The history of pollution is revealed through the Baltic Sea sediments
Decades of pollution from industry, agriculture, and traffic. A new dissertation measures metal contamination in Baltic Sea sediments from preindustrial times to the present. The results give hope for saving the marine environment, but also warns us about the risks of releasing metals from the seabed.
Bottom sediments consist mainly of tiny particles from mud, rock, and organic material. When this material falls to the seabed, new layers of sediment are created over time, each new layer on top of the previous ones.
A new dissertation by Sina Shahabi Ghahfarokhi examines the prevalence of different metal contaminants in the Baltic Sea seabed from preindustrial times to the present. The dissertation provides updated information on what contaminants have fallen to the seabed at different times in history and at what concentrations.
“The sediments can be seen as a timeline of the history of the Baltic Sea”, says Shahabi Ghahfarokhi, researcher at Linnaeus University.
The sediments bear witness to historical events
The sediment samples have been collected from 13 locations in the Baltic Sea, from the Bothnian Bay in the north to the Scanian coast in the south.
The samples consist of seabed sediments that have been encapsulated in cylinders to maintain the same structure as in the sea. The samples have been freeze-dried and then sliced into thin slices. Each slice represents a certain period of time in the history of the Baltic Sea, and its content can be seen as a testament to what contaminants fell to the seabed at that particular time.
“Events like the Chernobyl disaster help us navigate through time within the sediments. Because of the cesium contaminations from Chernobyl, I can look at a sediment slice and confirm that at a depth of 20 centimetres it is estimated to be the year 1986”, Shahabi Ghahfarokhi continues.
Helps us discover new contaminations
When we go deep enough, the sediments consist of layers that were formed before industrialisation and before humans had a clear impact on the Baltic Sea environment. These layers can be used as references. When sediments contain higher concentrations of metals than the natural values this means that they are contaminated, mainly as a result of human impact.
“Most elements are present in nature: aluminum, iron, cobalt, nickel, and rare earth elements are all there. However, humans sometimes influence these concentrations through our activities. When this happens, we create an imbalance in the environment”, Shahabi Ghahfarokhi explains.
Some metals are hazardous to our health already at very low concentrations. One such example is arsenic, which can be found at greatly increased levels in the Bothnian Sea in the north, most likely as a result of emissions from metal production. As shown in the dissertation, also cadmium can be found at higher levels in many places than has historically been the case. Other metals do not become hazardous to human health until they surpass a certain threshold value. Therefore, it is important to keep track of what can be found in the seabed. The dissertation provides updated statistics on a number of different metals and maps out the presence of uranium, which has previously been lacking in the Swedish Environmental Protection Agency’s database.
The 1970s saw the worst levels of pollution
The increase in metal contamination in the Baltic Sea was at its highest in the 1970s and 1980s. Increasing industrialisation and insufficient environmental legislation made the Baltic Sea the final destination for many contaminants. Lead additives from the fuels of that time are frequently occurring in old sediment layers, long after the use has stopped.
An important turning point in the health of the Baltic Sea took place in 1974, when all Baltic Sea countries united to reduce the emissions into the sea. They established the Baltic Marine Environment Protection Commission (HELCOM), a joint initiative to protect the marine environment and to promote biodiversity in the Baltic Sea.
The effects of HELCOM are evident in Sina’s research. All of a sudden, the levels of arsenic, cadmium, lead and other metal contaminants decrease in new-formed sediment layers.
“We still have a long way to go until the Baltic Sea is back to its natural state. However, the decrease in these values after 1974 shows that if we implement the correct strategies, it is possible to help the marine system recover, and even thrive.
Old pollutants are still present
So, what to do with the metal contaminants that are buried in the Baltic Sea seabed? Even if we limit future emissions, old contaminants are still present in the sediments. For every passing year, they are covered by an additional few millimetre of newly-formed sediment, but they are not going away.
The most important thing is that the metals do not leak out of the sediments and spread in the sea. As long as the contaminants remain in the sediments, separated from animals and humans, they do not cause much harm.
“It is far more dangerous to have metals in solution than in the sediments. Once they are released into the water, they risk coming into contact with animals and other living organisms”, Shahabi Ghahfarokhi explains.
Risk being released into the sea
This can be an issue when trying to save the Baltic Sea’s bottom environment. In his dissertation, Shahabi Ghahfarokhi shows that measures to counteract so-called dead zones, areas on the sea floor with no or almost no oxygen, risk releasing dissolved metal contaminants.
As part of his dissertation, he studies how bottom sediments in anoxic (greatly deficient in oxygen) areas of the Baltic Sea react when they are exposed to oxygen-rich water again. The experiment showed that when the sediment samples came into contact with oxygen, they started releasing soluble metals at a faster pace than normal.
“These results are a reminder that we have to be cautious and have the metal contaminations in mind in our efforts to save the Baltic Sea”, Shahabi Ghahfarokhi concludes.
Sina Shahabi Ghahfarokhi’s dissertation “Baltic Sea sediments: Source and sink for metal contamination”
Linnaeus University’s research group Environmental Geochemistry