Plutonium oxidation states in the southwestern Black Sea: Evidence regarding the origin of the cold intermediate layer

Water column profiles of reduced Pu (III + IV) and oxidized Pu (V + VI) species at two stations in the southwestern Black Sea reveal that oxidized Pu predominates even at the suboxic zone, between 50 and 100 m. Reduction of these oxidized species occurs at the base of the suboxic zone, where we noted a shift to the predominance of reduced Pu forms. At anoxic depths (below 150 m), we found only reduced Pu forms, at concentrations significantly less than the total Pu levels we found in the upper 150 m samples. Our data show that Pu oxidation states are useful in interpreting lateral advective processes that occur within the upper 150 m of the Black Sea water column. An additional interesting result of our measurements is that water at the cold intermediate layer (CIL) of the Black Sea is labeled with predominantly oxidized Pu. We believe this feature can be used to trace the origin of CIL water.