Anthropogenic 129I Traced in Environmental Archives by Accelerator Mass Spectrometry

Abstract: Since the beginning of the nuclear era, starting during the 1940s, large amount of radioactivity has been released into the environment. This thesis deals with the temporal and spatial distribution of the anthropogenic radioisotope 129I (T1/2= 15.7 Myr) in northern Europe. A routine sample preparation procedure for extraction of iodine from milligram amounts of solid materials has been developed and aimed for measuring the 129I concentration by the ultra-sensitive accelerator mass spectrometry method. The technique was further used for the analysis of 129I in sediments collected from two lakes in Sweden and one lake in Finland as well as sediments from two sites in the Baltic Sea. In addition, 129I concentrations in aerosol samples from northern and southern Sweden covering the period 1983 to 2000 have been measured. The results reveal a gradual increase in the anthropogenic 129I fluxes since the 1950s that are linked to emissions from the nuclear fuel reprocessing facilities in Sellafield (UK) and La Hague (France). A sharp increase coinciding with the Chernobyl accident is identified from the Swedish lakes located in areas characterised by relatively high Chernobyl fallout. Numerical modeling of the 129I deposition predicts that >50% of the flux to the lake sediments is related to the liquid emissions from the reprocessing facilities. The modeling also reasonably simulates the contribution of the Chernobyl event to the total 129I flux. The novel time series from northern Europe on 129I in aerosols show about one order of magnitude higher concentration in northern compared to southern Sweden. Estimate of 129I dry fallout based on the aerosol data suggests <25% contribution to the total fallout. The distribution of 129I in the sediment archives demonstrates the potential of the isotope as a new time marker for chronological and environmental investigations.