Submarine groundwater discharge (SGD) influences ocean chemistry,
circulation, and the spreading of nutrients and pollutants; it also shapes sea floor
morphology. In the Baltic Sea, SGD was linked to the development of terraces
and semicircular 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 sea floor morphological features discovered in the 1990s
are widespread and to further address the hypothesis linking their formation
to SGD. Sediment coring and sea floor photography conducted with a remotely
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
The influence of groundwater on sea floor morphology has been discussed for more than 80 years within the geoscientific community (Robb, 1990). Early examples include Stetson (1936), who suggested that the formation of submarine canyons in the southern flank of Georges Bank off the northeastern US coast was related to groundwater seeping aided by currents. Submarine canyons are today linked to a combination of geological processes including erosion by turbidity currents, slumping, and mass wasting (Harris and Whiteway, 2011; Shepard, 1981). However, groundwater as a shaping agent of sea floor morphology appears in several other marine geological settings (Robb, 1990). For example, mapped terraced walls and irregular courses of valleys in the continental slope off New Jersey have been interpreted to be formed by groundwater seeping during periods of lower sea level (Robb, 1984). Other examples of sea floor morphological expressions related to groundwater are depressions formed around submarine fresh and/or brackish springs in the Mediterranean, where extensive nearshore carbonate formations caused the development of submarine karstic aquifers (Rousakis et al., 2014). Depressions, which are similar in appearance to pockmarks formed by gas seeps, have also been found where freshwater escapes through the sea floor (Whiticar and Werner, 1981; Khandriche and Werner, 1995; Virtasalo et al., 2019). Groundwater discharge into the ocean is recognized as a widely occurring process and is commonly referred to as submarine groundwater discharge (SGD) (Moore, 2010). SGD is estimated to contribute about 6 %–7 % of the total hydrological discharge to the world oceans (Zektser, 2000), although different quantification methods have yielded varying results at specific localities (Prieto and Destouni, 2011). In the Mediterranean Sea, SGD has been shown to be a major source of nutrients (Rodellas et al., 2015) and the process has generally been raised as a potentially underestimated provider of chemical elements, including pollutants and nutrients, from land to coastal waters (Destouni et al., 2008).
In Sweden, terraces and semicircular depressions in the Baltic Sea floor were mapped in the 1990s along some islands in the southern Stockholm archipelago (Söderberg and Flodén, 1997) (Fig. 1). These features were interpreted to be formed by processes related to SGD (Söderberg and Flodén, 1997). The proposed mechanism was groundwater flowing through siltier permeable layers in glacial varved clay, and where the clay outcropped at the sea floor, the escaping fresh water led to the development of a terrace when the above layers of clay were undermined and collapsed. With respect to the semicircular depressions, Söderberg and Flodén (1997) found morphological similarities with sea floor features mapped near Kiel in Eckernförde Bay, southwestern Baltic Sea (Khandriche and Werner, 1995), where SGD is documented to occur from geochemical analyses of sea floor sediment and the water column (Schlüter et al., 2004).
Overview map showing the locations of the studied areas in this work with white dots. Corresponding figures for each area are shown with white text. The locations of previous studies showing SGD discussed here are shown with black dots. I and II: Söderberg and Flodén (1997); III: Schlüter et al. (2004); IV: Virtasalo et al. (2019). The bathymetry is from EMODnet 2018 (EMODnet Bathymetry Consortium, 2018)
More recently, about 1 m high sea floor terraces, extending from a few metres
to
Results are presented from four different regions in the Baltic Sea: (1) east of the island of Askö in the southern Stockholm archipelago; (2) west of the island of Kastellholmen in Stockholm Harbour; (3) the southern Blekinge archipelago; and (4) southeast of Tvärminne Zoological Station in the southern Finland archipelago (Fig. 1). Only a small subset of the acquired multibeam bathymetry is shown from the surveyed area in the southern Blekinge archipelago, and it is without geographic coordinates due to military restrictions of revealing detailed depth information. Geophysical mapping results from all other surveyed areas in this study are granted permission to be shown in their full extent and resolution.
