Sediment layers of different thicknesses lie at the bottom of the Baltic Sea. They were mainly created during and after the most recent Ice Age. Matter is still being deposited on the seabed, while waves and currents erode the seabed and shift around this matter.
The Baltic Sea’s sediments tell the tale of the sea’s evolution. The deposits can be used to determine when and how they were created: during or after the Ice Age.
The deposits formed during the Ice Age are called glacial sediments. They were deposited in the bottom or at the edge of the continental ice sheet, or carried there by meltwaters. Glacial deposits include moraines, eskers and glacial clays.
Post-glacial deposits are mainly fine sediments, such as silts, clays and muds. They were deposited on the seabed in the various lake and sea phases during the Baltic Sea’s evolution. In addition to fine sediments, this group includes such coarse sediments as sand that were carried away by erosion forces and subsequently deposited.
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.
Several different layers can be identified in sediment samples drilled from the seabed. Where deposition has been consistent after the Ice Age and the older deposits have not been eroded away, the layers have a typical order, known as stratigraphy. For example, this is the case in samples collected in the western Gulf of Finland.
Various glacial till deposits cover the bedrock on the seafloor. Deposits formed as the glacier retreated that were sorted and accumulated by the meltwaters may be found on top of them.
Clays were deposited outside the edge of the continental ice sheet in the deep waters of the ice lake. They are called varved clays as they reflect seasonal variations, similarly to the annual rings in trees. The bottom layer of the annual varve consists of course-grained deposits built up in spring and summer. Above this coarse layer is a fine winter layer.
The varved structures near the edge of the glacier may be tens of centimetres in thickness. Further away from the edge of the ice sheet, the varved clay layer has decreased to less than one millimetre.
Once the glacier had retreated completely from the Baltic Sea area, a large lake filled this basin. A homogeneous layer of clay was deposited at the lake bottom. Clay units with darker sulphide-containing bands or streaks were also formed at this time.
In the next phase seawater began to flow into the basin, which now contained brackish water with low salinity. During the brackish water stage, organic muddy clay was deposited in some parts of the seabed. Many basins and depressions had little or no oxygen at times. The low-oxygen periods can be identified in the clay as fine-layered structures.
The uppermost layers of bottom sediments consist of muddy clays and muds that were deposited on the seabed over the last few millennia. The clay layers may be topped by sand or silt that have been eroded and transported by currents and waves, especially along the coast.
Deposition has rarely been continuous, even in the deep waters of the sea, as the conditions have varied over the millennia. Benthic activity, such as organisms burrowing in the seabed, may also have mixed up sediments deposited on the seabed.
Deposition of materials on the seabed still continues. It is part of the natural development of the sea, as different solid particles are constantly entering and moving around in the sea.
Most of the matter deposited on the seabed comes from the continent. Rivers, wind and ice carry humus and inorganic matter to the sea, even across long distances from inland areas. Their volumes vary seasonally: the largest amounts of matter are transported into the sea during spring flooding and autumn rainfall, and the smallest amounts when the ground is frozen and covered with snow.
Part of the deposited materials originates in the marine ecosystem. Dead organisms and the droppings of marine animals sooner or later sink to the bottom. The primary production of the marine ecosystem is regulated by the seasons. Following the growth spurt of spring and summer, large amounts of organic matter sink to the bottom. Waves and currents also erode different materials from the seabed.
Whether they come from land, the seabed or marine organisms, the materials ending up in the sea are mixed together and transported by the sea. Waves and currents carry them further away from the shore and into deeper waters. When the currents finally slow down, the materials can sink to the bottom.
Among other factors, deposition is influenced by water depth, flow rate, distance from the shore and openness of the coast. Climate and ice cover also play a role. This means that there are major local variations in deposition; each site has its special dynamics.
Sinking rates are also influenced by the characteristics of the sinking particles: particle size, shape and density. Larger particles sink faster than smaller loose ones. Small particles often form larger clusters, however, and consequently sink to the seabed faster. Nevertheless, in deep water it takes a long time for the particles to reach the bottom.
The fluctuation in the sinking rate means that the further from the continent the sedimentation basin is, the finer the deposited material. On the other hand, the greatest volumes of organic matter produced by the marine ecosystem sink to the bottom in areas with the strongest algae production. Seabed erosion also causes local differences in the quantity and quality of the deposited material. Significant quantities of marine sediment may accumulate locally from mass movements of materials, such as landslides on steep undersea slopes.
Waves and ice erode the seabed, especially in shallow areas. In deep water the wave effect is weak and the sea state has little impact on the seabed.
On the shores of the northern Baltic Sea erosion is influenced by land uplift. The land rises 3 to 9 millimetres a year on the Finnish coast. The highest rate of land uplift is seen in the Kvarken and the Bothnian Bay, and the lowest in the Gulf of Finland. As the land rises, the sediments deposited on the seabed also rise and become exposed to erosion by shoreline forces, ice and bottom currents. As the coasts of the Gulf of Bothnia are mostly shallow, rather large areas can be exposed here in a relatively short time.
As the sinking and deposition processes are complex, the surface materials of the seabed vary in quality and age from place to place. Typical surface soils include sand, clay and moraine. Sand bottoms are more common near the southern coasts of the Baltic Sea, while clay bottoms are found in the basins further away from the coast. Exposed bedrock occurs especially in fragmented coastal areas.
