692 ABRHENIUS [CHAP. 25 



E. Silicates 



Skeletal structures of opaline silica are secreted by a number of marine 

 organisms, including the planktonic diatoms, radiolarians and silicoflagellates, 

 and some benthic sponges. Much of this opal is dissolved or peptized soon after 

 the death of the organisms; however, the remaining fraction of relatively 

 robust skeletons often forms a significant portion of the sediment, especially 

 below areas of high productivity such as the Subarctic Convergence, 

 the Equatorial Divergence, and the divergences along the west coasts of the 



Depth in 

 sediment 



400- 



5 10 15 m mole/I. 



Interstitially dissolved SiOo — > 



Fig. 21. Monosilicate in interstitial solution at various depths of the core Capricorn 

 38 BP, South Pacific. The interstitial solution was separated from the sediment by 

 adding a known mass of fresh sediment (with separately determined water content) 

 to a measured volume of filtered sea-water with a low and known silica content. After 

 dispersion, the suspension was centrifuged, and an aliquot of the supernatant 

 liquid was passed through a membrane filter and analyzed colorimetrically. (From 

 Arrhenius and Rotschi, 1953.) 



continents (cf. Fig. 1). The instability of silica in the interstitial water of the 

 sediment causes continuous dissolution of the siliceous fossils after deposition; 

 the silicoflagellates disappear first, followed by diatoms, then radiolarians, and 

 finally even the robust sponge spicules. Part of the dissolved silica reacts to 

 form authigenic aluminosilicates observed as overgrowths in partly dissolved 

 siliceous skeletons (see below). In some cases reprecipitation of the silica as opal 

 takes place, resulting in thin flakes of this mineral along the bedding planes in 

 phosphorite, or in laminae of chert, observed to occiu' at depth in pelagic 



