Magnetic properties of magnetosomal greigite and factors influencing its occurrence and preservation in Baltic Sea Littorina sediments

Abstract: The Baltic Sea is one of the world’s largest brackish water environments and is today suffering from, for example, eutrophication, spreading hypoxia, accumulations of contaminants and invasive organisms. These problems exist due to a combination of natural features that makes the Baltic Sea a sensitive ecosystem (such as long water residence time, limited water exchange, large catchment area), and anthropogenic pressures on land, (such as agriculture and land-use changes, large human populations and industries) and in the sea (such as transport, fishing and bottom-trawling). Modern hydrographical settings include two salinity gradients, one horizontal (south-west to north) and one vertical (a halocline, with denser more saline water close to the bottom). The permanent halocline at ~60-80 m prevents vertical mixing between oxic surface waters and bottom waters, which can result in benthic hypoxia or anoxia and the preservation of organic rich sediments (sapropels) that are laminated. Hypoxia (O2<2 mg/L) can destroy benthic communities, affects the entire food web and influences biogeochemical cycles of nutrients. Hypoxia/anoxia in the bottom waters has been present intermittently during the Littorina Sea stage (ca. last 8000 years), more or less coinciding with relatively warmer periods represented by the Holocene Thermal Maximum (HTM), the Medieval Warm Period (MWP), and approximately the last 200 years. The aim of this thesis is to increase our knowledge about the evolution of the Baltic Sea since the last deglaciation with a focus on the deeper basins during the Littorina Sea stage.Mineral magnetic enhancements of Littorina Sea laminated sapropels have been found to be due to magnetosomal greigite (Fe3S4). These ferrimagnetic grains are produced by magnetotactic bacteria (MTB) in a controlled process that creates specific shapes (cuboidal, elongate prismatic, roundish) and a narrow grain size distribution within the single domain window (mean 55x75 nm). The flux of iron (Fe) bound as magnetosomal greigite is low <0.7% in the Baltic Sea laminated sapropels compared to the total and reactive Fe flux, but might be of importance in other stratified waters suffering from hypoxia/anoxia. Magnetosomal greigite production (as indicated by mineral magnetic properties), hypoxia intensity (as indicated by Mo/Al), organic carbon content and Fe input correlates at a millennial time scale, but at a centennial time scale the trends can differ. Laminated sapropels are generally enhanced in magnetosomal greigite, elemental ratios and organic matter, which shows the link between hypoxic/anoxic condtions creasting an OATZ and the production of magnetosomal greigite. At a shorter decadal time scale, within a laminated interval, the presence of magetosomal greigite is linked to the intensity of hypoxia/anoxia and partly also to the organic matter input.