360 THE DEEP SEA 



include several per cent of rare earths, abnormal amounts of zinc, 

 copper, tin, and lead, and radioactive elements such as thorium 

 (Arrhenius et al., 1957). Figure 8a shows the tracks, in a photo- 

 graphic emulsion, of alpha particles from such a fish fragment. 

 The brown color from organic components in the fish fragments is 

 gradually lost with time in the oxidizing environment, and appears 

 more related to rate of sediment accumulation or burial than to 

 age. This material in Tertiary pelagic deposits has generally lost 

 the brown color, although this is not the case for many Tertiary 

 and older sediments from environments less oxidizing and more 

 rapidly accumulated than in the pelagic one. 



The pelagic (red) clay still poses many problems, such as the 

 proportion derived from pyroclastics and the various other sources. 

 Much of this information may come with more reliable methods 

 and knowledge on clay mineralogy. Occurring over the vast areas 

 with water depths of 4000-5000 m or more, it contains little 

 calcareous material, but commonly includes less than 1% of small 

 rhombs of dolomite, apparently formed within the sediment as 

 indicated by Correns (1939, p. 385) for the Atlantic Ocean. Even 

 the opal tests of plankton, especially of the delicate diatoms, tend 

 to be dissolved in the slowly accumulated clay, and at depth 

 within it. Figure 8^ shows in thin section one of the radiolarian 

 tests that has been largely dissolved within Tertiary sediment at 

 8 m below the top of a core. 



Although pelagic clay is generally highly oxidized, and high in 

 manganese, there is much variation in some features, with a 

 marked difference between some large regions. The pelagic clay of 

 the South Pacific might seem a type area, as it is here most distant 

 from major land sources of sediment. Accumulation in the area 

 seems slower than average from several characters, including the 

 exceptionally high manganese content with the resulting dark 

 brown color. Goldberg's ionium-thorium ratios also indicate the 

 slow rate, of about 3^ mm per thousand years. Pyroclastic material 

 of andesitic to basaltic composition is particularly abundant here, 

 though the original glass component has completely disappeared, 

 except in local areas of more rapid accumulation near some 

 volcanic islands. The hydrated glass, with the iron converted to 

 ferric state, known as palagonite is common. One of the zeolite 



