Maritime archaeologists are specialised in archaeological remains and finds in or near the water – at the bottom of the sea, in lakes, on beaches and on islands.
Using various types of finds, archaeologists try to work out as much as possible about people and communities that existed before our era. These finds might be wrecks, harbours, jetties, boat houses or other remains of people's living and working by the sea or on the shore of a lake. By studying a site or an object, an archaeologist tries to work out what happened there a long time ago.
Is there anything there?
In order for marine archaeologists to find out whether there is anything interesting on the sea bed, they often use an instrument known as a Side Scan Sonar. This is a type of echo sounder. The instrument is trailed behind a boat across the area being studied. On the boat is a computer that produces a picture showing the outline of irregularities on the sea bed. Wrecks and other objects that stick up show up in this picture. Archaeologists can then dive to find out if the objects they see on the scan are archaeological remains or not.
A small submarine for deep work
If the object to be investigated is too deep for divers to reach, archaeologists can instead send a ROV (Remotely Operated Vehicle) down there. A ROV is a small, unmanned submarine equipped with lights, a camera and mechanical arms. It is controlled from the surface. Marine archaeologists can be on a boat or on land, manoeuvring the ROV with a joystick and seeing the images it transmits via a cable on a computer screen. Simple mechanical tasks can be carried out using the arms.
Working as a diving archaeologist
Marine archaeologists often study remains that are still under water, which means they have to dive to the site. Diving archaeologists need to be trained divers as well.
Carrying out archaeological investigations under water requires time and patience. The deeper the divers go, the less time they can spend working on the bottom. The maximum permitted working depth for professional divers is 40 metres – but they can only stay at that depth for ten minutes before they have to surface again. For this reason, they usually don't work at depths greater than 30 metres, so that they can stay there a bit longer. A diver carries lot of equipment in order to be able to breathe and work under water.
How old is it?
In order to determine the age of a find, archaeologists use several different dating methods.
If they are studying a site where human traces have been present for a long time, they can use the stratigraphic method. This means that they investigate the site by excavating and studying one layer, or stratum, at a time. The topmost layer is the youngest, and the deeper you go, the older the remains will be.
Another way of determining the age of finds is to compare them with other similar finds made earlier, and whose age is known. That is called the typological method. With a wreck, you might look at the techniques used to build the ship, its shape, the system of rigging and what type of figurehead it had, and then compare that with other similar wrecks.
Anything that has ever been alive – e.g. a piece of wood or a bone from an animal – can be dated using the radiocarbon method. This measures a type of radioactivity which is present in all living matter. The radioactive substance breaks down very slowly when the plant or animal dies. The results of radiocarbon dating thus show how long ago the tree or animal lived.
Another method for dating trees is dendrochronology. This requires sawing off a section of the tree so that you can see the pattern of the annual rings and their varying thickness. The thickness is affected by how cold or warm winters and summers were when the tree was growing. The pattern is compared with sections from other trees whose age is known. When the pattern exactly matches a dated sample, it is possible to know precisely when the tree lived. Often it is also possible to determine in what region the tree grew.
Archives help archaeologists
Historical sources can be of great help to the marine archaeologist. Examples of historical sources include old navigation charts, descriptions of fairways or accounts of shipwrecks in books or archives.
Historical sources can provide clues as to where archaeologists might find remains. They can also help the archaeologist interpret a find. If an old wreck is discovered, marine archaeologists try to find out what ships are known to have sunk in that location. In this they can be helped by registrations in archives of shipwrecks that occurred a long time ago in that precise location.
The Fornsök database has data on all Sweden's ancient remains – on land and in water. Fornsök also contains 12 000 items of information or preserved accounts of shipwrecks.
Finds must be preserved
In the cold and dark waters at the bottom of the Baltic, objects can remain preserved for a very long time. If someone takes an object up to the surface, however, it begins to decay fairly rapidly. That is because the oxygen present in the air sparks chemical processes that allow biological organisms to begin breaking down the material.
Different materials can be preserved for different lengths of time, and their sensitivity to oxygen also varies. For example, wood from a wreck that has been lying in water for a long time becomes very brittle and may disintegrate when it is brought up on land and dries. Most of what is salvaged from water therefore has to be specially preserved in order not to be lost.
The preservation of seawater-saturated wood is a slow and expensive process, and the objects must then be kept in special climate-controlled rooms in order not to decay. The best way of preserving old wrecks and wooden finds is therefore to leave them at the bottom of the sea, where conditions are close to perfect – dark, cold and without oxygen.