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Temporal variations in plutonium and americium inventories and their relation to vertical transport in the northwestern Mediterranean Sea
Author(s) -
Fowler Scott W.,
Noshkin Victor E.,
Jacques La Rosa,
Gastaud Janine
Publication year - 2000
Publication title -
limnology and oceanography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.7
H-Index - 197
eISSN - 1939-5590
pISSN - 0024-3590
DOI - 10.4319/lo.2000.45.2.0446
Subject(s) - water column , sediment trap , environmental science , radionuclide , flux (metallurgy) , particulates , plutonium , biogeochemical cycle , sediment , scavenging , nuclide , hydrology (agriculture) , atmospheric sciences , chemistry , environmental chemistry , oceanography , geology , radiochemistry , nuclear physics , paleontology , biochemistry , physics , geotechnical engineering , organic chemistry , antioxidant
Vertical fluxes of 239+240 Pu and 241 Am 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 ( 239+240 Pu between 100–400 m; 241 Am 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 239+240 Pu inventory had not substantially changed between 1976–1990 whereas 241 Am had decreased by approximately 24%. Enhanced scavenging of 241 Am and a resultant, more rapid removal from the water column relative to 239+240 Pu 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 239+240 Pu and 241 Am 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 239+240 Pu loss and virtually all of the 241 Am removal from the water column. Upper water column (0–200 m) residence times based on direct flux measurements ranged from 20‐30 yr for 239+240 Pu and 5‐10 yr for 241 Am. The observation that 241 Am/ 239+240 Pu 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 241 Am 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 241 Am to the sediments.