ans2 The Black Sea so named for the colour of its deeps. It is a stratified sea, with significant anoxic water below the surface. The surface is largely fresh water, due to the net outflow in modern times.

The Deluge Thesis is one which considers that the Black Sea was a much smaller fresh water lake for some time after the last ice age.

Possibly about 5600 BC the rising sea levels allowed the Mediterranean Sea to overtop the land bridge and flood into the lake. [As had happened much earlier to the Mediterranean itself at Gibraltar.] There are certainly traces of older lake shorelines underwater in the Black Sea.

Some have argued that this impressive deluge was the origin of the Noah's Ark and similar tales.

Similar tales arise in other cultures, and an easier explanation, is that the observation of shell fossils at great elevations demands an explanation, and a great deluge or flood is the simple explanation.

[In Ice Age conditions, the world's oceans would have been much lower, and there would have been much less water flowing in the rivers. The climate in ice age conditions is cold and dry.]