The effects of the 2001 Barotropic intrusion of bottom-water upon the vertical distribution of inorganic iodine in the Gotland Deep

The distribution of iodide, iodate and total inorganic iodine in the water column of the Gotland Deep, in July 2001 and April 2002, is described. The general stratification of the water column is inferred from hydrographic measurements and nutrient analyses. These identify a well-oxygenated, estuarine surface layer (0–60 m), an oxy-cline (60–125 m), and a sulphidic zone (125–232 m). Fortuitously, the two sampling events occurred either side of a major barotropic intrusion which renewed the bottom water, thereby uplifting the earlier water column by about 20–30 m. The effects of this upon the iodine system is reported. The shapes of depth profiles for total inorganic iodine and their salinity counterpart are similar, with total inorganic iodine concentration rationalised to 35 salinity (RTiI) of 680 nM in waters between 100 and 232 m, and 75 and 232 m in July 2002 and April 2002, respectively. Meanwhile, between 0 to 60 m RTiI=440 nM at both samplings. As a result of this it is argued that upwards transport of iodine and salt in the Gotland Deep is closely coupled. A weaker co-variation between total inorganic iodine and phosphate concentration is taken to indicate that re-mineralisation of organic-I probably takes place mainly in the sediment, and not in the water column. In effect, re-mineralisation in the sediment produces a pool of salty, iodine rich bottom water, which by advection-diffusivity, moves up the water column. The uplift between the samplings was found to enrich the intermediate water (∼70–100 m) in iodide, which was subsequently oxidised to iodate in the period between the samplings. It is argued that the alternative possibility of an injection of iodate direct into this intermediate zone by means of a baroclinic intrusion, although possible, is unlikely. Greater integration of the mechanistic oceanographic approach underpinning this study into future 129I radiological assessments is advocated, to bring significant opportunities offered by the radio-chemistry, into marine chemistry.