The high-resolution multibeam bathymetric data shown here were acquired
using Stockholm University research vessels RV
Sub-bottom profiles were acquired with R/V
In 2013, we towed a Klein 3000
Post-processing of the multibeam bathymetry was done using the QIMERA
software by QPS version 1.7.2 (
In the Askö and Tvärminne areas, terraces were photographed and filmed using two different remotely operated vehicles (ROVs): (1)a Saab
Seaeye Falcon and (2) BlueROV2 by Blue Robotics. The photos shown from
Stockholm Harbour and Blekinge were captured by divers. A 3-D model over an
approximately 6 m long stretch of the terrace in Stockholm Harbour was
assembled using the Agisoft Metashape software (
Numerous sediment cores have been retrieved east of Askö from the areas
with terraces during the field component of a Stockholm University course in
marine geophysical mapping. This course used Askö yearly for fieldwork from 2009 to 2018. In Tvärminne, the mapped terraces were cored in 2017 and 2018. We present analyses of core Asko2018HT-2GC, retrieved at
58
Core Asko2018HT-2GC was subjected to high-resolution (1 cm) logging of sediment physical properties, including bulk density, magnetic susceptibility, and
A carbonate concretion found in core Asko2018HT-2GC was subjected to geochemical analyses at the Stable Isotope Lab of the Department of Geological Sciences, Stockholm University, for determining the water
source(s) during its formation. Two pieces of the concretion were analysed: one from the centre and one from the outer edge. The samples were milled to
a powder and analysed for total carbon using a Thermo Delta V mass
spectrometer and for phosphoric acid reactive carbon with a Gasbench
II MAT 253 mass spectrometer. The elemental analysis provides
Terraces in the sea floor were mapped by Jakobsson et al. (2016) using a multibeam echo sounder in the vicinity of the island of Skåren, located east of Askö (Fig. 2). The terraces were identified in shaded relief images of the processed multibeam bathymetry. These terraces have steps of about 1 m high, extend from a few metres to
Multibeam bathymetry of the studied area east of the island of Askö, southern Stockholm archipelago.
The multibeam bathymetry shows that the sea floor morphology of the terraces
in the area east of Askö, Tvärminne, and the southern Blekinge archipelago closely resemble one another (Figs. 3 and 4). There are only a couple of mapped terraces in the latter two areas preventing a meaningful statistical comparison of their depth distribution. However, the terraces we mapped outside the area of Askö occur in the deeper depth range: (1) Tvärminne at 15–20 m and (2) the Blekinge southern archipelago at 23–25 m. The
terrace in Blekinge is the longest we mapped; it is possible to trace
continuously for
Multibeam bathymetry of the studied area east of Tvärminne Zoological Station in the southern Finland archipelago.
Multibeam bathymetry of a terrace mapped in the southern Blekinge archipelago.
Multibeam water column information was logged and analysed for two-thirds of the surveyed area east of Askö. Seeps from the sea floor were found to be a common feature (Fig. 2a), and the question immediately arose if the seeps were related to either terraces or depressions in the sea floor. There is an abundance of seeps in the northern part where no terraces are identified. East and southeast of the island of Skåren, seeps begin to occur at about 20 m of water depth; i.e. with a few exceptions this is where the deepest terraces occur (Fig. 2b). The mean and median depths of the seeps are 21 and 20 m, respectively, while the shallowest is located at a water depth of 3 m and the deepest at 40 m. The multibeam backscatter shows that the terraces in the area east of Askö systematically appear in a relatively harder sea floor characterized by high backscatter, while the seeps generally occur in a softer seabed represented by lower backscatter (Fig. S1 in the Supplement). The side-scan data acquired in 2013 show high-resolution imagery of the terraces, with no apparent difference in signal intensity across them but with clear shadows present due to the bathymetric expressions (Fig. S2).
Sub-bottom profiles and bottom photographs portraying sea floor
terraces.
There is a semi-regular grid of sub-bottom profiles covering the entire area east of Askö (Fig. S3). These profiles were acquired during the field component of a Stockholm University course in marine geophysical mapping held yearly in this area since 2009. From this database two sub-bottom profiles, extending across terraces identified in the multibeam bathymetry, are shown in Fig. 5a and b. The terrace west of the island of Kastellholmen in Stockholm Harbour was mapped by a sub-bottom profile perpendicular to its extent (Fig. 5c). Common for all terraces imaged by sub-bottom profiles is that the acoustic stratigraphy indicates well-stratified sediments that outcrop at the sea floor where the terrace is formed. This is particularly clear in the profile from Stockholm Harbour (Fig. 5c). In the area east of Askö, where the sub-bottom profile coverage is most comprehensive, an acoustically semitransparent surface unit with few internal noticeable reflections is commonly found in sections with water depths deeper than the general occurrence of terraces (Fig. 5b).
Bottom photographs of sea floor terraces in the studied areas.
