656 ARRHENITJS [CHAP. 25 



Coagulation of unprotected colloids at high electrolyte concentration has 

 been demonstrated by von Smoluchovski (1917) to be a second-order reaction 

 with a rate constant of the order 2 x 10~2. If in the coarse ( > 0.5 \i) part of the 

 oceanic mineral suspensoid the average particle diameter is taken to be 1 y., in 

 agreement with the particle-size distribution found in the sediment, i the num- 

 ber of ])articles per ml is 7 x lO^. In 3 x lO^ sec (100 years, which is the maximum 

 passage time given above for this fraction), coagulation by such a reaction 

 would reduce the original particle concentration by a factor of 4 x lO^i, i.e. all 

 particles in 5 x 10^ ml of sea-water, or a 100 cm^ oceanic-water column, would 

 be aggregated together. 



If a similar reasoning is applied to the fine fraction of the suspensoid (0.01- 

 0.5 (jl), in which an average particle diameter of 0.05 [x is assumed, the initial 

 particle content of 3 x 10' per ml should be reduced to 6 x 10~i5 of the original 

 value in lO^o sec (300 years) which, as indicated above, is a probable passage 

 time for this fraction. This aggregation would comprise all particles in this 

 size range contained in a 400-cm2 water column. 



It is obvious that the rate of coagulation by such a reaction is far more rapid 

 than the rate of removal actually observed in the open ocean. Rates of the 

 order required by von Smoluchovski's theory prevail, however, in concentrated 

 suspensions such as in shallow seas and off river mouths (see e.g. Gripenberg, 

 1934). It is possible that one of the organic components of sea-water, present in 

 concentrations several orders of magnitude higher than that of the mineral 

 suspensoid, might decrease its rate of coagulation. Whether this is correct or not, 

 the low concentration of the suspensoid in the open ocean, its abnormally low 

 rate of coagulation, and, therefore, its long passage time, permitting wide areal 

 distribution, are observed properties which contrast with the properties of 

 the more concentrated, rapidly flocculating, and, therefore, locally varying 

 hydrosol observed in some coastal areas. Pelagic sediments may consequently 

 be defined on the basis of a maximum value for the rate of deposition of the 

 terrigenous component. This value seems to fall in the range of millimeters per 

 thousand years. Within the basins accumulating pelagic sediments, the terri- 

 genous deposition rate appears to vary not much more than one order of magni- 

 tude (ox 10~^ to 5 x 10~4 cm/year), whereas values much higher and varying 

 by several orders of magnitude are characteristic of the sediments fringing the 

 continents. An attempt to outline the area covered by pelagic sediments, as 

 defined above, is made in Fig. 1. 



The rapidly accumulating sediments on the continental slope are unstable, 

 and when the structures fail, coherent masses of sediment slide or slump. 

 When water infiltrates the sliding masses, the concentrated suspensions slide as 



particles have settled (1-1/e) of the height of the water column, or (1-1/2.72) x 4600== 2900 

 m on the average. 



1 The particle-size frequency distribution of the sediment is not identical with the 

 corresponding distribution in the suspensoid for reasons discussed in Sackett and 

 Arrhenius (1962), but this effect is too small to be considered in the order-of- 

 magnitude computation above. 



