Underwater archaeology

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Underwater archaeology is the study of past human life, behaviours and cultures using the physical remains found in salt or fresh water or buried beneath water-logged sediment. It is most often considered as a branch of maritime archaeology.

Underwater archaeological sites consist of wrecks (shipwrecks or aircraft), the remains of structures created in water (such as crannogs, bridges or harbours) or places where people once lived on land that have been subsequently covered by water due to rising sea levels or other phenomena.

Drawing to scale, underwater

1 Reasons
2 Challenges
3 Techniques
4 Interpretation and Presentation of Underwater Archaeology
5 See also
6 External links
- 6.1 Techniques
- 6.2 General references and organisations

[edit] Reasons

An individual shipwreck (such as the Titanic) can be of historical importance.
Shipwrecks (such as The Mary Rose) can also be important for archaeology because they can form a kind of accidental time capsule, preserving an assemblage of human artefacts at a moment in time i.e. when the ship was lost.
Materials are preserved differently under water than on dry sites on land. In anerobic, cold and dark conditions underneath waterlogged sediments, organics, such as plants, leather, fabric and wood may be preserved. These materials may still have evidence of how they were worked, such as tool marks on the surface of wood. This evidence can provide new insights into ancient crafts, cultures and lifestyles.
Changes in sea-level, because of local seismic events, or more widespread climatic or changes on a continental scale mean that some sites of human occupation that were once on dry land are now submerged.
Human societies have always made use of water; sometimes the remains of structures that these societies built underwater still exist (such as the foundations of crannogs, bridges and harbours) when traces on dry land have been lost.

Rock house settlement seen on left in 1927 while Lake Murray (South Carolina) was under construction, middle and right are two angles of aspect on Side-scan sonar in 100 ft of fresh water under the lake in 2005

[edit] Challenges

Underwater sites are inevitably difficult to access, and more hazardous, compared with working on dry land. In order to access the site directly, diving equipment and diving skills are necessary. The depths that can be accessed by divers, and the length of time available at depths, are limited. For deep sites beyond the reach of divers, submarines or remote sensing equipment are needed.

For a marine site, some form of working platform (typically a boat or ship) is needed. This creates logistics problems. A working platform for underwater archaeology needs to be equipped to provide for specialist remote sensing equipment, analysis of archaeological results, support for activities being undertaken in the water, storage of supplies, facilities for conservation for any items recovered from the water, as well as accommodation for workers.

Marine sites may be subject to strong tidal flows or poor weather which mean that the site is only accessible for a limited amount of time.

Underwater sites are often dynamic, that is they are subject to movement by currents, surf, storm damage or tidal flows. Structures may be unexpectedy uncovered, or buried beneath sediments. Over time, exposed structures will be eroded, broken up and scattered. The dynamic nature of the environment may make in-situ conservation infeasible, especially as exposed organics, such as the wood of a shipwreck, are likely to be consumed by marine organisms such as piddocks.

Underwater sites can be chemically active, with the result that iron can be leached from metal structures to form concretions. The original metal will be left in a fragile state.

Visibility may be poor, because of sediments or algae in the water and lack of light penetration. This means that the survey techniques that work well on land, generally can not be used effectively under water.

Equipment used for archaeological investigation, including water dredge and air lifts create additional hazards and logistics issues.

Artefacts recovered from underwater sites need special care.

Underwater sites do not provide good outreach possibilities and access for the general public.

[edit] Techniques

An important aspect of project design is likely to be managing the logistics of operating from a boat and of managing diving operations. Archaeological techniques and equipment applicable to underwater sites include:

Position fixing. Marine sites are typically located using GPS. Historically, sites within sight of the shore would have been located using transects. A site may also be located by visual surveying some form of marker (such as a buoy) from two known (mapped) points on land.
Remote sensing or Marine Geophysics[1]. Sensitive sonar, especially side-scan sonar or multi-beam sonar[2] may be used to image an underwater site. Magnetometry [3] can be used to locate metal remains such as metal shipwrecks, anchors and cannons. Sub-bottom profiling[4] utilises sonar to detect structures buried beneath sediment.
Remotely Operated Vehicle (ROV). Where it is not practical or safe for divers to physically visit a site, video cameras can be steered from the surface.
3D survey[5]. Three dimensional surveying is typically undertaken using depth gauges and tape measurements. Research[6] shows that such measurements are typically less accurate than similar surveys on land.
Photography and Photomontage or Photomosaic. Cameras, including video cameras can be provided with special housings that enables them to be used underwater. A series of photographs can be taken at adjacent points and then combined into a single image of the whole site.
Underwater Excavation. Where intrusive excavation is appropriate, silts and sediments can be removed from an area of investigation using a water dredge or airlift. When used correctly, these devices have an additional benefit in tending to improve the visibility in the immediate vicinity of the investigation.
Scale drawing can be undertaken underwater. Pencils will write underwater on permatrace, plastic dive slates, or matt laminated paper.
Archaeological Science. Dendrochronology is an important technique for dating the timbers of wooden ships. It may also provide additional information, including the area where the timber was harvested (i.e. likely to be where the ship was built) and whether or not there are later repairs or reuse of salvaged materials. Because plant and animal material can be preserved underwater, archaeobotany and archaeozoology have roles in underwater archaeology. For example, identification of pollen samples from sedimentary or silt layers can provide information on the plants growing on surrounding land and hence on the nature of the landscape. Information about metal artefacts can be obtained through X-ray of concretions. Geology can provide insight into how the site evolved, including changes in sea-level, erosion by rivers and deposition by rivers or in the sea.
Conservation [7], [8]. Artifacts recovered from underwater sites need stabilisation to manage the process of removal of water. The artifact either needs to be dried carefully, or the water replaced with some inert medium (as in the case of The Mary Rose). Artifacts recovered from salt water, particularly metals and glass need to be stabilised following absorption of salt or leaching of metals. In-situ conservation of underwater structures is possible, but consideration needs to be given to the dynamic nature of the site. Changes to the site during intrusive investigation or removal of artifacts may result in scouring which exposes the site.

[edit] Interpretation and Presentation of Underwater Archaeology

Diver trails can be used to allow scuba-divers to visit and understand archaeological sites that are suitable for scuba-diving. Otherwise presentation will typically rely on publication (book or journal articles, web-sites and electronic media such as CD-ROM). Television programmes can attempt to provide an understanding of underwater archaeology to a broad audience.