The bottom photos confirm that distinct terraces are formed in the sea floor in all mapped areas (Figs. 5d–f and 6a–f). It is also possible to identify that the terraces developed in stratified sediments outcropping at the sea floor, which was particularly evident in the acoustic stratigraphy of the sub-bottom profile from Stockholm Harbour (compare Fig. 5c with Fig. 5e and f). Holes with diameters between about 1 and 2 cm are abundant in the nearly vertical terrace walls in Stockholm Harbour and Tvärminne (Figs. 5d, e and 6c, d). Some of these holes appear to be cavities from stones that were embedded in the sediments and eventually fell out during the erosional process forming the terrace. However, we cannot confirm if this is always the case because the holes sometimes appear to extend rather deep into the terrace walls. Hence, they may be zones of piping and erosion that developed in response to focused groundwater flow.
The 4.3 m long core Asko2018HT-2GC, retrieved from a terrace in a water
depth of 16 m in the area east of Askö, consists of rhythmically
alternating 0.5–2.5 cm thick silty–clay layers (Fig. 7). The upper 3.5 m is composed of inclined (
Lithology and sedimentology of sediment core Asko2018HT-2GC. Image
includes a lithologic log
Geochemical analyses of the concretion found in the glacial clay unit of core Asko2018HT-2GC. See Fig. 2a for core location and Fig. 7 for an image of the concretion. The upper section shows elemental analyses made using the Thermo Delta V mass spectrometer, and the lower analyses were made on phosphoric reactive carbon with a Gasbench II MAT 253 mass spectrometer (see “Material and methods” for further details).
Cumulative grain size distributions for the laminated glacial clays in Asko2018HT-2GC. Sediments are exclusively composed of fine fraction
material, with 50 %–80 % of the sediments being
Two carbonate concretions were found at core depths of 1.77 m (diameter 1 cm) and 2.64 m (diameter 5.3 cm) (Fig. 7). The lowermost one has a
disc-shaped appearance, displaying a series of concentric rings growing outwards from a central spherical concretion. It is a classic
The results from the geochemical analyses on the concretion are shown in
Table 1 (
Schematic illustration showing how siltier layers in glacial clay
could act as a conduit for groundwater, eventually escaping at the sea floor as SGD and leading to the formation of a bathymetric terrace. The critical parameters
The first discoveries of terraces formed in the sea floor along the Swedish
coast were made in the 1990s with a conventional 30 kHz echo sounder and a
Our study generally supports the hypothesis put forward by Söderberg and Flodén (1997) on the terrace formation mechanism. They argued that groundwater flows through siltier layers in glacial clay to eventually escape at the sea floor. Erosion from the flowing water undermines the overlying layers, causing them to collapse and form a sharp terrace in the sea floor (Fig. 9). The underwater photos in Fig. 6 include examples in which we believe that this process can be readily envisioned. It is possible to see how cavities are formed in the varved clay at the bottom of some of the terraces as well as blocks of the overlying clay that have collapsed from being undermined. However, we cannot exclude the possibility that mechanisms other than SGD could produce terraces in the sea floor similar to those we mapped in this study, and therefore alternative formation mechanisms are discussed below.
We have also found from our geophysical mapping and coring results that the
terraces are systematically formed in glacial clay throughout the studied
areas. Glacial clay in the Baltic Sea sediment stratigraphy is commonly
found draped on top of till or glaciofluvial material; in some cases it
rests directly on bedrock (Andrén et al., 2011, 2015) (Fig. 9). This type of clay was for the most part deposited during the last deglaciation in front of the retreating Scandinavian Ice Sheet. Glacial clays left from previous older glaciations are extremely rare and found only at a few locations (e.g. Björck et al., 1990). Swedish geologist Gerard De Geer discovered that glacial clay is comprised of rhythmites, wherein the layers composed of varying proportions of clay and silt are annual depositions of erosional material from the retreating ice sheet (De Geer, 1912). He introduced the term “varve” for one annual layer of glacial clay and noted that its thickness and silt content varied depending on the proximity to the retreating ice margin; i.e. thicker and siltier varves were deposited close to the ice margin. He further proposed that there would be a higher degree of silt content in the part of the varve representing the meltwater-rich summer period. Thus, grain size variations in the glacial clays are found on a number of scales: from millimetre- to centimetre-scale variations across rhythmites, to longer-scale (decimetre) variations related to climatically driven variations in subglacial discharge, to even longer-scale (
It should be noted that the glacial clay sequences are time-transgressive throughout the Baltic basin, with older clay in the south and younger in the north because the Baltic basin was deglaciated from south to north (Hughes et al., 2016; Stroeven et al., 2016). The first small freshwater body in which glacial clay could be deposited during the last deglaciation formed in front of the ice margin around eastern Denmark and the northern coasts of Germany and Poland at about 16–15 ka (Houmark-Nielsen and Henrik Kjær, 2003). This water body grew as the ice sheet retreated northward to become the Baltic Ice Lake, which appears to have lasted until the end of the Younger Dryas cold period at about 11.7 ka, when it catastrophically drained westward north of Mount Billingen in south–central Sweden (Björck and Digerfeldt, 1986; Andrén et al., 2002; Swärd et al., 2015). Mapping of preserved palaeo-shorelines in the 1920s showed that the drainage occurred when the ice sheet margin reached north of the damming high terrain in the west (Lunqvist, 1921). A brackish water phase called the Yoldia Sea, likely constrained to the central Swedish side of the Baltic (Schoning, 2001), followed the Baltic Ice Lake (Björck, 1995); however, it would take several hundred years for the Baltic to become brackish after the drainage and deposition of varved glacial clay continued close to the retreating ice margin (Andrén et al., 2011). The Baltic Sea basin was completely ice free at about 10 ka (Hughes et al., 2016). As the ice retreated (Stroeven et al., 2016), conditions may have developed for terrace formation at different places around the Baltic Sea depending on local sea level in relation to glacial clay deposits. The mapped terraces in the different regions may therefore be of different ages; some may be inactive, while others are active today.
