PELAGIC SEDIMENTS 349 



The following observations of the present extent and rate of 

 accumulation of pelagic calcium carbonate deposits seem to offer 

 little help to this problem, but rather emphasize it more. 



The shells and their fragments from pelagic Foraminifera, 

 including Globigerina, clearly form most of the calcium carbonate 

 of modern pelagic deposits. The tiny coccoliths from certain 

 planktonic algae form a minor part of the present day calcareous 

 deposits. No inorganic crystals or other nonskeletal calcium 

 carbonate is found in pelagic sediments. 



Figure la shows an electron microscope photograph of the fine 

 part of such sediment, which consists of coccoliths of a few microns 

 that are also discernible with a light microscope, and their recog- 

 nizable debris in the finer fraction of less than 0.1 micron, with no 

 inorganic carbonate. 



Figure lb shows a core (Ch 40) of pelagic sediment of Miocene 

 age, with only about 10 cm of Quaternary cover and small amounts 

 of the upper part carried down in borings from benthonic organ- 

 isms. The Miocene clay grades down into coccolith ooze. The 

 shorter core (Ch 17), at different scale, is of Oligocene age that 

 has almost no later sediment cover. It is a coccolith ooze with a 

 few per cent of pelagic Foraminifera, and more than 80% calcium 

 carbonate. A small manganese nodule is evident, and the dark 

 upper surface is largely a surface residue from dissolution of the 

 calcium carbonate. It contains the same Radiolaria as in the lower 

 sediment, but here it is mixed with a few Recent ones. 



Figure 2a shows the abundant calcareous remains known as 

 discoasters and some of the larger coccoliths from the core of 

 Oligocene age. The discoasters, though generally associated in 

 occurrence with coccoliths, are only questionably related, as they 

 are now largely or entirely extinct forms. These average about 

 10 microns in size. Figure 2h shows another such assemblage of 

 Miocene age. 



Much recent data by Bradshaw (1959, p. 53, Fig. 37) on the 

 living populations of pelagic Foraminifera in the Pacific provide 

 information here interpreted in terms of present rates of accumu- 

 lation on the sea floor. Figure 3 shows his summarized total 

 population values, with the highest productivity areas indicating 

 more than 100 specimens per cubic meter of water. A large part 



