Environments of formation of ferromanganese concretions in the Baltic Sea: a critical review

Abstract

Ferromanganese concretions from the Baltic sea can be divided 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 Bothnia are most abundant in Bothnian Bay where the abundance reaches 15-40 kg m -2 in an area of about 200 km 2. This is equivalent to about 3 million tonnes of concretions and has led to these deposits being evaluated as an economic resource. These concretions are mainly spheroidal up to 25-30mm 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 -1 . In the Bothnian Sea, fiat crusts with low Mn/Fe ratios are widely distributed.Concretions from the Gulf of Finland are abundant in the eastern half of the gulf with a maximum abundance of 18-24kgm -2. One area of about 300 km 2 in the Russian sector of the gulf contains about 6 × 106 tonnes of spheroidal concretions and is receiving serious attention as an ore resource. These concretions frequently occur in the upper brown oxidized layer of the sediment.Concretions from the Gulf of Riga are most abundant (up to 17 kg m -2) around a central depression containing muddy sediments. Spheroidal concretions occur adjacent to the depression and discoidal concretions and crusts further away.Concretions from the Baltic Proper are found mainly around the margins of the deep basins in a depth range 48-103m. The concretions are mainly discoidal 20-150mm in diameter and crusts. Their abundance is mainly sporadic and more rarely common to abundant. Locally, abundances of 10-16 kg m -2 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 3) into the Baltic which occur on average once every 11 years, the anoxic waters are flushed out of the basins. Mn and Fe percipitate out as an unstable gel and are ultimately incorporated into the concretions. The concretions occur mainly on lag deposits in the vicinity of the halocline where strong bottom currents Occur.

Mn, Fe, Zn and As speciation in a fast-growing ferromanganese marine nodule

“…Low sedimentation rates, erosional conditions, and seasonal redox cycling favor the formation of ferromanganese concretions at shallow water localities of the SW Baltic Sea (Glasby et al, 1997;Hlawatsch et al, 2002a). These Baltic Sea ferromanganese nodules show an onion-like growth pattern of alternating Fe-and Mn-rich layers.…”

Land uplift effects on the phosphorus cycle of the Baltic Sea

“…In addition to controlling the Baltic Sea basin development, land uplift may exert profound influence on the cycles of particulate matter and of particle forming substances such as phosphorus (P). As the earth crust rises, more sediment areas are gradually exposed to the eroding force of wind and wave action (Jonsson et al 1990(Jonsson et al , 2003Glasby et al 1997;Meili et al 2000). Sediments of the Ö resund sound, which connects the Baltic Sea to the Kattegat, were once located above the sea level, but subsequently eroded down to bedrock during the course of 600 years, resulting in very high sedimentation rates in the adjacent Kattegat (Björck 1995).…”

“…Sediments of the Ö resund sound, which connects the Baltic Sea to the Kattegat, were once located above the sea level, but subsequently eroded down to bedrock during the course of 600 years, resulting in very high sedimentation rates in the adjacent Kattegat (Björck 1995). More than 80% of the matter on the seafloor of the Baltic Sea emanates from eroded shallow sediments (Pustelnikov 1977;Jonsson et al 1990;Glasby et al 1997;Jönsson et al 2005). In comparison, primary production appears to have a more limited impact on deepwater sedimentation in the Baltic Sea, since more than 97% of the biogenic particles are mineralised in the water column before they reach the seafloor (Pustelnikov 1977;Emelyanov 1988;Glasby et al 1997;Trimonis and Stryuk 2002).…”

“…More than 80% of the matter on the seafloor of the Baltic Sea emanates from eroded shallow sediments (Pustelnikov 1977;Jonsson et al 1990;Glasby et al 1997;Jönsson et al 2005). In comparison, primary production appears to have a more limited impact on deepwater sedimentation in the Baltic Sea, since more than 97% of the biogenic particles are mineralised in the water column before they reach the seafloor (Pustelnikov 1977;Emelyanov 1988;Glasby et al 1997;Trimonis and Stryuk 2002).…”

Modern concretions in intertidal flats of the Lower Saxonian Wadden Sea, Southern North Sea

“…They usually have a composition quite different from that of the surrounding material, and many are formed shortly after sediment deposition. Modern examples indude ferromanganese nodules (A> AGHA et al 1995;GLASBY et al 1996GLASBY et al , 1997 or siderite concretions found in salt marsh sediments (e.g. PYE et al 1990).…”