Thesis for the Degree of Doctor of Philosophy - DTU Orbit

129 I/ 127 I in all the samples investigated here are four – five orders of magnitude higher than the
reported natural isotopic ratio of 129 I/ 127 I (10 -12 , Fehn et al., 2007). The ratios of 129 I/ 127 I in
Fucus from Rømø were found to be in order of 10 -7 and 10 -8 in Fucus from Bornholm. Those
variations may be related to different factors as for example: i) Fucus from different locations
have integrated the iodine signal over different periods of time meaning that the seaweed
cannot be used as a monitor of water concentration, ii) short – term variations in the discharge
of radioiodine from nuclear reprocessing plants, iii) variations in transfer factors from the two
reprocessing plants in this case La Hague and Sellafield.
Even if fluctuating biological functions in seaweed may be compensated for by using the
129 I/ 127 I ratio the question still remains to what level the seaweed is reflecting an average
seawater ratio. This question is particularly interesting if the uptake of iodine depends on what
species it occurs in. Previous investigation (Hou et al., 2007; Hansen et al., 2011) have shown
that the two isotopes does not distribute among the two species, iodate and iodide, in a similar
way. This is an effect of the very slow kinetic exchange between iodide and iodate in
seawater. Iodine-129 in the form of iodide may thus remain as iodide even when the coastal
water from the English Channel mixes with more open ocean water where the main fraction
of stable iodine, I-127, exist in the form of iodate. If steady state conditions occur and the
iodine uptake in seaweed would be the same irrespectively of species the concentration factor
defined as ratio of 129 I/ 127 I (seaweed) relative 129 I/ 127 I (seawater) should be one. Using existing
129 I/ 127 I data in seawater from North Sea, Kattegat and Baltic Sea (Hou et al., 2007, Hansen et
al. 2011) and combining it with the 129 I/ 127 I data at corresponding years presented here yields
ratios of 0.5 for North Sea (2005), 0.7 (2006) for Southern Kattegat and ~1 (2007) for
Bornholm. Thus, the 129 I/ 127 I ratios in collected Fucus samples, closely match with
corresponding data from water. The somewhat larger difference at Rømø may be an effect of
this location being closer to the source and the relatively short residence time of water in the
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