Vertical and horizontal fluxes of plutonium and americium in the western Mediterranean and the Strait of Gibraltar

New data on the vertical distributions of plutonium and americium in the waters of the western Mediterranean and the Strait of Gibraltar are examined in terms of the processes governing their delivery to, transport in and removal from the water column within the basin. Residence times for plutonium and americium in surface waters of ;15 and ;3 years, respectively, are deduced, and it is shown that by the mid 1990s only ;35% of the 239,240Pu and ;5% of the 241Am deposited as weapons fallout still resided in the water column. Present 239,240Pu inventories in the water column and the underlying sediments are estimated to be ;25 TBq and ;40 TBq, respectively, which reconcile well with the time-integrated fallout deposition in this zone, taken to be ;69 TBq. The data show that there are significant net outward fluxes of plutonium and americium from the basin through the Strait of Gibraltar at the present time. These appear to be compensated by net inward fluxes of similar magnitude through the Strait of Sicily. Thus, the time-integrated fallout deposition in the western basin can be accounted for satisfactorily in terms of present water column and sediment inventories. Enhanced scavenging on the continental shelves, as evidenced by the appreciably higher transuranic concentrations in shelf sediments, supports this contention.