2023-06-15 18:54:37
The small North Sea plays quite a large role in the earth’s climate. Using soil samples, geologist Cecile Hilgen investigates how people have influenced the seabed in recent centuries. ‘Especially the upper decimetres are interesting.’
They moored once more on Wednesday in the harbor of ‘t Horntje on Texel, the researchers on the ship RV Pelagia. They were on the road for almost three weeks to conduct research at the bottom of the Norwegian Trough, a stretch of sea off the coast of Norway from 400 to 1,000 meters deep, on the threshold between the North Sea and the Atlantic Ocean.
Just before departure, PhD candidate Cecile Hilgen of Utrecht University is quite delighted. In addition to a large box with bags, containers and chemicals to preserve samples, there is also a huge auger on board. “A piston corer”, Hilgen corrects. “That is a kind of hollow drill, with which we can extract cores of 10 centimeters in diameter from up to 9 meters below the seabed. And that at a depth of hundreds of meters below the water surface.
“When I tell friends or family that I am going to collect samples from the bottom of the North Sea, they usually think of the sandbox off our coast. But in the shallow part of the North Sea, the influence of the tide is far too great. There the soil is mixed up a few times a day. And then there are all human influences, such as fishing with trawl nets.”
Influence of the industrial revolution on the seabed
In the Norwegian trough, the layers should still be neatly arranged in chronological order, like the pages of a history book, says Hilgen. “But whether that is really the case, I will find out in the lab,” says the PhD student. “By examining the drill cores layer by layer, I can see whether large fishing vessels, sea currents or benthic life have not messed things up too much here.”
Hilgen is particularly interested in the upper six meters of the seabed. “We know from previous research that this is roughly the layer that was deposited during the Holocene, so over the past 11,700 years. And within that, especially the upper decimeters are interesting, from the past 200 years. Because there we can see the influence of the industrial revolution on the composition of the soil and marine life.”
In the lab, Hilgen will look through a microscope for remnants of dinoflagellates, or ‘dinos’, as she affectionately calls her research objects. These are microscopic organisms that are a bit plant, with chlorophyll to convert sunlight into usable energy, and also a bit animal, with a swim tail to move through the water.
“The different types of dinosaurs are characteristic for a certain environment,” says Hilgen. You have species that belong to fresher water and others from saltwater. There are also differences in species depending on the nutrients and temperature. In this way, the remains of these organisms can tell a lot regarding the environment at a certain time.”
History book on the seabed
To explore the passage of time at the bottom of the world’s oceans, researchers can use a technique that can look at the layers in soil samples from the Norwegian Trench with unprecedented precision. By firing X-rays at the soil samples and observing how that radiation returns, the researchers can determine the chemical composition of a sample. This is done in the Nioz laboratory on Texel.
“And the great thing is: we can do that measurement in layers of a tenth of a millimeter at a time,” says geochemist Rick Hennekam of Nioz. “As a result, we can almost literally look from year to year at what has settled on the seabed. Hopefully, we will be able to pick up on human influence from that signal from regarding 200 years ago.”
Hilgen’s research is part of a larger project of institutes from the Netherlands, England, Norway and Germany, led by the Royal Netherlands Institute for Sea Research (Nioz). In that project, called North Sea – Atlantic Exchange, or NoSE for short, the researchers want to investigate the role of the relatively tiny North Sea in chemical and biological processes in the oceans.
“We already know that the North Sea plays a relatively important role,” says Nioz project leader Furu Mienis. “Shallow coastal seas such as the North Sea warm up relatively quickly and receive a lot of nutrients from the rivers. As a result, there is a lot of plankton growth and therefore CO2 sequestration. But what the exchange of that captured CO2 is between the sea and the ocean is still hardly known. The estimates vary from percentages between 0 and 40 percent of the amount of carbon that eventually ends up in the ocean via the North Sea. But that is of course a huge margin of uncertainty.”
‘A quarter of CO emitted by humans2 ends up in the seas’
The skeptics who now claim that with so much uncertainty things are not progressing at such a speed with the influence of CO2 on the climate, Mienis is happy to dissuade. “It is known that more than a quarter of the CO2 emitted by humans eventually ends up in the world’s oceans, as does most of the extra heat trapped by the greenhouse effect. So you might say that the oceans still solve a large part of our problems. What we don’t know is how exactly the exchange between sea and ocean takes place, and what influence humans have on carbon sequestration in the North Sea.”
Project leader Mienis does not rule out the possibility that more surprises will emerge from the seabed. “And that I wouldn’t be surprised regarding that is surprising enough in itself,” she laughs. “The Norwegian Trough is relatively close by, yet very little research has been done on it. Some parts of space have been better explored than many parts of the seafloor.”
Search for salt
Francesca Sangiorgi, her supervisor from the Department of Earth Sciences at Utrecht University, also looks over Cecile Hilgen’s shoulder. “It will be very exciting what the quality of the sediment archive will be,” she says. “If the layers are still clearly identifiable in order, we want to look in great detail at the composition of the various algae and molecules. Hopefully, we can also learn something regarding salinity in the past.
“Because unlike, for example, the CO2content, which we can even measure in gas bubbles from ice cores in Antarctica, we still don’t have a good way to also determine the salinity of ocean water from the past. In addition to the temperature, this salinity is an important motor behind the currents in the oceans. Under the influence of the changing climate, the salt content in the ocean can also change due to melting ice. That can have a drastic effect on the currents of warm and cold water in the oceans, and therefore on the climate.”
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