Data from two box cores show that the inventories of these isotopes 

 (activity/cm^) are 2.5 times higher in the sediments of the canyon axis than 

 they are on the open slope. The specific activities (activity/g) are much 

 higher in the canyon axis as shown in Figures 15 and 15. These observations 

 are a clear indication that the sediments in the canyon axis are accumulating 

 more of these sediment-reactive isotopes as a result of the collectively more 

 active processes operating in the canyon. If a correction were made for the 

 percentage of fine sediment, the relative enrichment in the canyon would be 

 even greater. 



The depth of penetration of these isotopes is a result of extensive 

 bioturbation in these sediments. This conclusion is based on the slow rates 

 of sediment accumulation in both areas and the presence of measurable 

 Plutonium and excess lead-210 activities at depths much greater than can be 

 explained by the sedimentation rate. Preliminary estimates of the rates of 

 sediment mixing by organisms suggest nearly the same mixing coefficients for 

 both locations (about 1 cmVy>^; Bothner and others 1987a). Biological mixing 

 of the sediments is another mechanism that influences the retention of 

 contaminants in sediments. 



SUMMARY 



The data we have collected in Lydonia Canyon suggest that it has higher 

 potential as a sink for contaminants than adjacent areas of the continental 

 shelf or slope. The most compelling evidence for greater scavenging in this 

 canyon is found in the limited data showing inventories and specific 

 activities of plutonium and lead-210. The possible mechanisms for the 

 enhanced scavenging are higher rates of sediment accumulation and more intense 

 and frequent sediment resuspension. 



Of the canyons in the North Atlantic OCS study area, Lydonia is the best 

 studied in terms of physical and geochemical measurements. While additional 

 information is needed in Lydonia Canyon to confirm some of the processes 



48 



