Measurements of 236U in Ancient and Modern Peat Samples and Implications for Postdepositional Migration of Fallout Radionuclides

236U was analyzed in an ombrotrophic peat core representing the last 80 years of atmospheric deposition and a minerotrophic peat sample from the last interglacial period. The determination of 236U at levels of 107 atoms/g was possible by using ultraclean laboratory procedures and accelerator mass spectrometry. The vertical profile of the 236U/238U isotopic ratio along the ombrotrophic peat core represents the first observation of the 236U bomb peak in a terrestrial environment. A constant level of anthropogenic 236U with an average 236U/238U isotopic ratio of (1.24 ± 0.08) × 10–6 in the top layers of the core was observed. Comparing the abundances of the global fallout derived 236U and 239Pu along the peat core, the post depositional migration of plutonium clearly exceeds that of uranium. However, the cumulative 236U/239Pu ratio of 0.62 ± 0.31 is in agreement with previous studies on the global fallout uranium and plutonium. In the interglacial peat samples a 236U/238U isotopic ratio of (3.3 ± 0.7) × 10–1...