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Earth Surface Dynamics An interactive open-access journal of the European Geosciences Union
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Discussion papers
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 12 Aug 2019

Submitted as: research article | 12 Aug 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Earth Surface Dynamics (ESurf).

Links between Baltic Sea submarine terraces and groundwater sapping

Martin Jakobsson1, Matt O'Regan1, Carl-Magnus Mörth1, Christian Stranne1, Elizabeth Weidner1,2, Jim Hansson4, Richard Gyllencreutz1, Christoph Humborg3, Tina Elfwing3, Alf Norkko5,3, Joanna Norkko5, Björn Nilsson6, and Arne Sjöström6 Martin Jakobsson et al.
  • 1Department of Geological Sciences, Stockholm University, and Bolin Centre for Climate Research, Stockholm, 10691, Sweden
  • 2Center for Coastal and Ocean Mapping, University of New Hampshire, USA
  • 3The Baltic Sea Centre, Stockholm University, Stockholm, 10691, Sweden
  • 4The Maritime Museum, Stockholm, Sweden
  • 5Tvärminne Zoological Station, University of Helsinki, Hanko, Finland
  • 6Department of Archaeology and Ancient History, Lund University, Sweden

Abstract. 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.

Martin Jakobsson et al.
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Martin Jakobsson et al.
Martin Jakobsson et al.
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Short summary
We studied coastal sea floor terraces in parts of the Baltic Sea using various types of sonar data, sediment cores and video. Terraces (~ 1 m high, > 100 m long) are widespread in depths < 15 m, and are formed in glacial clay. Our study supports an origin from groundwater flow through silty layers, undermining overlying layers when discharged at the sea floor. Submarine groundwater discharge like these may be a significant source of freshwater to the Baltic Sea that needs to be studied further.
We studied coastal sea floor terraces in parts of the Baltic Sea using various types of sonar...