is serious but might be resolved in shallow water areas through the use 

 of research submersibles. Near-bottom sensors and samplers might be 

 set in place and the accumulation of sedimentary material be observed 

 without washing out or exchanging with nearby sediments. If truly 

 representative samples could be obtained, and if the rate of sedimenta- 

 tion could be inferred from the collected material, the pathways of the 

 pollutants might be elucidated. 



PARTICULATE MATTER 



A first-order feature of the density field in the ocean is the shallow 

 pycnocline. This boundary coincides approximately with the bottom of 

 the photic zone and forms a barrier to mixing between the upper few 

 hundred meters and the deep sea. The water above the boundary is 

 the immediate recipient of all the continentally derived dissolved and 

 particulate material. Particles introduced or formed in this layer can 

 sink through the mixing barrier (a one-way process) into the deep water, 

 thereby depleting the surface water of their constituents. 



At the present time the particulate flux in the ocean is similar in many 

 respects to the "ether" of 19th century physics in that its nature is more 

 described by the eflFects attributed to it than by experimental observation. 

 These observational data on dissolved trace constituents are explained 

 in terms of scavenging. 



One of the central problems of marine geochemistry is to resolve by 

 direct measurement the source and composition of the particles con- 

 stituting the nepheloid layers as functions of space and time. The main 

 study areas can be categorized as follows: 



• Physics — Size frequency distribution 



— Density contrast with water 



— SettUng rates (Stokesian or not) 



— Effect of turbulence and shear. 



These factors determine the vertical flux of material and its concentra- 

 tion in the water column and how much water has to be collected to 

 obtain a statistically significant sample. It is necessary to calculate 

 reasonable sedimentation rates from measured fluxes to determine the 

 available surface area for reaction and the transit time through the 

 water column. 



• Chemistry — Identification of organic and inorganic phases and 



their temporal and spatial variations 

 — Radioisotope analyses to determine injection and 

 settling rates directly (Pb-210, Fe-55, Si-32, C-14, 

 Ra-226, Pu-238 and 239) 



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