Measurements of U-236 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^–12 was detected; although this measurement is an upper limit, it constitutes a significant step forward in the experimental determination of the natural ²³⁶U abundance and represents a true background sample for the ombrotrophic peat core.