32 



CHEMICAL RESULTS OF LAST CRUISE OF CARNEGIE 



than at the surface or at any other depth, owing to the 

 fact that oxygen is liberated by photosynthetic organ- 

 isms more rapidly than it is consumed, and mixing is 

 not sufficiently rapid entirely to prevent the accumula- 

 tion of excess quantities of oxygen. At some stations 

 this maximum oxygen layer was not found. 



3. A transition zone in which the oxygen content 

 decreases to a minimum. This decrease is no doubt 

 owing to a reduction of the amount of oxygen trans- 

 ferred by vertical mixing and to the consumption of oxy- 

 gen by animals and by oxidizing chemical substances. 



4. A deep layer in which the oxygen content in- 

 creases slowly with depth but never attains the same 

 concentration as at the surface. The water in this layer 

 receives no oxygen from the water strata or from the 

 atmosphere directly above, and contains only the oxygen 

 it had when descending from the surface, less the 

 amount consumed by respiration and oxidation. 



The decrease in the oxygen content in the transition 

 zone and the minimum oxygen values at the lower limit 

 are owing partly to a reduction in the amount of convec- 

 tive water movements and partly to the consumption of 

 oxygen by animals and decomposing organic material. 

 The relation to the circulation is indicated by the fact 

 that the oxygen minimum always occurs below the ther- 

 mocline. It is safe to assume, therefore, that the water 

 below the thermocline receives no appreciable quantities 

 of water from higher levels and consequently that its 

 oxygen supply cannot be replenishedfr om the atmosphere. 



The increase in the quantity of dissolved oxygen be- 

 low the minimum is probably owing to the fact that ani- 

 mals and organic matter become increasingly scarce 

 with increasing depth. However, if there were no oxygen 

 supplied to the stratosphere, even small quantities of 

 decomposing organic matter continuously supplied would 

 eventually result in an absolute absence of oxygen at the 

 level of the minimum oxygen content and this layer with- 

 out oxygen would gradually extend more deeply until it 

 reached the bottom. The water in the stratosphere, how- 

 ever, is continuously, although slowly, being replaced by 

 water from the surface and this water, of course, is ap- 

 proximately saturated with oxygen. This subject will 

 again be discussed under "Regional distribution" of 

 oxygen. 



That the vertical distribution of oxygen in the sea is 

 closely correlated with the distribution of other sub- 

 stances involved in biological activities, such as hydro- 

 gen ions and phosphates, is demonstrated in figure C20 

 which represents the distribution of these three sub- 

 stances at Carnegie station 137 which is representative 

 of the North Pacific. In the upper 200 meters where the 

 pH was high and the phosphate content low, the quantity 

 of dissolved oxygen was high. At the surface the oxygen 

 content was 4.64 ml per liter and at 50 meters over 5 ml 

 per liter. In the transition zone where the phosphate 

 content increased and the pH decreased with depth, the 

 dissolved oxygen content decreased rapidly. A minimum 

 oxygen content of 0.54 ml per liter or only 8 per cent of 

 total saturation occurred at about 700 meters, the same 

 depth at which the minimum pH values were found and 

 approximately at the same depth as for the phosphate 

 maximum. Below the oxygen minimum there was a slow 

 increase in quantity of oxygen with depth, concurrent 

 with an increase in pH and a decrease in the phosphate 

 content. The deepest observation, at 4343 meters, gave 

 3.46 ml per liter of oxygen which is seven times as 

 much as the minimum quantity found at 700 meters and 



about three-fourths the quantity of the surface water. 

 An examination of the graphs for the stations at which 

 oxygen was determined reveals that the correlation be- 

 tween oxygen, phosphate, and pH found at station 137 is 

 the usual condition. 



Of particular interest is the distribution of these 

 substances at station 152 in the North Equatorial Drift 

 where the vertical distribution differed from that at most 

 other stations because of unusual hydrographic conditions. 

 The curves for this station are shown in figure C21. It 

 will be noted that there was a marked increase in the 

 phosphate and a corresponding decrease in pH and oxygen 

 content in the upper 200 meters. The extensive layer of 

 low oxygen content at this station will be discussed later. 



Section V. stations 162 to 148 and 134 to 132: extends 

 northeastward from the Samoan Islands (latitude 14° 

 south, longitude 168° west) to San Francisco (latitude 37° 

 north). --The oxygen content at the surface from stations 

 130 to 132 is above 5.0 ml O2 per liter. South of station 

 133 it drops to between 4.0 and 5.0 ml per liter. This 

 can be expected from the higher temperature of the wa- 

 ter. South of station 158 it is usually below 4.0 ml per 

 liter. In the upper 100 meters at most stations in this 

 section, there is a layer of water of which the oxygen 

 content is higher than either above or below. At stations 

 130 and 131 the maximum oxygen content is 6.0 to 7.0 ml 

 per liter. South of station 151 it is between 4.0 and 5.0 

 ml per liter. In percentage saturation this appears as an 

 almost continuous layer between stations 130 and 151, 

 with over 100 per cent saturation. South of this the max- 

 imum is between 90 and 100 per cent saturation. 



The oxygen transition zone at depths above 500 me- 

 ters exhibits the same general features as the phosphate 

 transition. Below the transition zone there is a very 

 definite minimum of oxygen throughout the whole section. 

 This minimum changes in depth and in magnitude from 

 north to south. At the northernmost station, 130, it is 

 represented by 0.1 to 0.3 ml per liter or less than 5 per 

 cent of saturation. This condition exists to south of sta- 

 tion 133 where the absolute concentration of oxygen is 

 not quite so low but the percentage saturation does not 

 rise until past station 134. At station 150 the minimum 

 again drops below 0.3 ml per liter or 5 per cent satura- 

 tion. At this station the minimum is divided by a layer 

 of higher oxygen content, one branch of the minimum 

 occurring at 300 meters and the other at 500 meters. At 

 station 151, latitude 15° north, the minimum is repre- 

 sented by a thick layer lying between the levels of 100 

 and 450 meters, in which the oxygen content was only 

 0.05 ml per liter. This value lies so close to the exper- 

 imental error of the determination that it is possible 

 that in this layer there is no dissolved oxygen at all. 

 As one continues southward, two minima are again seen 

 at station 152 at about 100 and 400 meters, respectively. 

 The station curves of the vertical distribution of oxygen 

 (I-B, pp. 56-115) show that these minima persist to 

 station 154, south of which they converge at 500 meters 

 with a value increased to more than 0.5 ml per liter. 

 This minimum continues southward at about 400 meters, 

 rising somewhat at station 157 where it again sends off 

 a branch with values below 0.5 ml per liter at 500 me- 

 ters. It becomes less extreme southward with oxygen 

 values steadily increasing to more than 2.0 ml per liter 

 at station 162. A layer of high oxygen content can be 

 seen between the two minima coming in from the south 

 at station 161 at about 700 meters, where the oxygen 

 content is more than 3 ml per liter. Northward it rises 



