Nutrient and metal data for Jamestown North core 1 1 (a short core 

 collected on 7/13/76) and 12 (a long core collected on the same date) are given 

 in Tables 2-1 and 2-2. Metabolite concentrations vary in the manner discussed 

 earlier. 



The metal concentrations are striking. Mn and Fe values are similar to values 

 reported elsewhere for anoxic sediments (11). Cu and Cd concentrations, on 

 the other hand, are very low, being comparable to or less than Bay bottom 

 values of about 2 ppb (Cu) and 0.2 ppb (Cd) at the Jamestown North location. 

 There is some scatter in the Cu and Cd data. This is in part due to 

 contamination. For example, the 58-60 cm sample in Jn-12 has Mn, Fe and Cd 

 values which are all higher than values in surrounding samples, apparently due 

 to contamination. On the other hand, there may be real variations, such as a Cu 

 maximum at 3-6 cm in JNl 1. Nevertheless, the main conclusion is clear: of the 

 four metals we analyzed, those forming relatively soluble sulfide (Mn and Fe) 

 gave pore water concentration far above ambient bottom waters, whereas those 

 forming liighly insoluble sulfides (Cu and Cd) gave concentrations comparable 

 to or less than bottom water values. We would not expect such sediments to 

 release Cu and Cd to the bottom waters at significant rates. 



MODELLING SEDIMENT-WATER EXCHANGE 



To understand how fluxes of constituents between pore and overlying 

 waters at Jamestown North depend on pore water and bottom water 

 distributions, McCaffrey et al (8) constructed a simple model in which we 

 consider both exchange mechanisms — simple diffusion (transport along 

 gradients due to the thermal motion of ions and molecules) and advection 

 (transport in water which is moving as a result of the irrigation, feeding and 

 burrowing activities of the benthic fauna). 



Conceptually, the diffusive flux is easy to calculate — it is simply the 

 product of a diffusion coefficient and a concentration gradient. Diffusion 

 coefficients in Narragansett Bay sediments at 25^C were taken as half the value 

 at 25°C in deionized water. Concentration gradients are not well known: the 

 gradients are very steep in the top 1 centimeter and zero below, and the pore 

 water concentrations do not allow us to accurately estimate the gradients at 

 the interface. We have assumed that the concentration near the interface is as 

 sliown in Figure 2-2. Diffusive fluxes were calculated from the product of the 

 gradient and the diffusion coefficient. 



Aller (1) and others have stressed the importance of organisms in 

 sediment-water exchange. In calculating advective fluxes, it is necessary to 

 know the rate at which organisms move water. The activity of organisms may 

 either be modelled as a random, or "biodiffusion" process, or as an ordered, or 

 "biopumping" process. Since organisms pump water into the sediment to 



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