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.

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International audienceCore MD02-2508 retrieved from the plateau of the continental slope off Baja California recovered a 40-m-thick section of sapropel (up to 15% C(org)), calcareous clay, and diatom ooze with the age of 120 ka at the core bottom. The section is subdivided into three units: Unit I (Holocene) consists of the laminated sapropel; Unit II comprising oxygen isotope stages (MIS) 2, 3, and 4 is represented by homogeneous calcareous clay with interbeds of slightly siliceous sapropelic mud; and Unit III (MIS-5) is composed of laminated sapropelic diatom ooze. Laminated intervals are characterized by the intercalation of two lamina types: (1) dark-colored organic-rich laminae containing multi-species ``oceanic'' diatom assemblages, as well as radiolarians, coccoliths, planktonic and benthic foraminifera; (2) lighter-colored laminae consisting of diatom ooze with the neritic colonial diatom assemblages commonly composed of one to three species of a single genera. The dark laminae are assumed to be generated within a high productivity zone over the slope, whereas light ones likely represent diatom mats produced by short-term boisterous phytoplankton blooms, possibly in the coastal upwelling