At first glance it may seem odd to use drip stones from a cave just south of the Black Sea coast as a geological archive to reconstruct the sea's history. The research team led by Dominik Fleitmann and his graduate student Seraina Badertscher from the Oeschger Centre in Bern dated the stalagmites, which were as old as 670,000 years, and measured the content of a specific oxygen isotope. The isotope's value varies depending on whether the water stemmed from the Mediterranean Sea, the Caspian Sea or a tributary river of the Black Sea. The team showed that at least 19 exchanges between the Black Sea and either the Mediterranean Sea or the Caspian Sea must have occurred, leading each time to a dramatic change in the isotopic composition of the water. The evaporation subsequently also altered local precipitation, which is eventually reflected in the composition of the fossilised rain water captured in the drip stones analysed in Bern.
"It is remarkable how often the hydrology of the Black Sea has fundamentally changed," says Dominik Fleitmann. Over the past 670,000 years, there were at least twelve intrusions of water from the Mediterranean and seven exchanges with the Caspian Sea, the study showed. "Our data also proves that the depth of Bosporus Strait, which connects the Black Sea to the Mediterranean, has remained virtually unchanged over the years, despite the tectonic uplift in the region," Dominik Fleitmann added. The results of the study have now been published in the renowned journal "Nature Geoscience."
Interplay between fresh and salt water
The Bosporus, with a sill depth of about 35 meters, connects the Black Sea with the Mediterranean. With the inflows of salty water from the Mediterranean, the Black Sea is today a body of brackish water. But in the last glacial period, about 20,000 years ago, the Black Sea was a large freshwater lake, because there was no connection to the Mediterranean. The reason is that the global sea level – and with that also the level of the Mediterranean – was 120 meters lower than today. With the melting of the large ice masses and the resulting increase of the sea levels, water from the Mediterranean was able to spill into the Black Sea about 9,400 years ago, turning the freshwater lake into a semi-enclosed brackish sea. This occurrence – we believe today – is the basis for the Biblical story about Noah's Flood.
These occurrences have been outlined before, but so far there has been little precise information on when and how often the Black Sea had been connected with the Mediterranean. "Noah's Flood, if indeed it ever happened, was not a unique occurrence", Dominik Fleitmann states. The reconstruction at the Institute of Geological Sciences at the University of Bern managed to close that gap in knowledge.
