GEOCHEMICAL RECORDS OF SALT-WATER INFLOWS INTO THE DEEP BASINS OF THEBALTIC SEA

The estuarine circulation system of the Baltic Sea promotes stable str atification and bottom water anoxia in sedimentary basins of the Balti c proper. Ingressions of saline, oxygen-rich waters from the North Sea replace the oxygen depleted deep water. Timing and extent of the ingr essions vary on time-scales of years to decades, and are largely deter mined by wind-strength and storm frequency over the North Atlantic Oce an and Europe. Mn/Fe-ratios in sediments from a dated sediment core of the Gotland Deep (250 m water depth) record variations in redox condi tions that can be linked to historical observations of salt-water ingr essions. The sediment record of the dated core is marked by seven Mn/F e-excursions and suggests that major inflows terminating longer stagna tion periods have occurred more frequently during the last 250 years. This in turn suggests the more frequent generation of low-pressure are as over the North Atlantic in more recent times. The last three events have also been observed by hydrographic measurements. During the long time stagnation periods, Fe and Mn will be segregated into a particul ate phase (iron sulfide) which accumulates at the seafloor and a disso lved phase (Mn2+) accumulating in the deep, anoxic water body. Inflow of oxygenated water causes oxidation of Mn2+ to Mn4+ and precipitation of MnO2, which accumulates in Mn-rich layers at the sediment surface. When the bottom water becomes anoxic again, MnO2 degradation release Mn2+ into the pore water, and alkalinity increases as well during orga nic matter mineralization. Subsequently, Ca-rich rhodochrosite forms c lose to the sediment-water interface where pore waters are supersatura ted with respect to rhodochrosite. This mineral is stable under anoxic conditions and indirectly records redox Variations in the deep water body. Mn/Fe-ratios in longer sediment cores thus have a potential to r econstruct chemical regimes of the deep water of the Baltic Sea in the past and indirectly trace variability in the strength and frequency o f storms over the North Sea and the North Atlantic during the Holocene . Copyright (C) 1996 Elsevier Science Ltd