallout radionuclides, potential global indicators of the Anthropocene, raise questions about their preservation in bogs. This study evaluates the reliability of 241Am records in acidic, ombrotrophic peat cores. The 241Am source is the radioactive decay of the short-lived 241Pu (half-life 14.29 years) present in fallout from atmospheric nuclear weapons testing. We assessed 35 peat cores from 30 European and North American sites, collected over three decades for trace metal deposition studies. Each core underwent independent 210Pb dating. We excluded 10 cores due to significant 241Am peak quantification uncertainty. Of the remaining 25 cores, 22 displayed a single 241Am peak and 3 exhibited two peaks with similar activities. Among the 22 single-peak cores, 18 were within 10 years of AD 1963, with 10 within 5 years. For 10 of these 22 cores, 14C dates from the atmospheric bomb pulse curve were compared, showing ≤ 10 years of difference, with 7 cores ≤ 5 years from the 210Pb CRS model AD 1963. Factors contributing to age discrepancies included analytical limitations in 241Am measurements, peat accumulation rates affecting 241Am activities, post-depositional migration of 241Pu and/or 241Am, uncertainties in 210Pb age-modelling, and limited 14C samples. The utility of 241Am as an Anthropocene marker depends on the accepted uncertainty level in defining this epoch. The detection of 241Am in 35 cores, validated in 22 cores, suggests a hemispheric Anthropocene onset. The bomb pulse curve of 14C in bogs could serve as an alternative marker. Common 137Cs post-depositional migration in ombrotrophic peatlands contrasts with its immobility in minerotrophic peatlands, suggesting the latter may also retain nuclear weapons test fallout, including 241Am.

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