Marine Archaeology

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Marine Archaeology

Many of the most famous archaeological sites are those of the ancient Egyptians, Greeks, Romans, and Vikings. While most all of these archaeological sites, from Egypt's pyramids to Rome's Coliseum, are on land, these cultures had strong ties to the sea, engaging in frequent trade and exploration. They also used ships in wars. These civilizations left behind shipwrecks and the ruins of port cities that have been claimed by the sea due to erosion (wearing away of land). Technological advances in the late twentieth century permitted archaeologists to begin exploration and excavation of underwater archaeological sites. This branch of archaeology is called marine or underwater archaeology.

Archaeology explores how people lived in the past through excavation and survey. An excavation is a planned, careful exploration of ancient sites. It is sometime called a dig because digging (excavation) is the most well known method for discovering clues about the past at an archeological site. Wherever people live or work, they leave traces of their life. Pieces of pottery, metal, glass, wood, bricks, and cut stone can remain behind for hundreds or thousands of years after a dwelling (house), city, or civilization has vanished. Remnants that were made or used by humans are called artifacts. Artifacts and their surroundings, called context, give scientists clues to what past peoples valued, what they ate, where they lived, and how they worked.

Exploring underwater archaeological sites

When studying a site, marine archaeologists pay close attention to the context of each artifact, carefully noting and mapping exactly where each artifact or ruin was discovered. The location on site where an artifact is found is known as its provenience. Noting the provenience of each artifact helps marine archaeologists construct maps and computer models of the site. Artifacts and features are like pieces of a puzzle and such models permit scientists to see how the various pieces fit together.

Exploration of underwater archaeological sties is more difficult than studying land-based sites. Underwater archaeology requires special equipment. People who participate in under-water digs must be skilled in both scuba diving and archaeological field methods. Though often more challenging, underwater archaeologists follow the same scientific standards used on land to conduct careful studies of their sites.

Alexandria Submerged

Alexandria was an ancient port city in the western Mediterranean Sea on the Egyptian coastline. The city was first established by the Egyptians about 332 b.c.e., and later ruled by the Greeks, and then Romans. Because of its favorable location for trade, Alexandria was one of the wealthiest and most important cities in the ancient world. It was a center for trade, ship making, and scientific exploration. Alexandria's most famous buildings were its museum and library. Before being burned down in the third century, the library held the greatest collection of work of philosophy, literature, and science in the ancient world.

Because of its importance in the ancient world archaeologists have long been fascinated with exploring and studying the remains of the ancient city. Over the past two hundred years, most of this research took place on land. However, excavation of land-based sites is difficult because the modern city of Alexandria, Egypt, stands in the same location as the ancient city.

Some of the ancient city now lies underwater. Rising water levels and the changing flows of silt and water in the Nile River delta (the sedimentary deposits where the river meets the sea) deposits altered the coastline around Alexandria. In the 1960s, archaeologists began to explore the submerged areas of ancient Alexandria. Underwater excavation located the ruins of several temples, columns, and buildings. Marine archaeologists have unearthed pieces of pottery, stone statues, and metal artifacts. They have also studied the structures built by the ancient Alexandrians to improve their port. They have found ancient Alexandrians formed a large, protected harbor by connecting several manmade walls and barriers between small islands.

Often the first step in studying an underwater archaeological site is a surface survey, a study of the visible parts of the site. A surface survey can be as simple as an archaeologist diving to the site and looking over the structure, shipwreck, or smaller artifacts. A survey can be as complex as a carefully planned removal of visible, small artifacts from specific locations. Marine archaeologists often look for clues about the origin and age of the site from this initial survey.

Exploration of an underwater site usually requires excavation. Excavation requires both divers and a crew on board the research vessel. If a site is excavated, marine archaeologists first lay out a grid, a geometric plan for the site consisting of rows and columns of small squares. The small squares, or units, are then excavated by removing one layer of silt (tiny rock, soil, and other mineral particles) and artifacts at a time. Vacuum hoses are often used to remove silt layers and carry them back to the research vessel. The silt is then pushed through a screen or mesh and is examined for small artifacts. Larger artifacts are brought up by divers conducting the dig or by the research vessel's lifting mechanism.

As excavation progresses, marine archaeologists formulate detailed maps of the site. These maps can be drawn from precise notations of each artifact's location. Maps are usually drawn of each level of each unit both before and after a layer is silt is removed. They include drawings and notes about each significant artifact within that level. Features such as changes in the color or texture of sea floor sediments (particles of sand, silt, or clay) are also noted on such drawn maps.

Special maps of a site can also be made by using remote sensing. Remote sensing involves the use of equipment to discover and map buried or underwater sites from a distance. Sonar (which is short for Sound Navigation and Ranging) sends out sound pulses and interprets their echoes to penetrate the depths of the oceans and locate objects. Different remote sensing techniques vary in what they detect and the type of data they produce. Remote sensing techniques are typically used if a site is so deep that humans cannot dive to it, if a large layer of silt covers most of the site, or if archaeologists are trying to locate the precise location or boundaries of a site. Sidescan sonar can look over an 18-mile-wide (29-kilometer) swath of the ocean floor to locate natural features and archaeological sites. Underwater cameras with night vision features penetrate dark waters. A magnetometer, another remote sensing device, finds shipwrecks by detecting metal objects used in the ship's construction such as nails, brackets, decorative ironwork, or artillery (cannons and guns). The data from remote sensing apparatuses are processed by computers, which produce maps and charts that marine archaeologists can then interpret.

Some archaeologists have employed their basic senses while investigating underwater archaeological sites. In one project requiring the exploration of a two-thousand-year-old Roman shipwreck in the Mediterranean Sea, marine archaeologists even sampled the shipwreck's cargo. They tried a sip of the ancient wine and olive oil discovered in the ship's amphorae (large pottery jars)!

Marine archaeology and conservation

Excavation is sometimes not the preferred method to explore an underwater archaeological site. Mapping, remote sensing, and surface survey are often used without digging or removing artifacts from a site. Excavation is primarily used only when a site is in danger of destruction from modern vessels, severe storms, dredging (removing sediment from the bottom of a waterway, usually to make it wider or deeper), or rapid, natural decay. Leaving a site intact permits future archaeologists to study the same site.

Artifacts can be both helped and damaged by the water that surrounds them. Some artifacts, such as the wooden hull of ancient ships, are better preserved in the salt waters in which they sank. On the other hand, the iron and metal hulls of relatively modern vessels, such as the Titanic that sank in 1912, are destroyed by saltwater. The temperature of water, its salinity (salt content), and depth of a site also affect preservation. Cold water with low salinity does not destroy artifacts as rapidly as salty, warm water destroys artifacts. The hulls of shipwrecks in extremely deep ocean waters can be crushed by intense pressures equal to thousand of pounds per square inch.

Artifacts removed from underwater sites require special handling, preservation, and storage, also known as curation. The goal of proper conservation and curation is to clean and protect artifacts from further damage so that they may be studied by scientists or placed on display in museums. Various pieces of broken pottery jars are grouped together. Sometimes, the pieces are reassembled to restore the object, and sometimes they are left as individual fragments. Some artifacts recovered from the sea are kept moist in special wet or humid cases, other artifacts are carefully dried out once removed. Even a small artifact, such a sailor's wooden pipe, may take months or years of conservation work before ready for a museum display.

Adrienne Wilmoth Lerner