The influence of wind-induced resuspension on sediment accumulation rates : A study of archipelago and offshore areas in the NW Baltic proper

The processes of sedimentation such as erosion, resuspension, transportation and accumulation are of major importance when handling environmental problems such as pollutants and eutrophication. Redistribution of sediment can induce transport of pollutants, why a greater knowledge of the sediment processes and the factors giving rise to sediment transport is desirable. There are a multiple of factors that could affect sediment processes, such as currents, waves, wind and sea level changes. Wind conditions are one of the major factors influencing oceanographic conditions, why focus in this study is set on the relationship between wind frequencies and sedimentation processes. The aim of this study is to investigate gross accumulation rates (accumulation of dry substance), foremost in archipelago areas of the NW Baltic Proper, and to some extent an offshore area in the NW Baltic Proper. The thesis suggests time trends in gross accumulation rates to be affected by wind conditions. Former analyses of water content, Cesium-137, carbon and nitrogen in combination with dating from lamina counting resulted in data on dry substance deposition data from six enclosed bays in the archipelago of the NW Baltic Proper. In order to receive new materiel from an offshore area, field work were performed during the summer of 1999. Analyses were performed in the same way as above. The gross accumulation rates could be calculated and dated in a satisfactory way in the accumulation areas. The offshore area proved to be difficult to interpret, why no conclusions could be made concerning accumulation in deep offshore areas (180-203 m). Data on wind force and durability from SMHI together with accurate described annual gross accumulation rates allowed plotting the time trends of the two parameters. It proved that a good correlation was seen in wind speed ≥7-9 m/s (r2=0.5) when using annual core mean values. Former investigations has showed a good correlation at higher wind speeds, ≥14 m/s. It is suggested that in small enclosed bays, fetch are the limiting factor which does not allow wind speeds exceeding ≥7-9 m/s to lower the critical depth at which erosion/resuspension occur. Since great similarities were found between the inner and outer Stockholm archipelago and the archipelago of Södermanland it is indicated that the results are valid for a large part of the NW Baltic Proper archipelago area, provided that the predictions are based on a sufficient number of bays and cores.