SECT. 3] 



PELAGIC SEDIMENTS 



689 



meters. From the distribution of carbonate concentrations with depth in areas 

 with different productivities, the combined influence of hydrostatic pressure and 

 temperature and hkewise the influence of productivity are quite evident (Figs. 

 18 and 19). Further, at a higher total rate of deposition, providing rapid pro- 



Depth 



3000 



4000 - 



CaCO- 



20 



40 



60 



80 



100% 



-• — Average for 500 m intervals from 



all available data-, present sediment 



5000 



6000 

 m 



I 

 / 



j^f 



Pacific 

 135°W) 



Fig. 18. Concentration of calcium carbonate at the present sediment surface as a function 

 of depth in the Pacific Ocean. The average curve shows a pronounced effect of depth 

 on the rate of sokition of carbonate. In the central equatorial Pacific, characterized by 

 a wide regional variability in biological productivity, this latter factor dominates the 

 carbonate distribution. (From Bramlette, 1961.) 



tection of the carbonate, the disappearance of the carbonate occurs at greater 

 water depths (Fig. 20). Finafly, the influence of the removal of the dissolved 

 calcium ion is demonstrated by the carbonate distribution in the South Atlantic 

 basins; at equal depths the dissolution of calcite is far more extensive in the 

 Brazil Basin, which is flushed by Antarctic deep water, than in the neighboring 

 Congo Basin, which has a slower renewal of the bottom water (Wattenberg, 

 1933, 1937; Correns, 1939). 



At shallow and intermediate depths where the rate of dissolution of calcium 

 carbonate is low, calcite and aragonite are sometimes dolomitized. Outcrops of 

 dolomitized limestone, such as on the Nasca Ridge in the South Pacific (Bram- 

 lette, unpublished), are coated with manganese-iron oxide minerals, indicating 

 that the dolomitization was active in the past, and that it has since been re- 

 placed by oxide deposition. A similar sequence is observed in many oceanic 

 limestones and phosphorite deposits, which also display a coating of manganese- 

 iron oxide minerals (Fig. 14). Growth of sparse euhedral dolomite crystals in 



