14 



CHEMICAL RESULTS OF LAST CRUISE OF CARNEGIE 



in the tropical and South Pacific were off the coast of 

 Peru, where 103 mg PO4 per cubic meter were obtained 

 at station 70. In this region there is upwelling of sub- 

 surface water Similar to that off the coast of California 

 and it is possible that the Peruvian Current also brings 

 a supply of nutrients. 



Although the surface currents in the southern trop- 

 ics flow westward from the Peruvian Coast, the source 

 of the phosphate which causes the relatively high values 

 throughout the tropics cannot be attributed to the region 

 off Peru because there is not a continuous decrease in 

 the phosphate content with increased distance from the 

 coast. On the contrary there was a remarkably great 

 variability of the phosphate content at the surface 

 throughout the tropics. The complicated hydrographic 

 conditions in this region probably account for the irreg- 

 ular distribution of phosphate at the surface. For ex- 

 ample, Sverdrup (I-A,pp. 102,106) has shown that the cur- 

 rents in the southern tropics at depths of 100 and 200 me- 

 ters are irregular and intermittent and that eddies are 

 formed which bring about mixing of water at various depths. 



Surface Layer and Transition Zone 



Uiider the discussion of the vertical distribution of 

 phosphate, the phosphate surface and transition zones 

 were defined (see p. 8). The regional variation in the 

 thickness of these zones and in the quantity of phosphate 

 occurring in them is shown in figure CIO. In the prepa- 

 ration of this chart the depth of the lower limit of the 

 surface layer was taken as the depth to which the con- 

 centration of phosphate was almost uniform. This level 

 was usually very definite as may be seen by reference 

 to the curves of the vertical distribution of phosphate at 

 each Carnegie station. As indicated above, the transi- 

 tion zone was defined as the layer in which the increase 

 in phosphate was greater than 0.1 mg PO4 per cubic me- 

 ter per meter of depth but in some instances it was more 

 logical to place its lower limit at a slightly different 

 depth from the one so determined. Where the transition 

 zone was thick the increase in phosphate with depth was 

 less rapid than where the zone was thin, and often rep- 

 resented an increase of less than 0.1 mg PO4 per cubic 

 meter per meter. The phosphate content for each zone 

 is represented in the chart as the average for all levels 

 within the zone. 



Atlantic Ocean. --At the five stations south of Ice- 

 land both the surface layer and the transition zone were 

 extremely thin. It is remarkable, however, that at these 

 stations the phosphate content of the transition zone was 

 not so great as that in the Sargasso Sea. The depth of 

 the surface layer varied from to 25 meters; the lower 

 limit of the transition zone varied from 50 to 100 me- 

 ters. The concentration of phosphate in the surface 

 layer varied from 20 to 34 mg PO4 per cubic meter; 

 that in the transition zone from 32 to 45 mg PO4 per cu- 

 bic meter. 



The line of stations from stations 12 to 24 shows the 

 change in conditions from the Grand Banks of Newfound- 

 land, across the eastern edge of the Sargasso Sea, to the 

 North Equatorial Current. These stations are essential- 

 ly those used in construction of Section I (see p. 10). 

 At the two northern stations conditions resembled those 

 south of Iceland where the zones were thin; the lower 

 limit of the transition zone was above 50 and 100 me- 

 ters. Southward the zones expanded. The surface layer 

 reached a maximum depth of 350 meters at station 17, 



whereas the lower limit of the transition zone was mostly 

 well over 500 meters at stations between latitudes 20° 

 and 40° north. South of latitude 20° north the thickness 

 of .the zones decreased until at station 24 the surface 

 layer was only 50 meters thick and the lower limit of 

 the transition zone was only 150 meters below the sur- 

 face. The concentration of phosphate in the surface layer 

 varied from 19 to 27 mg PO4 per cubic meter at the two 

 northern stations but was below 12 mg PO4 per cubic 

 meter everywhere to the south. The mean concentration 

 of phosphate in the transition zone was fairly uniform 

 in this series of stations but showed a slight tendency to 

 increase southward. The values varied from 40 to 114 

 mg PO4 per cubic meter. 



The third series of stations in the Atlantic are those 

 included in Section n, stations 24 to 34 (see p. 10). The 

 depth of the surface layer was rather uniform, varying 

 from 25 to 100 meters. The concentration of phosphate 

 was uniformly less than 10 mg PO4 per cubic meter. 

 The depth of the transition zone was greatest (625 me- 

 ters) at station 31 just inside the Caribbean Sea. East 

 and west of this it decreased, particularly to the east 

 where it extended only to 150 meters at station 26. The 

 concentration of phosphate in the transition zone through- 

 out this section was the same or slightly higher than in 

 the southern part of Section I; the values ranged from 

 48 to 144 mg PO4 per cubic meter. 



The variations in the thickness of the surface layer 

 and transition zones in the Atlantic closely paralleled 

 variations in the thickness of the thermocline. North of 

 station 14 there was a thin layer of warm water, with a 

 sharp density gradient to the underlying cold water. In 

 these regions the increase in phosphate is confined 

 chiefly to this narrow thermocline. Farther south, 

 particularly between latitudes 20° and 40° north, 

 although the surface water was warmer, the warm water 

 extended to greater depths and there was a much more 

 gradual change in temperature and density to the colder 

 water below. Here, too, the increase in phosphate was 

 confined chiefly to the thermocline but since the latter 

 extended over several hundred meters of water, so like- 

 wise the phosphate transition zone was very deep. The 

 narrowing of the phosphate zones toward the equatorial 

 regions and their variations in Section II followed simi- 

 lar changes in the density gradients in those regions. 



The distribution of phosphate reported by Seiwell 

 and Seiwell (1934) for their three stations west of Car - 

 negie Section I agrees well with Carnegie data, although 

 the transition zone at their station 1169, latitude 16° 22' 

 north, longitude 41° 10' west, extends to about 375 me- 

 ters, which is somewhat lower than that at Carnegie sta- 

 tions in this latitude. At their three stations in the west- 

 ern edge of the Sargasso Sea, however, between latitudes 

 22° and 34° north, longitudes 65° and 68° west, they 

 show a much more pronounced expansion of both the sur- 

 face layer and the transition zone than was found by the 

 Carnegie in the eastern edge of that area. In one case 

 their surface layer extended to more than 550 meters. 

 Although their observations extended to 1000 meters, 

 this was not deep enough to show the lower limit of the 

 transition zone in all cases. 



Pacific Ocean. --In the Pacific we shall first consid- 

 er the line of stations between Japan and California, sta- 

 tions 113 to 130, all of which are north of latitude 35° 

 north. At these stations the phosphate content was high 

 in the transition zone as well as in the surface layer. 

 The surface layer was everywhere narrow or entirely 



