48 REDFIELD, KETCHUM AND RICHARDS [CHAP. 2 



The composition of sedimentary rocks indicates that quantities of oxygen 

 equivalent to those now present in the atmosphere may have been liberated in 

 this way. Since this process would operate only to the extent that the supply of 

 oxygen was inadequate, it may have served to regulate the composition of the 

 atmosphere. 



E. Silicon in the Biochemical Cycle 



As a nutrient silicate silicon differs from the inorganic compounds of nitrogen 

 and phosphorus in not being a universal requirement of living matter. It is 

 present, however, in large quantities in the tests of diatoms, which dominate 

 the phytoplankton in the cooler ocean waters. Analyses of diatoms show Si/P 

 ratios varying between 16 and 50, depending on the species (Vinogradov, 1953). 

 Silicon is transported from the surface to the depths by the sinking of these 

 organisms and is liberated there in the course of regeneration. 



Studies on the seasonal changes in nutrient concentrations in the water of 

 the English Channel show that the silicate cycle corresponds in many ways 

 with the cycle of phosphorus; silicate and phosphate being depleted or re- 

 generated simultaneously (Atkins, 1930). 



The relationship of the distribution of silicate to other elements involved in 

 the biochemical cycle is less obvious in the open oceans than in locally restricted 

 waters. However, Richards (1958) has demonstrated a linear relationship in 

 the simultaneous changes in the concentration of silicate silicon, inorganic 

 phosphate and nitrate nitrogen in much of the water column at stations in the 

 Western Atlantic. The ratios of change were approximately JO:ZlSi:zlN:JP = 

 — 272: 15: 16: 1. In these waters, silicon appears to enter the biochemical 

 cycle in about the same proportions as nitrogen. 



In the oceans, in general, the concentrations of silicate vary greatly in their 

 proportion to the phosphate and nitrate present. Such variation arises from 

 the fact that in different parts of the oceans the proportions of diatoms to other 

 phytoplankton, which does not require silicon, differ greatly. Consequently, 

 the statistical composition of the plankton is variable in respect to silicon. 

 Moreover, the solution of silicate from the diatom tests may be expected to 

 follow a different course from the regeneration of nitrogen and phosphorus, 

 which is related more directly to the oxidation of the organic matter. Con- 

 sequently, the silicate may be set free at different depths than are nitrate and 

 phosphate. Finally, animals which feed on phytoplankton have no use for 

 silicon. The tests of diatoms will be rejected and will tend to sink to dissolve at 

 depth while nitrogen and phosphorus will be retained by the animal and be 

 regenerated from its excretions and decomposition products in the upper 

 layers of water. 



For these reasons, the fractionation of silicate by biological action probably 

 follows a different course from that of the other nutrients. A better analysis of 

 the factors responsible for the distribution of silicate in the oceans might add 

 greatly to our understanding of the biochemical circulation . 



