Geochemical records of salt-water inflows into the deep basins of the Baltic Sea

Abstract

The estuarine circulation system of the Baltic Sea promotes stable stratification and bottom water anoxia in sedimentary basins of the Baltic proper. Ingressions of saline, oxygen-rich waters from the North Sea replace the oxygen depleted deep water. Timing and extent of the ingressions vary on time-scales of years to decades, and are largely determined by wind-strength and storm frequency over the North Atlantic Ocean and Europe. Mn/Fe-ratios in sediments from a dated sediment core of the Gotland Deep (250 m water depth) record variations in redox conditions that can be linked to historical observations of salt-water ingressions. The sediment record of the dated core is marked by seven Mn/Fe-excursions and suggests that major inflows terminating longer stagnation periods have occurred more frequently during the last 250 years. This in turn suggests the more frequent generation of low-pressure areas 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 particulate phase (iron sulfide) which accumulates at the seafloor and a dissolved phase (Mn 2+) accumulating in the deep, anoxic water body. Inflow of oxygenated water causes oxidation of Mn 2+ to Mn 4+and precipitation of MnO 2, which accumulates in Mn-rich layers at the sediment surface. When the bottom water becomes anoxic again, MnO 2 degradation release Mn 2+ into the pore water, and alkalinity increases as well during organic matter mineralization. Subsequently, Ca-rich rhodochrosite forms close to the sediment-water interface where pore waters are supersaturated 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 reconstruct chemical regimes of the deep water of the Baltic Sea in the past and indirectly trace variability in the strength and frequency of storms over the North Sea and the North Atlantic during the Holocene.