Buried Beneath the Black Sea: Cities and Ships Submerged
Discovering volcanoes old and new
According to Greek mythology, on the eastern shores of the Black Sea, there was once a kingdom called Kolchis. Located in what is today the Republic of Georgia, this kingdom was said to be the home of the Golden Fleece, the legendary treasure that Jason and his Argonauts set out to find and bring back to Greece.
The Argonauts’ quest across the previously unexplored Black Sea to bring back treasure is an allegory of the Greeks’ own exploration, and eventual colonization, of the region, says Robert Ballard, an archaeological oceanographer at the University of Rhode Island (URI). Searching not only for gold, but also for fish, the Greeks “went in both friendly and as a raiding party,” he says. “They made it into a wonderful journey.”
For more than two decades, Ballard has been on his own epic quest to merge oceanographic tools and techniques with archaeological methods, in an effort to uncover long-buried historical treasures — such as the R.M.S. Titanic, which Ballard discovered in 1985, hidden under 3,800 meters of water in the North Atlantic Ocean.
In the summer of 2006, Ballard continued this quest in the Black Sea, spearheading one leg of a series of expeditions in the region that aimed to uncover geological and oceanographic evidence of classical human history buried under the waters bordering Greece, Turkey and Ukraine.
Fed by several major European rivers, the surface waters of the Black Sea are also only half as salty as the ocean. To a depth of about 50 meters, these waters support abundant marine life, from zooplankton and mollusks to fish and dolphins. Below that surface layer, however, lies denser, saltier water. From about 200 meters all the way down to the sea’s bottom — about 2,000 meters at its deepest point — the water is almost completely oxygen-free, or anoxic. This layer gives the Black Sea the distinction of being the largest anoxic body of water in the world: Because the density difference between the two layers is so large, they rarely mix, and the colder, denser waters of the bottom rarely receive any influx of oxygen from above.
These conditions make the Black Sea a unique laboratory for scientists to study its physical, chemical and biological properties. But the low-oxygen conditions also offer something to archaeologists and historians: Many organisms that consume and break down organic materials cannot survive in that environment. Therefore, Ballard and other scientists suggest, ships’ hulls and other often-fragile clues to ancient civilizations are likely still well-preserved and intact under the sea’s dark waters. “It’s a veritable museum there,” Ballard says.
The labors of Hercules
The team used Hercules and IFE’s “towfish” Echo, which carried sidescan sonar and sub-bottom profiling instruments to map the seafloor, to scan more than 650 square kilometers of underwater terrain in the hunt for archaeological treasures from ancient shipping routes to paleoshorelines. Hercules is part of a coupled robotic surveying and sampling system. The ROV is connected by a 30-meter-long tether to Argus, a “towsled” that is connected to the ship by a cable and is equipped with multiple lights, high-definition cameras and temperature, salinity and pressure sensors. Hercules also has its own high-definition cameras and sensors, as well as a pair of manipulator arms to collect samples.
The 2006 expedition, which was conducted in collaboration between URI and the Department of Underwater Heritage of Ukraine, was looking for evidence of ancient deep-water trade routes from the Bronze Age, at about 1500 B.C., to the Byzantine period, about A.D. 1000, Ballard says.
One principal question that historians ponder is how well the ancient mariners were able to navigate far from land. Some argue that rather than cutting directly across the Black Sea along the shortest route, captains in antiquity would have preferred to stay close to the shoreline to avoid navigating through the tricky currents and winds out in the middle of the sea. Others, however, suggest that ancient mariners had no difficulty venturing far from shore. Given the Black Sea’s unique preservation ability, Ballard thought that if the team searched along the suspected trade routes, it might be possible to find evidence of Bronze Age ships lying intact on the seafloor — supporting the idea that some ancient captains did take the shortest route, and were willing to rely on their navigation skills.
To test this, the team focused on trade routes that may have led directly from the ancient Greek colony of Chersonesos — established about 2,500 years ago in the southwestern part of Crimea, near modern-day Sevastopol — to the Bosporus Strait. At a key location on the peninsula and possessing deep-water harbors, Chersonesos was a primary transit point for maritime trade from the Bronze Age through the Byzantine era.
To study the site, the team used “traditional archaeological survey techniques” that Ballard modified for deep-sea work, says Katy Croff, a URI graduate student in archaeological oceanography. Previous underwater archaeology studies, limited by technology and time, have focused on studying a specific wreck. The 2006 expedition, however, aimed to do a large-scale archaeological survey of the area to figure out which sites might be of most interest, she says. “We wanted to find as much as we could first, rather than camping out at one place and then finding out that 100 feet away there is something more interesting.”
When surveying a site on land, archaeologists generally create a grid, demarcated by lines of string, to keep track of the distribution of artifacts within the site. Underwater, the task requires a bit more hardware, but the principle is the same, Croff says. Instead of lines, “we used side-scan [sonar] to find targets to go back to,” she says. “This is one of the first large-scale underwater surveys of its kind.”
