Shallow-Marine Ferromanganese Concretions

Shallow-water continental margin ferromanganese concretions have been reported in a number of areas such as the Baltic Sea, Black Sea, Kara Sea, Loch Fy ne, Scotland, and Jervis Inlet, British Columbia (Calvert and Price 1977). In fact, these were the first type of marine Mn deposits to be discovered, during the 1868 Sofia expedition to the Kara Sea led by A.E. Norden-skioldfEamey 1990). 

Baltic Sea concretions can be classified into three main types based on their abundance, morphology, composition and mode of formation those from the Gulfs of Bothnia, Finland and Riga, from the Baltic Proper and from the western Belt Sea. 

Concretions from the Gulf of Botlmia are most abundant in Bothnian Bay where the abundance reaches 15-40 kg m in an area of about 200 kml This is equivalent to about 3 million tons of concretions and has led to these deposits being evaluated as a possible economic source of Mn. These concretions are mainly spheroidal up to 25-30 mm in diameter and are formed in the uppermost water-rich sediment layers at well-oxidized sites. They are most abundant where sedimentation rates are 0.4 mm a.  

Concretions from the Baltic Proper are found mainly around the margins of the deep basins in a depth range 48-103 m. The concretions are mainly discoidal 20-150 mm in diameter and crasts. Their abundance is mainly sporadic and more rarely common to abundant. Locally, abundances of 10-16 kg m are attained. Their formation is the result of the build up of Mn and Fe in the anoxic waters of the deep basins of the Baltic Proper. During major inflows of North Sea water ( 100 km ) into the Baltic which occur on average once every 11 years, the anoxic waters are flushed out of the basins. Mn and Fe precipitate out as an unstable gel but are ultimately incorporated into the concretions as 

Concretions from Kiel Bay in the western Belt Sea occur in a narrow depth range of 20-28 m at the boundary between sands and mud in zones of active bottom currents. They occnr as coatings on molluscs and as spheroidal and discoidal concretions. The formation of the concretions is influenced by the development of summer anoxia which leads to the diagenetic remobilization and lateral transport of Mn. This accounts for the high Mn/Fe ratios of these concretions. 

---