Pycnocline and Deep Mixing in the Black Sea: Stable Isotope and Transient Tracer Measurements

The peculiar mixing processes at the pycnocline and deep regions of the Black Sea are reviewed. In addition to the wind stirring and convective mixing, active in the upper pycnocline, the other important mechanism that results in limited ventilation of the anoxic waters of the Black Sea is the Mediterranean dense water inflow from the Bosphorus, modified by the entrainment of surface and intermediate waters, introduced into the interior through double-diffusive intrusions. This inflow, aided by the surface Ekman flux divergence, boundary processes and internal wave breaking, is the main mechanism for the mixing and renewal of the sub-pycnocline waters in the Black Sea interior. A review of these mixing processes is complemented by results from isotope measurements with improved accuracy and reduced noise compared to earlier experiments. Measurements of the stable isotopes oxygen-18 (18O) and deuterium (2H) confirm the origin of the water masses in the Black Sea and in the Turkish Straits, including the sea of Marmara, to be a continuous mixture with variable fractions of salty Mediterranean waters with inflowing fresh waters. Tritium (3H) measurements confirm very little penetration of the transient signal to the sub-pycnocline and deep waters of the Black Sea, in comparison to the better ventilated Mediterranean waters filling the lower layer of the Marmara Sea. The comparison of stable and transient tracer isotopes shows the effects of fresh waters originating from the north-western shelf, and the difference between the renewal mechanisms of the Marmara and Black seas, as well as those between the upper and lower pycnocline of the Black Sea.