"...the fountains of the great deep [were] broken up, and the windows of the heavens were opened. And the rain was upon the earth forty days and forty nights."
This quote from the Book of Genesis is part of a familiar tale — the story of Noah's flood. Scholars have known for a long time that the Bible isn't the only place this story is found — in fact, the biblical story is similar to a much older Mesopotamian flood story in the epic of Gilgamesh. Scholars usually attribute things like the worldwide occurrence of flood stories to common human experiences and our love of repeating good stories, but recently scientists have started to uncover evidence that Noah's flood may have a basis in some rather astonishing events that took place around the Black Sea some 7,500 years ago.
The scientific version of Noah's flood actually starts long before that, back during the last great glaciation some 20,000 years ago.
This was a time when the earth looked very different from what we are used to today. Thick ice sheets extended down from the North Pole as far as Chicago and New York City. All that water had to come from somewhere, so ocean levels were about 400 feet lower than they are today. In essence, water that evaporated from the oceans fell as snow (which was compacted into glacial ice) rather than rain (which would flow back and replenish the oceans as it does now). The East Coast of the United States was 75 to 150 miles farther out than it is today, and places like Manhattan and Baltimore would have been inland cities. During this period, meltwater from the European glaciers flowed down to the Black Sea basin, then out through a river channel into the Mediterranean. Because the Mediterranean is connected to the world ocean at Gibraltar, it was also 400 feet lower than it is today, so this flow of fresh water through the Black Sea was downhill.
Two geologists at Columbia University's Lamont-Doherty Earth Observatory have offered a new theory of what happened next. William Ryan and Walter Pitman, in Noah's Flood (Simon & Schuster), postulate that as time went on, the world warmed, the glaciers retreated and meltwater from the European glaciers began to flow north into the North Sea, depriving the Black Sea of its main source of replenishment. The level of the Black Sea began to drop, and most of the area around its northern boundary — the area adjacent to present-day Crimea and the Sea of Azov — became dry land. At this point, the level of the Black Sea was several hundred feet below that of the Mediterranean, and the two were separated by the barrier of the Bosporus, then dry land. This situation, with the world ocean rising while the Black Sea was falling, could not last forever. Eventually, like a bathtub overflowing, the Mediterranean had to pour through into the Black Sea basin.
The idea that ocean basins can flood catastrophically during periods of rising sea levels is nothing new in geology. Five million years ago, long before there were any humans around, just such an event occurred. The level of the Atlantic Ocean had dropped, or some tectonic event had occurred, with the result that water could no longer get through, and the Mediterranean gradually shrank down to a desert spotted with a few salty bits of ocean. Subsequently, when either the Atlantic rose again or another geological change took place, ocean water began pouring back into the former sea. The basin filled, and the present-day Mediterranean was created.
We know such things because sediments reveal history. Ryan and Pitman began taking cores of the present-day Black Sea. The cores seemed to be telling a strange story indeed, particularly in the northern areas. At the very bottom of the cores, dozens of feet below the present seafloor, they found layered mud typical of river deltas.
Carbon-dating of shells in this mud indicates that it was laid down between 18,000 and 8,600 years ago. This data showed that an area of the Black Sea about the size of Florida might have been much like the lower Mississippi Delta today — rich farmland with an abundant supply of fresh water.
Directly above the layers of mud is a layer of what Pitman calls "shell hash" — an inch-thick layer of broken shells — overlain by several feet of fine sediment of the type being brought into the Black Sea by rivers today. The shells in the "hash" are typical of what was in the Black Sea when it was a body of fresh water. The fine sediments contain evidence of saltwater species previously unknown in the Black Sea. It is the interpretation of these layers that tells us what happened on that inevitable day when rising sea levels in the Mediterranean reached the base of the sediments at the bottom of the Bosporus — and all hell broke loose.
When the Mediterranean began to flow northward, it "popped the plug" and pushed those sediments into a "tongue" of loose sediment on the bottom of what would become the present-day Black Sea (this tongue can still be seen in cores taken from the ocean bottom in that area). As the flow of water increased, it began to cut into the bedrock itself. The rock in this area is broken — Pitman calls it "trashy" — and even today rockslides are a major engineering problem for roads cut into the cliffs alongside the Bosporus. The incoming water eventually dug a channel more than 300 feet deep as it poured into the Black Sea basin, changing it from a freshwater lake to a saltwater ocean. In this scenario, the mud beneath the shell hash represents sediments from the rivers that fed the freshwater lake, the shell hash the remains of the animals that lived in that lake, and the layers above it the result of the saltwater incursion.