and DWD at larger depths, but DWD does also occur in some protected depressions where bottom depths may be 20-30 meters or less. DWD represents Postglacial deposits, whereas SWD represents more or less reworked tills. SWD largely consist of little altered rock detritus and are rich in silty, sandy or coarser fractions. DWD contains much clay sized material, but silts and sand may occur occasionally. In general SWD are rich in quartz and feldspars, whereas the DWD are poor in these minerals, but instead show higher contents of X-ray amorphus (background producing) matter, and clay-minerals. The relation Quartz-Feldspar also varies latitudinally, the feldspar-richest sediments occurring towards the North. DWD are enriched in Ti, Fe, Mn, Ba and probably some trace elements (e.g. Cu, Cr) whereas SWD are relatively rich in Si. Al, Ca, Na, and some traces occur in the same concentrations in both sediment types. DWD are identical to average shale in composition and SWD to average granite. This is probably due to a mechanical action, the panning of sediments by waves, bottom currents etc. preferentially moving clay and mica minerals into the deep basins, whereas coarse weathering residues, rich in quartz and feldspar, remain in shallow waters. Iron-manganese concretions occur extensively on the bottom areas that are protected from excessive abrasion and rapid accumulation. The nodules in the Bothnian Bay, are richer in Mn, Cu, Ni and Co than in other parts of Gulf of Bothnia. It is concluded that, in addition to organic matter, hydroxides and other fines may be important adsorbers of pollutants in sediments
Merentutkimuslaito , 1978. 8-35 p.