Maritime Archaeology - Preservation of Material Underwater

Preservation of Material Underwater

There are significant differences in the survival of archaeological material depending on whether a site is wet or dry, on the nature of the chemical environment, on the presence of biological organisms and on the dynamic forces present. Thus rocky coastlines, especially in shallow water, are typically inimical to the survival of artifacts, which can be dispersed, smashed or ground by the effect of currents and surf, possibly (but not always) leaving an artifact pattern but little if any wreck structure.

Saltwater is particularly inimical to iron artefacts including metal shipwrecks, and sea organisms will readily consume organic material such as wooden shipwrecks. On the other hand, out of all the thousands of potential archaeological sites destroyed or grossly eroded by such natural processes, occasionally sites survive with exceptional preservation of a related collection of artifacts. An example of such a collection is the Mary Rose. Survival in this instance is largely due to the remains being buried in sediment

Of the many examples where the sea bed provides an extremely hostile environment for submerged evidence of history, one of the most notable, the RMS Titanic, though a relatively young wreck and in deep water so calcium-starved that concretion does not occur, appears strong and relatively intact, though indications are that it has already incurred irreversible degradation of her steel and iron hull. As such degradation inevitably continues, data will be forever lost, objects' context will be destroyed and the bulk of the wreck will over centuries completely deteriorate on the floor of the Atlantic Ocean. Comparative evidence shows that all iron and steel ships, especially those in a highly oxygenated environment, continue to degrade and will continue to do so until only their engines and other machinery project much above the sea-floor. Where it remains even after the passage of time, the iron or steel hull is often fragile with no remaining metal within the layer of concretion and corrosion products. The USS Monitor, having been found in the 1970s, was subjected to a program of attempted in situ preservation, for example, but deterioration of the vessel progressed at such a rate that the rescue of her turret was undertaken lest nothing be saved from the wreck.

Some wrecks, lost to natural obstacles to navigation, are at risk of being smashed by subsequent wrecks sunk by the same hazard, or are deliberately destroyed because they present a hazard to navigation. Even in deep water, commercial activities such as pipe-laying operations and deep sea trawling can place a wreck at risk. Large pipelines can crush sites and render some of their remnants inaccessible as pipe is dropped from the ocean surface to the substrate thousands of feet below. Trawl nets snag and tear superstructures and separate artifacts from their context.

The wrecks, and other archaeological sites that have been preserved have generally survived because the dynamic nature of the sea bed can result in artifacts becoming rapidly buried in sediments. These sediments then provide an anaerobic environment which protects from further degradation. Wet environments, whether on land in the form of peat bogs and wells, or underwater are particularly important for the survival of organic material, such as wood, leather, fabric and horn. Cold and absence of light also aid survival of artifacts, because there is little energy available for either organic activity or chemical reactions. Salt water provides for greater organic activity than freshwater, and in particular, the shipworm, terredo navalis, lives only in salt water, so some of the best preservation in the absence of sediments has been found in the cold, dark waters of the Great Lakes in North America and in the (low salinity) Baltic Sea (where the Vasa was preserved).

While the land surface is continuously reused by man, the sea bed was largely inaccessible until the advent of submarines and scuba equipment in the twentieth century. Salvagers have operated in much earlier times, but much of the material was beyond the reach of anyone. Thus the Mary Rose was subject to salvage from the sixteenth century and later, but a very large amount of material, buried in the sediments, remained to be found by maritime archaeologists of the twentieth century.

While preservation in situ is not assured, material that has survived underwater and is then recovered to land is typically in an unstable state and can only be preserved using highly specialised conservation processes. While the wooden structure of the Mary Rose and the individual artifacts have been undergoing conservation since their recovery, the Holland 1 provides an example of a relatively recent (metal) wreck for which extensive conservation has been necessary to preserve the hull. While the hull remains intact, its machinery remains inoperable. The SS Xantho engine that was recovered in 1985 from a saline environment after over a century underwater is presently considered somewhat anomalous, in that after two decades of treatment it can now be turned over by hand.

A challenge for the modern archaeologist is to consider whether in-situ preservation, or recovery and conservation on land is the preferable option; or to face the fact that preservation in any form, other than as an archaeological record is not feasible. A site that has been discovered has typically been subjected to disturbance of the very factors that caused its survival in the first place, for example, when a covering of sediment has been removed by storms or the action of man. Active monitoring and deliberate protection may mitigate further rapid destruction making in situ preservation an option, but long term survival can never be guaranteed. For very many sites, the costs are too great for either active measures to ensure in situ preservation or to provide for satisfactory conservation on recovery. Even the cost of proper and complete archaeological investigation may be too great to enable this to occur within a timescale that ensures that an archaeological record is made before data is inevitably lost.