MIXING BETWEEN OXIC AND ANOXIC WATERS OF THE BLACK-SEA AS TRACED BY CHERNOBYL CESIUM ISOTOPES

The Chernobyl nuclear power station accident in 1986 released readily measureable quantities of fallout Cs-134 and Cs-137 to Black Sea surface waters. This pulse-like input of tracers can be used to follow the physical mixing of the surface oxic waters, now labeled with the Chernobyl tracers, and the deeper anoxic waters, which were initially Chernobyl free. By 1988, there is clear evidence of Chernobyl Cs penetration below the oxic/anoxic interface at deep water stations in the western and eastern basins of the Black Sea. This rapid penetration of surface waters across the pycnocline cannot be explained by vertical mixing processes alone. Data from profiles at the mouth of the Bosporus suggest that significant ventilation of intermediate depths can occur as the outflowing Black Sea waters are entrained with the inflowing Mediterranean waters, forming a sub-surface water mass which is recognized by its surface water characteristics, i.e. initially a relatively high oxygen content and Chernobyl Cs signal. The lateral propagation of this signal along isopycnals into the basin interior would provide a rapid and effective mechanism for ventilating intermediate depths of the Black Sea. This process could also account for the lateral injection of resuspended margin sediments into the basin interior. The temperature and salinity data suggest that the entrainment process occurs at depths of 50-80 m, mixing waters from the Cold Intermediate Layer with the incoming, denser Mediterranean waters.