How long does it take for Black Sea water to be completely recycled?

It is estimated that a complete recycling of the Black Sea's waters takes approximately 2,500 years. This slow rate of exchange is primarily due to the limited water movement through the Bosporus, which serves as the sea's connection to the Sea of Marmara and the wider ocean system.1

Furthermore, the vertical intermixing of water within the Black Sea is restricted by its stratified nature, meaning that bringing water from the depths to the surface can take hundreds of years. This weak ventilation of the deep layers is also responsible for the lack of oxygen below the upper water levels, creating a "dead" zone contaminated by hydrogen sulfide.12