The Baltic Sea basin, with the limits of the Fennoscandian ice sheet and source and sink areas used
The Baltic Sea basin, with the limits of the Fennoscandian ice sheet and source and sink areas used in the calculations.


The Baltic River

Adrian sets the scene: “If we go back 5 or 10 million years, there’s little sign that the Baltic Sea existed then. Back then, the coastline lay well to the south in what is now Germany and the Netherlands. Fennoscandia held a subcontinental scale river system, equivalent in size to the Orinoco River today in South America. The main tributaries extended from Finnish Lapland down the axis of the Bothnian Sea and from western Russia along the Gulf of Finland.” 

Geologists have dubbed this the Eridanos river, named after the river god of Hyperborea who, in Greek mythology, delivered amber to the shores of the Baltic. For some scientists, the origins of the Baltic basin lie in erosion by this huge river system. For others, the basin of the Baltic was excavated by the great ice sheets of the Pleistocene Ice Age. So the obvious question to ask is where has the sediment gone?

The Baltic sediment

Mikis takes up the tale: “Around the southern edge of the Baltic we have a sediment trap walled by the Valdai ridge in Belarus, the Carpathian and Sudetes mountains in Poland and the Harz massif in Germany. This sink leaked, with some sediment lost to the North Sea. But the sink traps most of the sediment dumped in it from the source area of the Baltic basin to the north. So if we know the age of the sink sediment and its volume we can say a lot about when the Baltic basin was eroded.”

So Mikis set himself to gather data from all the maps of the geological surveys of the countries around the southern shore of the Baltic from Denmark in the west to Belarus in the east. And to calculate the total volume of sediment and then apply various corrections to convert loose sediment back into the rock that has been eroded out of the Baltic basin. It was immediately obvious that there was a lot of Pleistocene sediment in the sink – the sediment was young, with no traces of marine sediment from before the Ice Age.

Mikis continues: “The Baltic Sea basin has an average depth of around 50 m. The volume of rock lost from below present day sea level in the basin, and around its shores, is roughly equivalent to the Pleistocene sedimentary volume in the sink to the south. This is strong evidence that the Baltic basin was mainly excavated by ice sheets”.

The emerging glacial surfaces on the Baltic shore at Forsmark
The emerging glacial surfaces on the Baltic shore at Forsmark.


Deep glacial erosion of the Baltic basin but when?

To find out more about just when the erosion happened means delving into the complex stratigraphy of the sink area. That’s not easy, because due to thrusting and erosion by the Fennoscandian ice sheet, the sediment sequences are folded and faulted. But there are a few places in Poland and Belarus where boreholes penetrate long, relatively undisturbed sediment sequences.

And the origin and age of the sediments in those boreholes is revealing. Mikis explains: “The sediments were mainly laid down by ice or meltwater. The oldest sediment maybe goes back to 1.2 million years but a whopping 87% of the sediment in Poland is younger than 500 hundred thousand years old. And the first marine sediments are younger still. The Baltic is a geological juvenile.”

This makes sense in terms of what we know of climate-driven fluctuations of the ice sheets. Around 1.2 million years ago, there was a transition from glaciation under 40 thousand year orbital cycles towards extended 100 thousand year cycles. This change coincided with a growth in ice sheet size across the Northern Hemisphere. But the largest ice sheets reached their maximum limits south of the Baltic over the last 500 thousand years. 

Adrian comments: “Ice sheets have transformed the landscape of NW Europe. Where the ice sheet beds were flowing fast across soft sedimentary rocks, there was enormous capacity for erosion. Before 500 thousand years ago, the North Sea was a closed basin to the south. Then the English Channel formed as meltwater from the ice sheet cut through the chalk ridge between Dover and Calais. The deeps of the Norwegian Trench and the Skagerrak are submarine glacial valleys. The dense networks of meltwater-cut tunnel valleys beyond the present southern Baltic shoreline are in places hundreds of metres deep.”

In the Baltic basin, the three sub-basins of the Bothnian Bay, the Bothnian Sea and the Baltic Sea proper each occupy depressions in sedimentary rocks. The resistant granites of the Kvarken and Åland sills remain close to present sea level. The shallow chalk sill at Darss at the entrance to the Kattegat controls seawater inflow today to the Baltic and maintains its low salinity. Adrian observes: “Rivers cannot erode much below sea level. Even when we allow for remaining isostatic rebound, it’s clear that the Baltic sub-basins are not products of river erosion. It seems clear to us that the Baltic occupies a series of glacially-overdeepened rock basins. Just like the Great Lakes of North America.”

Our mediterranean sea is young

As we head for the shores of the Baltic this summer, it’s hard to think that the vastness of the Baltic, the literal mediterranean sea of Northern Europe, not being there at all. But until just over 1 million years ago, the Eridanos river system was still carrying sediment southwards. The excavation of the Baltic basin has happened mainly in the last half million years, a blink of a geological eye. The answer to an obvious question turns out to be rather intriguing.

Scientific article

Hall, A. M. & Van Boeckel, M., Accepted: Origin of the Baltic Sea basin by Pleistocene glacial erosion. GFF.

Contact

Adrian Hall
adrian.hall@natgeo.su.se