NO. 1 EMERY AND HULSEMANN: SUBMARINE CANYONS 57 



cent of the samples. A best-fit straight h'ne through the plotted values 

 for these samples yields an average C/N ratio of 8.9, nearly the same 

 as the average for the surface sediments of the basins (Emery, 1960a, 

 p. 276). 



When plotted against median diameter, the nitrogen (Fig. 20) as 

 w^ell as the organic carbon exhibits a close relationship. Percentages of 

 nitrogen decrease from an average of about 0.4 per cent for sediments of 

 5 microns median diameter to less than 0.05 per cent for sediments of 

 median diameter coarser than 100 microns. This relationship to grain 

 size is typical and it results from the similarity in settling velocity of 

 organic matter and of fine-grained silts or clays and from adsorption 

 of organic matter on clay minerals. Average total organic matter is 2.16 

 per cent when computed from organic carbon (1.7 times the average 

 of 1.27 per cent organic carbon) and 1.87 per cent vv^hen computed from 

 nitrogen (17 times the average of 0.11 per cent nitrogen). Perhaps the 

 best figure for average total organic matter is the average of the two 

 values, or 2.0 per cent. 



Comparison with Sediments of Adjacent Areas 

 Sediments of the canyons reveal differences which depend upon the 

 degree of isolation from sources of detrital material. These differences 

 are best illustrated by a comparison of sediments from canyons cutting 

 the mainland shelf, the island shelves, and the bank tops (Table 3). 

 Most pronounced is an increase in average percentage of calcium car- 

 bonate from mainland canyons to island canyons to bank canyons. The 

 average median diameter exhibits little change, except for an increase 

 in Tanner Canyon, the only one off a bank. Percentage of organic matter 

 increases from mainland to island canyons probably because the slower 

 rate of deposition of similar average grain sizes of detrital sediment in 

 the latter permits less dilution of organic matter. 



When compared with sediments of the source areas (mainland shelf, 

 island shelves, and bank tops) and with those of the sites of final deposi- 

 tion (basin floors), the sediments of the canyons are found to be inter- 

 mediate in nearly all the averages (Table 3). Sediments of the canyons 

 are finer grained than those of the shelves and coarser than those of the 

 basin floors. Sorting coefficients are also intermediate, except at Tanner 

 Canyon where only six samples are available, most of which are coarse 

 grained. The average content of calcium carbonate also is intermediate 

 between values for shelf and basin sediments except for the mainland 

 canyons, which have a very low content for some unknown reason. Aver- 

 age contents of organic matter are intermediate in all instances. These 



