Younger seabed deposits

The seabed of the Baltic Sea has many sediment deposits, which can vary widely between regions. Let’s take a closer look at the bottom deposits of the Baltic Sea!

Seabed composition

There have been several ice ages over the past 2.6 million years. Ice sheets have eroded away part of the sediment deposits on the seafloor of the Baltic Sea. As a result, those sediments deposited during warm phases between glacial periods have not been preserved much or have not yet been found in marine areas. These warm phases between the ice ages are called interglacial periods.

The younger deposits of the Baltic Sea seafloor have been formed mainly during and after the last glaciation, i.e. over the past 25,000 years. As a result, these younger seabed deposits have survived better than older deposits. These preserved sediment deposits can be divided into two types according to their origin and environment, i.e. glacial and post-glacial.

Glacial sediments are deposited on the bottom or at the edge of the ice sheet. In addition, glacial meltwater may also have collected at the sides and front of the ice sheet. Glacial sediments include features such as moraines and till material, including glaciofluvial deposits, such as eskers, as well as glacial clays including varved clays.

Postglacial deposits include lake and marine sediments formed at different phases of the Baltic Sea, which are generally fine sediments such as silt, clay, and mud. Coarse-grained sediments like erosional sand also belong to this group.

In the accumulation areas, the topmost unit of the seabed is composed of organic clays and muds, which have been deposited over the last few hundred years. In some places, these clay and mud deposits are covered with sand and silt, which have been eroded from harder bottoms by currents and waves. Such places are especially found on the coast.

The typical sediment stratigraphy

This is the typical sediment stratigraphy in areas where deposition has been consistent after the Ice Age and where previously deposited material has not been eroded away:

  1. Homogenous muddy clay (uppermost)
  2. Laminated muddy clay
  3. Iron sulphide mottled sediments
  4. Homogenous clay
  5. Varved clay from the Yoldia Sea phase
  6. Varved clay from the Baltic Ice Lake phase
  7. Till (lowest)
 Seabed sediment stratigraphy in the Gulf of Finland, a place where it is assumed that there has been continuous sedimentation over millennia.
Typical sediment stratigraphy in the Gulf of Finland. Kaskela et al., 2017.
 Explanation of the sediment stratigraphy for the different substrate types shown in the figure.
A pictorial explanation of the sediment stratigraphy in the Gulf of Finland. Kaskela et al., 2017.

Bedrock is covered with different units of till. When the ice sheet retreated, sediments deposited by meltwater could have accumulated on top of till. Clays were deposited outside the edge of the continental ice sheet into deep waters  of the ice lake. These clays are called ‘varved’ clays, i.e. showing an annual deposition of sediments. Their structure shows evidence of seasonal variation in glacial melting. For example, lower down in the annual varve, the clay is coarse-grained (even silt and sand) and corresponds to a spring and summer layer, when melting was at its peak. Above this coarse layer is a fine winter layer when meltwater was at a minimum and thus, only capable of transporting the finest of sediment deposits.

The varved structures in the front of the glacier may have been tens of centimetres thick. Further from the ice sheetedge, the thickness of these varved clays has decreased to less than one millimetre. Far from the ice sheet edge, under calm flow conditions, clay was deposited on the bottom of the basin.

It is estimated that the continental ice sheet would have deglaciated from the Baltic Sea basin some 10,000 years ago. As the amount of meltwater from the ice sheet decreased, homogenous clay was deposited at the bottom of the then large lake. Clay units with darker sulphide-containing bands or streaks were also formed.

When the sea level began to rise, it crossed the threshold of the Danish straits and saltwater began to flow into the Baltic Sea basin. This was the beginning of the brackish water phase. In some places, organic matter containing sediments accumulated on the seabed and it was during this phase that many basins and depressions in the Baltic Sea were at times low in oxygen. The sediments from such periods can be distinguished due to their fine-layered structures.

The uppermost units of bottom sediments consist of organic muddy clays and muds that have been deposited on the seabed over the last few thousand years. Some areas have coarser material such as sand and silt and such locations are especially found on the coast.

Deposition is rarely continuous, even in the deeps of the sea. Conditions have changed over the millennia. There are major temporal and spatial differences in the seabed deposits and nowhere is sedimentation exactly alike.

The erosion, transportation, and deposition of material

The erosion, transportation and deposition of sediment materials are influenced by many different factors. These include water depth, bedrock type, current velocity, distance from the coast, shore exposure, climate, ice cover, benthic fauna (bottom living animals), and land uplift.

The typical seasonal fluctuation of the Baltic Sea climate regulates both the primary production of the marine ecosystem, as well as the amount of minerals washed into the sea by rivers.

The northern Baltic is characterised by rapid land uplift, rising about 5 to 9 millimetres per year. As the land rises, older sediments are exposed to sources of energy for eroding seabed, such as waves, as well as bottom currents.

 Rate of land uplift in the Baltic Sea basin area in mm/year.
Land uplift in the Baltic Sea region. Harff & Meyer, 2011.

Rivers, wind, and ice can carry humus, i.e. organic material, as well as inorganic matter from the continent to the sea. Organic material and inorganic sediments eroded from the seafloor are transported to deeper water by the force of waves and currents. This transport from the shore to the depths continues until currents slow sufficiently for the material to sink to the seabed. Mass movements of material, i.e. submarine landslides, can also occur on steep slopes, resulting in the transportation and deposition of sediments. 

Sediment particles begin to sink to the seabed as the water flow rate slows. Sinking rates are affected by the particle size, shape, density and by how fast the water flows. The larger the particle size, the faster the particle settles to the bottom. Small particles often form larger aggregates, which accelerates their descent to the seabed. The deeper the water body, the longer it will take to sink. Water depth is also important for wave erosion.

Waves are capable of eroding the seafloor. Such abrasion is strongest in shallow areas and weakens with depth. In addition, the local seafloor topography causes fluctuations in bottom currents and hence, the subsequent erosion and transport of material. The farther the sedimentation basin is located from the continent, the finer the sedimentation material deposited. Where primary production is high, the amount of organic matter deposited also shows a general increase. Certain activities of benthic organisms, such as burrowing, can mix the sediments deposited on the seabed.

In the Gulf of Bothnia, land uplift is rapid, rising 5-9 millimetres per year. Thus, this process reveals previously deposited sediments, which become exposed to shoreline forces, such as ice, wave action and bottom currents. The highest rate of land uplift occurs in the Bothnian Bay. Since the shores in this area are already shallow, large submerged areas will become exposed in a relatively short period of time.

Due to varying processes, seafloor substrates of different ages from the Ice Age and earlier to younger, laminated mud or sand layers, may be exposed. There may also be exposed bedrock on the seabed. As a result, bottom substrate types are very unevenly distributed in different areas. The erosion, transport and deposition of material vary with time and place. Moreover, sedimentation is rarely continuous, even in the deepest parts of the sea, and nowhere is it exactly the same.

 Occurrence of seabed surface substrate types in the Baltic Sea.
Seabed substrate map of the Baltic Sea. Kaskela & Kotilainen, 2017. Data: EMODnet Geology, 2016.