Temporal evolution of 137Cs, 237Np, and 239+240Pu and estimated vertical 239+240Pu export in the northwestern Mediterranean Sea

摘要

The evolution of Cs, Np and Pu at the DYFAMED station (NW Mediterranean) is discussed in relation to physical processes, downward fluxes of particles, and changes in the main input sources. The data set presented in this study represents the first complete Np vertical profiles (0.12–0.27 μBq L), and constitutes a baseline measurement to assess future changes. A similar behavior of Cs and Np has been evidenced, confirming that Np behaves conservatively. While the Cs decrease has been driven by its radioactive decay, the vertical distribution of Np has not substantially changed over the last decade. In the absence of recent major inputs, a homogenization of their vertical distribution occurred, partly due to deep convection events that became more intense during the last decade. In contrast, Pu surface levels in the NW Mediterranean waters have fallen in the past four decades by a factor of 5. This decrease in surface has been balanced by higher concentrations in the deep-water layers. A first estimate of the downward Pu fluxes in the NW Mediterranean Sea is proposed over more than two decades. This estimation, based on the DYFAMED sediment trap time-series data and published Pu flux measurements, suggests that sinking particles have accounted for 60–90% of the upper layer (0–200 m) Pu inventory loss over the period 1989–2013. The upper layer residence time of Pu is estimated to be ~ 28 years, twice as long as the residence time estimated for the whole western Mediterranean (~ 15 years). This difference highlights the slow removal of Pu in the open waters of the NW Mediterranean and confirms that most of the Pu removal occurs along the coastal margin where sedimentation rates are high.