Excess of 236U in the northwest Mediterranean Sea

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 100 m depth to about 1.5 × 10^− 9 at 2350 m depth, indicate that anthropogenic ²³⁶U dominates the whole seawater column. The corresponding deep-water column inventory (12.6 ng/m² or 32.1 × 10¹² atoms/m²) exceeds by a factor of 2.5 the expected one for global fallout at similar latitudes (5 ng/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 0.2 pg/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.