Why Are Scientists from the University of Rhode Island Exploring the Aegean and Black Seas?
A key purpose of NOAA’s Ocean Exploration Program is to investigate the more than 95 percent of Earth’s underwater world that until now has remained virtually unknown and unseen. Such exploration may reveal clues to the origin of life on Earth, cures for human diseases, answers on how to achieve sustainable use of resources, links to our maritime history, and information to help protect endangered species. In addition, exploration of active volcanoes provides important information on potential hazards such as risks to shipping from shallow eruptions, as well as the generation of dangerous tsunamis from large submarine eruptions and landslides.
The geographic region surrounding the Aegean and Black Seas has been the stage for many spectacular performances in Earth’s geologic and human history. According to the theory of continental drift, one of these performances began about 225 million years ago when Earth’s landmasses were concentrated into a single supercontinent known as Pangaea. Over the next 25 million years, Pangaea began to split into two new supercontinents; Laurasia to the north and Gondwanaland to the south. The eastern portions of the two supercontinents were separated by a body of water called the Tethys Ocean. As the splitting process continued, Laurasia rotated counterclockwise to the south, while Gondwanaland rotated counterclockwise to the north. This motion slowly closed the Tethys Ocean, eventually forming the Mediterranean, Aegean, Black, Caspian and Aral seas.
Human activities on the region’s stage began during Paleolithic times; artifacts discovered near Istanbul are believed to be at least 100,000 years old. Well-known Aegean cultures include the Minoans (ca 2,600 - 1,450 BC), Mycenaeans (ca 1,600 - 1,100 BC), Ancient Greeks (776 - 323 BC), and Hellenistic Greeks (323 - 146 BC). Istanbul - the only city that spans two continents - has been a crossroads of travel and trade for more than 26 centuries. Mariners have traveled the Aegean since Neolithic (Stone Age times; 6,500 - 3,200 BC), probably for a combination of purposes, including trading, exploration, and warfare.
Interactions between these cultures and many others were often violent and destructive. So, too, were interactions with geological processes. Some of these processes are directly related to the same forces that are believed to have caused the breakup of Pangaea (see Volcanoes, below). One of the most dramatic and destructive events was the eruption of a volcano near a small Aegean island called Thera (also known as Santorini), sometime between 1,650 and 1,450 BC. Estimated to be four times more powerful than the Krakatoa volcano of 1883, the eruption left a crater 18 miles in diameter, spewed volcanic ash throughout the Eastern Mediterranean, and may have resulted in global climatic impacts. Accompanied by earthquakes and a tsunami, the volcano destroyed human settlements, fleets of ships, and may have contributed to the collapse of the Minoan civilization on the island of Crete, 110 km to the south. On Thera, the ancient city of Akrotiri was completely buried beneath the ash. Excavation of the city began in 1967, and is ongoing. The Bronze Age eruption of the Theran volcano was by no means its last. In fact, the volcano erupted at least 12 times between 197 BC and 1950; and most geologists agree that a violent eruption will happen again.
Interactions with other geological processes may have been equally disastrous. In 1997, geologists William Ryan and Walter Pitman published a theory in which the Black Sea was inundated around 5,600 BC by flood waters from the Mediterranean passing through the Straits of Bosporus at Istanbul. Such a deluge, if it occurred, would have been disastrous for human settlements along the Black Sea shoreline; and might have provided an origin for accounts of cataclysmic floods in Christianity and other cultures. Subsequent research has neither proved nor disproved the Black Sea deluge theory; but in 2000, Robert Ballard discovered remains of a wooden structure that may have been part of an ancient seaport 95 meters below the surface of the Black Sea (see http://news.nationalgeographic.com/news/2000/12/122800blacksea.html). This may be one of the best places in the world to look for remains of ancient civilizations, because the deep waters of the Black Sea contain almost no oxygen; so the biological organisms that normally attack such relicts cannot live in this environment.
Finding well-preserved marine archaeological sites, studying ancient maritime trade, and exploring the history of the Theran volcano are the primary goals of the Aegean and Black Sea 2006 Expedition.
The Aegean and Black Sea 2006 Expedition is focused on using modern oceanographic tools and techniques in archaeological investigations of unexplored regions of the Aegean, Black, and Eastern Mediterranean Seas. Key questions concern technology, archaeology, and volcanology. The basic technological question is: How can geological, physical, chemical, biological, and archaeological data be integrated in oceanographic models to predict the locations of well-preserved archaeological sites (primarily ancient shipwrecks)? When promising sites are located using these models, the primary archaeological questions guiding further investigations are: what do these sites reveal about the organization and nature of ancient maritime trade and communication; and how have these activities matured through time? Finally, questions about volcanology are focused specifically on the Theran volcano: What recent hydrothermal and volcanic activity has occurred at this volcano, and what new details can be discovered concerning its massive eruption during the Greek Bronze Age?
