The Darss Sill, hydrographic threshold in southwestern Baltic: Late quaternary geology and recent sediment dynamics

About 73% of the water exchange between the Baltic and the North Seas is via the Darss Sill. Joint Danish and German marine geological investigations, including shallow seismic surveys and sediment sampling, have been carried out in the Darss Sill area between 1989 and 1991. Results from these studies and unpublished archive data from the Institute of Baltic Sea Research, Warnemünde, are presented. These show that the entire Quaternary sedimentary sequence in most of the area is at least 40 m thick. A zone of glacial till outcropping between Denmark and Germany belongs to the Late Weichselian ice marginal line G (“Velgaster Staffel”). The formation of this line caused the damming of a pre-existing (sub)glacial meltwater discharge system. At the beginning of the Baltic Ice Lake formation, a deep and presently buried channel incised in this ice marginal line was part of a (glacio)fluvial drainage system with discharge towards the northeast, i.e. into the Baltic Ice Lake. During the Baltic Ice Lake transgression damming of the Darss Sill occurred due to accumulation of sandy sediments within this channel. On the other hand, the recent Kadet Channel was only occasionally overflown during the Baltic Ice Lake highstand maximum, which was 18 m BSL (below present sea level) in the Darss Sill area. The main erosion and deepening of the Kadet Channel began during the Ancylus Lake highstand maximum. At present, sediment transport on the Darss Sill is governed by northeasterly inflow of saline bottom waters in the entire area from the Kadet Channel to the German coast. Baltic outflow affects the seabed at shallower depth in the Danish sector. Large-scale current-induced bedforms in the area include, amongst others, sandwaves with a height of 5 m. Changes of the bedforms observed indicate a maximum bottom flow speed of 70–100 cm s−1, both for inflow and for outflow

Sedimentologische Auswirkungen der Grundfischerei in der Kieler Bucht (Westliche Ostsee)

Sedimentological effects of trawl fishery are little investigated compared to the biological effects. The objectives for the present investigation concern the mechanical disturbance of the sediments and the reworking induced by the ploughing effect of the trawl-net otter boards. For investigation of the distribution of trawl-net tracks on the sediment surface and their morphological analysis highresolving side-scan Sonar and video instruments (ROV) were used, occasionally supported by scuba divers. Manifold mechanical effects on the sediments observed up to 23 cm sediment depth were studied on X-ray graphs from video-controlled box core sampling. The reworking of sediments is due to the suspension cloud during trawling as well as to erosion which acts preferredly at small-scale topography formed by the ploughing effect of the otter boards. Oblique action of the otter boards and their frequently jumping rnotion reduce the surface disturbed by trawling. Sedimentologische Auswirkungen der Grundfischerei sind bisher im Vergleich zu biologischen Effekten wenig untersucht. Die Fragestellung der vorliegenden Arbeit setzt hauptsächlich an den mechanischen Störungen und den Konsequenzen für die Sedimenturnlagerung durch den Einfluß der Grundfischerei, vor allem durch die Pflugwirkung der Scherbretter, im Gebiet der Kieler Bucht an. Die Verbreitung der Grundfischereispuren am Meeresboden wurde mit Hilfe eines hochauflösenden Seitensichtsonars untersucht, für die morphologische Analyse wurden zusätzlich Video- und Tauchbeobachtungen eingesetzt. Die mechanischen Störeffekte wurden an Röntgen-Radiographien untersucht. Die an Radiographien von video-gesteuerten Kastengreiferproben beobachtete Tiefenwirkung der mechanischen Störungen des Sedimentgefüges - Faltungs- und Wickelgefüge, Zerscherungen sowie Laminierung durch Pflugsohleneffekte werden beschrieben - beträgt bis zu 23 cm. Umlagerung erfolgt über die beim Grundfischen erzeugte Suspensionswolke sowie über Erosion, die an den durch die Scherbretter entstandenen Unebenheiten bevorzugt ansetzt. Die Verbreitung der Suspensionswolke erfolgt deshalb mehr lokal, während das erodierte Material durch episodische Bodenströmungen weiter verfrachtet wird. Eine beträchtliche Reduktion der Flächenwirkung der Scherbretter auf Schlickböden erfolgt dadurch, daß diese zum weitaus überwiegenden Teil nur schräg "über Eck" den Grund berühren oder, durch elastische Reaktion des Geschirrs auf den Sedimentwiderstand bedingt, sich hüpfend fortbewegen

Power Spectrum Analyses of Storm Layers in Marine Silty Sediments: A Tool for a Paleoclimatic Reconstruction?

Climatic events are imprinted on the sedimentary record of the Baltic Sea. In these sediments, storm events are documented by sandy horizons within the mud sequences which dominate the deeper basins. The spectral characteristics of the storm layer sequences were determined through analyses of the optical density of sediment X-radiographs. Previously the evaluation of X-radiographs was done visually and provided qualitative rather than quantitative information. In order to enable a more objective analysis, an optical scanner was developed. Subsequently the digitized sequence was analysed using Fast Fourier Transformation procedures. The results show that the observed cycles which lead to the formation of alternating layers can be attributed to calendar and solar band frequency variations