In depressions of the Baltic Sea, where the bottom is periodically marked by stagnation, silt contains as much as 5% Mn (up to 17% in some layers) and 9-10% Corg. Silt in such depressions is laminated. The marine sediment sequence is stratified due to influx of ocean water into the sea: upper layers are oxic, while lower (near-bottom) layers are hydrosulfuric. The boundary between them is represented by the transitional O2-H2S layer. This zone (redox barrier) is marked by drastic variation in Eh. The zone below this barrier is characterized by accumulation of huge amounts of dissolved manganese Mn(II) and iron Fe(II), which diffuse from the hydrosulfuric layer into the oxic layer under influence of the gradient and precipitated as suspension with as much as 15% Fe and 45% Mn. When fresh oxygenated saline water is transported to the depressions the hydrosulfuric setting gives way to oxic one and dissolved elements are transformed into particulate phases as hydroxides and geologically instantly precipitated at the bottom. After 5-10 yr setting changes; hydrogen sulfide is again delivered to water column from the bottom. This is accompanied by supply of dissolved Mn(II) and Fe(II) previously accumulated as gel-type sediment on the bottom. Thus, the cycle of elements is repeated. The latter, however, is not completely dissolved. Some portion remains on the bottom as black rhodochrosite microlayers (laminas) that contain as much as 29% Mn. The black laminas accumulated during aeration include remains of bottom foraminifers. In addition, the bottom comprises pale diatom laminas and brownish gray varieties composed of clayey and organic substances. Bulk samples of the laminated silt contain as much as 12.9% Mn or 26.9% MnCO3. Depressions in the Baltic Sea represent an unique site of the Earth marked by accumulation of carbonate-manganiferous sediments at present. We believe that Oligocene manganese carbonate-oxide ores described by N.M. Strakhov and co-authors were accumulated in the same manner. Compositions of manganiferous sediments in the Baltic region and some ancient ores in Europe are compared. The author studied five stages of Mn accumulation and sediment transformation into ores.

Supplement to: Emelyanov, Emelyan M (2011): Ferromanganese ore process in the Baltic Sea. Translated from Litologiya i Poleznye Iskopaemye, 2011, 3, 227-248, Lithology and Mineral Resources, 46(3), 199-219