sediment, where barium concentrations increased only about 35 percent above 

 pre-drilling levels; maximum concentrations were 380 ppm. Near the drilling 

 rig in Block 410, the barium concentrations in the fine fraction of bottom 

 sediments reached 10,000 ppm in post-drilling samples. 



TRACE METALS AND RADIONUCLIDES 



Twelve metals were measured in bottom sediments at 15 stations on the 

 continental slope off Georges Bank, and all were found at levels expected for 

 uncontaminated fine grained sediment (Bothner and others 1987b). Lead was the 

 only metal showing slight enrichment in the surface sediments within the 

 cores, compared to deeper sediments. This trend is seen in many off-shore 

 areas and is thought to reflect the use of lead in gasoline and industry. 



The metal concentrations in sediments from a station in Lydonia Canyon 

 and from the adjacent slope (both at 550-m water depth) were compared as a 

 test for differences in scavenging. The analysis was carried out on only the 

 sediment fraction finer than 60 microns. The heavy metal concentrations were 

 divided by Aluminum concentrations in order to normalize for textural 

 variability (Figure 14). For perspective, the resulting concentration ratios 

 were compared to those in world average shales (Krauskopf 1967). 



Four metals (Figure 15) showed a consistently higher enrichment factor in 

 the canyon axis compared to the slope in each of the three sampling periods. 

 This is taken as supportive evidence, although certainly not dramatic, that 

 the potential for scavenging is greater in the canyons. 



The hypotheses for greater scavenging in the canyons is more clearly 

 supported by the distribution of radioisotopes plutonium-239/-240 and lead- 

 210. Plutonium has been introduced to the ocean surface from nuclear weapons 

 testing in the atmosphere, and had a peak fallout about 1963. Lead-210 is 

 continuously generated from the natural decay of uranium. It has a half-life 

 of 22.3 years and is introduced to the ocean both from the atmosphere and from 

 in situ decay of dissolved radium-226. 



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