Links Between Baltic Sea Submarine Terraces and Groundwater Sapping

TitleLinks Between Baltic Sea Submarine Terraces and Groundwater Sapping
Publication TypeJournal Article
AuthorsJakobsson, M, O'Regan, M, Mörth, C-M, Stranne, C, Weidner, E, Hansson, J, Gyllencreutz, R, Humborg, C, Elfwing, T, Norkko, A, Norkko, J, Nilsson, B, Sjöström, A
JournalEarth Surface Dynamics
Date PublishedJanuary 3

Submarine Groundwater Discharge (SGD) influences ocean chemistry, circulation, spreading of nutrients and pollutants, and shapes seafloor morphology. In the Baltic Sea, SGD was linked to the development of terraces and semi-circular depressions mapped in an area of the southern Stockholm Archipelago, Sweden, in the 1990s. We mapped additional parts of the Stockholm Archipelago, areas in Blekinge, southern Sweden, and southern Finland using high-resolution multibeam sonars and sub-bottom profilers to investigate if the seafloor morphological features discovered in the 1990s are widespread and to further address the hypothesis linking SGD to their formation. Sediment coring and seafloor photography conducted with a Remote Operated Vehicle (ROV) and divers add additional information to the geophysical mapping results. We find that terraces, with general bathymetric expressions of about 1 m and lateral extents of sometimes > 100 m, are widespread in the surveyed areas of the Baltic Sea and are consistently formed in glacial clay. Semi-circular depressions, however, are only found in a limited part of a surveyed area east of the island Askö, southern Stockholm Archipelago. Our study supports the basic hypothesis of terrace formation initially proposed in the 1990s, i.e. groundwater flows through siltier permeable layers in glacial clay to discharge at the seafloor, leading to the formation of a sharp terrace when the clay layers above seepage zones are undermined enough to collapse. By linking the terraces to a specific geologic setting, our study further refines the formation hypothesis and forms the foundation for a future assessment of SGD in the Baltic Sea that may use marine geological mapping as a starting point. We propose that SGD through the sub-marine seafloor terraces is most likely intermittent and linked to periods of higher groundwater levels, implying that to quantify the contribution of freshwater to the Baltic Sea through this mechanism, more complex hydrogeological studies are required.