THE FLUX AND COMPOSITION OF RESUSPENDED SEDIMENT 



IN TWO SUBMARINE CANYONS FROM THE WESTERN NORTH ATLANTIC: 



IMPLICATIONS FOR POLLUTANT SCAVENGING 



by 



Michael H. Bothner 



U.S. Geological Survey 



Woods Hole, MA 02543 



ABSTRACT 



Sediment traps were used to estimate the flux of resuspended 

 sediments in Lydonia and Oceanographer Canyons and on the adjacent U.S. 

 North Atlantic continental shelf and slope. The axes of both canyons are 

 sites of much more resuspension activity than are areas of comparable 

 depth outside of the canyons. The highest resuspended flux (157 g/m^/day) 

 was measured in Lydonia Canyon axis 5 m off the bottom at 300 -m water 

 depth. At the head of Lydonia Canyon, in water depths of 100 to 125 m, 

 the variability in the flux and texture of the trapped sediment correlates 

 with the timing and strength of major storms. Sediments trapped deeper 

 along the canyon axis show textural differences over time that are 

 similar to those observed at the head of the canyon, but the record is 

 more complicated. The greater variability in sediment textue at deep 

 sites suggests that additional processes, such as internal waves, cause 

 resuspension in the canyons. 



The hypothesis that shelf -derived material is being transported into 

 Lydonia Canyon is supported by two recent data sets. First, the 

 concentration of barium, a major element in drilling mud, increased in the 

 resuspended sediment collected in Lydonia Canyon during the period in 

 which eight exploratory wells were drilled on Georges Bank. Second, 

 carbon-14 dating of piston cores from the head of Lydonia Canyon 

 indicates that sediment is accumulating at an average rate of 60 cm/1, 000 

 yr. The core locations are down gradient and down current from the 

 southern flank of Georges Bank. 



The more intense resuspension and higher accumulation rates in this 

 canyon, as compared to those on the adjacent continental slope, suggests 

 that the canyon sediments have a higher potential for the adsorption of 

 sediment-reactive pollutants. This hypothesis is based on the 

 observations that sediments in the canyon axis have higher specific 

 activities and higher inventories of lead-210 and plutonium-239 , 240 than 

 sediments on the continental slope. In addition, concentration ratios 

 of Cd/Al, Cr/Al , Cu/Al , and Pb/Al are higher in surface sediments of the 

 axis of Lydonia Canyon than they are on the adjacent slope. 



Frequent resuspension of fine-grained sediments in canyons 

 increases the opportunity for particulates to adsorb dissolved materials 

 and strip them from the water column. This process may make some canyons 

 a more effective sink for pollutants than the open slope. 



B-9 



