Raudan esiintymismuodoista pohjoisen Itämeren sedimenteissä

The forms of iron, especially iron oxides, are significant in the geochemical cycle of phosphorus and many trace elements in the sediment. In aerobic conditions, the iron oxides of the sediment bind phosphorus and trace elements. In anaerobic conditions, on the other hand, iron oxides dissolve, in which case the bound trace elements and phosphorus are released. When phosphorus is released from the sediment and migrates to a water column, this may further aggravate the eutrophication problem of the Baltic Sea through internal load. Thus, the goal of this thesis is to clarify and gain new information on how the forms of iron and their amount vary in the sea areas of the Baltic Sea with different oxygen conditions and what significance this may have with regard to the eutrophication of the Baltic Sea. Water and sediment samples were collected in April 2006 on board the research vessel R/V Aranda and, in October 2006, on board the research vessel Geomari. The sample stations were situated in the Bothnian Bay, the Bothnian Sea, the northern part of the Baltic Sea Proper, and the Gulf of Finland. Water and sediment samples were collected from several different depths. Of the water samples, temperature and salinity as well as the concentrations of oxygen, hydrogen sulphide and phosphate phosphorus were determined. In all the sample stations, the oxidation-reduction potential and pH of the sediment were measured. Of the sediment samples, the overall concentrations of iron, manganese, phosphorus, sulphur, nitrogen, and carbon as well as the forms of iron were analysed. The analysis of the forms of iron was carried out with a sequential leaching method, in which the four separated forms were: 1) iron bound in carbonates (siderite and ankerite) or iron bound in acid-volatile sulfide (AVS), 2) easily reduced iron oxides (ferrihydrite and lepidocrite), 3) reducing iron oxides (goethite, hematite, akaganeite) and 4) magnetite. The results show that the different oxygen conditions in the water near the bottom in the studied stations can be seen as variation of the forms of iron in the sediment, especially regarding iron oxide concentrations. The sediments of the Bothnian Bay and Bothnian Sea have, due to good oxygen conditions in the water near the bottom, plenty of easily reduced and reducing iron oxides, especially in the sediment's surface layers. Instead, in the sulfidic sediments of the occasionally anaerobic Gulf of Finland and especially the northern part of the Baltic Sea Proper, which has been anaerobic for a long time, there are few easily reduced and reducing iron oxides. From the point of view of internal load and the eutrophication of the Baltic Sea, the aerobic sediments containing plenty of iron oxides in the stations of the Bothnian Bay and the Bothnian Sea could act as binders of phosphorus and other trace elements. Instead, in the station of the Gulf of Finland the amount of iron oxides in the sediment can vary due to the occasional anaerobic state and, in the station in the northern part of the Baltic Sea Proper that has been anaerobic for a long time, there are few phosphorus binding iron oxides, in which case phosphorus is released from the sediment to the water and can mix with the productive layer to be used by algae.