10-15 



of biologically mixed material and the deeper portion as reflecting the 

 increasing contamination of Long Island Sound sediments by human intro- 



duction of metals. Assuming no mixing below the top few centimeters, 

 they used the distribui 

 on the metal profiles. 



210 

 they used the distribution of Pb to date the core and put a chronology 



We now know that this was an oversimplified interpretation. 



210 

 Benninger et al (1979) have shown that the trace-metal and Pb data 



can be interpreted as due to diffusion-like biological mixing to depths 

 of 15 cm and episodic deep burrowing to depths of one meter or more. 

 Thus the exponential decrease of trace metals is the result of two 

 processes, one an increasing metal flux to the sediments with time, and 

 the other a redistribution based on a diffusion-like mixing process near 

 the top of the core. It is not possible to reconstruct the time- 

 scale of metal pollution accumulations without unravelling the contri- 

 butions of the sediment mixing processes. 



The Rote of Atmospheric Transport in Tvace Metal Input to 

 Long Island Sound 



The Historical Record in a Salt Marsh 



Unlike sediments in Long Island Sound, salt marsh deposits are 

 not subject to major bioturbation. Therefore, if they continue to grow 

 upward, each layer should preserve a record reflective of the deposi- 

 tional environment at that point in time. As sea level rises over 

 geologic time the saltmarsh elevation will grow upward to keep pace. 

 Tidal gauge data (Figure 10-10) show that during the past 100 years sea 

 level has been rising relative to the Connecticut coast at a rate of 

 about 2 mm per year. This is probably related to a world-wide rise in 

 sea level resulting from climate-induced changes on the earth's water 

 balance. 



McCaffrey (1977) and McCaffrey and Thomson (1979) have shown 



210 

 that the Pb (22 year half life) concentration at each level in a 



vertical profile of the salt marsh predicts the rate of sea level change 



