Archives help archaeologists
Historical sources can be of great help to the marine archaeologist. They 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.
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.
Side scan sonar
To find out whether there is anything interesting on the sea bed, maritime archaeologists 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.
The boat carries a computer that produces a picture showing the outline of irregularities on the sea bed. Wrecks and other objects that stick up appear in this picture. Archaeologists can then dive to find out whether or not the objects they see on the scan are archaeological remains.
A small robot for deep water
If the object to be investigated is too deep for divers to reach, archaeologists can instead send a remotely operated vehicle (ROV) down there. A ROV is a small, unmanned submarine equipped with lights, a camera and mechanical arms. It is controlled from the surface. Maritime archaeologists can be on a boat or on land, maneuvering 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.
How old is it?
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. With this method, 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.
With the typological method, the finds can be compared with other similar finds discovered earlier, and whose age is known. For 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 – like 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.
With dendrochronology, a section of the wood is sawed off so that you can see the pattern of the annual rings and their varying thicknesses. The thickness is affected by how cold or warm the 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, we can know precisely when the tree lived. Often, we can also determine the region where the tree grew.