356 THE DEEP SEA 



sharp decrease with depth, and a compensation depth near 

 4500 m. The lower curve is from a more southern area, between 

 latitudes 40° and 50° S, where the compensation depth is greater 

 (near 5000 m), and comparable with that common in the Atlantic 

 Ocean. 



Figure 5 represents an area at the north edge of the Pacific 

 equatorial belt of high productivity and carbonate deposition. 

 The carbonate percentage is here controlled more largely by the 

 abrupt change northward in carbonate supply from the counter- 

 current belt of very high productivity, and with little change in 

 depth of water. E\en here, however, the depth relation is also 

 indicated, with a compensation depth near 4700 m. The other 

 curve is an average of all available samples from the Pacific, 

 affected by all the variable factors, but ne^'ertheless showing a 

 marked change between 4000 and 5000 m. The points are based 

 on averages for all samples between 500-m inter\'als. 



Figure 6 shows the distribution of calcium carbonate from all 

 available data for the central and eastern Pacific, and its relation 

 to the depth contours of 4000 and 5000 m. Despite the inadequate 

 control on both, the relation of carbonate content and depth is 

 apparent in the southern Pacific. It would be closer if additional 

 depth contours, such as 4500 m, were shown, as indicated by 

 individual core depths. Most of the North Pacific is below depths 

 of 4000 m, but factors other than depth are apparent here. Pro- 

 ductivity of calcareous plankton is relatively low over large areas, 

 including part of that of less than 4000 m depth to the southwest 

 of Mexico. Rapid clastic accumulation and high organic matter 

 content have been indicated for the large region ofif Alaska. 

 Increased supply of CO2 to the bottom waters thus seems probable, 

 and may influence the compensation depth above 4000 m here. 



Statements commonly imply that the solution largely occurs 

 during the descent of calcareous shells to great depth. This time, 

 however, is insignificant compared with that possible on the ocean 

 floor, with the slow rates of accumulation. Foraminiferal tests 

 settle at rates requiring only a few days to reach 5000 m depth, but 

 might remain near the sediment-water interface for hundreds of 

 years in areas of no net accumulation of carbonate. Coccoliths, 

 averaging 5 microns, would settle much more slowly (requiring 



