A Lake Becomes a Sea
Around 7,600 years ago, the Black Sea was a vast, freshwater lake. Its surface lay hundreds of feet below the level of the Mediterranean Sea, separated by a natural dam of land at the Bosporus strait. As the last ice age ended, melting glaciers caused global sea levels to rise. The theory, proposed by geologists William Ryan and Walter Pitman in 1997, suggests that the rising Mediterranean eventually broke through this barrier.
The result was a cataclysmic flood. An estimated 50 cubic kilometers (10 cubic miles) of saltwater poured through the breach each day, a volume equivalent to 200 times the daily flow of Niagara Falls. The roar of the cascade might have been audible over 60 miles away. This deluge raised the level of the Black Sea by up to six inches per day, submerging about 100,000 square kilometers (39,000 square miles) of land. The influx of seawater continued for at least 300 days, permanently transforming the freshwater lake into the saltwater sea it is today.
Evidence from the Deep
The primary evidence for this rapid inundation comes from sediment cores drilled from the seafloor. These cores reveal a sharp, distinct boundary line. Below it are fossilized freshwater mollusks, cracked mud and plant roots, indicating a previously dry or freshwater environment. Directly on top of this layer, the cores show saltwater mollusk species native to the Mediterranean. Radiocarbon dating of the freshwater shells just below the boundary and the saltwater shells just above it places them at the same approximate age—around 5600 BCE—suggesting a sudden, not gradual, transition.
Support comes from sonar mapping of the seafloor, which has identified ancient shorelines, drowned river valleys, and even human-worked timbers about 100 meters (330 feet) below the current sea level. The theory is debated. Some studies suggest the water level difference was much smaller—perhaps only 30 meters—leading to a more modest flood. Other geologists propose that water had always flowed from the Black Sea to the Mediterranean, and no single catastrophic flood occurred. Despite the debates, the evidence shows a significant environmental change in the region's deep past.