Oceanographic processes and the preservation of sedimentary structure in Eckernförde Bay, Baltic Sea

Abstract

The sedimentary structure preserved within the seabed of Eckernförde Bay was investigated together with the oceanographic processes influencing that structure. A series of four cruises were undertaken during winter to summer conditions. An instrumented tetrapod was deployed to monitor boundary-layer processes controlling sediment transport. Coring devices recovered sediment to examine the benthic biological community, to measure rates of sedimentological processes, and to document sedimentary structure. During fair-weather conditions, the dominant mechanism for supplying sediment to Eckern-förde Bay is import from the Baltic Sea associated with internal waves. Earlier work has documented the erosion of shallow deposits during storms and the transport of this material to deeper sites in the Bay. Bottom shear stresses exerted in the Central Basin during all conditions are below critical stresses, which makes the Bay an excellent sediment trap. Sediment from both distant and local origins is reworked in the Central Basin of Eckernförde Bay by a pioneering community of benthic organisms, which is maintained by seasonal hypoxia/anoxia. The population is characterized by few species, small body sizes, young ages, and limited depth of mixing (∼1 cm). However, the community effectively pelletizes most of the sediment reaching the seabed. The very restricted thickness for the surface mixed layer (∼1 cm) and the substantial sediment accumulation rates (mean of 0.39 cm yr -1 for the Central Basin) give sediment a short exposure to modern oceanographic processes before being buried. These conditions allow for partial preservation of sediment deposited as storm layers, thus forming laminations of unpelletized sediment. These laminations separate thick beds of pelletized sediment deposited during fair weather or as thin storm layers (i.e., <1 cm thick). In general, the oceanographic processes in Eckernförde Bay allow for preservation of a high-resolution record of environmental processes. For example, changes recorded for the past half century indicate that slower sediment accumulation rates previously characterized some portions of the study area.