The team crisscrossed the northern Black Sea with the sidescan sonar, which mapped strip after strip of seafloor. To “see” what the seafloor looks like, the sonar sends out acoustic pulses to the seafloor that bounce off irregularities on the bottom, such as rocks and ripples. It then records the reflected energy. The pulses reflecting off sand ripples — or half-buried shipwrecks — will create a very different sonar picture from that of a smooth, muddy bottom.
With the sonar, the researchers discovered several sunken ships from the region’s thousands of years of history, including a World War II-era Russian ship sunk by a German torpedo, says Dwight Coleman, a marine geologist also at URI, who was a co-chief investigator on the expedition.
The team also discovered a late-Byzantine-era vessel from the 10th century, loaded with ancient one-handled clay jars called amphorae, Coleman says. “It lies right along a suspected trade route between Chersonesos and Istanbul,” he says. The ship was found about 24 kilometers offshore — still within sight of land, but not hugging the coast. That suggests that Byzantine-era captains did indeed know how to navigate across the open waters and were familiar with the wind and current patterns, he says. Although the ship does fall along the suspected trade route, however, “it’s hard to scientifically say ‘it’s on this route’ until we collect more data,” he adds.
Although armed with Hercules, which can collect not only images but also artifacts, the team has not yet begun to excavate their find. “Everything we did was non-intrusive — we just took pictures,” Croff says. Although the team is curious about what the amphoras might hold, which could include “wine, olive oil or fish guts,” she says, “collecting artifacts requires conservation,” which the recent expedition wasn’t set up to do. “This material has been down there for thousands of years and it’s still well-preserved. If we know where it is and how to get back to it, the best place for it is really on the seafloor,” Croff says. Conservation of the clay artifacts, once removed from their underwater environment, requires first and foremost that they must be kept damp, and then only very slowly dried, to avoid cracking.
The team plans to return to the site in August to begin its excavation in earnest. “This is really just the beginning,” Ballard says. “We’re happy to really move forward next year — once you start excavating, that’s when the discoveries really come.” And upon the team’s return, Hercules will also take more than just pictures, he says.
After the flood
In 1997, Walter Pitman and Bill Ryan of Columbia University proposed a controversial hypothesis: that the sea level abruptly rose about 7,200 years ago, due to salty Mediterranean water breaking through a natural dam across the Bosporus Strait and flooding the freshwater Black Sea — timing that they note coincides roughly with the biblical story of Noah’s Flood (see Geotimes, February 2004). The hypothesis is still disputed, and archaeologists, historians and oceanographers continue to search for evidence of Neolithic sites that might have been inundated by the rising sea.
“Most of looking for these archaeological sites is an exercise in geology — looking for evidence of ancient shorelines,” Coleman says. Additionally, he says, geological training is necessary to sort out those ancient shorelines from modern sedimentary environments on the shelf, which are likely due to the density gradient in the Black Sea.
That strong density difference, which keeps the sea’s layers mostly stable and in place, can still produce some interesting effects, Coleman says. For example, sonar pulses sent from the upper layer can bounce off the higher-density layer. Additionally, internal “waves” that never see the surface can travel along the interface between the high- and low-density layers of water, he says. Once those waves reach the shelf, they break. “At 150 meters deep, we see evidence of sedimentary bedforms that look like sand waves,” he says. Those bedforms can easily be confused for buried shorelines.
Correctly interpreting evidence of ancient shorelines, therefore, requires understanding “landscape archaeology, or environmental archaeology” as well, Coleman says. “We’ve mostly identified places where paleoriver channels have flowed across the shelf, evidence of terrestrial flora such as grass growing, and peat deposits and deposits within cores that are evidence of an arid and subarid landscape,” he says. The researchers also identified sites with shoreline features such as smooth, well-rounded beach stones and intact bleached mollusk shells — a freshwater species dating to more than 7,000 years ago, he says. Such mollusks could only have lived in nearshore environments that became inundated and were preserved by rising sea levels.
The team has also mapped the layers of sediment that lie underneath parts of the Black Sea floor, using a “sub-bottom profiler,” also mounted on the towfish Echo. The sub-bottom profiler sends sound pulses toward the seafloor. As in seismic reflection surveys, some of the sound reflects off the seafloor and some of it may penetrate it, providing information about the thickness, composition and slope of the seafloor’s layers. These data, collected about 15 to 30 kilometers off the coasts of northern Turkey and southern Ukraine and up to 50 kilometers off the coast of Bulgaria, could potentially be interpreted as representative of coastal and nearshore structures and paleo-shoreline deposits, Coleman says.
The data also reveal that the sea hides other features fascinating to geologists and oceanographers, Coleman says. “We have compiled some geologic cross sections that show active slumping going on off the slope, and other geologic processes,” such as underwater landslides likely triggered by earthquake activity and turbidity currents, he says.
Opening up the sea
“There are so many places still to be explored,” Croff says. “Especially underwater.”