The Aegean and Black Sea 2006 Expedition is divided into two segments. In the first segment, side-scan sonar, subbottom profiling, and multibeam bathymetric technology are used to survey selected portions of the Aegean, Black, and Eastern Mediterranean Seas. The second segment uses remotely operated vehicles (ROVs) for direct visual observation of promising sites located during the first segment.
Sonar (which is short for SOund NAvigation and Ranging) systems are used to determine water depth, as well as to locate and identify underwater objects. In use, an acoustic signal or pulse of sound is transmitted into the water by a sort of underwater speaker known as a transducer. The transducer may be mounted on the hull of a ship, or may be towed in a container called a towfish. If the seafloor or other object is in the path of the sound pulse, the sound bounces off the object and returns an echo to the sonar transducer. The system measures the strength of the signal and the time elapsed between the emission of the sound pulse and the reception of the echo. This information is used to calculate the distance of the object, and an experienced operator can use the strength of the echo to make inferences about some of the object’s characteristics. Hard objects, for example, produce stronger echoes that softer objects. This is a general description of active sonar. Passive sonar systems do not transmit sound pulses. Instead, they listen to sounds emitted from marine animals, ships, and other sources. Subbottom profiler systems are another type of sonar system that emits low frequency sound waves that can penetrate up to 50 meters into the seafloor. Visit http://ocean.noaa.gov/technology/tools/sonar/sonar.html for more information about sonar systems.
Side-scan sonar systems use transducers housed in a towfish, usually dragged near the sea floor, to transmit sound pulses directed toward the side of the ship, rather than straight down. Return echoes are continuously recorded and analyzed by a processing computer. These data are used to construct images of the sea floor made up of dark and light areas. These images can be used to locate seafloor features and possible obstructions to navigators, including shipwrecks (visit http://oceanexplorer.noaa.gov/technology/tools/sonar/sonar.html for more information). The side scan sonar towfish used on the Aegean and Black Sea 2006 Expedition is called ECHO, and belongs to the Institute for Exploration (visit http://oceanexplorer.noaa.gov/projects/thunderbay01/echo/echo.html for more information).
Multibeam sonar systems are used to make bathymetric maps and create three-dimensional images of the seafloor. Multibeam sonars send out multiple, simultaneous sonar beams in a fan-shaped pattern that is perpendicular to the ship's track. This allows the seafloor on either side of the ship to be mapped at the same time as well as the area directly below (visit http://oceanexplorer.noaa.gov/technology/tools/sonar/sonar.html for more information).
Scientists will use the Institute for Exploration ROV Hercules and Argus during the expedition. Hercules is designed primarily to study and recover artifacts from ancient shipwrecks, and always operates in tandem with a second vehicle, the camera and light sled Argus. The two vehicles are connected by a 30-meter (100 foot) fiber optic tether, and Argus is connected to the surface ship by a much longer fiber optic cable. This arrangement allows Hercules to move around without being affected by movements of the surface ship. The dual ROV system is capable of operating to depths of 4,000 meters, and carries a high-definition (HD) video cameras and lights; sensors for measuring pressure, water temperature, oxygen concentration, and salinity; and a pair of still cameras to accurately measure the depth and area of the research site as well as create multi-image "mosaics" of the site. At sea, the Argus-Hercules system generally operates 24 hours a day, operated by a six-person team: a Watch Leader who makes sure that the scientific goals of the dive are being addressed; a Pilot who controls Hercules’ thrusters, manipulator arms, and other functions; an Engineer who controls the winch that moves Argus up and down and assists the Pilot; a Navigator who relays positions of the vehicles and ship and communicates with the ship’s crew to coordinate ship movements; and Video and Data watch-standers who record and document all the data that the vehicles send up from the deep. Visit http://oceanexplorer.noaa.gov/technology/subs/hercules/welcome.html for more information about the Argus-Hercules ROV system.
More about Volcanoes
Many geologists consider the breakup of Pangaea as a part of the theory of plate tectonics. According to this theory, the Earth’s crust consists of large segments known as tectonic plates. These plates are portions of the outer crust (lithosphere) about 5 km thick, as well as the upper 60 - 75 km of the underlying mantle. Tectonic plates move on a hot flowing mantle layer called the asthenosphere, which is several hundred kilometers thick. Heat within the asthenosphere creates convection currents (similar to the currents that can be seen if food coloring is added to a heated container of water) that cause the tectonic plates to move several centimeters per year relative to each other.
The junction of two tectonic plates is known as a plate boundary. Where two plates slide horizontally past each other, the junction is known as a transform plate boundary. Movement of the plates causes huge stresses that break portions of the rock and produce earthquakes. Places where these breaks occur are called faults. A well-known example of a transform plate boundary is the San Andreas Fault in California.
