Distribution of 137 Cs and other radioactive tracers in the eastern Mediterranean: Relationship to the deepwater transient

Abstract

{[}1] The time evolution of Cs-137 distribution in the easternMediterranean in the period 1977 - 1999 is discussed in relation towater circulation and, in particular, to the Eastern MediterraneanTransient (EMT). Tritium data are used to reconstruct the spatialdistribution of Cs-137 before the EMT and the Chernobyl accident.Relatively high concentrations characterized the surface andintermediate waters of the whole basin, and the levels graduallydecreased toward the bottom. Tracer concentrations in the deep watersdecreased from west to east, reflecting the spreading of the core of theEastern Mediterranean Deep Water formed in the Adriatic Sea. In the1990s, a significant change was induced by the intrusion of new deepwater of Aegean origin in the bottom layers of the Ionian and Levantinebasins, marked by a relatively high Cs-137 concentration (around 2.5 Bqm(-3)). As a result, in 1999, the levels were almost homogeneous in thewhole deep Ionian Sea. In the past the ``oldest'' waters of the easternMediterranean were found around 1500 m, characterized by a tracerminimum layer. During the EMT this layer was displaced upward to 750 -1500 m and was also much thinner than previous. The analysis of verticalprofiles and ratios of Cs-137 and Sr-90 shows that a ``Chernobyl''contribution is present in the surface, intermediate and deep waters ofthe Ionian Sea, but not in the tracer minimum layer and in the deepAdriatic Sea. The analysis of tritium/ helium ages in this areaindicates a decreased convective renewal of basin waters in the period1987 - 1999, that also explains the absence of a Chernobyl signal.