marine and underwater archaeology

marine and underwater archaeology. Humankind's artefacts litter the seabed, partly as a result of mercantile and naval activities, but also because landscapes have become submerged. This submergence is not only the result of the sea level rising as the ice caps melted at the end of last glaciation 20,000 years ago, but also a result of tectonic movements. For example, the southern half of the North Sea was only inundated by the sea about 7,000 years ago. Up until then it had been sparsely populated by bands of stone age hunter-gatherers and in 2003 two stone-age settlements dating back to the Mesolithic period, between 10,000 and 5,000 years ago, were found off the coast of Tyneside. The seashore always offered a rich abundance of resources for these early peoples. In historical times tribes of North American indigenous peoples such as the Haida of the Vancouver region had lifestyles intimately adapted to life on the coast. On islands in the Caribbean the refuse tips or middens left by the Caribs, when excavated, have shown how the diets of these coastal peoples changed, partly because of climate change, but also because of their impact on the resources they were exploiting. Sea levels also change locally because of tectonic movements. These can be slow, gradual shifts: for example in south-east Britain the land is sinking about 1 mm per year, fast enough to turn several coastal archaeological sites into marine sites over the centuries. However, in many places siltation has meant that harbours used in Roman and medieval times are now some distance from the sea. In the Mediterranean the impact of the African tectonic plate pushing up against the Eurasian plate has been rotating the island of Crete so that Graeco-Roman harbours are now high and dry above sea level on one side of the island but submerged on the other.

Some tectonic movements are sudden and violent. On Christmas Day 1968 a strong earthquake near Anchorage in Alaska resulted in vertical shifts of some parts of the coast by over a metre, and Charles Darwin was greatly influenced by witnessing the impact of a metre shift in sea level near Santiago in Chile. But perhaps the most significant archaeological event was the explosive end of the island of Santorini in the Mediterranean that brought the Minoan civilization centred on Crete to an abrupt end. Such events add to the archaeological record not only by dropping local sea levels but also through the destructive power of the tsunamis they generate, which wash immense numbers of artefacts into the sea.

Perhaps one of the most remarkable prehistorical underwater sites is the Cosquer Cave near Marseille, which is at the end of a 175-metre- (575-ft)-long tunnel 37 metres (120 ft) below sea level. The cave is not accessible to the general public, but it is now possible to make a virtual visit to see the remarkable paintings of horse, ibex, aurochs, jellyfish, great auks, and human hands, which were probably painted during the last glaciation when the sea was 70 metres (230 ft) lower than today.

However, in most people's minds marine archaeology is associated with shipwrecks. The long history of seafaring stretches well back into prehistory and, inevitably, where there has been a high density of shipping, accidents have occurred. Before the advent of effective navigation instruments, seafarers tended to hug coastlines, where so many shipwrecks occurred. Warfare at sea has also added to the profusion of shipwrecks on the seabed. Examples range from the great Mediterranean naval battles of early times, to sinkings of the Mary Rose off Portsmouth, the Swan, a Cromwellian warship, off Mull, the destruction of the Spanish Armada by Atlantic storms in 1588, and the sinking of 20 million tonnes of shipping during the Second World War (1939–45). Some of the latter are now conserved as war graves, such as the battleship HMS Royal Oak in Scapa Flow and the USS Arizona in Pearl Harbor. Shipwrecks are a magnet to those whose techniques are destructive and aimed at maximizing financial profit, rather than maximizing the recovery of archaeological data. There is a fine dividing line between shipwrecks suitable for salvage and those that should be treated as historical monuments.

Marine archaeology began in earnest once diving technology had advanced sufficiently for the diving bell to be invented, although Roman ‘urinators’, or free divers, had salvaged cargoes from sunken ships in the Mediterranean. But the greatest advance, so far as marine archaeology was concerned, came with the aqualung, so long associated with Jacques Cousteau, supplemented by the earlier invention of flippers. Since then technologies based on satellite navigation, both above and under water, air-lifts, dynamic positioning of recovery vessels, use of gas mixtures, sonars, and robotics have all added to the array of techniques used by marine archaeologists to solve the particular challenges presented by working in the hostile marine environment.

The first task is to locate a site. Recorded historical evidence and artefacts washed up or found in fishermen's trawls can sometimes provide an approximate position. In shallow water or in the inter-tidal zone, shipwrecks may be discovered by chance by recreational divers or by walkers. In deeper water, beyond the safe limits for diving, side-scan sonars are now being used, supplemented by observations with underwater vehicles.

Before any excavation is carried out, a fine-scale survey needs to be conducted, using traditional land-based techniques, of laying out grid lines with tapes, drawing, photographing, and mapping features on the seabed. Sediment is removed and sieved for tiny finds. The exact positions of artefacts as they are uncovered are plotted, and possibly removed for conservation. The lifting of whole sections of hull, such as occurred with the Mary Rose and the Vasa, requires the heavy lifting gear used in salvage operations, but the vast majority of wrecks are left in situ.

Artefacts are dated using various isotope techniques and dendrochronology—the use of tree rings, which yield a precise dating signature if the location where the original tree grew is known. Once in the air and light, deterioration is rapid unless immediate conservation action is taken. Metal objects are often X-rayed and treated by electrolysis, attaching them to anodes to reverse their degradation. Organic items, wood, fabric, and leather, need to be consolidated by infusing them with polyethylene glycol or freeze dried, a process that, in the case of the timbers of the Vasa and the Mary Rose, is ongoing and requires decades of treatment. Such artefacts are rarely preserved in sites on land and provide direct evidence of trading practices, woodworking techniques, food storage, medicinal preparations, weaving techniques, and so on. Human remains are particularly interesting because they are the remains of people who died at the peak of their active lives. Their physical attributes are often very informative. For example, the archers drowned on the Mary Rose had physiques that made them able to draw weights greater than those of modern Olympic athletes; and the seamen on the Swan had suffered from rickets.

Bibliography

Delgado, J. (ed.), Encylopaedia of Underwater and Maritime Archaeology (1997).
Marx, R. , An Introduction to Marine Archaeology: The Underwater Dig (1976).www.decadevolcano.net/santorini/minoaneruption.htmwww.adp.fsu.eduwww.maryrose.orgwww.culture.fr/culture/archeosm/en

M. V. Angel