25. PELAGIC SEDIMENTSi 



G. Arrhenius 



1. Concept of Pelagic Sedimentation 



The term pelagic sediment is often rather loosely defined. It is generally 

 applied to marine sediments in which the fraction derived from the continents 

 indicates deposition from a dilute mineral suspension distributed throughout 

 deep-ocean water. It appears logical to base a precise definition of pelagic 

 sediments on some limiting property of this suspension, such as concentration 

 or rate of removal. Further, the property chosen should, if possible, be reflected 

 in the ensuing deposit, so that the criterion in question can be applied to ancient 

 sediments. 



Extensive measurements of the concentration of particulate matter in sea- 

 water have been carried out by Jerlov (1953); however, these measurements 

 reflect the sum of both the terrigenous mineral sol and particles of organic 

 (biotic) origin. Aluminosilicates form a major part of the inorganic mineral 

 suspension; aluminum is useful as an indicator of these, since this element 

 forms 7 to 9% of the total inorganic component, 2 and can be quantitatively 

 determined at concentration levels down to 3 x lO^i^ (Sackett and Arrhenius, 

 1962). Measurements of the amount of particulate aluminum in North Pacific 

 deep water indicate an average concentration of 23 [xg/1. of mineral suspensoid, 

 or 10 mg in a vertical sea- water column with a 1 cm^ cross-section at oceanic 

 depth. The mass of mineral particles larger than 0.5 [x constitutes 60%, or less, 

 of the total. From the concentration of the suspensoid and the rate of fallout of 

 terrigenous minerals on the ocean floor, an average passage time (Barth, 1952) 

 of less than 100 years is obtained for the fraction of particles larger than 0.5 [i. 

 For the finer particles the average passage time is longer, such as more than 

 200 (but considerably less than 600) years. 



A mechanism which possibly contributes significantly to the removal of 

 coarse suspensoid is aggregation in the gut of filter-feeding animals; this 

 phenomenon has been observed by Rex and Goldberg (1958). Gravitative 

 settling of single grains could account for the deposition of most particles 

 larger than a few microns, but for smaller grains this mechanism is inadequate 

 since the settling time required by Stokes' law^ is several orders of magnitude 

 larger than the passage time actually observed ( < 10^ years). ^ 



1 Much of the information presented in this chapter is the result of research partly 

 carried ovit under Contract No. AT (ll-l)-34 with the U.S. tomic Energy Commission, 

 partly sponsored by the Petroleum Research Fund of the American Chemical Society 

 (ACS PRF Unsolicited Award 875-C6). The generous support from these agencies is 

 gratefully acknowledged. 



2 Data from the Pacific Ocean (Sackett and Arrhenius, 1962); determinations from the 

 English Channel give similar values (Armstrong, 1958). 



3 7800 years for a 0.1 ^ spherical particle of density 2.6 through 2900 m of the oceanic- 

 water column at 10°C. 



4 The passage time is defined as the time at which the mass of particles originally present 

 in the water column has been reduced to 1/e of the original value. This occurs when the 



[MS received February, 1961] 655 



