XUTRIEXT BUDGETS 



305 



phorus is deposited annually. This is very nearly the same as the calculated 

 contribution from rivers. 



SILICON 



The geochemical cycle of silicon also has its uncertainties. Few data 

 are available for computing an average silicon content of phytoplankton ; 

 moreover, the silicon in sediments is so great, owing to the abundance 

 of silicate minerals, that the contribution by organisms is not determin- 

 able by ordinary chemical analysis of sediments. 



Following the method used for nitrogen and phosphorus, the total 

 dissolved silicon content of the ocean was found to be 4.0 x 10^- tons 

 on the basis of an average concentration of 105 /xg-a/L, taken from 

 silicate-depth curves of Sverdrup, Johnson, and Fleming (1942, p. 245). 

 According to Clarke (1924, p. 119) SiOa averages 11.67% of the 

 2.735 X 10^ tons of dissolved solids carried by rivers to the ocean 

 annually. Thus, the annual contribution of dissolved silicon is 150 x 10^ 

 tons. The annual increment for dissolved silicon is about 0.004% of 

 the total oceanic reserve, roughly approximating the 0.009% for nitrogen 

 and 0.002% for phosphorus. Using an average of 26% silicon (Clarke, 

 1924, p. 518; Gould, 1953, p. 168) in the 16 x 10^ tons of suspended 

 sediment of rivers, we find that 4150 x 10^ tons of silicon are con- 

 tributed to the ocean annually in the form of mineral grains. The total 

 annual contribution of dissolved and suspended silicon is thus 4300 x 10® 

 tons. 



The approximate rate of deposition of all forms of silicon can be 

 computed from Clarke's (1924, pp. 516, 518) values of Si02 in near- 

 shore sediment, 57%, and in pelagic sediment weighted for type and 

 area, 49%. Using these percentages, the areas of nearshore and pelagic 

 sediments, and the estimated rates of deposition of these sediments, com- 

 putation shows that about 3800 x 10® tons of silicon are deposited 

 annually. As was shown for phosphorus, the annual supply of silicon 

 is approximately the same as the annual loss to the bottom. 



TABLE 1 

 Nutrient Budget 



Reserve in ocean 



Annual use by phytoplankton 



Annual contribution by rivers( dissolved) 



(suspended) 



by rain 



Annual loss to sediments 



Millions of Metric Tons 



Nitrogen \ Phosphorus 



920,000 



9,600 



19, 



Q 



59 



9 



'V 



120,000 



1,300 



2 



12 







13 



Silicon 



4,000,000 



150 s 

 4,150 > 





 3.800 



lA' 



