SECT. 3] PELAGIC SEDIMENTS 677 



preservation, separation, and diagnosis of spherules in sediments, polar ice is a 

 promising source for recovery and quantitative study of Pleistocene cosmic 

 accretion (Thiel and Schmidt, 1961). 



A material of geophysical significance found on the deep-ocean floor is 

 maghemite (R. Mason, unpublished), which is a diagenetic alteration of 

 magnetite (Hagg, 1935; B. Mason, 1943). The presence of maghemite increases 

 the magnetic susceptibility of the solid. The martite reported by Mellis (1952, 

 1959) might possibly also be a pseudomorph of maghemite after magnetite. 



B. Sulfates 



The high concentration of sulfate ion in sea-water, occasionally increased in 

 the interstitial water of the sediment by oxidation of proteinaceous matter, 

 probably limits the solubility of strontium, barium, radium, and lead (Arrhenius, 

 1959). Radioactive solid solutions of celestite (SrS04) and anglesite (PbS04) in 

 barite (BaS04) thus constitute geochemically important mineral species on the 

 ocean floor (Arrhenius, Bramlette and Picciotto, 1957; Arrhenius, 1959) (Fig. 

 13). A sample consisting of a large number of celestobarite crystals, obtained 

 from equatorial North Pacific sediments, showed the average amount of sub- 

 stitution to be 5.4 mole per cent celestite and 0.05 mole per cent anglesite. 



Comparatively high concentrations of barium, strontium, and lead are found 

 in some marine planlitonic organisms, which also contain considerable quantities 

 of other heavy-metal ions (Table III). This suggests that biological extraction 

 from surface sea-water and subsequent sinking is an important mechanism in 

 accreting these elements to the sediment. Among the organisms notable in 

 this respect are some species of Foraminifera, pteropods and heteropods. The 

 acantharid Radiolaria (Schewiakoff, 1926) are particularly efficient in extracting 

 strontium; the celestite (SrS04) skeletons of these protozoans contain in 

 addition 0.4 mole per cent of barite in sohd solution. The skeletons consist of 

 dart-like spines of radially arranged celestite microcrystals with Y oriented in 

 the radial direction of the spine. At decomposition of the protoplast, the spines 

 become detached and presumably settle at a high rate owing to their shape 

 and high density. In contact with sea- water they are, however, rapidly 

 corroded and dissolve entirely before burial in the sediment occurs. 



Thus, marine organisms provide a conveyor mechanism for certain elements 

 from the surface of the ocean to the deep water or the bottom, where the 

 dissolved elements are released by dissolution. Phosphorus, silicon and nitrogen 

 have been found in higher concentrations in intermediate and deep water than 

 in surface water (ref. in Sverdrup et ah, 1946). Still higher gradients are found 

 from the near-bottom water and the interstitial water of the sediment for 

 sihcon and phosphorus (Koczy, 1950, fig. 2.3; and Table VI of this chapter), for 

 radium (Koczy, 1958), for barium (Chow and Goldberg, 1960), and for nitrate 

 (3 to 15 times excess over bottom-water concentration in interstitial water in 

 South Pacific sediments, Arrhenius and Rotschi, 1953, fig. 29). Part of the ions 

 released into the bottom water or interstitial water are returned to the sediment 



