The opening of the connection between the Mediterranean and Black Seas as sea level rose above the Bosporus sill has long been associated with the formation of the most recent, Holocene, sapropel deposit (S1) in the eastern Mediterranean, but the mechanism has remained elusive. We present a model for the opening of the Black Sea, based on hydraulics arguments, which demonstrates that increased freshwater flux out of the Black Sea began 500--1000 years after sea level reached sill depth and that the timescale for the increased freshwater flux that drained the freshwater reservoir of the Black Sea is about 2500--3500 years. We argue that the increased freshwater discharge out of the Black Sea would lead to decreased deep water formation and higher productivity in the surface waters in the eastern Mediterranean, two conditions generally associated with sapropel formation. The delay in increased freshwater flux after the opening of the Black Sea and the period of increased freshwater discharge appear to match the onset and duration of sapropel deposits in the eastern Mediterranean.¿ 1997 American Geophysical Union