Once a lake, now a sea. The waters of the Baltic hide many secrets. One of them was discovered by researchers from the University of Kiel and the University of Leipzig, who found a wall at the bottom of the Baltic Sea that testifies to human activity in the region. Dr Aleksandra Brodecka-Goluch from the Department of Chemical Oceanography and Marine Geology at the University of Gdańsk tells us what else is hidden in the depths of our sea.

Marcel Jakubowski: - Recently, scientists from the University of Kiel and the University of Leipzig discovered a wall made of almost 1500 stones at the bottom of the Baltic Sea. What do we know about the Baltic area before it was flooded about 12,000 years ago?

Dr Aleksandra Brodecka-Goluch: - The Baltic Sea has evolved strongly over the last thousands of years. At certain times, it was connected to the ocean through the open Danish Straits, while at other times it functioned as a lake. The shoreline of this body of water has also changed - in the southern part of the Baltic, especially off the German coast, some areas that are underwater today were land a few thousand years ago. This is also the explanation that has been proposed in the context of the discovered wall, which, according to scientists from Kiel and Leipzig, was once on land. This theory sounds as plausible as possible. A good example of the evolution of the coastline in the built-up areas of the southern Baltic is the area around Pobierowo, in the western part of the Polish coast. The Trzęsacz church located there was part of a village about 2 km from the shoreline until a few hundred years ago. As a result of abrasive processes, the sea has ‘swallowed up’ part of this building. Today, there is only one wall of this church left. Nowadays, we are able to trace changes in the coastline, but what was taken away several thousand years ago remains to be discovered.

- Do these dynamic shoreline changes occur everywhere, or only in the case of the Baltic Sea basin?

- In general, the coastlines of many different bodies of water have evolved over thousands of years. The Baltic Sea stands out because these processes have strongly altered the physical and chemical conditions in the water. In the past, the Baltic was alternately a sea and a lake. As a consequence of the decreased salinity and oxygen deficits resulting, among other things, from the large input of organic matter, in certain areas of the Baltic there are conditions in which objects such as sunken ships can survive at the bottom for longer periods in a virtually unchanged state; they are, in a sense, preserved. The chances of a wooden wreck surviving for hundreds of years in the ocean or in the open sea are slim, whereas in the Baltic we can expect this type of situation and, consequently, later discoveries. In such a 'maritime museum' rests, for example, the wreck of the ship 'Solen', which sank in the 17th century.

- As part of your scientific work at the Faculty of Oceanography and Geography, you have explored the bottom of the Baltic Sea on many occasions. Has anything surprised you during such scientific cruises?

- Some time ago, during a non-invasive survey of the seabed of the central Gulf of Gdańsk, which had long been known to be a typical example of an accumulation seabed, i.e. there is a successive accumulation of material and very fine-grained sediments, we found a kind of ‘island’ of very hard, poorly sorted material of different fractions. When we lowered the ROV to the bottom, to a depth of about 90 m, we found, to our great surprise, stones of about 50 cm in size and a large amount of hard post-glacial material in general - something we would never expect to find in this area. Similarly, there could be wrecks and all sorts of other 'surprises' on the bottom. For example, in 2019 we accidentally discovered - the most characteristic in the Gulf of Gdańsk - a very active pockmark with a relative depth of almost 10 m, and on this occasion, we also came across several wrecks. Some of them were known about, but one was not marked on any map. Despite what was believed just a few years ago - that the Baltic is known inside out - it turns out that this unique sea can still surprise us.

- Pockmarks are one of your main scientific interests. Among other things, you studied them during a month-long SEA-EU cruise from Gdańsk to Cádiz. What are these objects?

- Pockmarks are characteristic depressions in the seabed, usually cone-shaped. They are also found in the Baltic Sea, both close to shore, at a depth of a few metres or so, and at great depths tens of nautical miles from land. Gases, mainly methane and carbon dioxide, escape from active pockmarks; fresh water often also flows out of them, affecting the entire water column above them and even the atmosphere. It has sort of always been known that such structures could exist in the Gdańsk Basin, and earlier publications indicated that they were located at the bottom of the Gdańsk Deep, among other places. However, it was not until 2018, thanks to the research capabilities of the University of Gdańsk vessel r/v Oceanograf, that we were able to take a close look at and map the pockmarks found at the bottom of the central Gulf of Gdańsk. We are currently conducting a geochemical and microbiological study of the pockmarks together with scientists from the AGH University of Science and Technology in Krakow. When analysing sediment samples from inside the pockmarks during microbiological studies, we accidentally came across living algal cells that need light to function. The problem is that it is dark at 90 metres depth, which means these organisms should not be living in such conditions. This is one of the issues we want to look at closely. Perhaps the presence of these algae on the bottom has a simple explanation - but the Baltic may as well surprise us again.

