Researchers drill six metres down into the seabed off Hanko to find out more about the state of the archipelago a thousand years ago.

It is the first of June and the first day of summer, but on board the research vessel Electra it is cold. A stiff breeze ripples the waters off Tvärminne Zoological Station in Hanko—ten metres per second, to be exact. We are standing on the upper deck, shivering slightly in the northwest wind. Or are we shivering in anticipation as we wait for the results of the latest sediment sampling?

“It’s clay, all the way up to the weight,” marine biologist Joanna Norkko says as she stands next to me.

“That means the tube has gone all the way down, just like it’s supposed to,” she explains.

On the lower deck of the Electra, three researchers are handling a six-metre steel tube. They are extracting a sediment sample from the seabed with a piece of equipment called a piston corer. Earlier today, their first attempt failed. The tube only penetrated half a metre into the sediment. Perhaps it struck a rock.

The second attempt looks like it’s more successful. Technicians Draupnir Einarsson and Mattias Murphy hoist the sample out of the water with the aid of a winch.

 

The piston corer is a tube-shaped instrument that can extract sediment cores up to six metres in length from the seabed.

“The steel tube is plunged into the seabed with the help of gravity. The piston corer weighs half a tonne,” Tina Elfwing explains, while keeping an eye on her staff on the deck below us. Elfwing is director of the Stockholm University Baltic Sea Centre, and the vessel Electra is the property of the Centre.

Sediment, i.e., mud, gravel and clay, is collected in a white plastic tube inside the steel outer tube. Once the steel tube has been hoisted out of the water, the white plastic tube is cut into smaller, one-metre sections that are easier to transport to a lab for analysis. 

piston corer electra

 

Link:  Read more about what researchers learned from sediment cores from the Bothnian Sea

Environmental archive

For researchers, sediment cores are environmental archives that reveal the historical development of the Baltic Sea. The bottom layers reveal historic algal blooms and hypoxia, among other things, and they are an important instrument for distinguishing between natural and anthropogenic changes in the Baltic Sea.

“In this part of the Baltic Sea, uplift is approximately 50 centimetres per hundred years,” says Christoph Humborg, professor in coastal biogeochemistry.

This means that the oldest layers of sediment in the six-metre-long sample that is now being winched onto the deck by the researchers were the surface of the seabed some 1,000 years ago. Why do seabed levels rise in this fashion?

“Everything that sinks to the bottom of the sea, such as phytoplankton and zooplankton, gathers there over the years and forms new layers,” Professor Alf Norkko explains.

The interesting thing about the Baltic Sea is that its seabed levels rise faster than the floors of the deep oceans.

“In the deep oceans, organisms decompose or are eaten before they reach bottom. In the shallow waters of the Baltic Sea, however, a large part lands on the seabed,” Norkko says.

Bridging the Baltic

The research vessel Electra’s visit to Tvärminne is part of Baltic Bridge, a strategic collaboration between the University of Helsinki and Stockholm University in the field of Baltic Sea research.

“Research units in the Nordic countries are so small that it makes sense for us to collaborate. By pooling our resources, we can benefit from each other’s expertise and infrastructure,” Christoph Humborg says.

The collaboration between the two countries is enhanced by the recent appointment of Christoph Humborg to a visiting professorship at Tvärminne, while Alf Norkko is a visiting professor at the Stockholm University Baltic Sea Centre (in English here).

The collaboration seeks to combine the Centre’s mathematical modelling expertise with Tvärminne’s species knowledge and understanding of ecological processes.

“Our current models and scenarios primarily concern the open seas of the Baltic. Our joint objective is to learn more about the Baltic Sea archipelago areas and the shallow parts of the sea,” Christoph Humborg says.

“A key issue is the capacity of the shallow coastal areas to act as filters, capturing nutrients from land areas and thus preventing nitrogen, phosphorus and the like from spreading out to sea,” says Norkko.

Echo sounder on testosterone

When I’ve had enough of the wind on the outer deck, it’s time to visit the bridge. I ask Commander Thomas Strömsnäs how long it took the Electra to travel from Askö in Sweden to Tvärminne in Finland.

“Forty-eight hours. But we stopped several times to chart the seabed, because two of our researchers wanted to search for the Salpausselkä ridge that runs across the Baltic Sea, through Sweden and all the way up along the coast of Norway.”

“And did you find it?” I ask.

“Well, it looks like we did. On land, the ridge is fully visible, but the part of Salpausselkä that runs across the Baltic Sea can’t be detected using regular sonar because it’s hidden in sediment. In order to detect the formation, you have to use a penetrating echo sounder that reaches into the seabed,” Strömsnäs explains.

Alf Norkko says Electra is equipped with an “echo sounder on testosterone”.

Thomas Strömsnäs

Photo: No ship’s wheel on this bridge. Commander Thomas Strömsnäs steers the Electra with the help of a joystick that rotates 360 degrees.

During the week-long stay in Tvärminne, Strömsnäs will steer the Electra up and down along the coast in order to chart the seabed. Several computer programs will be running. One will register the topography of the seabed, its surface. Another will penetrate the seabed, displaying formations that are not visible on the surface. Yet another can collect data on the water column, registering what goes on between the surface of the sea and the surface of the seabed, such as the release of bubbles of methane or carbon dioxide gases from the seabed.

Storfjärden bay in 3D

All the data collected in this fashion will then be combined to create a 3D model of the seabed that the researchers at Tvärminne will be able to use in their work.

“The 3D map will be an important tool for us. The map will help us determine in advance whether a certain spot is suitable for sampling, and what the type of sediment is at that particular spot,” says Joanna Norkko.

Alf Norkko describes the importance of charting thus:

 “When you take samples on land, you can see what type of spot you are in, whether it is in a field or in the woods. You know what conditions to expect. When you’re at the bottom of the sea, it’s not as easy to choose a spot that’s relevant to the sampling you want to do. With the help of this new charting tool, we can know in advance whether we’ll be in a field or in the woods, and plan accordingly.”