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

Abstract

The time evolution of 137Cs distribution in the eastern Mediterranean in the period 1977–1999 is discussed in relation to water circulation and, in particular, to the Eastern Mediterranean Transient (EMT). Tritium data are used to reconstruct the spatial distribution of 137Cs before the EMT and the Chernobyl accident. Relatively high concentrations characterized the surface and intermediate waters of the whole basin, and the levels gradually decreased toward the bottom. Tracer concentrations in the deep waters decreased from west to east, reflecting the spreading of the core of the Eastern Mediterranean Deep Water formed in the Adriatic Sea. In the 1990s, a significant change was induced by the intrusion of new deep water of Aegean origin in the bottom layers of the Ionian and Levantine basins, marked by a relatively high 137Cs concentration (around 2.5 Bq m−3). As a result, in 1999, the levels were almost homogeneous in the whole deep Ionian Sea. In the past the “oldest” waters of the eastern Mediterranean were found around 1500 m, characterized by a tracer minimum layer. During the EMT this layer was displaced upward to 750–1500 m and was also much thinner than previous. The analysis of vertical profiles and ratios of 137Cs and 90Sr shows that a “Chernobyl” contribution is present in the surface, intermediate and deep waters of the Ionian Sea, but not in the tracer minimum layer and in the deep Adriatic Sea. The analysis of tritium/helium ages in this area indicates a decreased convective renewal of basin waters in the period 1987–1999, that also explains the absence of a Chernobyl signal.