Because the Bosporus is the only connection between the fresh to brackish Black Sea and the saline Mediterranean Sea (via the Sea of Marmara and the Dardanelles), it has played a critical role in controlling fluctuations in salinity, oxygen content, sedimentation, sea level, and life in the Black Sea during the Quaternary. The original Bosporus valley apparently was cut by a south-flowing river which continued through the Dardanelles and was partly controlled by north-south faults. Upon drowning, the valley became a marine strait, although at times of glacial lowering of sea level it acted as a marine spillway or resumed its subaerial-valley condition with a central drainage divide. Its bedrock channel eventually was cut to at least 100 m below present sea level.

During the marine-strait stage, the interface between the northward, saline bottom current and the southward, freshwater surface current slopes northward at an angle which is a function of the two densities and the hydraulic head. When sea level is only moderately high, the interface touches bottom south of the northern exit, and the saline bottom water is returned southward by the surface current. Only when sea level is higher than today can large quantities of Mediterranean water enter the Black Sea and bring salt and oxygen to the deep Black Sea bottom.

A marine spillway on the south would cause gradual desalinization of the Black Sea and permit euxinic conditions at the deep bottom. When the Bosporus becomes a stream valley, the Black Sea becomes a lake which, depending on climatic factors, could be saline or fresh and euxinic. If the "lake" bottom is lowered abruptly by tectonism, rapid sedimentation and oxygenation by frequent turbidity currents may occur.

These theoretical predictions can be tested by examining Black Sea bottom sediments and their interstitial waters, as well as fossils and shoreline terraces and deposits formed during the Quaternary. Tentative evidence suggests the hypothesis that, as a result of abrupt depression of the Black Sea bottom during Wurm II glaciation, sea level dropped several hundred meters, causing the north-flowing Bosporus stream to displace its head southward and to capture the old tributaries of the original south-flowing stream. This hypothesis would reconcile the evidence of initial southward drainage with Penck's argument that, in its later stages, the Bosporus river was the lower course of the south-flowing Kiathane River, which doubled back to the north via the Golden Horn. This hypothesis requ res further testing by deep-sea coring.