Seafloor deposits and sediments reveal the past and present state of the Baltic Sea

A great deal of information about the Baltic Sea and its ecosystems has been stored in the sediment deposits of the seabed. Sediments can be read like a history book. They show not only the ancient changes that have occurred in the sea but also the more recent events which have taken place in the catchment area.

Marine geology opens the door to both the history and the current state of the Baltic Sea. Understanding the different phases of the sea will help solve its current problems.

In Finland, the seabed and its sediment layers are studied by several universities and governmental research institutes. Seabed research is also being carried out at Coastal Business, Transport, and Environment Centres, i.e. ELY Centres, as well as in many private companies.

Environmental changes over millennia

The ancient history of the Baltic Sea has been marked by postglacial changes. By contrast, the recent history of the sea is largely man-made. Drainage of the catchment area, deforestation, and land cultivation, as well as settlements and industry, have markedly increased the amount of sediment material transported to the sea.

It can be seen from the older sediments how the Baltic Sea has responded to various natural changes in the past, such as to fluctuations in salinity or climate. For example, sediments show how the oxygen status of the depths and marine life has changed in different periods.

Knowledge of this history helps us to predict future developments, including the effects of current climate change.

Sediments participate in the circulation of substances

Nutrient levels, particularly of nitrogen and phosphorus, are also studied in the sediments. An increased nutrient load in the sea causes eutrophication, which is one of the worst environmental problems in the Baltic Sea. Sediments also play an active role in eutrophication because nutrients can circulate between the water and bottom deposits.

The accumulated bottom sediments also reveal another major problem in the Baltic Sea, i.e. harmful and dangerous substances that have been released into the sea. These substances can also be released from the sediment back into the water, especially if the seabed is excavated or disturbed.

Marine geology studies these sediment processes, i.e. how the sediment deposits affect the water quality and thereby the marine ecosystems also.

Seabed sediment sample from a depression in the Gotland Basin, the Baltic Sea.

A varied landscape hides beneath the surface

The seafloor is a highly variable environment. Therefore, an important part of seafloor research involves investigating the geological diversity of the seabed, i.e. geodiversity, as well as its regional variations.

Knowledge of the geodiversity can help us to understand changes in marine life or biodiversity. For example, such information can be used to find ecologically valuable habitats or locate suitable areas for construction. Geological knowledge also helps to choose those construction methods that cause the least harm to nature.

Overall, seafloor mapping provides an important knowledge base for the planning of marine use. Marine geological data can also be used to inform us about the seabed's natural resources, such as marine sand and gravel.

Models fill information gaps

In Finland, the geology of the seabed has been studied for decades. However, the geological structure and sediment composition are still only known for less than a quarter of all Finnish marine areas.

Gaps in information can be filled using statistical models. These models, based on the data collected and their interpretation, provide an estimate of the geological features of unexplored areas. The models can also be used to predict future developments and assess the impact of different actions on the sea.