Manganese cycling in the Baltic Sea over the last ~8000 years: the influence of redox conditions on mineral formation and burial

Sammanfattning: The Baltic Sea is one of the largest brackish water bodies in the world and is suffering extensive environmental issues caused by eutrophication and warming climate. A halocline in the water column divides more saline bottom water fed by inflows through the narrow connection with the North Sea from the surface waters and inhibiting exchange of oxygen between the two water masses. With the nutrient input and the warmer climate primary productivity is increased and the surplus of organic matter results in higher consumptions of oxygen in the water column. As a consequence widespread oxygen deficiencies occur in the bottom waters. Although the present spreading of hypoxia is mostly caused by the influence of human activity on land surrounding the Baltic Sea hypoxia has occurred intermittently before. Since the transition from the freshwater Ancylus Lake to the brackish/marine Littorina Sea ca. 8,000 years ago, the Baltic Sea has experienced three major intervals of bottom water hypoxia. These intervals occur in relation with warmer climates during the Holocene Thermal Maximum (HTM), the Medieval Climate Anomaly (MCA) and during the last decades. This study shows that these hypoxic intervals had a major impact on redox-dependent dynamics of manganese (Mn). Previous studies link the occurrence of Mn enrichments in the high organic sediments of the deep anoxic basins to short time changes in bottom water redox conditions caused by inflows of oxygenated salt water from the North Sea. However, sediment and pore water data of the modern hypoxic interval indicate that this process can be overturned by other factors. In the near surface sediments of the Gotland Basin a clear decline in Mn sequestration can be seen although inflows of oxygenated waters occur. This decline coincides with a major expansion of hypoxia and the occurrence of anoxic, sulfidic bottom waters obvious from monitoring data and sediment geochemical proxies e.g. molybdenum contents. A rise in sulfate reduction due to eutrophication and a decline in Fe input results in higher sulfide availability at the sediment-water-interface shortly after inflows causing a more rapid reductive dissolution of Mn oxides. As a consequence, Mn carbonates may no longer form. In contrast to most deep basins in the Baltic Sea the Landsort Deep as the deepest pit shows major enrichments of Mn to present. The steep geometry channels Mn oxides into the basin and the high alkalinity and sulfide concentration in the bottom waters at the sediment-water-interface allow frequent formation of Mn carbonate as well as sulfides at the sediment surface. With the transition to brackish/marine conditions ca. 8,000 years ago and the onset of the first hypoxic interval major amounts of Mn were buried over vast areas of the deep Baltic basins. The likely reason is a major input of dissolved Mn either from the underlying formerly oxic fresh-water sediments, where Mn oxides may have been present in the surface sediments or from land by fluvial transport or coastal erosion during a period of rather large relative sea level changes. After this first impulse, Mn burial reduced and varied with depth with no burial at the shallowest site (93 m) investigated and nearly continuous in the Fårö Deep (194 m). Although Mn sequestration occurred during intervals with hypoxic bottom waters in the deep basin, the intensification of hypoxia during the later intervals of the HTM und the MCA as observed during the last decades of the modern interval led to a reduction of Mn burial. In the Gotland Basin as well as at the Fårö Deep this decline in Mn content in the sediments coincides with indicators for enhanced hypoxia and euxinic bottom waters.