The Baltic Sea is geologically diverse

Geodiversity means geological complexity, which consists of the bedrock and soil, as well as changes in geological processes and surface landforms. Geodiversity also contributes in part to biodiversity, i.e. biological complexity, since geological features influence habitat structures. For example, various aquatic organisms commonly thrive on bedrock and sand bottoms.

The geological diversity of the Baltic Sea has been examined based on the variations found in bedrock geology, seabed substrates, and seabed structures. The most geologically diverse areas occur in the northern Baltic Sea in an area of crystalline basement bedrock, as well as near the coastline, and particularly in archipelago areas.

The seabed of Finnish coastal regions is a geologically diverse and unique.
Archipelago areas within crystalline rocks have been identified as geodiversity hotspots.

Compared to the northern Baltic Sea, the southern parts are topographically smoother. Several factors influence the diverse landscape there. These include the so-called tectonic lineaments of the crystalline basement bedrock, i.e. long features visible in the terrain, which are an expression of an underlying geological structure, as well as shear zones formed during the early stages of bedrock development. A shear zone is a zone of strong deformation (with a high strain rate) surrounded by rocks with a lower state of finite strain.

In addition to the bedrock characteristics, the location of substrate types and seabed structures has been influenced by both glacial erosion and sediment accumulation, as well as modern processes. Therefore, both the geological characteristics and past geological processes have determined the features of the Baltic Sea seafloor.

 Geodiversity is presented here according to quantiles: Q1 (< 25%), Q2 (25%–50%), Q3 (50%–75%), and Q4 (> 75%). Q4 is the most diverse.
The seabed geodiversity of the Baltic Sea is highest on the northern coasts of the Baltic Sea. The Geodiversity Index describes the distribution of geological diversity. Q4 is the most diverse. Kaskela & Kotilainen, 2017. Coastline: European Environment Agency, 2013.

Geodiversity in the Baltic Sea is threatened by many factors

Anthropogenic pressures on coastal and marine areas have increased dramatically in recent years. Urban growth and land scarcity have increased construction on the shorelines and on the water. In addition, other construction activities on the seabed have increased, such as the construction of harbours and shipping lanes, various cable and gas pipeline projects, as well as the construction of renewable energy facilities, such as offshore windfarms.

In addition, the natural conditions on our coasts indirectly increase this need. Land uplift and river-borne sedimentation are constantly reducing the depth of our coastal waters, such that the maintenance of harbours and shipping lanes requires repeated dredging of the seabed.

The exploitation of the seabed's natural resources, such as the need for rock aggregates for construction will also increase in the future. All the above operations are closely related to dredging and dumping.

Geohazards can be caused by Nature or by humans

Geohazards are geological risks related to the seabed. They can be separated into risks caused by natural circumstances and those caused by human-related activities associated with the seabed.

Seabed construction may involve a variety of geological problems, such as dangerous situations caused by natural conditions. Of these, the best known globally include earthquakes, submarine mass movements, tsunamis, and volcanic eruptions, or even meteorite impacts.

These types of geohazards can cause serious human loss, as well as financial damage. However, in the Baltic Sea the greatest construction problems are manly geotechnical in nature, such as structural subsidence on soft seafloors.

Coastal erosion and underwater mass movements are also possible in some areas of the Baltic Sea. However, such risk factors should be identified and taken into consideration before any construction work is undertaken.

 Seabed shaded relief map of Neugrund meteorite impact crater, situated off the coast of Estonia.
A bathymetric map of the seabed from the Neugrund meteorite impact crater site off the coast of Estonia. The structure was formed circa 535 million years ago and is considered one of the best-preserved seabed asteroid collision structures in the world. Kaskela et al., 2016. Data: Estonian Maritime Administration.

Seabed deposits contain harmful substances

Seabed deposits present a different type of environmental risk. Within these bottom deposits, the sediments may record harmful substances from both natural sources, as well as from anthropogenic loads.

Increases in anthropogenic pollutants, such as heavy metal loads from the 1950s to the 1970s and the 1980s are reflected in the seabed sediment records of the Baltic Sea. Although the concentrations of lead, mercury, cadmium, and radioactive cesium in the seabed surface sediments have declined markedly over earlier decades, harmful substances continue to be abundant deeper in the sediments.

