Origin and environment of manganese-rich sediments within black-shale basins

Sediments with unusually high manganese contents occur in anoxic basins of the central Baltic Sea. We present a new model for the enrichment of manganese which is based on sedimentological and geochemical analyses of Holocene sediments from the Gotland Deep, and on published hydrographic data. Due to salinity stratification and high primary production, bottom waters of the central Baltic Sea are anoxic and rich in dissolved Mn2+. Episodically, bottom waters become oxygenated by inflows of denser water from the North Sea. Mixing results in oxidation of Mn2+ to fine-grained particulate oxides which accumulate in the deepest parts of the basins where currents are mostly absent. Here, anoxic conditions are re-established within a few months or years, and manganese oxides are again dissolved causing high concentrations of Mn2+ in the near-bottom layer. Alkalinity is high as a consequence of bacterial reduction of manganese oxides and sulphate. Under these conditions, manganese is reprecipitated and forms a layer of Ca-rich rhodochrosite on the top of laminated sapropels. Occasionally, periods of such extremely and rapidly changing redox-conditions are followed by long-term oxygenation of bottom waters. In that case, a layer rich in manganese oxides forms on the top of a bioturbated layer due to dissolution of rhodochrosite and oxidation of manganese in the near-surface sediment. Return of anoxic conditions results again in the transformation of manganese oxides into rhodochrosite. Varying rhodochrosite contents in the basinal sediments therefore record a detailed history of redox-conditions in bottom waters. The manganese-rich sediments of the Baltic Sea share many features with some occurrences of Lower Jurassic and Lower Carboniferous manganese ores in western and central Europe. We therefore suggest that these manganese enrichments were formed by similar processes.