Status of the Black Sea Flood Hypothesis

Exploration of the Black Sea shelf reveals two major shelf-crossing unconformities. The older unconformity separates mostly-barren deposits of late glacial age from overlying Neoeuxinian sediment containing fresh to brackish fauna. This unconformity can be traced over the shelf edge to depths beyond –140 m. The substrate below is dry, firm, and contains unchallenged evidence of subaerial exposure at least to depths of –110 m. The Neoeuxinian cover is present on the outer shelf and is preserved, though incompletely, in depressions on the middle and inner shelf. It is even found as subsurface valley fill in the coastal limans and Sea of Azov. The Neoeuxinian on the shelf represents a transgression leading to a highstand at ~ –20 m below today’s sea surface, which was reached by 10,000 BP (uncorrected). Sediments with marine fauna lie above the Neoeuxinian and are separated from it by a sand to gravel layer that represents a younger unconformity. In the limans, the hiatus between the Neoeuxinian and overlying Bugazian is called “peririf” and on the shelf a “washout.” Dune fields between –65 and –80 m and wave-truncated terraces with beach-like berms at –90 to –100 m contain shell material dated between 9500 and 8500 BP, suggesting that the younger unconformity represents a post-Younger Dryas regression that took the surface of the Black Sea’s lake below the level of the global ocean. Strontium isotopes document the first arrival of saltwater at 8400 BP. Objections to the rapid flooding hypothesis in which Mediterranean water initially poured into a low-lying enclosed lake are centered on the interpretation of the younger unconformity as evidence of either (1) subaerial erosion (and thus a major early Holocene regression) or (2) underwater erosion that does not require a regression. When examined, specific criticisms appear to be based on different interpretations of observations but do not as yet present a concrete refutation of a lowstand of the lake prior to the Mediterranean connection. The flooding hypothesis is today just as vulnerable as when it was first formulated. It serves to best account for the ubiquitous nature of the younger unconformity that not only appears in sediment cores but is also widely mapped by high-resolution reflection profiling. Greater attention needs to be paid in the future to a more comprehensive investigation to find the cause of the younger unconformity.