Although the external phosphorus loading that causes eutrophication has been reduced, the sea’s recovery may be delayed by the release of phosphorus from bottom sediments under anoxic conditions.

 Sediment samples from the Bay of Bothnia and the Gulf of Finland containing harmful substances such as lead, copper, zinc and arsenic.
Harmful substances in sediment. Concentrations of lead (Pb), copper (Cu), zinc (Zn), and arsenic (As), in sediment samples from the Bothnian Bay and the eastern Gulf of Finland, in 50 cm long sediment core (mg / kg dry weight). The figure also shows the toxicity limits: Lower toxicity limits (“effects range-low”; ERL) and mid-range toxicity limits (ERM). Vallius et al., 2014

Dredging can spread harmful buried substances

During dredging, seabed materials disperse in the water. Fine fraction of sediments clouds the water and may be carried far from the dredging site with the help of currents. Such fine sediments can absorb harmful substances, such as heavy metals. 

Dredging affects seabed deposits with elevated levels of harmful substances, causing the resuspension and further transportation of previously deposited and buried material. The disposal of contaminated sediments to the seabed has similar adverse effects on the environment.

For example, during major construction work in the Neva Bay from 2006 to 2008, fine sediments and their associated harmful substances were transported into the Gulf of Finland, up to tens of kilometres off the coast.

Seafloor construction projects sometimes expose contaminated sediments

Large seabed construction operations, such as harbour projects, can reveal unpleasant surprises from the bottom deposits. For example, during the dredging of Vuosaari Harbour, contaminated sediments containing toxic tributyltin (TBT) were found on the seabed.

These TBTs, which had previously been dumped into the sea from a shipyard in the area, had to be dredged out and stabilised. These added activities affected the construction planning and altered the order of construction of the port. In addition, munitions, mines, and chemical weapons on the seabed can pose risks to the environment and to construction activities.

 The fine material transported to the Gulf of Finland in 2006, 2007 and 2008 in connection with major construction works on the Gulf of Neva can even be seen from space.
Satellite imagery showing the fine sediment transported to the Gulf of Finland during major construction work in the Gulf of Neva. Sukhacheva ja Orlova, 2014; Ryabchuk et al., 2016.

The extraction of natural resources may cause damage to marine habitats and sensitive biotopes

Making the water turbid by spreading fine sediments can also result from the extraction of natural seabed resources, such as marine sand excavation. Marine sand retrieval alters seabed habitats, and results in seabed habitat loss. In some locations, such changes persist over a long time.

Fine sediments resuspended by activities such as dredging, spoil dumping, and seabed resource extraction may sink to the bottom as a fine sludge and cover fish reproduction areas. 

Thus, seabed sedimentation may in some places be harmful to the marine habitats and sensitive biotopes of the Baltic Sea, such as in seaweed- and mussel-reef communities. Moreover, in sheltered bays with poor water exchange, even small-scale dredging can have significant environmental impacts.

Contaminated bottom sediments can cause great environmental risks

When contaminated seabed sediments are resuspended, they are associated with major environmental risks, such as polluting marine life and contaminating the sea. There is a relatively good understanding of the harmful substances found in seabed sediments.

However, there is still a lack of detailed knowledge about the concentrations of pollutants in the bottom sediment, as well as their regional location and distribution. Also, little is still known about the migration of toxic substances in sediments caused by dredging.

Geological research and knowledge protect the Baltic Sea environment

The Baltic Sea is a treasure trove of shipwrecks. On the seafloor lies a wealth of well-preserved wrecks, some of which are up to several centuries old. The destruction of these historically significant wrecks through seabed construction works must be prevented by investigating their location in conjunction with preliminary investigations.

Human activities involving the seafloor place a burden on the environment and if poorly designed and implemented, may involve environmental risks. Potential risks, such as the presence of harmful substances in the bottom sediments of an area, must be resolved before any construction work is undertaken.

Geological surveys of the seabed provide information on the structure and composition of the seabed and, for example, on factors affecting construction. Research data can be used to direct construction to the most suitable location and to choose the right method for its safe implementation, while minimising the environmental risks. The fundamental knowledge of seabed geology is essential to successful marine spatial planning, which guides the sustainable use and development of marine areas.

Climate change will bring its own challenge to the geodiversity of the seabed and will shape environmental conditions in ways we do not yet know the exact effects of.