688 ARRHENIUS [CHAP. 25 



Owing to relatively rapid and easily distinguished evolutionary changes of 

 the coccolithophorids, it has been possible to use their fossil remains for indica- 

 tion of geological age by highly precise biostratigraphic correlations (Bramlette 

 and Riedel, 1954). The mass of coccohths in the carbonate fraction of deep-sea 

 sediments sometimes exceeds that of foraminiferal tests; such coccolith oozes 

 were particularly abundant in the Cretaceous and the Tertiary, Coccolith ooze 

 also constitutes the glacial maximum productivity stage below the Pacific 

 Equatorial Divergence (Fig. 36). 



In sediments older than the Middle Mesozoic, identifying coccoliths is usually 

 difficult, owing to gradual recrystalHzation of the calcite; in fact, many fine- 

 grained limestones in older geological formations may be recrystallized cocco- 

 Hth oozes (Bramlette, 1958). Although the calcareous planktonic Foraminifera 

 did not evolve until Mesozoic time, calcium from the continents might thus have 

 been deposited as carbonate over the deep-ocean floor by planktonic organisms 

 early in the history of the Earth. 



Aragonite shells are secreted by some groups of planktonic organisms, of 

 which pteropods are the quantitatively most important. Because of the re- 

 latively high rate of dissolution of aragonite in sea-water at low temperatures 

 and high pressures, pteropod ooze is accumulating only in comparatively 

 shallow areas. 



Extensive substitution in the marine calcium-magnesium carbonate struc- 

 tures is essentially limited to strontium in aragonite. A concentration of other 

 trace elements such as lead, copper, barium, strontium, titanium, and iron 

 is to be found in the protoplasts of some species of Foraminifera (Table III) 

 and in pteropods, whose organic shell structures interlayered with aragonite 

 provide a possible site for heavy-metal ions concentrated from sea-water (Curl 

 and Nicholls, unpublished). Thus, a number of trace elements can be related 

 to calcareous fossils or their dissolution residue (Correns, 1937; Graf, 1960). 

 Several of these organism groups might consequently play an important role 

 in the vertical-transport mechanism mentioned in Section 2-B of this chapter. 



Pelagic sediments, as defined in the present work, include shallow-water 

 pelagic carbonate sediments which constitute bioherms of various types and 

 particularly coral reefs. This group of sediments is described elsewhere in this 

 volume (Chapter 22). 



Calcite and aragonite are highly soluble, so the original distribution of these 

 minerals over the ocean floor is extensively modified by post-dej)Ositional 

 dissolution favored by low temperatures, high hydrostatic pressures, rapid 

 removal of the solution, and high partial pressures of CO2, generated by oxida- 

 tion of organic matter at the sediment surface. The dissolution process in a 

 given stratum is slowed as new sediment accumulates above it, producing 

 a diffusion barrier of increasing thickness, separating the carbonate from the sol- 

 vent. The ultimate rate of accumulation of carbonate at a given depth and circu- 

 lation is consequently determined by the rate of deposition of carbonate minerals 

 and by the total rate of accumulation of sediment. The regional distribution of 

 calcium carbonate in ocean sediments illustrates the influence of these para- 



