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

Abstract

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 25–30 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 −1 . In the Bothnian Sea, flat 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 18–24 kg m −2 . One area of about 300 km 2 in the Russian sector of the gulf contains about 6 × 10 6 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 48–103 m. The concretions are mainly discoidal 20–150 mm 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. 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 occur as coatings on molluscs and as spheroidal and discoidal concretions. In Lübeck-Mecklenburg Bay, the concretions are restricted to limited areas where glacial till is exposed through the mud. 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. Increasing attention is being directed to the use of concretions for the long-term monitoring of heavy-metal pollution in the Baltic. The method shows considerable promise. Zn profiles have already been used to monitor such pollution in concretions from Kiel Bay. However, there remain a number of difficulties such as the lack of knowledge of concretion growth rates, the fragmentary knowledge of the input and fate of heavy metals in the Baltic and the different periods of industrialization in the various pats of the Baltic. Such a monitoring techniques would be of considerable value if the Baltic is going to be cleaned up over the next century. A more detailed evaluation of the mode of formation of the ferromanganese concretions and their uptake of heavy metals would therefore appear to be an important next step in developing a strategy for monitoring pollution in the Baltic Sea.