Black Sea deluge hypothesis

The Black Sea deluge is the best known of three hypothetical flood scenarios proposed for the Late Quaternary history of the Black Sea. One other flood scenario proposes a rapid, even catastrophic, rise in sea level of the Black Sea.

History
In 1997, William Ryan, Walter Pitman, Petko Dimitrov, and their colleagues first published the Black Sea deluge hypothesis. They proposed that a catastrophic inflow of Mediterranean seawater into the Black Sea freshwater lake occurred around 7,600 years ago, c. 5600 BCE.

As proposed, the Early Holocene Black Sea flood scenario describes events that would have profoundly affected prehistoric settlement in Eastern Europe and adjacent parts of Asia and possibly was the basis of oral history concerning the myth of Noah's flood. Some archaeologists support this theory as an explanation for the lack of Neolithic sites in northern Turkey. In 2003, Ryan and coauthors revised the dating of the early Holocene flood to 8,800 years ago, c. 6800 BCE.

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 global sea levels to rise.

The rising Mediterranean finally spilled over a rocky sill at the Bosporus. The event flooded 100000 km2 of land and significantly expanded the Black Sea shoreline to the north and west. According to these researchers, 50 km3 of water poured through each day, two hundred times the flow of Niagara Falls. The Bosporus valley roared and surged at full spate for at least 300 days. They argued that the catastrophic inflow of seawater resulted from an abrupt sea-level jump that accompanied the Laurentide Ice Sheet collapse and the ensuing breach of a bedrock barrier in the Bosporus strait.

Popular press accounts
Popular discussion of this early Holocene Black Sea flood scenario was headlined in The New York Times in December 1996 and later published as a book. In a series of expeditions widely covered by mainstream media, 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 m of water off the Black Sea coast of modern Turkey.

Late Pleistocene Great Flood hypothesis
In 2003 and 2007, a more ancient catastrophic flood scenario was proposed by Andrei L. Chepalyga for the Late Quaternary sea level rise of the Black Sea. The hypothesis for a Late Pleistocene Great Flood argues that brackish Neoeuxinian Lake, which occupied the Black Sea basin, was rapidly inundated by glacial meltwater overflow from the Caspian Sea via the Manych-Kerch Spillway shortly after the Late Glacial Maximum, about 17,000–14,000 BP. These extensive meltwater flooding events linked several lacustrine and marine water bodies, starting with the southern edge of the Scandinavian and southward, through spillways to the Manych-Kerch and Bosphorus, ultimately forming what has been referred to as the Cascade of Eurasian Basins. This event is argued to have caused a rapid, if not catastrophic, rise in the level of the Black Sea. It might have imposed substantial stresses upon contemporary human populations and remained in cultural memory as the Great Flood. The authors also suggested that the event might have stimulated the beginning of shipping and horse domestication.

Black Sea gradual inundation hypothesis
In addition to the early Holocene "Noah’s Flood" scenario proposed by Ryan, Pitman, Dimitrov, and their colleagues and the Caspian Sea overflow scenario of Chepalyga,  the non-catastrophic progressive flood model (or gradual inflow model) has been proposed to explain the Late Quaternary sea level history of the Black Sea.

About 8,000 YBP, the level of the Marmara Sea would have risen high enough for two-way flow to start. The evidence used to support this scenario includes the disparate ages of sapropel deposition in the eastern Mediterranean Sea and Black Sea; buried back-stepping barrier islands observed on the Black Sea shelf; and an under-water delta in the Marmara Sea, near the Bosporus Strait, composed of Black Sea sediments.

Counter arguments
Criticisms of the deluge hypothesis focus on the magnitude and pace of the water level rise in the Black Sea. With enough moderation of these features, the catastrophe hypothesis is voided. However, a few key points should be noted:
 * Since the ending of the last glacial period, the global sea level has risen some 120 m.
 * The flood hypothesis hinges on the geomorphology of the Bosporus since the end of the glacial age. The Black Sea area has been isolated and reconnected many times during the last 500,000 years.

Opponents of the deluge hypothesis point to clues that water was flowing out of the Black Sea basin as late as 15,000 years ago.

In this alternative scenario, much depends on the evolution of the Bosphorus. According to a study from 2001, the modern sill is 32 – below sea level and consists of Quaternary sand over-lying Paleozoic bedrock in which three sills are found at 80 – below sea level. Sedimentation on these sills started before 10,000 years ago and continued until 5,300 years ago.

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 Bosphorus or enough of a difference in water levels, if at all, between the two water basins.

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.

According to a 2009 study by Liviu Giosan, Florin Filip, and Ștefan Constatinescu, 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. Since the depth of the Bosphorus, 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 might never have been broken during the Holocene, and hence there could have been no sudden waterfall-style transgression. The flooding could have been "not so big".

In 2011, several authors concluded that "there is no underwater archaeological evidence to support any catastrophic submergence of prehistoric Black Sea settlements during the late Pleistocene or early Holocene intervals".

A 2012 study based on process length variation of the dinoflagellate cyst Lingulodinium machaerophorum shows no evidence for catastrophic flooding. Geophysical, geochronological, and geochemical evidence points to a "fast transgression" of the submergence lasting between 10 and 200 years.

A 2022 literature review concluded that there was insufficient evidence for a flood scenario. It was more likely that the waters of the Black Sea itself gradually outflowed to the Mediterranean. There was also no archaeological evidence of humans evacuating the area during the relevant time frame.