DISTRIBUTION OF CALCIUM CARBONATE IN AREA INVESTIGATED BY CARNEGHE 



123 



temperature, salinity, and calcium content which 

 characterize surface waters, whereas high carbon diox- 

 ide and low temperature, salinity, and calcium, together 

 with large .hydrostatic pressure, tend to bring about so- 

 lution, and to inhibit precipitation. The latter are the 

 properties of deep water. 



The percentage of calcium carbonate in a bottom 

 sediment depends, first, on its relative rate of deposi- 

 tion with respect to that of other materials, and second, 

 on the relative rates of removal, by solution or other- 

 wise, of calcareous and noncalcareous materials after 

 deposition. In a terrigenous deposit the increment of 

 detrital noncalcareous material may be so great as to 

 mask the calcium carbonate even though the rate of dep- 

 osition of the latter may be quite rapid. Large amounts 

 of detrital material, however, are not favorable for the 

 extensive growth of lime-secreting organisms, hence 

 the rate of precipitation of carbonates from sea water in 

 many near-shore regions is not rapid. On the other 

 hand, the detrital material of terrigenous deposits is 

 often calcareous and therefore adds to (rather than re- 

 ducing) the total amount of lime in the sediment. 



The rate of deposition of noncalcareous materials in 

 pelagic deposits is presumably more or less constant, 

 so that only the absolute rate of deposition of CaC03 is 

 of importance in these sediments. The rate of deposi- 

 tion is dependent on the rate of precipitation in the sur- 

 face waters and the rate of solution at depth. 



Precipitation of CaC03 in Surface Waters . Although 

 the biological precipitation of CaC03 undoubtedly plays a 

 far greater role than does purely inorganic precipitation, 

 which may occur only in certain shallow regions, the 

 amounts of both are doubtless partly controlled by the 

 physicochemical conditions. The fact that removal of 

 lime takes place from waters near the surface is shown 

 by the curve for the titratable base-chloride ratios^ of 

 Atlantic and Pacific waters, given in figure 46. In the 

 Atlantic a marked decrease in the amount of base takes 

 place at about 100 meters below the surface. In the Pa- 

 cific the minimum zone is somewhat lower, but an even 

 greater amount of removal has apparently occurred. 

 This diminution in the base must be almost entirely ow- 

 ing to the removal of lime. It is generally believed 

 (Vaughan [1924], Wattenberg [1933]) that there are two 

 types of biogenetic lime precipitation; the building up of 

 calcium carbonate in the cells of benthonic and plankton- 

 ic plants and animals, and precipitation outside the cells. 

 In plants this is brought about by reduction of CO2 ten- 

 sion through assimilation and also, in bacteria, through 

 NH3 production, etc. The possible effects of one physi- 

 cal factor, namely, temperature, on lime-secreting or- 

 ganisms are indicated by Murray and Hjort (1912) who 

 state, with reference to the horizontal distribution of 

 calcium carbonate organisms, "that they are most abun- 

 dant both at the surface and at the bottom in warm tropi- 

 cal regions where the annual range of surface tempera- 

 ture is least." Furthermore, "Of the pelagic deposits, 

 the Globigerina and pteropod oozes of tropical regions 

 probably accumulate the most rapidly, from the greater 



^The titratable base, defined as the equivalent con- 

 centration in milU -equivalents per liter of strong bases 

 balanced against weak acid radicals (principally carbon- 

 ate, bicarbonate, and borate) bears a more or less con- 

 stant ratio to the total lime content, and varies with the 

 amount of precipitation and solution of calcium carbonate 

 'see Moberg, Greenberg, Revelle, and Allen [1934]). 



variety of tropical pelagic species of foraminifera and 

 molluscs, and the larger and more massive shells se- 

 creted in tropical as compared with extra-tropical re- 

 gions." 



Wattenberg (1931), on the basis of Hentschel's data 

 (1931) from the Meteor expedition, has pointed out that 

 the proportion of calcium carbonate -forming organisms 

 in the nannoplankton of the Atlantic, that is, the Cocco- 

 lithophoridae, decreases markedly as the physicochemi- 

 cal conditions for precipitation of calcium carbonate 

 become less favorable. 



Trask (1932) has adduced statistical evidence that 

 CaC03 content, other things being equal, is a function of 

 the salinity of the surface waters. This holds especially 

 for the two sides of the Central American isthmus. On 

 the east side there is much more calcium carbonate at 

 any given depth than on the west side, yet the water 

 temperatures of both regions are about the same. The 

 salinity, on the contrary, as may be seen from Schott's 

 (1928) map, is much lower on the western side. Simi- 

 larly, in the southeastern Pacific, the area of high car- 

 bonate is an area of salinity over 36.0 per mille; as 

 already stated, the boundary of high CaCOs content north 

 of the equator closely parallels the 34.5 per mille iso- 

 haline. On the other hand, the salinity off the east coast 

 of South America is very high (37.0 per mille) in com- 

 parison with that of the eastern south Atlantic, and yet 

 the bottom deposits of the eastern south Atlantic are 

 much higher in calcium carbonate than are those of the 

 western south Atlantic. As will be shown, however, this 

 is probably to be explained by greater solution at depth 

 in the western Atlantic. That there is little noticeable 

 correlation between calcium carbonate content and sa- 

 linity in the north Pacific is probably related to the 

 great average depth of that ocean. 



A rise in salinity effects an increase in the degree 

 of saturation of surface water, especially when it is in 

 equilibrium with the atmosphere, but the effect of in- 

 creasing salinity is much less than that of a comparable 

 rise in temperature or loss of carbon dioxide. The large 

 apparent effect of salinity found by Trask probably is 

 owing to some other factor whose variations may be cor- 

 related with varying salinity. 



An important consideration affecting the rate of pre- 

 cipitation of calcium carbonate is the length of time dur- 

 ing which surface water remains at the surface. After 

 chemical precipitation has occurred from supersaturated 

 surface water, obviously no more precipitation will occur 

 until the conditions in the water are changed, that is, 

 usually until deep water of high titratable base content 

 has risen to the surface. Similarly, after the quantity of 

 nutrient salts in surface water has been depleted by or- 

 ganisms, lime-secreting organisms, especially plants, 

 will no longer be present in appreciable amounts. 



Solution at Depth . The degree of solution of solid 

 particles of calcium carbonate settling from the surface 

 which will take place in deep water is dependent on the 

 degree of undersaturation of that water and on the amount 

 of water which is brought into contact with the solid par- 

 ticles of calcium carbonate. Over great depths these 

 particles will thus be more likely to dissolve than in 

 shallow water simply because they are brought in contact 

 with more water during the period of settling and, simi- 

 larly, particles which are kept in suspension by turbulent 

 currents will tend to be dissolved more completely. Mur 

 ray and Irvine (1891) have set up the following mechanism 

 for the correlation between calcium carbonate content 