- Is it easy to find remnants of human activity at the bottom of the Baltic Sea?

- In the sediments of the Gulf of Gdańsk, especially those near the river mouths, there is such a high sedimentation rate that even if a hypothetical wall built by humans in the past were found on the bottom, it would be completely buried within thousands or sometimes only hundreds of years. On the other hand, further west, in the region of the central coast of Poland, a different type of bottom and sediment is present. If any structures from ancient times have survived in such a place, there is a chance that they can still be discovered. In addition, storm conditions in the Baltic Sea occur several dozen days a year, which in the past contributed to the sinking of many ships - the wrecks of some of them are also waiting to be discovered.

- They can be discovered but, on the other hand, they can pose a danger...

- Yes, as a remnant of the Second World War, wrecks with conventional or chemical weapons still lie on the bottom of the Baltic Sea. While the geographical positions of these sites are known for the most part, such post-war ‘surprises’ are not always located where we would expect them to be, for example, due to the activity of sea currents. As recently as the 1950s, incidents were recorded in which the sea carried ashore an arsenal that had been sunk shortly after the war.

- Instead of amber...

- ...there were barrels of chemical weapons. The sea takes and the sea brings back to surface. There have been cases where divers have lost some objects underwater a few hundred metres from the shore and found them on the beach a few days later. On the other hand, some objects are buried for a long time or even forever, especially in areas with accumulation bottoms and muddy sediments, which can be compared to on-land marshes. Over time, however, it may become apparent that in certain places the currents will uncover part of something larger, such as a fragment of a mast, which informs researchers of the ship's location. For such discoveries to occur, you also need a bit of luck and to be in the right place at the right time.

- So even if we scanned every square centimetre of the Baltic seabed, we still wouldn't discover all its secrets?

- Absolutely, it may turn out that in certain areas a particular object will only be uncovered in a few years' time, and objects that sank to the bottom just a while ago may no longer be found. It is worth adding that surveying and mapping the seabed itself is very expensive. Even if we are talking about a relatively small area of a few square kilometres, such a process can take tens to even hundreds of hours of work by vessels equipped with specialised tools. Currently, different institutions are mapping the Baltic Sea with different projects, but each of them is looking for different objects and not all of them share information. We are usually looking for more specific forms and shapes on the bottom and we may miss something that would interest someone else.

- Is there any common map of objects on the bottom of the Baltic Sea?

- There is, for example, a common register of wrecks, which is maintained by the Hydrographic Office of the Navy. On the other hand, not everyone reports their findings, especially if they are interested in objects of a different type or are not aware that what they see on the screen while conducting a bottom survey is something of value to another group of scientists.

- The Baltic Sea is relatively shallow - nevertheless, it has its depths. Do these sites have different characteristics from shallower parts of the sea?

- The average depth of the Baltic Sea is about 52 metres, while the depths of individual bodies of water within the Baltic can vary widely. In the Polish Exclusive Economic Zone, we have, for example, the Gdańsk Deep, which has its lowest point at 118 metres below sea level. More to the north, we have the Gotland Deep, further to the west the Bornholm Deep, and in the northern part of the Baltic Sea, the Landsort Deep. The bottoms of the Baltic Deeps are covered by silty sediments with a high proportion of organic matter, so, in simple terms, they function in a way similar to the previously mentioned marshes. Both the sediment and the water column in these areas are very characteristic environments in many respects. Among other things, the deep regions are prone to oxygen deficits, i.e. there is often, for example, hydrogen sulphide instead of oxygen in the bottom water, especially towards the end of the growing season. Due to the presence of this gas, some areas of the Baltic hinterland were called azoic deserts because there was virtually no life there. Under such anaerobic conditions, the decomposition of large amounts of incoming organic matter can lead to the formation of methane. Over hundreds, thousands of years of such processes, microbial methane builds up in the bottom sediments and at some point, once the pore waters are saturated, it begins to bubble intensely into the pelagic water. Last year, Swedish scientists reported that the bottom of the Landsort Deep is heavily gassing and that gas bubbles containing methane are entering the water column virtually all the time.

To assume that in the Baltic Sea, everything we could discover, we have discovered, and everything we could see, we have seen, is a major overstatement. Secrets are hidden in the water, on the seabed, and even under the seabed, thanks to which the Baltic Sea has continued to surprise us chemically, physically, and biologically for decades, and there is hardly any end in sight to the unheard-of discoveries.

- Thank you for the interview.