406 



LAUGHTON 



[CHAP. 18 



It is evident that the higher percentage of disturbance on the continental 

 sl()i)e and seamounts reflects the larger po])nlati()ns in shallower water. Particu- 

 larly low ])ercentage disturbance was found on the red-clay sediments in the 

 deep basins. 



The vertical diffusion of surface material can be estimated in the following 

 way. Assume that within an area A, an area a is disturbed so that the surface 

 material is uniformly mixed to a dei)th z. If mo is the mass of surface material 

 per unit area before disturbance, then the mean concentration c of surface 

 material at a depth z after n such disturbances is given by 



at 2 



c/wo = ya„lznA. 



(1) 



^^ 



Fig. 37. 41" 18'N, 14° 23..5'W. Depth 5341 in. Iberia abyssal plain. Area of picture 0.7 by 

 0.7 m. (Photo by N.I.O.) 



A mound bearing the imprint of a star and tracks radiating from a point possibly 

 made by repeated excursions of the tentacles of a buried animal in search of food. 



The summation can be carried out over the four ty])es of disturbance des- 

 cribed above, each of which can be given a series of values of ::„ and ((„ estimated 

 from the photographs of the bottom and from cross-sections of cores. Only 

 surface disturbances with dimensions greater than 3 cm were used in the 

 analysis and a constant term, used for the disturbance by internal galleries, 

 was estimated from sections of cores. This term provides the major contribution 

 to disturbance. 



The results of this analysis for three deep-basin stations are showni in Fig. 38 

 at depth intervals of 0.1 ft (3 cm). It will he seen that the surface material is 



