DISTRIBUTION OF OXYGEN IN THE PACIFIC OCEAN 



35 



the layer of maximum oxygen content varied in a simi- 

 lar manner, usually being found at some 30 to 90 meters 

 at the northern stations but at much higher levels at the 

 other stations. This maximum was observed at practi- 

 cally every station but it was more pronounced at the 

 northern stations. North of latitude 20° north the maxi- 

 mum oxygen values represented saturation at all but 

 three stations, whereas south of this latitude there was 

 only one station where supersaturation was observed. 

 This decrease from north to south in the maximum oxy- 

 gen values parallels the concentration of oxygen found at 

 the surface. 



Transition Zone and Oxygen Minimum 



The lower limit of the transition zone was charac- 

 terized by a minimum of oxygen and there was usually a 

 more or less uniform decrease in the oxygen content 

 from the lower limit of the surface layer to this mini- 

 mum. The oxygen transition zone did not coincide with 

 the temperature transition zone for the minimum oxygen 

 layer always occurred below the thermocline. If the 

 boundary between the troposphere and stratosphere be 

 defined by the 10° isotherm, as Sverdrup suggests (I- A, 

 p. 83), following Wust (1929), then the oxygen minimum 

 layer was usually a feature of the stratosphere but in 

 some instances of the troposphere. 



In the transition zone the distribution of oxygen was 

 more variable, both vertically and horizontally, than in 

 any of the other zones. ~ The lower limit of this zone, 

 that is, the depth of the minimum oxygen values, oc- 

 curred at from 650 to 1495 meters in latitudes north of 

 20° north but much nearer the surface in the tropics and 

 especially in the North Equatorial Drift and the Equato- 

 rial Countercurrent. These differences can be correlat- 

 ed with similar differences in the thermal stratification 

 of the water but in some cases there is evidence that 

 horizontal water movements also played a part. A con- 

 dition such as existed at station 144 (see station graphs, 

 I-B, pp. 110-111), where there was a marked increase 

 in the oxygen content between 300 and 400 meters, 

 strongly indicates the presence of a narrow layer of 

 water recently descended from near the surface. This 

 layer was observed at most of the Carnegie stations 

 north of latitude 20° north but it was best developed, 

 hence of most recent origin, in the area between lon^- 

 tudes 150" and 160° west. In the Bushnell stations this 

 layer can be detected only as far as station Bm, lati- 

 tude 30° 20' north, but at the stations farther north 

 there were similar layers of relatively high oxygen 

 content at higher levels. 



The minimum oxygen values increased gradually 

 from about 0.2 ml per liter at the most northerly sta- 

 tions to about 0.8 ml per liter near latitude 20° north. 

 Between this latitude and 6° north the quantities were 

 0.3 ml per liter or less. At station 151 where conditions 

 were more extreme than at the other stations, the oxy- 

 gen content decreased from over 5 ml per liter at 25 

 meters to less than 0.1 ml per liter at 100 meters. Be- 

 tween 100 meters and 400 meters there was a layer of 

 water in which there was practically no oxygen, the 

 amount observed ranging from 0.03 to 0.06 ml per liter 

 or from 0.5 to 1 per cent of saturation. At 500 meters 

 the quantity was only slightly higher, or 0.15 ml per liter. 



The low concentration or almost complete absence 

 of oxygen at most levels above 500 meters at the sta- 

 tions in the North Equatorial Drift and the Equatorial 



Countercurrent is very striking but must be considered 

 merely as an accentuation of a condition found at all the 

 other oxygen stations in the North Pacific. This accen- 

 tuation is owing to the peculiar hydrographic conditions 

 of the region (see I-A, pp. 105, 106). At stations 151 

 and 152 there was a strong stratification of the water 

 above 100 meters as shown by the temperature and den-- 

 sity profile^ and consequently no appreciable quantities 

 of water rich in oxygen could sink from the surface 

 layer. A similar stratification occurred above 200 me- 

 ters at nearby stations. Sverdrup points out that there 

 was a strong upward movement at the borders of the 

 Equatorial Countercurrent, particularly at the northern 

 border, that is, at stations 151 and 152. The effects of 

 this upward movement are quite evident in all the above 

 profiles. It must be realized that the rising water comes 

 from a level at which the oxygen content is already very 

 low. As it moves into this region and rises, it is sub- 

 jected to a further depletion of its oxygen while ascend- 

 ing through the depths at which the oxygen consumption 

 is most rapid. 



The oxygen profile for Section V shows that at the 

 southern stations there were two- oxygen minima, sepa- 

 rated from each other by a layer of water of fairly high 

 oxygen content occurring at about 700 meters at station 

 161 and rising to 400 meters at station 157. The salinity 

 profile for the section shows two layers of water coming 

 in from the south, the Intermediate Antarctic Current 

 with low salinity at 700 meters, and another layer of 

 water with high salinity, probably recently descended 

 from the surface, coming in at about 200 meters. The 

 water of both these currents would be expected to be 

 well supplied with oxygen and it is probable that they, at 

 least in part, accoimt for the higher oxygen values in the 

 transition zone and the less extreme minimum at the 

 southern stations. 



Stratosphere 



In the water below the transition zone the oxygen 

 content was found to increase slowly with increasing 

 depth. This increase is summarized in table C4, which 

 shows the average quantities of oxygen at several depths 

 in four zones of the Pacific for which data are presented 

 in this report. Only stations at which samples were 

 taken to a depth of at least 3000 meters are included in 

 this table. 



Table C4. Summary of oxygen content in deep water 

 of the Pacific Ocean 



Latitudes 



No. 

 sta- 

 tions 



Oxygen in ml per liter 



2000m 2500 m 3000 m 3500 m 



The water in the stratosphere receives practically 

 no oxygen from the troposphere but since it is certain 

 that the water in the stratosphere originally descended 

 from the surface, it is safe to conclude that its oxygen 

 content was originally fairly high, that is, at least 4.5 ml 



