A new ground-level fallout record of uranium and plutonium isotopes for northern temperate latitudes
Plutonium and uranium isotope ratios can be used to differentiate the sources of nuclear contamination from nuclear weapon establishments (Environ. Sci. Technol. 34 (2000) 4496; Internal Report for AWRE Aldermaston, UK (1961)), weapon fallout (Geochim. Cosmochim. Acta 51 (1987) 2623; Earth Planet. Sci. Lett. 63 (1983) 202; Earth Planet. Sci. Lett. 22 (1974) 111; Geochim. Cosmochim. Acta 64 (2000) 989), reprocessing plants, reactor or satellite accidents (Science 105 (1979) 583; Science 238 (1987) 512) and in addition they provide markers for post-1952 geochronology of environmental systems. A good record of plutonium and uranium isotope ratios of the background resulting from atmospheric nuclear testing is essential for source characterisation studies. Using recently developed mass spectrometric techniques (J. Anal. At. Spectrom. 16 (2001) 279) we present here the first complete records between 1952 and the present day of northern temperate latitude 240Pu/239Pu and 238U/235U atom ratios for atmospheric deposition. Such information was not derived directly during the period of atmospheric testing because suitable mass spectrometric capability was not available. The currently derived records are based on an annual herbage archive and a core from an Alpine glacier. These studies reveal hitherto unseen fluctuations in the 238U/235U atmospheric fallout record, some of which are directly related to nuclear testing. In addition, they also provide the first evidence that plutonium contamination originating from Nevada Desert atmospheric weapon tests in 1952 and 1953 extended eastwards as far as northwestern Europe. The results presented here demonstrate that we now have the capability to detect and precisely identify sources of plutonium in the environment with implications for the development of atmospheric transport models, recent geochronology and environmental studies.