28 



CHEMICAL RESULTS OF LAST CRUISE OF CARNEGIE 



concentration brought about by organism can be removed, 

 within a reasonable length of time, only by water move- 

 ments. 



In the transition zone the pH decreased rapidly with 

 depth and then more slowly until at most stations a min- 

 imum, ranging from 7.5 to 7.9, was reached at a depth 

 which varied from 200 to 1500 meters. At some stations 

 the minimum was lacking, pH remaining practically con- 

 stant from below the transition zone to the greatest 

 depth at which values were obtained. 



Below the depth of niinimum value, at the stations 

 where it occurred, the pH increased slowly with in- 

 creasing depth. At no station was there conclusive evi- 

 dence of a decrease in pH near the bottom as was some- 

 times found by Wattenberg (1927b, 1933). At 3000 me- 

 ters the pH varied from 7.7 to 7.9 and at 4000 meters 

 usually from 7.8 to 7.9. 



The variation in pH with depth can be correlated 

 with biological activity and with the circulation of the 

 water. Remembering that pH varies inversely as the 

 carbon dioxide content, the high pH at the surface can be 

 accounted for by the fact that the carbon dioxide tension 

 of the water is nearly in equilibrium with that of the at- 

 mosphere, either because part of the carbon dioxide has 



escaped from the water or has been consumed by the 

 plants, and the low values in the deeper water are ow- 

 ing to the excess of carbon dioxide produced by animal 

 respiration or decomposition of organic material. The 

 increase in pH below the depth at which minimum values 

 were found indicates that decomposition or animal res- 

 piration reaches a maximum at an intermediate depth. 

 Such a conclusion is corroborated also by the distribu- 

 tion of phosphate and of oxygen. 



The striking correlation between the vertical distri- 

 bution of phosphate and of pH is shown in figures C12and 

 C13. Figure C12 represents a region (station 137) of 

 relatively undisturbed hydrographic conditions, whereas 

 figure C13 (station 71) represents a region where the 

 circulation is more active. In the latter region, off the 

 coast of Peru, upwelling of subsurface water occurs, 

 causing an upward displacement of both the phosphate 

 and pH curves in the upper 500 meters. Further, it is 

 of interest to note that at station 71 both the pH and 

 phosphate curves show the effect of the Intermediate 

 Antarctic Current, the pH values in the water of this 

 current being lower and the phosphate values higher than 

 would probably be the case if this, current were lacking. 



REGIONAL DISTRIBUTION 



Surface. --Since the solubility of carbon dioxide in 

 sea water increases with decreasing temperature, one 

 would expect the carbon dioxide content of the surface 

 water to be greater, and, therefore, the pH to be lower, 

 in the higher latitudes than in the lower. In general, the 

 Carnegie found this to be the case but evidently the ex- 

 planation is not to be found in the variation in the solu- 

 bility of carbon dioxide but rather in the nature of the 

 circulation of the water. 



In the Atlantic the pH values at the surface north of 

 latitude 40° north were between 7.9 and 8.2 (see fig. 

 C14), whereas south of this latitude they ranged between 

 8.2 and 8.4. In the Pacific the observations in the trop- 

 ics and intermediate latitudes usually gave values be- 

 tween 8.2 and 8.4. At the most northerly station, about 

 latitude 50° north, the pH of the surface water was about 

 8.0, whereas at the most southerly stations, in latitude 

 40° south, it was about 8.1. These variations are too 

 great to be owing to differences in the solubility of 

 carbon dioxide. According to Buch (1933), sea water of 

 average salinity and with the same carbon dioxide ten- 

 sion as the atmosphere has a pH of 8.14 at a tempera- 

 ture of 0° and a pH of 8.22 at 30°--a difference of only 

 0.08 for a temperature range greater than that encoun- 

 tered by the Carnegie . On the other hand, the variation 

 in pH with depth has a much greater magnitude than that 

 directly attributable to variation in temperature, and 

 the differences in pH found by the Carnegie at the sur- 

 face were imdoubtedly caused by differences in vertical 

 water movements. This assumption is further strength- 

 ened by the fact that in the Atlantic Wattenberg (1933) 

 found the lowest pH values at the equator near the coast 

 of Africa and from there low values extended well out to 

 the center of the Atlantic. Along the coast of Africa up- 

 welling of subsurface water occurs, and this unquestion- 

 ably accounts for the low pH values found there. 



That the pH at the surface of the sea is not prima- 

 rily a function of the temperature of the water is further 

 indicated by the strong negative correlation between 



Carnegie values of pH and phosphate content which, of 

 course, is not affected by temperature. This relation is 

 shown graphically in figure C16. For these two varia- 

 bles the coefficient of correlation was calculated to be 

 0.621, with a probable error of 0.032. With the temper- 

 ature, the pH showed a positive correlation of about the 

 same magnitude but apparently the temperature varia- 

 tions found cannot entirely be accounted for by differ- 

 ences in latitude, notably in regions where abnormally 

 low temperatures occurred, as, for example, off the 

 coasts of California and South America and near the 

 Aleutian Islands. In these regions the presence of sur- 

 face water in the convection layer was undoubtedly part- 

 ly responsible for the relatively low temperature as well 

 as for the high phosphate content and the low pH. It may 

 therefore be concluded that, in general, the differences 

 in the pH of sea water caused by the solubility of carbon 

 dioxide are masked by the differences caused by verti- 

 cal circulation and biological activity. 



Subsurface. --Below the surface layer, also, there is 

 a close correlation between pH and phosphate, not only 

 vertically but horizontally. The variations in pH at var- 

 ious depths along the entire course coyered by the Car - 

 negie are represented graphically in pH sections I to XVI 

 (I-B, pp. 56-115) and are described on pages 25 to 27. 



The variation in pH at the surface has already been 

 discussed. As would be expected, at the depths falling 

 within the transition zone, the variability in pH was the 

 greatest, depending on the thickness of this zone and on 

 the nature of the vertical gradient. At greater depths 

 the pH, like the phosphate content, varied slightly from 

 station to station, although these variations gave no def- 

 inite indication of regional differences either in the At- 

 lantic or in the Pacific. There was a significant differ- 

 ence, however, between the pH of the deep water of the 

 Pacific and the deep water of the North Atlantic. At 

 2000 meters in the Atlantic (see fig. C15) the pH was 

 about 7.9, whereas in the Pacific it usually ranged be- 

 tween 7.7 and 7.8. At 4000 meters a few observations 



