Postglacial water level changes of a drowned lake and river system in northeastern Mecklenburg Bay, southern Baltic Sea

Abstract. The postglacial geological evolution of the southwestern Baltic Sea is marked by dynamic water level developments, accompanied by pronounced coastline displacements and associated changes in coastal geomorphological features. This study describes preserved remnants of paleo-landscapes in northeastern Mecklenburg Bay, dating to the period from the Baltic Ice Lake to the regression following the Ancylus Lake stage. The Late Pleistocene to Early Holocene geologic development of the area is reconstructed based on multibeam echo sounder, sidescan-sonar, and sediment echo sounder data; sediment core analysis; and visual seafloor imagery. The spatial distribution of Baltic Ice Lake deposits suggests a coastline at depths of 20 m below present sea level at the northeastern margin of Mecklenburg Bay during the Baltic Ice Lake's initial transgressive phase. The deposits are truncated by a regional erosional unconformity associated with the final regression following the Baltic Ice Lake highstand that marks the onset of lowstand conditions in the study area. A developing terrestrial environment was subsequently flooded by the Ancylus Lake. The Ancylus Lake highstand is constrained by peat and fine-grained limnic deposits in water depths of 19 m below present sea level. The acoustic data reveal a lacustrine environment throughout the Ancylus Lake stage until at least 10.2 ka cal BP. The regression at the end of the Ancylus Lake period is preserved by a northward-draining channel system cutting into Ancylus Lake sediments. The geometry of these channels suggests a water level drop of 6–7 m in northeastern Mecklenburg Bay, supporting a minor Ancylus Lake regression west of the Darss Sill. The findings provide insights into the water level development of a key threshold area in the southwestern Baltic Sea and therefore contribute to the reconstruction of the postglacial evolution of the entire Baltic Sea.