DISTRIBUTION OF CALCIUM CARBONATE IN AREA INVESTIGATED BY CARNEGIE 



125 



contact with them, and thereby aids the process of 

 solution. 



That some recrystallization and reprecipitation of 

 CaC03 takes place on the sea bottom is indicated by the 

 presence in many of the Globigerina oozes collected by 

 the Carnegie, as also in certain of the Valdivia samples 

 (see Murray and Philippi, 1905) of recrystallized tests 

 of several species of pelagic foraminifera, notably 

 Globorotalia tumida. and also by the distribution of 

 CaC03 in the various size grades of some of the sedi- 

 ments. It is shown in the section on mechanical analy- 

 ses that both the sand and clay fractions of certain Glo- 

 bigerina oozes contain more CaCOs than the interme- 

 diate silt grades. X-ray analyses show the fine-grained 

 calcium carbonate to be calcite, and not aragonite, as 

 would be expected if it were formed under conditions 

 comparable with those of the Bahama Banks (see 

 Vaughan, 1924). It seems reasonable, therefore, that if 

 these fine-grained materials are the result of chemical 

 precipitation, their formation might have taken place 

 under cold water conditions, probably at the bottom. 



Summary of Theoretical 

 Considerations 



In summarizing the above discussion, we may repeat 

 the statement that the percentage of calcium carbonate 

 in a bottom sediment depends on its relative rate of dep- 

 osition with respect to that of other materials, and on 

 the relative rates of removal by solution or otherwise of 

 calcareous and noncalcareous materials after deposi- 

 tion. Besides being affected by the rates of deposition of 

 noncalcareous materials, the relative rate of deposition 

 of CaC03 is also dependent on the rate of precipitation 

 in waters near the surface, and the rate of solution at 

 depth. Precipitation at the surface, though largely or- 

 ganic, is probably favored by certain physical and chem- 

 ical conditions in the water, such as high salinity and 

 temperature, and low carbon dioxide content, and it is 

 also affected by the length of time during which surface 

 water remains at the surface. The amount of solution 

 of solid particles of calcium carbonate which will take 

 place in deep water is dependent on the degree of under- 

 saturation of the water due to high CO2 content, low 

 temperature and salinity, and possibly also to hydrostat- 

 ic pressure; and on the amount of water which is brought 

 into contact with the solid particles of calcium carbon- 

 ate. The depth of the water and the intensity of turbu- 

 lence are thus factors of importance. The amount of 

 solution of calcium carbonate on the bottom itself is de- 

 pendent on the rate of interchange between the intersti- 

 tial waters of the sediments and the supernatant bottom 

 water and on the degree of undersaturation of the bottom 

 water. The velocity of the bottom currents and the 

 quantity of the bottom fauna affect these factors. En- 

 richment in CaC03 also perhaps takes place at the bot- 

 tom, due to removal by currents of the finer noncalcar- 

 eous parts of the sediments and, in certain cases, per- 

 haps to reprecipitation. 



Theoretical Explanation of Observed 

 Facts of Distribution 



Many of the facts summarized on pages 4 and 5 with 

 respect to the distribution of CaC03 in the sediments of 



the Pacific may be explained in terms of the general 

 factors affecting the percentage of CaC03 which have 

 been outlined above. The large amount of lime in the 

 sediments of the southeast Pacific, thus, is probably to 

 be correlated in part with the moderate depths of this 

 region. The irregular band of sediments poor in car- 

 bonate along the South American coast is probably owing 

 to at least three factors: (1) the masking effect of ter- 

 rigenous materials; (2) the relatively low temperature 

 of the surface waters of the Peruvian Current; (3) the 

 large amount of organic matter in the water and on the 

 bottom, which allows the development of a large bottom 

 fauna, and the decomposition of which produces carbon 

 dioxide. The small lime content of the sediments of the 

 north Pacific probably is owing to the great average 

 depth of this region. The abrupt decrease in calcium 

 carbonate in the region of the Counter Equatorial Cur- 

 rent may be related to (1) the decrease in the salinity, 

 temperature, and pH of the waters near the surface 

 which occurs here, (2) the turbulence of the water, (3) 

 the rich bottom faunas underlying the current. The de- 

 crease of average calcium carbonate content with in- 

 creasing depth is probably to be explained by a mechan- 

 ism similar to that proposed by Murray and Irvine (1891), 

 although the enormous hydrostatic pressure of deep wa- 

 ter may play some part. The irregularity of the rela- 

 tion of bottom depth and CaCOs content may be owing to 

 the varying intensities of turbulence and the varying de- 

 grees of undersaturation of the deep waters of different 

 regions, as well as to the varying rates of precipitation 

 of calcium carbonate in the subsurface waters. The de- 

 crease of average calcium carbonate content with in- 

 creasing latitude, which is observed for certain depth 

 intervals, may be explained by the low temperatures and 

 high CO2 contents of the waters of high latitudes. The 

 excess of calcium carbonate in pelagic sediments with 

 reference to terrigenous sediments of the same depth 

 and latitude may be owing to the rapid rate of accumula- 

 tion of noncalcareous materials in terrigenous deposits 

 and also to the relatively rapid rate of accumulation of 

 organic matter in terrigenous deposits. 



The differences between the average carbonate content 

 of Atlantic and Pacific pelagic sediments of the same depth 

 and between those of the south and north Pacific may also 

 be shown in terms of the above-mentioned factors. The 

 marked excess of calcium carbonate in Atlantic sediments 

 of any depth when compared withPacific sediments of the 

 same depth, thus, might be explained by the fact that the 

 deep water of the Pacific is moreundersaturated with cal- 

 cium carbonate than is the deep water of the Atlantic. For 

 example, according to the Carnegie results, the pH of 

 the Pacific deep water averages about a tenth less than 

 that of the Atlantic. Similarly, the data of Thompson, 

 Thomas, and Barnes (1934) on the vertical distribution 

 of CO2 for a deep station off the coast of Washington, 

 gave values for carbon dioxide in deep water far in ex- 

 cess of any of thope obtained by Wallenberg (1933) in 

 the Atlantic. Furthermore, the total amount of dissolved 

 oxygen in the deep water of the north and east Pacific, 

 according to the results of the Carnegie (Graham and 

 Moberg [1935], see also Moberg fl930l) and of the 

 Bushnell (Revelle, unpublished data) is much lower than 

 the amounts of oxygen at corresponding depths and lati- 

 tudes in the Atlantic. Since the sum of the dissolved ox- 

 ygen and carbon dioxide for any given water mass is 

 usually more or less constant, the low oxygen content 

 in the Pacific must be balanced by a correspondingly 



