129I in archived seawater samples

Anthropogenic 129I (t1/2=15.7 My) discharged by the nuclear fuel reprocessing facilities at Sellafield (UK) and La Hague (France) is a promising tracer of physical and biogeochemical processes in the North Atlantic and Arctic Oceans. To improve understanding of its releases and dispersal, 129I was measured in archived seawater samples that had been collected as part of previous tracer studies, thus allowing direct comparison of 129I with other anthropogenic radionuclides. A sample collected in the eastern subtropical North Atlantic in 1969 was selected in order to directly measure the impact of weapons-fallout 129I in the oceans. The measured 129I/127I ratio of the sample is 0.53±0.08×10-10 (0.53±0.08IU: 1IU≡129I/127I=10-10), compared to the pre-anthropogenic ratio of ∼10-12 (0.01 IU). The ratio of 129I to 137Cs is 2.0±0.6 (atom ratio), 1.6±0.6 when corrected for 7 yr of radioactive decay from the bomb-input peak. This observed ratio is ten times higher than predicted from fission yields, possibly reflecting the greater volatility of iodine relative to cesium. 129I/137Cs ratios in Scottish and Norwegian Coastal waters, sampled in 1976 and 1978, are in good agreement with predictions based on the available release data. Reprocessing 129I is clearly seen in the northern Greenland Sea (>6 IU) and in Denmark Straits Overflow Water (1.4 IU) in samples collected during the TTO/NAS program in 1981. The strength of the tracer signal in the overflow water – the ratio of the concentration in the overflow core to its minimum value in the station profile – is approximately four times higher for 129I than for the other tracers measured (137Cs, 90Sr, and 3H).