10-17 



especially as the rate has not been constant over the past 100 years 



(Figure 10-10) . In addition, these researchers showed that the calculated 



Pb flux, as determined by the standing crop of unsupported Pb in 

 the salt marsh, equaled the atmospheric flux as determined for New Haven 

 by Benninyor (1978). This implies that:' (1) the traco metal distribu- 

 tion vertically in the salt marsh reflects the changing flux over time 

 and that no vertical migration of the trace metals is expected by diffusion 

 or biological activity; and (2) that there should be an atmospheric flux 

 of trace metals recorded in the growing salt marsh. Indeed the calcu- 

 lated flux of Cu, Pb and Zn (Figure 10-11) appears to be almost solely 



atmospheric as the predicted flux of these metals is in agreement with 



210 

 the estimated atmospheric flux (Table 10-1) . That both the Pb and trace- 

 metal fluxes to the salt marsh can be ascribed almost completely to an 

 atmospheric source is not surprising since the surface of a salt marsh, 

 on which Spartina alterniflora grows, is inundated by sea water only 

 about 5% of the time. The remainder of the time it behaves like an 

 atmospheric collector. 



The Intei'pretation of the Trace Metal Record in Long Island Sound Coves 



We can now test the following question: how much of the trace 

 metal content of Long Island Sound sediments may be explained by an 



atmospheric source? To answer the question, we integrate the total 



210 

 excess Pb m a core with the integrated excess metal content. Bennin- 



210 

 ger (1978) has shown that the Pb content in Long Island Sound sediment 



is due predominantly to atmospheric supply and that there is no loss 



from the Sound. We then compare the ratio of integrated trace-metal 



210 

 content to integrated Pb content found in a sediment core from the 



area under consideration to the ratio found by McCaffrey (1977) and 

 McCaffrey and Thomson (1979) in the salt marsh. This normalization 

 overcomes the problem of both horizontal and vertical mobility in the 

 sediment. When such a calculation is made for the Long Island Sound 

 long core (core 1148, Fig. 10-9) analyzed by Thomson, Turekian and McCaffrey 

 (1975) it shows that all the Pb could be explained as of atmospheric 

 origin, probably also most of the Zn and a smaller fraction of the 



