The Times They Are A Changin’ – and the landscape too

“The Times They Are A Changin’ “, wrote Bob Dylan a song once, a long time ago. But it is very true still today, when the current coronavirus situation is changing the world and our societal behaviour. Also, the environment is changing, partly due to our own activities, partly by natural forces.

SEAmBOTH project started three years ago, and today it will end. In three years so many things happened, but somehow those years went quickly. And in geological timescales it is a very short time indeed. SEAmBOTH project provided a huge amount of new information on the northern Bothnian Bay environment, its biology and geology.

Geological Survey of Sweden (SGU) and Geological Survey of Finland (GTK) collected together 5000 km of survey line data from the seafloor with acoustic-seismic methods and seabed sampling. The produced new geological maps cover approximately 500 km2 of the seabed. SGU produced also the seabed substrate models for the whole SEAmBOTH area. The results of the project more than doubled the amount of existing geological map data of the area.

Map
The main geological study areas in Sweden and Finland (dark blue rectangles), in SEAmBOTH area, in the Bothnian Bay. The seabed substrate data (at a scale of 1:100 000) is shown with EMODnet Folk 5 classification. The map shows also existing previous seabed substrate data from the area. Maps: SGU and GTK

How does the seabed look like? The geological maps from the pilot areas show that the seafloor consists mainly of soft sediments, mud and silty clay. However, modern accumulation areas, areas where soft muddy sediments are deposited at the seabed, cover only small areas of the seafloor. Till material, those substrates that ice ages left us, cover around one third of the seabed. Bedrock outcrops exist very seldom in the area. However, the seabed substrate types are very unevenly distributed in different areas, e.g. in some pilot areas sand covers around 20% of the seabed, but in some areas not at all. In the shallow areas, which are not sheltered, seabed substrates are dominated by the hard substrates. In those large areas, forces like ice and wave action erode the seabed, and the eroded and resuspended material will be transported (by currents and ice), and finally accumulated in more sheltered and deeper areas.

Block of clay with sand on top and measuring stick at the side
Seabed sediment sample from the erosional seabed area. The sample reveals that clay is covered with a thin residual sediment (mainly sand). Geologian tutkimuskeskus (GTK).

In the broad scale, the most striking morphological features at the seafloor are canyons or canyon-like seabed features. Those probably ancient river channels cover large areas of the SEAmBOTH area. The power of the high-resolution survey is that many fine scale seabed features can be discovered, features which are not visible in coarser data. One of those fine scale features are erosional hard clay structures. In these features the clay is very compact, and it can create complex reef like structures. These features are very similar to “clay labyrinth” feature found earlier in the Finnish coastal areas by Metsähallitus, and discussed also in the earlier SEAmBOTH blogs

The influence of human activities can be seen underwater and also under the seafloor, in this area too. Geochemical analysis of soft muddy sediment indicate, that concentrations of some harmful substances of anthropogenic origin, like cadmium, lead, zinc and mercury are recorded in the seabed sediments. The concentrations of these heavy metals in the surface sediments have generally declined over the last decades. However, in some areas, concentrations of cadmium and zinc in the subsurface sediments are still relatively high.

Chart
Cadmium (Cd) concentrations in the seabed sediment cores offshore Kemi. Geologian tutkimuskeskus (GTK).

SEAmBOTH area, and the whole Bothnian Bay, is an area of rapid land uplift, which leads to the continuous erosion of seafloor. It exposes new seafloor regimes (i.e. older deposits to seafloor) and gradually brings these to shallower depths and finally to shore. The land uplift will shape slowly but surely the coast and the seabed.

In the long run, in approximately 2,000 years, the land uplift continues and if the sea level does not rise significantly, the water connection between the Bothnian Sea and Bothnian Bay will close, and the Bothnian Bay will become the largest inland lake in Europe, i.e. the Bothnian Lake. So, this unique and beautiful area in under a constant change. We can’t help the change, but we can, and we must make sure, that our seas remain in a good health for the future generations. The SEAmBOTH project ends, but the journey continues. No matter what, there’s nothing permanent, except the change, even underwater.

Map where Bothnian Bay is almost a lake and Kvarken sea area has shrunk
The Gulf of Bothnia, circa 2,000 years in the future. Harri Kutvonen, Geologian tutkimuskeskus (GTK).  

Aarno Kotilainen, GTK

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