16 



CHEMICAL RESULTS OF LAST CRUISE OF CARNEGIE 



97 33'west, to Callao, Peru, the effects of theupwelling 

 along the coast of Peru are more pronounced. There is 

 also a decrease in the thickness of both the surface lay- 

 er and transition zone with decreasing distance from 

 the coast. Near the offshore end of the line the surface 

 layer was 100 meters or more in depth at several sta- 

 tions, but near the coast only 25 meters deep. The low- 

 er limit of the transition zone was at 375 to 475 meters 

 at the outer stations and was at 75 to 100 meters near 

 the coast. As Callao was approached, there was a con- 

 siderable increase in the quantity of phosphate ranging 

 from 30 mg PO4 per cubic meter in the southern part of 

 the line to 106 mg per cubic meter at station 70 near 

 Callao. Similarly, the average phosphate content of the 

 transition zone ranged between 94 and 159 mg PO4 per 

 cubic meter at the stations remote from land but was 

 above 200 mg per cubic meter along the coast. 



The last line of stations on this chart to be discussed 

 extends from station 60, latitude 40° 24' south, longitude 

 97° 33' west, more or less northward and then eastward 

 to near the coast of South America, slightly south of the 

 equator. In this line the phosphate distribution south of 

 Easter Island, stations 60 to 55, resembled that at the 

 same latitudes to the east. From stations 55 to 45, lati- 

 tude 4° 35' south, longitude 105° 03' west, the layers 

 were deeper and their phosphate content less than at the 

 stations to the east. In this part of the series the con- 

 centration of phosphate was mostly under 20 mg P04per 

 cubic meter in the surface layer which extended to 200 

 meters in several places. The transition zone extended 

 to about 500 meters and had an average phosphate con- 

 tent less than 100 mg PO4 per cubic meter. Station 47, 

 where observations were made in November, in the 

 same vicinity as station 79, latitude 12° 36' south, longi- 

 tude 112° 14' west, was occupied three months later. At 

 the latter station the concentrations of phosphate in both 

 the surface layer and transition zone were considerably 

 greater and the zones somewhat thinner than at station 

 47. The temperature at the two stations also showed a 

 wide difference. Very little seasonal variation in the 

 hydrographic conditions of the water can be expected at 

 this latitude except as caused by differences in the cir- 

 culation which, in turn, may be caused by seasonal 

 changes in other localities. At the stations running near 

 the equator from longitude 105° 03' west to near the 

 coast, remarkably high phosphate values were observed 

 at the surface. At the three stations nearest the coast 

 the transition zone began immediately at the surface. 

 At these stations, as at other stations along the South 

 American coast, the high phosphate content near the 

 surface probably was owing to upwelling of subsurface 

 water and to the Peruvian Current. 



The 2000-meter Level 



The 2000-meter level was selected for a comparison 

 of the phosphate content of the deep water of the various 

 parts of the oceans. It would have been preferable to use 

 a lower level had sufficient observations been available 

 from below 2000 meters. This level, however, is fairly 

 representative of the deep water as it is well below the 

 transition zone and the tropospheric circulation. Figure 

 Cll shows the scaled values of phosphate in mgP04 per 

 cubic meter at 2000 meters for each station at which ob- 

 servations extended to that depth. The mean phosphate 

 values at 2000 meters for certain latitudinal areas in the 

 Atlantic and the Pacific are given in table CI. 



Table CI. Comparison ot concentrations of phosphate in 



Pacific and Atlantic at 2000 meters according to 



Carnegie observations 



Region 



Pacific 



No. 

 sta- 

 tions 



mg/m3 PO4 



Atlantic 



No. 

 sta- 

 tions 



mg/m3 PO4 



tent of the deep water increased from north to south 

 practically as far as the observations extended. As will 

 be seen from the table, north of latitude 40° north the 

 average value at 2000 meters was 79 mg PO4 per cubic 

 meter, between 40° and 20° north it was 111 mg, and 

 south of 20° north 120 mg PO4 per cubic meter. The 

 explanation of this distribution must be sought in the 

 nature of the circulation of the North Atlantic (I-A, 

 pp. 79, 81). 



This subject will be discussed further in connection 

 with the discussion of the phosphate content at the 2000- 

 meter level in the Pacific. 



Pacific Ocean. --In the Pacific no consistent regional 

 variation in phosphate at 2000 meters is evident from an 

 inspection of the scaled station values. There was a 

 considerable variation from station to station but a pro- 

 gressive change in any direction is difficult to discern. 

 The means for different latitudinal areas (see table CI), 

 however, show a progressive increase from south to 

 north in the concentration of phosphate, the values in- 

 creasing from 220 mg PO4 per cubic meter between lati- 

 tudes 20° and 40° south to 260 mg PO4 per cubic meter 

 north of latitude 40° north. This south-north phosphate 

 gradient is in accord with the assumption that practically 

 all the deep water of the Pacific originates in the antarc- 

 tic and not to any important extent at the surface in the 

 warmer areas and that this water is slowly moving north- 

 ward. 



Comparison of Oceans. --The most interesting fea- 

 ture of the phosphate distribution at 2000 meters is the 

 great difference in the values obtained in the Pacific and 

 in the North Atlantic. The mean value for all Carnegie 

 Atlantic stations was 106 mg PO4 per cubic meter, 

 whereas for the Pacific it was 242 mg per cubic meter, 

 or more than twice as high as in the Atlantic. The high- 

 est value obtained in the North Atlantic, 166 mg PO4 per 

 cubic meter, was only slightly above the lowest value, 

 154 mg, obtained in the Pacific. Thomsen (1931a) also 

 found twice as much phosphate in the Pacific at 2000 me- 

 ters as in the Atlantic at that depth. Furthermore, he 

 found that the deep water of the Indian Ocean contained 

 approximately the same amount of phosphate as that of 

 the Pacific. 



If regions of similar latitude in the North Atlantic 

 and the North Pacific be compared, the difference is 

 equally striking. This is clearly shown in table CI. 



At 2000 meters in the eastern North Atlantic, Atkins 

 (1926b) found a phosphate content corresponding to the 



