Source 



239 



derived directly from pelagic and benthonic 

 organisms and for that which is reworked 

 from shallow-water areas; however, prob- 

 ably nearly all the latter is also of organic 

 origin, mostly comminuted mollusk shells. 

 A somewhat greater rate of deposition of 

 calcium carbonate may exist on the shelves 

 and bank tops owing to the additional contri- 

 bution of calcareous algae in the euphotic 

 zone, but negligible deposition of detrital 

 sediments in these areas causes the calcar- 

 eous sediments to be exposed to the overly- 

 ing water for so long that resolution is im- 

 portant. One result of solution is the ob- 

 served high concentration of coarse shell 

 fragments in these areas, the more delicate 

 tests having been largely or wholly dis- 

 solved. Another result is the frequent ex- 

 posure of ancient rocks. Where rocks 

 have not been buried the net rate of accum- 

 ulation of calcium carbonate is zero. Else- 

 where on the shelves the rate may be greater 

 or less than that in the basins by factors 

 that are unknown but doubtlessly highly 

 variable. 



That solution of calcium carbonate is pos- 

 sible during its period of settling through 

 the water column or during exposure on the 

 bottom before burial is suggested by the ap- 

 parently unsaturated condition of the water. 

 The percentage saturation of calcium carbo- 

 nate in sea water is given by the ratio of the 

 ion product and the apparent solubility 

 product: 



[Ca++] X [C0,=] _^ 

 Per cent saturation = ~ — — x 100 



The carbonate ion concentration may be 

 computed from temperature, chlorinity, hy- 

 drogen ion concentration, and alkalinity, 

 using equations given by Harvey (1955). 

 Measurements of these parameters were 

 made at various depths in the water over 

 several of the basins in connection with a 

 study of the dissolved nutrients in the water 

 (Rittenberg, Emery, and Orr, 1955). Cal- 

 cium ion in sea water bears a constant ratio 

 to chloride ion and was so computed. The 

 apparent solubility product was based on 

 Smith's (1941) values corrected to the ob- 



served chlorinity and temperature by Wat- 

 tenberg's coefficients (Sverdrup, Johnson, 

 and Fleming, 1942, pp. 206-207). The re- 

 sulting values of percentage saturation of 

 calcium carbonate (Fig. 199) show typical 

 oversaturation at the surface and undersatura- 

 tion below 100 meters. Below the basin sills 

 the water is 30 to 60 per cent saturated, and 

 it exhibits a sUght increase in percentage 

 saturation at the bottom as would be ex- 

 pected from solution of calcium carbonate 

 at the water-sediment interface. Great reli- 

 ance, however, should not be placed on these 

 curves, owing to uncertainty of the solubility 

 product and the great dependence of the 

 computations on measurements of pH — 

 never as precise as desirable. 



Siliceous organic remains are present in 

 quantities that are unknown because of the 

 difficulty of making adequate separations. 

 They probably constitute a good deal less 

 than 1 per cent of the total weight of sedi- 

 ment. The organic matter in the basin sedi- 

 ments averages about 7 per cent by dry 

 weight of total sediment, actual percentages 



% SATTURATION CaCOa 



50 100 150 



Figure 199. Curves showing computed percentage satu- 

 ration of calcium carbonate in the water above several 

 basins. Arrow indicates depth of basin sills and cross- 

 hatched line shows the bottom depth at the samphng sites. 



