The deep-water sediments deposited during the last 25,000 years in the Black Sea are distinguished by three sedimentary units: unit 1--carbonate-rich microlaminated sediment consisting mainly of the coccolithophore Emiliania huxleyi, commonly about 30 cm thick, deposited within the last 3,000 years; unit 2--organic-rich microlaminated sediment commonly about 40 cm thick, deposited between 3,000 and 7,000 years B.P.; and unit 3--alternately light and dark lutite, not completely penetrated by any of our cores, deposited between 7,000 and at least 25,000 years B.P.

Along the Anatolian coast and in the eastern part of the Black Sea, this sequence is interrupted in places by turbidites or extended as a result of a relatively high terrigenous sedimentation rate. Generally, the sedimentation rates ranged from about 40 to 90 cm/1,000 years during the deposition of unit 3 (7,000 to at least 25,000 years B.P.), and averaged about 10 cm/1,000 years during the deposition of units 1 and 2 (from the present to 7,000 years B.P.). These changes in sedimentation rate coincide with major changes in the environmental conditions of the Black Sea. Unit 3 was deposited during a period when the Black Sea was changing (because of Pleistocene sea-level changes) from a marine to a freshwater aerobic, lake, reaching the latter about 22,000 years B.P. The lake phase las ed about 12,000-13,000 years. Sedimentation rates were high during this time interval, but started to decrease as sea level rose during the last 5,000 years of this phase (9,000-14,000 years B.P.). At about 9,000 years B.P. and continuing to 7,000 years B.P., seawater occasionally spilled over the Bosporus and entered the Black Sea. The entrance of this water began the change from the freshwater well-aerated phase to the marine stagnant phase. At about 7,000 years B.P. the H₂S zone was well established in the deep basin, and deposition of unit 2 began there. On the basin slope the deposition of unit 2 began about 6,200 years B.P., because of the rising H₂S-oxygen boundary. Sedimentation rates dropped as a result of rising sea level in the Black Sea, which permitted iver-carried sediment to be trapped in the estuaries or on the shelf. About 3,000 years ago the conditions that presently characterize the Black Sea were reached.

The three sedimenatray units can be correlated with the stratigraphic zonations established by Russian workers.