SECT. 3] 



PELAGIC SEDIMENTS 



713 



maximum rate of production (glacial)" in the middle diagram, and the dashed 

 curve marked "Pleistocene maximum rate of dissolution (glacial)" have been 

 constructed. The production appears to have increased greatly, especially in the 

 Equatorial Divergence, and the dissolution seems to have increased somewhat, 

 owing to respiration of benthonic animals, although not enough to counter- 

 balance the greatly increased rate of deposition of calcareous skeletal remains. 

 Thus, the rate of accumulation of calcium carbonate reaches a maximum during 

 this time. The greatest increase occurs at the equator, where values of 5 to 10 g 

 of calcium carbonate per cm^ in a thousand years are found. 



Although the amount of calcium carbonate relative to the inorganic com- 

 ponents of the sediment is an analytically convenient measure of past variations 



Equatorial Pacific 



Relative rate of biotic 



accumulation 



Equatorial Atlantic and Caribbean 



Generalized surface water 

 30 25''C paleotemperature 



Fig. 39. Age relations and stratigraphic designation of climatically controlled strata in the 

 equatorial Pacific and Atlantic Oceans. (Data from Arrhenius, 1952; Emiliani, 1955; 

 and Emiliani, in lift., 1961.) 



in organic productivity, it is not the only one available. The variations 

 described above are accompanied by still more marked changes in the relative 

 amounts of phosphate from fish debris and of opaline silica from diatoms. The 

 diatoms also display characteristic changes in their reproductive cycle (Fig. 37). 

 As is seen from the planktonic paleotemperature data at the curve "Pleisto- 

 cene maximum rate of production (glacial)" in the middle graph of Fig. 36 

 (11.9°C at the equator, 12.9°C at 2°N and 14.1°C at 7°N), the surface tempera- 

 ture gradient away from the equator displayed by the oxygen isotope ratio in 

 the foraminiferal tests is even steeper during the time of circulation maximum 

 (Riss [Illinoian] Glacial Age) than under the present conditions of interglacial 

 type. This further confirms that an increased rate of upwelling in the Equatorial 

 Divergence is the direct cause of the increased productivity. A displacement 

 toward the Equator of the high pressure centers now found between latitudes 



