72 RKDFIELD, KETCHUM AND RICHARDS [CHAP. 2 



this layer, its northward flow must produce a substantial part of the inorganic 

 phosphorus maximum. 



The distribution of phosphorus of oxidative origin is shown in Fig. 17c. The 

 distribution depends directly upon that of the apparent oxygen utilization from 

 which it is estimated (see page 33). Maximum concentrations are found in the 

 equatorial region of the intermediate layer, from which the concentrations 

 diminish both northward and southward. This maximum must arise from the 

 regeneration of phosphorus from organic matter derived from the surface 

 layers of the tropical ocean. 



The sub-surface water in the subantarctic region contains substantial con- 

 centrations of phosphorus of oxidative origin. The northward flow of antarctic 

 intermediate water will carry this fraction to the intermediate layer of lower 

 latitudes along with the preformed phosphorus. Allowance for the contribution 

 of this fraction of subantarctic origin to the total phosphorus of oxidation can 

 be made (see Redfield, 1942). When this is done, the occurrence of phosphorus 

 of local oxidative origin becomes restricted in the South Atlantic to the tropical 

 region. 



The origin of the phosphate maximum is evidently complex, being due to the 

 combined effects of fractions introduced by the transport of the water of 

 the intermediate layer from its origin in the subantarctic regions, supplemented 

 by local regeneration in the tropics and the subsequent redistribution of the 

 latter by the horizontal circulation. The maximum local regeneration occurs at 

 somewhat lesser depths than the maximum of the preformed phosphate. 

 Accordingly the oxygen minimum lies somewhat above the phosphate 

 maximum. 



The depths between the sub -surface (200 m) and the upper boundary of the 

 intermediate layer, in which at < 27.0, are occupied by the North and South 

 Atlantic Central Water-masses. In the equatorial region these water-masses 

 are separated by the density distribution except for a zone of limited thickness 

 (ca. 200 m), so that water can move between them only by passing through 

 the sub -surface layer. In these isolated pools, which circulate as great eddies, 

 the concentrations of the several phosphorus fractions decrease toward the 

 surface depending on the degree of mixing of the rich water of the intermediate 

 layer and the water of the surface layer which is impoverished by the synthesis 

 and sinking of organized matter. The resulting patterns of distribution depend 

 on the circulation which is peculiar to the eddies occupied by the central water- 

 masses, and which also results in gradients in the temperature and salinity 

 distribution. 



The depths below the intermediate layer, where at is greater than 27.5, are 

 occupied by the North Atlantic Deep Water. This water mass originates in 

 high northern latitudes and flows southward. The concentration of inorganic 

 phosphorus increases slowly from values of about 1 mg atom/m 3 in the north 

 to about 1.5 mg atoms/m 3 at 32°S. Beyond this point the phosphate concentra- 

 tions increase rapidly as the North Atlantic Deep Water mingles with deep 

 water of antarctic origin, in which the concentrations are more than 2.6 mg 



