Excess of 236U in the northwest Mediterranean Sea

Abstract

In this work, we present first 236U results in the northwestern Mediterranean. 236U is studied in a seawater column sampled at DYFAMED (Dynamics of Atmospheric Fluxes in the Mediterranean Sea) station (Ligurian Sea, 43°25′N, 07°52′E). The obtained 236U/238U atom ratios in the dissolved phase, ranging from about 2×10−9 at 100m depth to about 1.5×10−9 at 2350m depth, indicate that anthropogenic 236U dominates the whole seawater column. The corresponding deep-water column inventory (12.6ng/m2 or 32.1×1012 atoms/m2) exceeds by a factor of 2.5 the expected one for global fallout at similar latitudes (5ng/m2 or 13×1012 atoms/m2), evidencing the influence of local or regional 236U sources in the western Mediterranean basin. On the other hand, the input of 236U associated to Saharan dust outbreaks is evaluated. An additional 236U annual deposition of about 0.2pg/m2 based on the study of atmospheric particles collected in Monaco during different Saharan dust intrusions is estimated. The obtained results in the corresponding suspended solids collected at DYFAMED station indicate that about 64% of that 236U stays in solution in seawater. Overall, this source accounts for about 0.1% of the 236U inventory excess observed at DYFAMED station. The influence of the so-called Chernobyl fallout and the radioactive effluents produced by the different nuclear installations allocated to the Mediterranean basin, might explain the inventory gap, however, further studies are necessary to come to a conclusion about its origin.