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The Black Sea deluge is a hypothesized catastrophic rise in the level of the Black Sea circa 5600 BC due to waters from the Mediterranean Sea breaching a sill in the Bosporus Strait. The hypothesis was headlined when The New York Times published it in December 1996, shortly before it was published in an academic journal. While it is agreed that the sequence of events described did occur, there is debate over the suddenness, dating and magnitude of the events. Two opposing hypotheses have arisen to explain the rise of the Black Sea: gradual, and oscillating.:15 The oscillating hypothesis specifies that over the last 30,000 years, water has intermittently flowed back and forth between the Black Sea and the Aegean Sea in relatively small magnitudes, and does not necessarily presuppose that there occurred any sudden "refilling" events.
In 1997, William Ryan and Walter Pitman published evidence that a massive flooding of the Black Sea occurred about 5600 BC through the Bosphorus, following this scenario. Before that date, glacial meltwater had turned the Black and Caspian Seas into vast freshwater lakes draining into the Aegean Sea. As glaciers retreated, some of the rivers emptying into the Black Sea declined in volume and changed course to drain into the North Sea. The levels of the lakes dropped through evaporation, while changes in worldwide hydrology caused overall sea level to rise. The rising Mediterranean finally spilled over a rocky sill at the Bosporus. The event flooded 155,000 km2 (60,000 sq mi) of land and significantly expanded the Black Sea shoreline to the north and west. According to the researchers, "40 km3 (10 cu mi) of water poured through each day, two hundred times the flow of the Niagara Falls... The Bosporus flume roared and surged at full spate for at least three hundred days."
Samplings of sediments in the Black Sea by a series of expeditions carried out between 1998 and 2005 in the frame of a European Project ASSEMBLAGE and coordinated by a French oceanographer, Gilles Lericolais, brought some new inputs to Ryan and Pitman's hypothesis. These results were also completed by the Noah Project led by Petko Dimitrov from the Bulgarian Institute of Oceanology (IO-BAS). Furthermore, calculations made by Mark Siddall predicted an underwater canyon that was actually found.
While some geologists claim it as fact that the sequence of events described did occur, there is debate over their suddenness and magnitude. In particular, if the water level of the Black Sea had initially been higher, the effect of the spillover would have been much less dramatic. A large part of the academic geological community also continues to reject the idea that there could have been enough sustained long-term pressure by water from the Aegean to dig through a supposed isthmus at the present Bosporus, or enough of a difference in water levels (if at all) between the two water basins.
Countering the hypothesis of Ryan and Pitman are data collected prior to its publication by Ukrainian and Russian scientists including Valentina Yanko-Hombach, who claims that the water flow through the Bosporus repeatedly reversed direction over geological time depending on fluctuation in the levels of the Aegean Sea and the Black Sea. This contradicts the hypothesized catastrophic breakage of a Bosporus sill. Likewise, the water levels calculated by Yanko-Hombach differed widely from those hypothesized by Ryan and Pitman.
In 2007, a research anthology on the topic was published which makes much of the earlier Russian research available in English for the first time, and combines it with more recent scientific findings.
A February 2009 article reported that the flooding might have been "quite mild".
According to a study by Giosan et al., the level in the Black Sea before the marine reconnection was 30 m below present sea level, rather than the 80 m, or lower, of the catastrophe theories. If the flood occurred at all, the sea level increase and the flooded area during the reconnection were significantly smaller than previously proposed. It also occurred earlier than initially surmised, ca. 7400 BC, rather than the originally proposed 5600 BC. Since the depth of the Bosporus, in its middle furrow, at present varies from 36 to 124 m, with an average depth of 65 m, a calculated stone age shoreline in the Black Sea lying 30 m lower than in the present day would imply that the contact with the Mediterranean may never have been broken during the Holocene, and hence that there could have been no sudden waterfall-style transgression.
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Although neolithic agriculture had by that time already reached the Pannonian plain, Ryan and Pitman link its spread to people displaced by the postulated flood. More recent examinations by oceanographers such as Teofilo A. "Jun" Abrajano Jr. at Rensselaer Polytechnic Institute and his Canadian colleague Ali Aksu of the Memorial University of Newfoundland have cast some doubt on this linkage. Abrajano's team, finding sapropel mud deposits in the Sea of Marmara which are today associated with freshwater outflow over top of salt-water inflow, have concluded that there has been sustained fresh water outflow from the Black Sea to the Mediterranean for at least 10,000 years. In 2003, Michael Sperling  concluded that the Black Sea was not a major freshwater source contributing to formation of the Marmara Sapropel S1. Aksu found an underwater delta south of the Bosporus; evidence for a strong flow of fresh water out of the Black Sea in the 8th millennium BC. Nevertheless, Erkan Gökaşan and later Kadir Eris demonstrated that the development of the delta is clearly associated with the Kurbağalı Stream on the east coast, and not with the Black Sea outflow through the strait.
In a series of expeditions, a team of marine archaeologists led by Robert Ballard identified what appeared to be ancient shorelines, freshwater snail shells, drowned river valleys, tool-worked timbers, and man-made structures in roughly 100 metres (330 ft) of water off the Black Sea coast of modern Turkey. Although radiocarbon dating of freshwater mollusc remains indicated an age of about 7,500 years, radiocarbon dating in freshwater molluscs in particular can be inaccurate. Such inaccuracies, however, are always in the direction of objects appearing older than they actually are (containing less 14C than expected), so the time given is a maximum age of a freshwater shoreline at that location.