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Huge amounts of anthropogenic 129I have been and still are released to the environment through liquid and gaseous discharges from the nuclear fuel reprocessing facilities worldwide and in particular the ones in Europe. Most of this 129I signal has been accumulated in the marine environment which plays a major role in the iodine natural pool.  In this thesis, an overview of available 129I concentrations in waters of the oceans and marginal seas together with new data about 129I and 127I spatial distribution in surface seawater along a transect between the North Sea and the northeastern Atlantic Ocean are presented. After comprehensive chemical separation, the concentrations of iodine isotopes (127I and 129I) and their species (iodide and iodate) were analysed using accelerator mass spectrometry and inductively-coupled plasma mass spectrometry. The results show that, generally, changes in the 127I and 127I-/127IO3- are comparable to data from other marine waters which are related to natural distribution patterns. A considerable variation of 129I along the transect is observed with the highest values occurring in the eastern English Channel and relatively low values obtained in the northeastern Atlantic Ocean. Inventory estimations of 129I in the North Sea and the English Channel are 147 kg and 78 kg, respectively, where more than 90% resides in the Southern Bight and the eastern English Channel. Iodate is the dominant iodine species for both 127I and 129I in most seawater samples from the North Sea to the Atlantic Ocean. 129I species variability suggests a slow process of 129I- oxidation in the open sea. It takes at least 10 years for the 129I-/129IO3- pair to reach their natural equilibrium as the water is transported from the English Channel. The results suggest a main transport of 129I from the western English Channel via the Biscay Bay into the northeastern Atlantic Ocean. Further, high 129I/127I and distinctive 129I-/129IO3- values south of 40°N indicate possible contribution of 129I through Mediterranean Outflow Water. The environmental radioactive impact of 129I and possible applications in ecosystem studies are also discussed.

Shelf seas represent an important source of iron (Fe) to the open ocean. Additionally, shelf seas are highly productive environments which contribute to atmospheric carbon dioxide drawdown and support large fisheries. The work presented in this thesis describes the seasonal cycle of Fe in the Celtic and Hebridean Shelf Seas, and determines the physico-chemical speciation of Fe supplied from oxic margins. The results from repeated field surveys of the central Celtic Sea showed a nutrient type seasonal cycling of dissolved Fe (< 0.2 µm; dFe), which is surprising in a particle rich shelf system, suggesting a balance of scavenging and remineralisation processes. Coincident drawdown of dFe and nitrate (NO3-) was observed during the phytoplankton spring bloom. During the bloom, preferential drawdown of soluble Fe (< 0.02 µm; sFe) over colloidal Fe (0.02-0.2 µm; cFe) indicated greater bioavailability of the soluble fraction. Throughout summer stratification, it is known that NO3- is drawn down to < 0.02 µM in surface waters. This study revealed that both dFe and labile particulate Fe (LpFe) were also seasonally drawn down to < 0.2 nM. Consequently, it is hypothesised that the availability of Fe seasonally co-limits primary production in this region. At depth both dFe and NO3- concentrations increased from spring to autumn, indicating that remineralisation is an important process governing the seasonal cycling of dFe in the central Celtic Sea. In spring, summer and autumn, distinctive intermediate nepheloid layers (INL) were observed emanating from the Celtic Sea shelf slope. The INLs were associated with elevated concentrations of dFe (up to 3.25 ± 0.16 nM) and particulate Fe (up to 315 ± 1.8 nM) indicating that they are a persistent conduit for the supply of Fe to the open ocean. Typically > 15% of particulate Fe was labile and 60-90% of dFe was in the colloidal fraction. Despite being < 50 km from the 200 m isobath, the concentration of dFe was < 0.1 nM in surface waters at several stations. Broadly, the concentration of nutrients in surface waters described an oligotrophic environment where co-limitation between multiple nutrients, including Fe, appears likely. Over the Hebridean shelf break, residual surface NO3- concentrations (5.27 ± 0.79 µM) and very low concentrations of dFe (0.09 ± 0.04 nM) were observed during autumn, implying seasonal Fe limitation. The dFe:NO3- ratio observed is attributed to sub-optimal vertical supply of Fe relative to NO3- from sub-surface waters. In contrast to the shelf break, surface water in coastal regions contained elevated dFe concentrations (1.73 ± 1.16 nM) alongside low NO3-. Seasonal Fe limitation is known to occur in the Irminger and Iceland Basins; therefore, the Hebridean shelf break likely represents the eastern extent of sub-Arctic Atlantic seasonal Fe limitation, thus indicating that the associated weakening of the biological carbon pump exists over a wider region of the sub-Arctic Atlantic than previously recognised. These key findings demonstrate that the availability of Fe to phytoplankton may seasonally reach limiting levels in temperate shelf waters and that oxic margins persistently supply Fe dominated by colloidal and particulate fractions to the ocean.