Where tectonic plates are moving apart, they form a divergent plate boundary. At these boundaries, magma (molten rock) rises from deep within the Earth and erupts to form new crust on the lithosphere. Most divergent plate boundaries are underwater (Iceland is an exception), and form submarine mountain ranges called oceanic spreading ridges.
If two tectonic plates collide more or less head-on, they produce a convergent plate boundary. Usually, one of the converging plates moves beneath the other in a process called subduction. Subduction produces deep trenches, and earthquakes are common. As the sinking plate moves deeper into the mantle, increasing pressure and heat release fluids from the rock causing the overlying mantle to partially melt. The new magma rises and may erupt violently to form volcanoes that often form arcs of islands along the convergent boundary. These island arcs are always landward of the neighboring trenches. This process can be visualized as a huge conveyor belt on which new crust is formed at the oceanic spreading ridges and older crust is recycled to the lower mantle at the convergent plate boundaries.
The tectonic setting of the Aegean/Black/Mediterranean Sea area is complex, and includes two major plates (the Eurasian and African Plates) as well as several minor ones (including the Hellenic, Turkish, Arabian, and Van Plates). Boundaries between these plates are not always clear, but motion at plate boundaries is undoubtedly responsible for earthquakes and volcanoes throughout the region. This is not considered a highly volcanic area, but even one Thera-type volcano can do plenty of damage!
Thera and its neighboring islands are the most active volcanic center in the Aegean Arc, and are part of a single volcano that has erupted many times. At one time, there were fewer than the five islands visible today. In fact, one of its older names is Strongyle, which means circular or rounded, suggesting that the overall appearance was different in the past. During some of its eruptions, the floor of the volcano’s crater collapsed into the magma chamber below, forming a caldera. Over time, the caldera was partially re-filled by other volcanic activity. Before the eruption during Minoan times, the rim of the caldera was an almost-continuous ring, with only a single entrance. The floor of the caldera collapsed again during the Minoan eruption, destroying part of the above-water ring, thereby creating two new channels.
Studies of volcanic ash deposits suggest that the Minoan eruption happened in at least five stages. The first was a series of precursory volcanic events that took place over several months; and may have provided the inhabitants with a warning of things to follow, perhaps accounting for the absence of human remains at Akrotiri. The next stage should have left no doubt about what was coming: An eruption column 36 km high was ejected into the atmosphere, blanketing the Eastern Mediterranean with hot pumice and ash. Then came an even more violent stage. Cracks in the crater floor allowed seawater to contact hot magma, producing explosions of steam that hurled large blocks of rock and clouds of steam and ash over the islands and adjacent ocean. These explosions were followed by a classic pyroclastic flow; a river of ash, pumice, gases, and possibly mud that flowed down the sides of the volcano at speeds of 60 miles per hour or more. In the final, and most violent stage of all, the pyroclastic flow reached the coast with temperatures approaching 400°C. Visit http://www.immersionpresents.org/ and click on Ancient Eruptions! for more information about the Minoan eruption, including photographs of other volcanoes in similar eruptive stages.
More About the Black Sea
The Black Sea is the world’s largest water body in which the bottom waters never mix with shallower waters (a condition known as meromictic). As a result, the deeper waters are completely anoxic (devoid of oxygen). Seawater flows into the Black Sea basin from the Mediterranean via the Straits of Bosporus, while freshwater enters from several European rivers including the Danube. As a result, salinity gradually increases with depth from about 18 ppt at the surface to about 22 ppt in deeper waters. A water mass known as the Cold Intermediate Layer (CIL) separates surface waters from deeper waters, and is the major reason for deep-water isolation. Below about 200 m, bacterial decomposition of biomass sinking from shallow water consumes all available oxygen, while the anaerobic metabolism of other bacteria causes the formation of hydrogen sulfide.While such conditions are not favorable for many biological species, they are excellent for preserving human artifacts from normal processes of degradation. The discovery of what may have been an ancient shoreline 95 m below the present surface of the Black Sea may support Ryan and Pitman’s suggestion that a catastrophic flood transformed the Black Sea from a freshwater lake to its present condition. Additional support for the idea comes from radiocarbon dating of the shells of freshwater molluscs sampled at the ancient shoreline site. These analyses show the age of the freshwater molluscs to be about 7,500 years, while saltwater species from the same area appeared about 6,900 years ago. In other words, the transition from fresh to saline conditions was fairly rapid. More recent analyses of other data conclude that while this flood did occur, it was not as catastrophic as suggested by Ryan and Pitman, and a more severe flooding event took place 16,000 - 13,000 years ago (see http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_58733.htm). Notwithstanding debate about the relative significance of ancient floods, the anoxic waters of the Black Sea may still reveal a great deal about seafaring activities of Stone Age peoples.
For More Information
Contact Paula Keener-Chavis, national education coordinator for the NOAA Office of Ocean Exploration, for more information.
Other lesson plans developed for this Web site are available in the Education Section.