The Baltic Sea is the youngest sea in the world. It acquired its current features only a couple of thousand years ago. In the millennia preceding this, many rapid changes occurred in the Baltic Sea basin. These have left their mark on the present Baltic Sea.
The Baltic Sea has formed in an ancient depression in the bedrock. Over millions of years, geological processes have shaped this basin. Among other things, repeated ice ages have eroded away some of the early sediments.
20,000 years
The Baltic Sea is a very young sea. As recently as 20,000 years ago, the Baltic Sea basin was covered by a thick continental glacier. The glacier melted away over the next 10,000 years.
5 stages
The developmental history of the Baltic Sea includes five stages, during which the basin has alternated between fresh water, sea water, and finally brackish water.
Before the last Ice Age, there was a warmer interglacial period called the Eemian warm period, which occurred about 130,000 to 115,000 years ago.
During the Eemian warm period, the ocean level was elevated, making Fennoscandia an island. Wide straits connected the Baltic Sea basin to both the North- and White Seas. The Eemian Sea was more saline than the present Baltic Sea.
Although no Eemian deposits have been found in the present-day sea areas of Finland, they have been discovered in the southern Baltic Sea.
The last ice age began about 115,000 years ago and ended around 11,700 years ago. During this Weichsel glaciation, the entire Baltic Sea basin was at least periodically covered by an ice sheet. The glacier was at its largest about 20,000 years ago, when the Baltic Sea basin was covered by an ice sheet several kilometers thick.
The ice sheet began to retreat from the southern parts of the Baltic Sea basin about 18,000 years ago. The basin was completely exposed from the ice about 10,000 years ago. Since then, there has been no continental ice in the basin area.
As the climate warmed and the edge of the ice sheet began to retreat around 18,000 years ago, water began to accumulate in the Baltic Sea basin. This formed the Baltic Ice Lake. Its water level was significantly higher than that of the ocean.
Varved clays drilled from the bottom of the Baltic Sea indicate that the seasons varied greatly at that time. In summer, the ice melted rapidly, and fine-grained material settled on the lake bottom in thick layers. In winter, much less material was deposited.
The Baltic Ice Lake phase lasted over 6,000 years. It ended about 11,700 years ago when a connection to the North Sea opened through central Sweden. This created the first post-glacial brackish water phase, the Yoldia Sea. Its water was saltiest near the sea connection. The Yoldia Sea phase ended less than a thousand years later when the sea connection closed.
The most recent lake phase of the Baltic Sea is called the Ancylus Lake. This phase began 10,700 years ago and lasted just under 2,000 years. The clay layers of the Baltic Sea, containing darker, sulfide-rich streaks, date back to the Ancylus Lake period.
The next brackish water phase, the Littorina Sea, began when the Baltic Sea basin reconnected with the North Sea, this time through the Danish straits. This occurred about 9,000–10,000 years ago. The actual brackish water phase is considered to have started in the middle of the sea about 8,000 years ago, slightly earlier in the southern parts and slightly later in the northern parts. The water of the Littorina Sea was saltier at the beginning of this period than the current water of the Baltic Sea.
The transition from the Ancylus Lake to the Littorina Sea was rapid and dramatic. The freshwater mass changed to low-salinity brackish water in a short time. The change in the chemical and physical properties of the water affected the living conditions of aquatic organisms.
The Baltic Sea has been a brackish water basin similar to its current state for a few thousand years. During this time, it has also changed. The land has risen, and the water level has dropped in some places. The connection to the North Sea has weakened, reducing the salinity of the water.
At this rate, the Gulf of Bothnia will eventually close off at the Kvarken. This would happen in about 2,000 years, and the Bay of Bothnia would become the largest inland water body in Europe, the Bothnian Lake. However, the sea level rise caused by climate change may slow down or completely prevent the formation of the Bothnian Lake.
You can also read about the geological history of the Baltic Sea at Geologia.fi (in Finnish)(avautuu uuteen ikkunaan, siirryt toiseen palveluun)
