The research focuses on the area around the Darss Sill, a shallow submarine threshold that separates Mecklenburg Bay from the Arkona Basin and still acts as a bottleneck for water exchange in the modern Baltic Sea. Because the sill sits close to the isostatic zero line, the zone where the crust is neither rising nor sinking, it preserves an unusually clean record of past water levels. Yet despite its importance for understanding the entire Baltic’s postglacial evolution, the area had not been examined with modern high-resolution acoustic methods for roughly fifteen years. The new study closes that gap by combining multibeam echo sounder bathymetry, sidescan sonar mosaics, sediment echo sounder profiles, five sediment cores and underwater video footage into a single integrated reconstruction.

The story begins around 17,000 years ago, when the retreating Scandinavian ice sheet opened the Baltic basin to meltwater and the vast proglacial Baltic Ice Lake began to form. The researchers identified seismic units corresponding to two transgressive phases of this lake, separated by an erosional unconformity. Coarse-grained, organic-flecked sediments sampled in cores, including a piece of wood radiocarbon-dated to roughly 13,700 years before present, indicate a shoreline remarkably close to the study site during the lake’s initial phase. The spatial distribution of these deposits places a Baltic Ice Lake coastline at about 20 meters below present sea level along the northeastern margin of Mecklenburg Bay, confirming that the lake spilled across the Darss Sill and connected the region with the Arkona Basin to the east.

What followed was one of the most dramatic hydrological events in European prehistory. When the ice margin retreated from the Mt. Billingen area in central Sweden at the end of the Younger Dryas, the natural dam holding back the Baltic Ice Lake failed, and the water level plunged by an estimated 25 to 30 meters, possibly within just one to two years. Around 7,800 cubic kilometers of freshwater surged toward the North Sea, shrinking the lake’s surface area by nearly a fifth. In the study area, this catastrophe is written into the seafloor as a widespread erosional unconformity that truncates the Baltic Ice Lake deposits, marking the onset of lowstand conditions when Mecklenburg Bay and the Arkona Basin were separated by exposed land on the Darss Sill.

As global sea level crept upward and isostatic rebound in central Sweden once again cut the Baltic off from the ocean, a new dammed freshwater body, the Ancylus Lake, formed around 10,800 years ago. Its transgression flooded the developing terrestrial environment of the sill, and the researchers found its highstand fingerprint in an unexpected place: a low-backscatter platform of laminated gyttja and peat lying in water depths of about 19 meters below present sea level. Underwater video revealed the platform as a smooth, erosionally truncated surface more than two meters high in places, with wood fragments protruding from its base. Radiocarbon dates on plant remains from the platform cluster between roughly 10,800 and 10,200 years ago, constraining the Ancylus Lake highstand and showing that lacustrine conditions persisted in the area until at least 10,200 calibrated years before present.

The most visually striking discovery, however, came from the period after the Ancylus Lake peak. The acoustic data revealed three continuous, roughly north–south-trending channel branches incised into the lake sediments, draining northward toward deeper water. The western channel averages about 280 meters wide, while the central and eastern branches are around 120 meters across, and where their bases are visible in seismic profiles they cut 6 to 7 meters deep. Because the channel infill consists entirely of Ancylus-period deposits dated to about 9,000 years before present, the channels record a water level drop of at least 6 to 7 meters following the Ancylus highstand, a moderate regression rather than the catastrophic drainage some earlier hypotheses proposed.

That finding matters because the outlet of the Ancylus Lake has been one of the longest-running controversies in Baltic Sea geology. Some researchers once argued that erosion of the Kadet Channel through the Darss Sill triggered a catastrophic outburst, possibly lowering the lake to global sea level within two to three years and carving a hypothetical mega-river dubbed the Dana River. Later work found no evidence of deeply incised channels or major delta deposits to support that scenario, and the new study adds further weight to the skeptics: the team found no trace of a large east–west-trending Dana River channel in their data. Instead, the modest 6-to-7-meter drop supports a gentler scenario, consistent with earlier suggestions of a calm drainage in which fluvial erosion and rising global sea level together accounted for the change.

The reconstruction also carries implications far beyond geology. From the Baltic Ice Lake through the Ancylus Lake stage, the region’s coastlines swept back and forth across areas that are now several meters underwater, and these were precisely the landscapes favored by Mesolithic hunter-gatherer communities. Previous discoveries in the western Baltic, including a submerged Stone Age hunting structure found in 2024 at depths of up to 21 meters, have shown how rich the flooded archaeological record can be. By pinning down where shorelines actually stood during each stage, the new paleogeographic maps give archaeologists a targeting system for locating drowned settlements, and the authors emphasize that similar preserved landscapes likely extend along the entire southern Baltic coast from the coastline down to about 20 meters of water depth.

Technically, the study demonstrates how much information modern marine surveying can extract from a modest patch of seafloor. Ten-meter bathymetric grids exposed meandering sand dunes up to two kilometers long, probably shaped by present-day bottom currents, alongside circular depressions and small cliffs carved into soft organic sediments. Parametric sediment echo sounders resolved seismic units only a few meters thick, while laser diffraction grain-size analysis and accelerator mass spectrometry radiocarbon dating, calibrated with the IntCal20 curve, anchored the sequence in time. Even the geometry of channel rims told a story: where erosion cut through cohesive, organic-rich gyttja into weaker sandy layers beneath, the resistant upper edges were left standing as ridges on the seafloor, mimicking the peat ridges that mark ancient riverbeds elsewhere on the Darss Sill.

Questions remain. The team could not determine exactly where the post-Ancylus channels met the remaining lake body, nor whether the widespread Holocene erosion they observed occurred during the regression itself or later, during the early Littorina Transgression, when rising brackish seas finally drowned the sill and established the modern Baltic. The drainage route of the Ancylus Lake, whether through the Great Belt or some other pathway, is still unresolved, and the authors call for future work directly east of the Darss Sill to close the gap. But the broader picture is now much sharper: a threshold region that was flooded, drained, flooded again and finally submerged, its entire 13,000-year water level history preserved in a few meters of sediment just beneath the waves, waiting for sonar, cores and cameras to bring it back to light.

Subject of Research: Postglacial water level changes and paleolandscape evolution of the Darss Sill region in the southern Baltic Sea

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

Article References: Granzow, J., Tauber, F., Arz, H. W., Geersen, J., & Feldens, P. (2026). Postglacial water level changes of a drowned lake and river system in northeastern Mecklenburg Bay, southern Baltic Sea. E&G Quaternary Science Journal, 75(1), 107-129. https://doi.org/10.5194/egqsj-75-107-2026

Image Credits: AI Generated

DOI: 10.5194/egqsj-75-107-2026

Keywords: Baltic Sea, Mecklenburg Bay, Darss Sill, Baltic Ice Lake, Ancylus Lake, sea level change, paleogeography, marine geology, sediment cores, radiocarbon dating, submerged landscapes, Mesolithic archaeology