outcrops on canyon walls; and (d) winnowing and re-suspension of fine sediment 

 from sand, silt, and clay deposited on canyon floors. 



Deposition of sediment occurs as follows: (a) sand, silt, and clay on 

 the upper slope below 300 m; (b) fine sediment around canyon heads; (c) sand 

 (containing little fine sediment) on the floors of large canyons; and (d) 

 sand, silt, and clay on the floors of medium and small canyons. 



The physical environment of the canyon is reflected in the fauna. The 

 largest canyons contain the most heterogenous bottom (habitat) types, and the 

 epibenthic fauna there exhibits the highest diversity and greatest biomass. 

 Medium canyons are less diverse, and the fauna of the smallest canyons 

 resembles that of the upper slope due to the absence of hard substrate for 

 attached organisms. However, at deeper depths, these smaller canyons may 

 support high concentrations of "canyon indicator" taxa. This is true, for 

 example, in Heezen Canyon off Georges Bank and Hendrickson Canyon off New 

 Jersey. 



Sediment Resuspension and Potential for Pollutant Transport on Particles 



The processes of sediment resuspension are more frequent and much more 

 intense in the axes of Lydonia and Oceanographer Canyons than on the adjacent 

 continental shelf, or at comparable depths of the continental slope. Sediment 

 traps placed 5 m above the bottom in the canyon axis collected about 8 and 60 

 times more sediment than on the continental shelf and slope, respectively. 

 This greater resuspension activity in canyons increases the opportunity for 

 particles to adsorb and transport dissolved contaminants from the water column 

 to the bottom sediment. Measurements of lead-210 and plutonium-239/-240 

 distributions in sediment cores from the axis of Lydonia Canyon and from the 

 open continental slope (both locations at the 630-m water depth) support this 

 hypothesis. The behavior of these isotopes in the marine environment, by 

 virtue of their affinity for particulates, is analogous to the behavior of 

 many contaminants. Both isotopes have inventories (disintegrations per minute 

 per cm^) that are 2.5 times higher in the canyon axis than on the slope. 



96 



