Nutrient and metal data for Jamestown North core 11 (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 JN11. 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 highly 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 
shown in Figure 2-2. Diffusive fluxes were calculated from the product of the 
gradient and the diffusion coefficient. 
Allei (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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