We estimate that about 12 TBq y− 1 of 137Cs presently leaves the Black Sea with the upper layer flow through the Bosphorus and only 2 TBq y− 1 is returned with the lower layer inflow of Mediterranean water from the Marmara Sea. Accounting for river fluxes, estimated on the order of 2 TBq y− 1 few years after the Chernobyl accident, and possibly decreased by now, we can thus estimate a net rate of loss of about 8–10 TBq y− 1.
Investigating the effective redistribution in the upper water column, the supply by the inflowing Mediterranean water alone does not explain the increase of 137Cs concentration and inventory at intermediate depths in the Western Black Sea. The most important mechanism transferring 137Cs and dissolved contaminants from the surface water to the sub-pycnocline layer appears to be the turbulent entrainment of a larger quantity of Black Sea water into the inflowing plume of Mediterranean water through mixing processes on the southwestern shelf and continental slope following its exit from the Bosphorus. This process produces an extra export of some10 TBq y− 1 of 137Cs from the surface to the sub-pycnocline depths of the Black Sea, a quantity comparable in magnitude to the total export out from the basin. It is the entrainment flux resulting from the mixing, and the further advection and penetration of this water into the Black Sea deeper layer (200–600 m) that seems to maintain the inventory with little change over time. Through these two processes the Black Sea surface layer (0–50 m) loses every year about 4% of its total inventory of 137Cs.