The geochemical analyses of the concretion from core Asko2018HT-2GC do not
irrefutably determine whether or not groundwater flow occurred through the
glacial clays, but they provide valuable insights into the formation environment.
The
The
Precipitation in the Stockholm area has a yearly mean
Isostasy plays an important role in estimating the potential hydrogeological
connection in the Baltic region between land and sea through glacial clay.
The Swedish terrain was isostatically depressed by the Scandinavian Ice Sheet, which was several kilometres
thick (Lambeck et al., 2010), and the highest coastline is therefore found in several areas far inland of the present coast (Björck, 1995). Isostatic rebound eventually caused glacial clay sequences to be lifted above the level of the Baltic Sea, thereby creating a hydraulic head between the landward end of permeable glacial clay layers and their seaward continuation (Fig. 9). To estimate SGD into the Baltic Sea through these permeable layers one could apply Darcy's law according to
The SGD through the terraces will most likely be intermittent considering the relatively large variation of the seasonal groundwater table around the Baltic Sea, as shown by geological survey monitoring on land. Looking at data from monitoring stations provided by the Swedish Geological Survey in the vicinity of the Stockholm archipelago, we note that the seasonal variation in the groundwater table of some locations exceeds 3 m, and the highest groundwater table is generally found from the late fall to late spring (Fig. S5). Groundwater discharge to the Baltic Sea through the sea floor is known to occur through geological formations other than the terrace formations discussed here, in particular where glaciofluvial aquifers on land connect with the sea floor (Peltonen, 2002). However, the prevalence of the potentially SGD-related terraces in the mapped regions suggests that SGD to the Baltic Sea is likely underestimated and remains an unconstrained source of pollutants and nutrients, as previously argued by Destouni et al. (2008). The ecological influences of the terraces is therefore unknown. It is also clear that there is a general lack of data and understanding of SGD emission mechanisms for proper assessments of emission rates and/or volumes (Taniguchi et al., 2002).
There are other mechanisms that potentially could have played a role in the
formation of the sea floor terraces mapped in this study. For example,
sliding and slumping of glacial varved clays has been suggested to occur due
to the liquefaction of layers during palaeoseismic events (Hutri and Kotilainen, 2007; Virtasalo et al., 2007). This could leave behind terraces at the sea floor formed in glacial clay. However, we do not observe any morphological evidence of sliding, and most of the terraces we mapped occur in areas where the sea floor slopes at
The multibeam bathymetry and sub-bottom profiles presented in this work have been granted public release by the Swedish Maritime Administration, apart from the data shown from Blekinge without coordinates and precise location information. The released data are available for download from the Bolin Centre Database:
The supplement related to this article is available online at:
MJ prepared the paper with input from all co-authors. MJ analysed and processed the multibeam bathymetry and sub-bottom profiles, MO measured and analysed the sediment cores, CMM did the geochemical analyses, and EW identified seeps in the multibeam water column data. JH and AS captured photos of terraces while diving.
The authors declare that they have no conflict of interest.
We thank the crew and captain of R/V
Further funding was provided by the Academy of Finland (project ID 294853) and the University of Helsinki/Stockholm University strategic fund for collaborative research (the Baltic Bridge initiative). The article processing charges for this open-access publication were covered by Stockholm University.
This paper was edited by Susan Conway and reviewed by Joonas Virtasalo and one anonymous referee.