86 



MERRILL AND DUCE 



TABLE 5 



Mean Atmospheric Concentrations 

 of Trace Metals at Enewetak* 



TABLE 6 



Estimates of Annual Atmospheric Deposition 

 of Trace Metals to the Ocean at Enewetak" 



more uniformly distributed throughout the year. Thus their 

 smaller change in concentration from spring to the rest of 

 the year may largely reflect the changes in atmospheric cir- 

 culation patterns for those time periods. 



From measurement of these trace metals in rain and 

 dry deposition, estimates can be made of their atmospheric 

 deposition to the ocean surface at Enewetak (Duce et al., 

 1981). The temfxsral variation in atmospheric concentra- 

 tions shown in Table 5, the monthly rainfall amounts at 

 Enewetak (Fig. 2a), and the measured concentrations of 

 these metals in rain and dry deposition were taken into 

 consideration when the total deposition rates given in 

 Table 6 were calculated. Note that the data in Table 6 

 suggest that both wet and dry deposition arc important for 

 all elements. There is evidence, however, that much of the 

 measured dry deposition of at least some of these metals 

 may be the result of metals being recycled from the sea 

 surface on sea salt aerosols (Duce, 1982; Settle and 

 Patterson, 1982). This would mean the dry deposition 

 values do not represent a net input of these metals to the 

 ocean. Thus the numbers presented in Table 6 probably 

 represent an upper limit relative to net inputs from the 

 atmosphere to the ocean. 



'From Duce et al , 1981. 



tEstimated from Al. 



tFrom Settle and Patterson, 1982 



HEstimated from average marine clay composition. 



§Sum of 0.3 authigenic plus 0.3 silicate lattice. 



"Estimated from Pb. 



Marine sedimentation rates for these metals are 

 presented in Table 6 and have been determined from the 

 chemical analysis of surface sediments collected near 29°N 

 159°W. An estimate of the overall sedimentation rate near 

 Enewetak was determined from mapping measured sedi- 

 mentation rates over the entire North Pacific (M. Leinen, 

 personal communication). Where the surface sediments 

 were not analyzed for a psarticular metal, crustal ratios to 

 Al were used for elements present in crustal abundance in 

 the atmosphere, and average marine clay composition was 

 assumed for the atmospherically enriched elements. 



It is apparent that the atmospheric deposition to the 

 ocean and the marine deposition to the sediments are very 

 close for Al, Fe, V, Sc, Cr, Eu, Cs, Th, Ta, Hf, Rb, and 

 Cu, suggesting atmospheric transport is very important for 

 marine sedimentation of these metals near Enewetak. 

 Atmospheric input accounts for only a small part of the 

 Mn and Co in the sediments. However, the atmospheric 

 input of Pb, Zn, Cd, Se, and Sb to the ocean is apparently 

 considerably greater than the deposition of these elements 

 to the sediments. There are at least two pxjssible explana- 

 tions for these latter results. First, this would be expected 

 if the atmospheric concentrations and deposition rates of 



