Temporal variations in plutonium and americium inventories and their relation to vertical transport in the northwestern Mediterranean Sea

Vertical fluxes of Pu239+240 and Am-241 and temporal changes in their inventories in the northwestern Mediterranean Sea have been examined through high-resolution water column sampling coupled with direct measurements of the vertical flux of particle-bound transuranics using time-series sediment traps. Water column profiles of both radionuclides showed well-defined sub-surface maxima (Pu239+240 between 100-400 m; Am-241 at 100-200 m and 800 m), the depths of which are a result of the different biogeochemical scavenging behavior of the two radionuclides. Comparison of deep water column (0-2,000 m) transuranic inventories with those derived from earlier measurements demonstrate that the total Pu239+240 inventory had not substantially changed between 1976-1990 whereas Am-241 had decreased by approximately 24%. Enhanced scavenging of Am-241 and a resultant, more rapid removal from the water column relative to Pu239+240 was also supported by the observation of elevated Am/Pu activity ratios in sinking particles collected in sediment traps at depth. Direct measurements of the downward flux of particulate Pu239+240 and Am-241 compared with transuranic removal rates derived from observed total water column inventory differences over time, show that particles sinking out of deep waters (1,000-2,000 m) could account for 26-72% of the computed total annual Pu239+240 loss and virtually all of the Am-241 removal from the water column. Upper water column (0-200 m) residence times based on direct flux measurements ranged from 20-30 yr for Pu239+240 and 5- 10 yr for Am-241. Th, observation that Am-241/Pu239+240 activity ratios in unfiltered Mediterranean seawater are six times lower than those in the north Pacific suggests the existence of a specific mechanism for enhanced scavenging and removal of Am-241 from the generally oligotrophic waters of the open Mediterranean. It is proposed that atmospheric inputs of aluminosilicate particles transported by Saharan dust events which frequently occur in the Mediterranean region could enhance the geochemical scavenging and resultant removal of Am-241 to the sediments.