A tale of two seas
The Bosphorus Strait is a 30-kilometer-long channel of water with a secret. On the surface, a swift current flows south, carrying less-salty water from the Black Sea into the Sea of Marmara. But beneath this, a second, hidden current flows in the opposite direction. This dense, salty undercurrent travels north from the Mediterranean basin, hugging the seabed like a submarine river. This two-layer system is driven entirely by differences in water density.
The Black Sea receives vast amounts of freshwater from major rivers like the Danube and Dnieper, making its surface water brackish, with a salinity of about 18 practical salinity units (psu). The Mediterranean water entering the strait is much saltier due to high evaporation, registering around 38 psu. The less dense Black Sea water stays on top, while the heavier Mediterranean water sinks and flows northward along the bottom. The volume of this exchange is immense; the upper layer carries an average of 600 cubic kilometers of water per year southward, while the lower layer transports about 300 cubic kilometers northward.
A river on the seafloor
The salty undercurrent is a powerful flow of water. It behaves as a powerful, channelized flow, an underwater river complete with its own banks and meanders. This submarine channel was carved by the current itself and can reach speeds of over a meter per second. The flow is governed by the strait's complex underwater topography, which includes two sills—underwater ridges—at the northern and southern entrances. These sills act as hydraulic controls, causing the current to accelerate as it passes over them.
This deep, salty inflow is the only source of ventilation for the deep waters of the Black Sea. Without it, the entire basin would be devoid of oxygen. Even with this inflow, the Black Sea remains the world's largest anoxic basin, with over 87% of its volume being a "dead zone" containing no oxygen and high levels of hydrogen sulfide. This anoxic layer starts at a depth of about 90 to 160 meters, confining most marine life to the upper, oxygenated waters. The steady supply of denser Mediterranean water is therefore maintains the delicate chemical balance that supports life in the upper Black Sea.