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

The sedimentary structure preserved within the seabed of Eckernforde Bay wa s investigated together with the oceanographic processes influencing that s tructure. 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. Goring devices recovered sedimen t to examine the benthic biological community, to measure rates of sediment ological processes, and to document sedimentary structure. During fair-weather conditions, the dominant mechanism for supplying sedime nt to Eckernforde Bay is import from the Baltic Sea associated with interna l waves. Earlier work has documented the erosion of shallow deposits during storms and the transport of this material to deeper sites in the Bay. Bott om shear stresses exerted in the Central Basin during all conditions are be low critical stresses, which makes the Bay an excellent sediment trap; Sedi ment from both distant and local origins is reworked in the Central Basin o f Eckernforde Bay by a pioneering community of benthic organisms, which is maintained by seasonal hypoxia/anoxia. The population is characterized by f ew species, small body sizes, young ages, and limited depth of mixing (simi lar to 1 cm). However, the community effectively pelletizes most of the sed iment reaching the seabed. The very restricted thickness for the surface mixed layer (similar to 1 cm) and the substantial sediment accumulation rates (mean of 0.39 cm yr(-1) fo r the Central Basin) give sediment a short exposure to modern oceanographic processes before being buried. These conditions allow for partial preserva tion of sediment deposited as storm layers, thus forming laminations of unp elletized sediment, These laminations separate thick beds of pelletized sed iment deposited during fair weather or as thin storm layers (i.e., <1 cm th ick). In general, the oceanographic processes in Eckernforde Bay allow for preservation of a high-resolution record of environmental processes. For ex ample, changes recorded for the past half century indicate that slower sedi ment accumulation rates previously characterized some portions of the study area. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.