The nature of ferromanganese concretions from the eastern gulf of Finland, Baltic Sea

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Abstract
As a result of favorable environmental conditions, ferromanganese concretions in the eastern part of the Gulf of Finland achieve the highest levels of abundance and variability found in shallow‐water environments. The abundance of the Mn‐rich spheroidal concretions concentrated in the most prospective fields in the Russian sector of the Gulf of Finland reaches 50 kg/m2, and the total weight of these concretions is calculated to be about 6 million tonnes. This weight is considered to be relatively constant with time because of the cyclic processes of growth and dissolution of the concretions. The spheroidal concretions contain on average 33.8% MnO2, 20.2% Fe2O3, and 2.7% P2OS. The concentrations of Cu, Ni, Zn, Mo, etc., in the concretions are near the regional background levels. The contents of these elements have increased during the last few decades as a result of the anthropogenic input of the metals into the Baltic. The growth rate of the spheroidal concretions is calculated to be about 0.03–0.08 mm/yr. The technology of extraction of Mn from the Mn‐rich spheroidal concretions has been developed by the Hypronikel Institute in St Petersburg. This will permit the commercial recovery of these concretions in the future.