Title: Excess Freshwater Outflow from the Black Sea-Lake during Glacial and Deglacial Periods and Delayed Entry of Marine Water in the Early Holocene Require Evolving Sills
Description: The Black Sea becomes periodically isolated from the global ocean during each glacial period. This occurs when the elevation of the global ocean is lower than the Bosporus sill, putting a stop to inflow of salt water to the Black Sea. This phenomenon allows the Black Sea to evolve from a marine environment to a freshwater one. It is also evident that the depth of the Bosporus sill does not remain at the same elevation, and instead is dynamic. The sill becomes filled with sediments during periods of its sub-aerial exposure but is subsequently eroded to its bedrock during periods of outflow from the Black Sea-Lake to the global ocean. This interpretation comes from the observations that during the last glacial period, the Black Sea-Lake was in a positive hydrological balance, fresh, and predominantly outflowing to the global ocean over a deep Bosporus sill, at approximately 80 meters below sea level (mbsl). It is highly likely that there were brief periods when the lake froze and the outflow suspended, such as during the extreme stadial conditions associated with the North Atlantic iceberg-discharge Heinrich Event 2 (HE 2) at ~24 kyr before present, when there is also no evident carbonate accumulation in stalagmites that receive water from evaporated Black Sea surface water.

Upon the onset of deglaciation, large floods originating from the Fennoscandinavian Ice Sheet and the Alps, delivered meltwater so as to fully ventilate the Black Sea-Lake and even potentially replace all of the water in the basin. These floods occurred near the time of the deglacial iceberg-discharge Heinrich Event 1 (HE 1 at ~17 kyr before present), and left pulses of red-colored sediment everywhere on the western half of the Black Sea basin.