24 



CHEMICAL RESULTS OF LAST CRUISE OF CARNEGIE 



The curve in figure C18 represents the average of 

 unpublished data for five series of samples taken at a 

 station 10 miles off La Jolla. The curve for this station 

 shows that the concentration of silicate was between 460 

 and 770 mg Si02 per cubic meter from the surface to a 

 depth of 40 meters. Below this it gradually increased, 

 attaining a value of 1320 mg Si02 at 100 meters and 3040 

 mg Si02 per cubic meter at 500 meters. The average 

 for the Carnegie stations shows a silicate content be- 

 tween 500 and 800 mg Si02 per cubic meter to a depth 

 of more than 200 meters. In fact, the increase in con- 

 centration with depth does not become marked until be- 

 low 300 meters. At 500 meters the average value is 

 approximately the same as for the La Jolla stations but 

 lower than at the Monterey stations. It may be noted 

 that for all three series of stations the quantity of sili- 

 cate at the surface was approximately the same; at all 

 depths between the surface and 500 meters the Carnegie 

 stations contained considerably less silicate than did the 

 other two series of stations. The differences can prob- 

 ably be explained by the fact that both at Monterey and 

 at La Jolla there is a considerable amount of upwelling 

 of water from intermediate depths where the silicate 

 content is high. Similarly, the higher silicate content of 

 the water at Monterey than at La Jolla can probably be 

 accounted for by the difference in the rate of upwelling 

 in the two localities. 



The low concentration of silicate in the upper levels 

 of the open ocean are in accordance with the low phos- 

 phate content in the upper water layers in the same re- 

 gion. As explained in discussing the distribution of 

 phosphate, the water in the photo synthetic zone, which 

 has been depleted of nutrients, is carried to depths of 

 several hundred meters by convective currents. As 

 shown in figure CIS, the three localities represented in 

 this figure have approximately the same silicate con- 

 tent at 500 and 600 meters. This is to be expected since 

 the upwelling water along the coast comes from above 

 this depth (I-A, pp. 102, 103). 



Thoulet (1889) found that distilled water dissolves 

 more silica from rocks than does sea water, whereas 

 Joly (1901) obtained the opposite result. In this connec- 

 tion it is of interest to note that the deep water of the 

 Pacific contains more silicate per unit volume than do 

 many rivers and lakes. Clark (1916) gives an extensive 

 compilation of analyses of natural water from many 

 parts of the world. For many of the rivers and lakes of 

 the world the mean values of silicate are below 8000 

 mg Si02 per cubic meter, a value frequently observed 

 in the Pacific. 



From the information available, it seems evident 

 that the Pacific is much richer in silicate than the Atlan- 

 tic. We have previously pointed out that the concentra- 

 tion of phosphate in the two oceans shows a similar dif- 

 ference (p. 16). Atkins' (1926a) figures for the North 

 Atlantic show a concentration of 160 mg Si02 per cubic 

 meter at the surface, 400 mg at 500 meters, 620 mg at 

 2000 meters, and 1560 mg per cubic meter at 3000 me- 

 ters. The mean values for these depths for all Carnegie 

 stations at which silicate was determined are, respec- 

 tively: 420, 2790, 7070, and 7330 mg Si02 per cubic me- 

 ter. Not enough information is available for either the 

 Atlantic or Pacific to permit definite conclusions re- 

 garding the difference in the concentration of silicate in 

 the two oceans but the evidence points to a situation 

 similar to that in the case of the distribution of phos- 

 phate. 



As in the case of phosphate, one would expect to 

 find higher silicate content in the Antarctic than in the 

 Arctic. The maximum silicate values found by Bruje- 

 wicz (1931) in the Barents Sea were about 1000 mg Si02 

 per cubic meter, indicating that in the Arctic the con- 

 centration of silicate is relatively low. Unfortunately, 

 however, there are no observations available regarding 

 the quantity of dissolved silicate in the Antarctic Ocean. 



Since silica is required by diatoms for the forma- 

 tion of their frustules, the question has often been 

 raised as to whether the concentration of silicate in sea 

 water is ever sufficiently low to limit the production of 

 diatoms. It is known that the silicate content of sea 

 water can be reduced to much below any of the values 

 found at the Carnegie stations. For example, Atkins 

 (1926a) reports for the North Sea values as low as 40 mg 

 Si02 per cubic meter, the lowest concentration that can 

 be determined by the Dienert and Wandenbulcke method. 

 At the Carnegie stations there were very few values be- 

 low 250 mg Si02 per cubic meter whereas the phosphate 

 in a large area of the North Pacific was scarcely meas- 

 urable by a method that is vastly more sensitive than 

 that used for the determination of silicate. Consequent- 

 ly, it appears highly probable that in the region of the 

 Pacific investigated by the Carnegie , as well as by 

 others previously referred to, silicate does not limit 

 the production of diatoms. It is realized, however, that 

 the concentration to which silicate may be extracted 

 from sea water undoubtedly varies with different spe- 

 cies of diatoms and with different physical and chemi- 

 cal conditions of the water. 



