Thesis for the Degree of Doctor of Philosophy - DTU Orbit

Although the oxygen concentration decreases with increasing depth in the Baltic Sea and at some locations
reaches anoxic conditions (unpublished data), an opposite trend occurs for the concentration of 129 IO3 - and
127
IO3 - (Figs. 9 and 10 and Paper III). A possible explanation for the high concentration of 129 IO3 - in
oxygen depleted deep water of the Baltic Sea may be the transport of iodate enriched Kattegat water which
flows along the bottom of the Baltic Sea and is subsequently transported to the Arkona, Bornholm and
Gotland Basins. The enrichment of iodate in anoxic bottom water of the Baltic Sea may however be
explained not only by the effect of saline water intrusion. Another source of iodate under such anoxic
conditions can be the release of iodine as iodide and iodate from sediments by digenetic processes (Yuita,
1992; Muramatsu et al., 1996). Our results show that the reduction of iodate in the oxygen deficient bottom
water of the Baltic proper (Figs. 9 and 10) is a slow process since higher concentrations of iodate (both
isotopes) was found in bottom water when comparing with surface and intermediate waters of Baltic Sea for
both seasons.
The results of this work shown that (i) reduction of iodate and oxidation of iodide in Skagerrak and Kattegat
may be a slow process since insignificant change in 129 I and 127 I speciation was found (ii) reduction of iodate
to iodide seems to be relative fast process in surface water of the southern Baltic Sea; (iii) although suboxic
or anoxic condition occur in some of the Baltic Sea deep waters, the concentration of 129 IO3 - increases with
water depth indicating that the reduction of iodate in the anoxic bottom water of Baltic Sea seems to be a
slow process. The main mechanism of iodine reduction may thus be biotically driven rather than abiotic.
3.3.2 Spatial and temporal variation of 129 I and 127 I by analysis of archived Fucus Vesiculosus samples
(in paper V). Over the past decade, liquid and gaseous release of 129 I from reprocessing facilities such as
La Hague (France) and Sellafield (UK), has significantly increased the natural environmental concentrations
by several orders of magnitude (Alfimov et al., 2004; Fehn et al., 2007; Aldahan et al., 2007). The relatively
long half life of 129 I (15.7 myr) and the long residence time (30 kyr) of iodine in the marine environment as
well as continuous releases from nuclear fuel reprocessing facilities make this isotope a suitable
oceanographic tracer. Apart from marine tracer aspects we may also gain insight into the important
production of volatile iodine species created by marine algae and which are released to the atmosphere.
Marine algae play an important role (Leblanc et al., 2006) in the global cycle of iodine in the environment in
the sense that they accumulate seawater iodine at high concentration levels and have the ability to transform
a part of it into volatile organic iodine (VOI), such as methyl iodide (CH3I) or diiodomethane (CH2I2;
Carpenter et al. 2007) and release them into seawater. From the seawater surface the volatile organic iodine
species are released into the atmosphere and are subsequently broken down by photolysis and reactions with
ozone (O3) (Jones & Carpenter, 2005; Martino et al. 2006) forming a reactive pool of iodine species which
afterwards contribute to the ozone depletion, particle formation and cloud condensation (Küpper et al., 2008;
O’Dowd et al., 2002). Despite the significant role of marine algae in the iodine cycle in the environment,
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