TRANSURANIC RADIONUCLIDES IN MARINE ENVIRONMENT 533 



the plutonium is being retained by alumina; the remainder is fixed to glass-fiber prefilters 

 with an effective pore size of 0.3 ^im. More recently, Holm et al. (1977) measured the 

 particulate plutonium and americium retained by 0.45-/um MilUpore filters from open 

 ocean surface Mediterranean waters (Tables 1 and 2). Substantially lower fractions of 

 plutonium were found on the filters than was reported by Livingston and Bowen or by 

 Silker. The average percent of plutonium retained for 14 samples (1700 to 7680 liters) 

 was only 3.8 ± 0.2%. By contrast, the average percent of the ^^'Am retained by the 

 filters was 10 ± 1%. 



That fractionation of ^'*'Am and plutonium isotopes is occurring in the water 

 column is inferred from tlie profile data of Livingston and Bowen (1976a) by the 

 frequency with which the ^^ ' Am/*^^"^''^^°Pu ratio in deep waters from the Atlantic 

 exceeds those ratios observed either on land (Krey et al., 1976) or in coastal sediments 

 (Livingston and Bowen, 1976a). This ratio at present is calculated to be 0.22, which is in 

 good agreement with undisturbed soil measurements (0.22 to 0.25) and shallow coastal 

 sediments (0.20) (Krey et al., 1976; Livingston and Bowen, 1976a). Deep-water samples 

 from the North Atlantic, however, appear to have '^^ Kml'^'^'^^^Vxx ratios significantly 

 different (higher) from this range of values. Profile data from the North Pacific are fewer 

 in number but do not appear to evidence the same trend. Although the actual mechanism 

 for this apparent fractionation is yet to be proven, it is conceivable, as suggested by 

 Livingston, Bowen, and Burke (1976), that ^^' Am is preferentially more associated with 

 inorganic particles than are plutonium isotopes. Certainly the data from the Mediter- 

 ranean make the hypothesis an attractive one. Fukai, Ballestra, and Holm (1976) have 

 shown that ^'^'Am is depleted in Mediterranean surface waters relative to ^^^'■^^^Pu. 

 The mean ratio was 0.055 ± 0.007 from nine stations throughout the Mediterranean (July 

 to September 1975), a value significantly different from 0.22. From core samples taken 

 from the Mediterranean by Livingston, Bowen, and Burke (1976), the surface-sediment 

 ratios averaged 1.2 ±0.4 and are equaled only by values from the northeast South 

 American slope sediments off the Guiana coast (0.7 to 1.2). The Millipore filter data of 

 Holm et al. (1977) shown in Tables 1 and 2 evidence a distinct enrichment of 

 ^'^^ Am/^^^"^'*'^Pu; this ratio in unfiltered seawater is 0.055 ±0.007, whereas the same 

 ratio on 0.45-/jm filters is 0.13 ±0.05 (la level). 



It is well known that the Mediterranean has a high proportion of terrigenous detritus 

 in its waters and sediments (Emelyanov and Shimkus, 1972), and biological productivity 

 is known to be low (Brouardel and Rink, 1956). These facts, coupled to the data already 

 in hand from the Mediterranean basin, appear to us to be increasingly compelling 

 evidence that, in fact, fractionation of ^^^Am and plutonium isotopes can occur, 

 depending on the nature of the particulate matter in the water column. Slightly damaging 

 to the argument, however, is the fact that ^"^ ' Am/'^'^^''^'**^Pu ratios in open-ocean 

 plankton appear to be very nearly those observed for the mixed-layer waters (Livingston 

 and Bowen, 1976b). The same appears to be true iox plankton samples taken from Lake 

 Michigan (Wahlgren et al., 1976). One piece of critical information that is still missing, 

 however, is the measurement of '■*' Am/^^^'^'*°Pu ratios in zooplankton metabolic 

 particulate products (molts and fecal pellets), which would be invaluable in helping to 

 clarify this question. 



That molts and fecal pellets from zooplankton can transport plutonium to depth has 

 been clearly shown by the recent laboratory data of Higgo et al. (1977), in which 

 239,240p|j nieasurements on the molts and fecal pellets of the euphausiid Me^a/n'cr/- 



