122 



MARINE BOTTOM SAMPLES OF LAST CRUISE OF CARNEGIE 



Summary of Facts of CaCG3 Distribution 



We may now summarize the chief facts as to the dis- 

 tribution of CaC03 in the north and southeast Pacific 

 which have been brought out in the above discussion. 



1. Sediments of high lime content cover most of the 

 bottom of the south Pacific in the area investigated (that 

 is, east of 150° west longitude, and north of 50° south 

 latitude), but there is an irregular band of sediments 

 poor in carbonate along the South American coast, and, 

 also, the sediments at depths greater than 4000 to 4500 

 meters, depending on the latitude, are usually poor in 

 lime. 



2. Except for the areas surrounding oceanic islands 

 and for certain others at depths less than approximately 

 3500 meters, almost the entire bottom of the Pacific 

 north of latitude 10° north is covered by sediments of 

 low carbonate content. 



3. Sediments of intermediate carbonate content, be- 

 tween 15 and 50 per cent, cover a much smaller total 

 area than do those either higher or lower in lime, and 

 generally form narrow borders of varying width around 

 areas of sediments rich in carbonate. 



4. The rather sharp boundary between sediments of 

 high and low carbonate content in the central eastern 

 Pacific does not follow the depth contours but lies close- 

 ly parallel to the average southern limits of the Counter 

 Equatorial Current and the Peruvian Current, approxi- 

 mately along the 34.5 per cent surface isohaline. 



5. Within any given latitude limits the average cal- 

 cium carbonate content of all sediments taken together 

 tends to be at a maximum value between 1500 and 3000 

 meters, and then decreases irregularly with increasing 

 depth, reaching a minimum between 4500 and 5000 me- 

 ters, depending on the latitude. Pelagic sediments, in 

 general, exhibit a continuous but irregular decrease of 

 average calcium carbonate with increasing depth. For 

 most depth and latitude intervals, however, the carbonate 

 contents of individual samples are widely and unevenly 

 distributed. 



6. For depths less than 1000 meters and greater 

 than 4000 meters the average carbonate content of all 

 sediments is at a maximum near the equator and tends 

 to decrease with increasing latitude toward both the 

 south and the north; for depths between 1000 and 4000 

 meters there is a more or less continuous decrease in 

 the amount of CaCOs from south of latitude 40° south to 

 latitude 60° north. On the whole, sediments of any depth 

 in the south Pacific are higher in CaC03 than sediments 

 of the same depth in the north Pacific. 



7. The average carbonate content of pelagic sedi- 

 ments is higher than that of terrigenous sediments of the 

 same depth and latitude. 



8. The average carbonate content of Atlantic pelagic 

 sediments is higher than the average of Pacific pelagic 

 sediments of the same depth. The difference between the 

 two oceans is particularly marked for pelagic deposits of 

 depths between 4000 and 5000 meters. Pacific sediments 

 of these depths contain, on the average, only one-half to 

 one-third the amount of carbonate present in similar 

 Atlantic sediments. 



9. There is an inverse relation between the contents 

 of organic matter in sediments and of CaC03, and be- 

 tween the latter and the ratio of benthonic to pelagic 

 foraminifera. These relations are interpreted to mean 

 that regions in which there is a relatively rapid rate of 

 accumulation of available organic material, and hence a 



rich bottom fauna, tend to be, other things being equal, 

 somewhat poor in CaCOs. 



Although several facts about the distribution of 

 CaCOs in the area of the Pacific investigated have been 

 summarized above, doubtless many other and more de- 

 tailed relations would be discovered on further explora- 

 tion of the physical, chemical, and biological conditions 

 of the water and of the bottom sediments, such as those 

 which have been determined for the Atlantic and which 

 are mentioned in the following discussion. 



Theoretical Considerations 



The Cycle of CaC03 . Before attempting to explain 

 the causes of the relations summarized above, let us 

 first briefly discuss the general conditions which deter- 

 mine the amounts of CaC03 in marine sediments. In the 

 last analysis, the rate of accumulation of CaC03 on the 

 sea bottom depends on the rate of increment from rivers 

 of dissolved calcium to the ocean as a whole. The dis- 

 solved salts of rivers are usually relatively very high in 

 dissolved calcium (Clarke, 1920, p. 125), whereas in sea 

 water itself the amount of calcium is relatively small 

 and bears a more or less constant ratio to the tota' S'llt 

 content. It is probable that the excess of lime carried in 

 by rivers, as indicated by the difference in the calcium 

 salinity ratios of river and ocean waters, is largely re- 

 moved by deposition of solid calcium carbonate on the 

 sea twttom and that, as suggested by Murray and Irvine 

 (1890), the sea as a whole is about in equilibrium with 

 respect to CaC03. Both accumulation from river waters 

 and withdrawal by precipitation are exceedingly slow, 

 however, and little is known of the absolute rates of these 

 processes, so that it is impossible to say whether a bal- 

 ance is actually preserved. For example, Murray and 

 Irvine estimate that the discharge of rivers would re- 

 quire 680,000 years to make up an amount of lime equal 

 to that already present in the sea. 



There is also a cycle of CaC03 in the sea itself. Car- 

 bonates are precipitated from solution in certain regions 

 and redissolved in others. Although most known fossil 

 limestones were probably deposited in shallow warm 

 waters, at the present time by far the greatest surfaces 

 where lime sediments are being deposited are covered 

 by deep water (Vaughan, 1924). The general process of 

 lime deposition in deep water is usually as follows: (1) 

 the sinking of lime shells and skeletons formed near the 

 surface toward the bottom, perhaps accompanied, as 

 Heim (1924) suggests, by CaCOs particles chemically 

 precipitated from supersaturated water; (2) the solution 

 of some or all of these at depths where the water is not 

 saturated with CaC03, and (3) further solution, together 

 with some reprecipitation, on the sea bottom. The phys- 

 ical and chemical factors which accompany and control 

 these processes are chiefly the carbon dioxide and cal- 

 cium contents, which control the hydrogen-ion concen- 

 tration, and the temperature, salinity, and hydrostatic 

 pressure of the water. 



In another paper (1934) the writer has discussed the 

 significance of these physical and chemical factors and 

 has shown that their order of importance is, first, car- 

 bon dioxide content and, then, temperature and salinity, 

 whereas the known effects of hydrostatic pressure and 

 the calcium content are of relatively minor significance. 

 In general, precipitation and absence of solution are 

 favored by the low carbon dioxide content and high 



