Anthropogenic ²³⁶U and ¹²⁹I in the Mediterranean Sea: First comprehensive distribution and constrain of their sources

The first basin-wide distribution of ²³⁶U/²³⁸U atom ratios and ¹²⁹I concentrations is presented for the Mediterranean Sea. During the GEOTRACES GA04S-MedSeA expedition in 2013 seawater was collected from 10 vertical profiles covering the principal sub-basins of the Mediterranean Sea. The main objective was to understand the distributions of ²³⁶U and ¹²⁹I in relation to the water masses, and to constrain their sources in this region. The ²³⁶U/²³⁸U atom ratios and the ¹²⁹I concentrations ranged from (710±40)×10^-12 to (2220±60)×10^-12 and from (4.0±0.1)×10⁷ to (13.8±0.3)×10⁷at·kg^-1, respectively. The results show that radionuclide-poor Atlantic Water is entering at the surface through the Strait of Gibraltar whereas comparably radionuclide-enriched Levantine Intermediate Water is sinking in the Eastern Basin and flowing westward at intermediate depths. Low radionuclide levels were found in the oldest water masses at about 1000-2000m depth in the Eastern Basin. At greater depths, waters were relatively enriched in ²³⁶U and ¹²⁹I due to dense water formation occurring in both, the Eastern and Western Basins. The inventories of ²³⁶U and ¹²⁹I cannot be explained only by global fallout from atmospheric nuclear bomb testings carried out in the 1950s and 1960s. We estimate that the liquid input of ²³⁶U from the nuclear reprocessing facility of Marcoule (France), via the Rhône river, was of the same order of magnitude than the contribution from global fallout, whereas liquid and gaseous releases of ¹²⁹I from Marcoule were up to two orders of magnitude higher than global fallout. For both radionuclides, the contribution from the Chernobyl accident is found to be minor.