202 COOKE 



concentration increased, or any combination of these, to bring the system to 

 steady state and prevent the disappearance of the stable phase by dissolution. If 

 the solid solution is lower in magnesium than is required for steady state at a 

 given temperature, pressure, and seawater composition, then the pressure must 

 be raised, the temperature dropped, or the magnesium concentration in liquid 

 solution decreased, or all of these, to bring the system to steady state and 

 prevent the formation of a stable higher magnesium phase by precipitation. 



The effect of temperature is now being investigated. It is a classical variable 

 in this type of thermodynamic analysis and is not expected to behave unusually, 

 but this remains to be seen. 



In the natural system, if these observations are correct, the temperature and 

 the pressure and the seawater composition determine the depth at which calcites 

 of different composition can be stable, where they will become unstable, how 

 rapidly the differing compositions will dissolve, and what range of solid-solution 

 compositions can exist in the sea. As the calcite-seawater system is only in 

 equilibrium or steady state when the chemical potential of each component in 

 the one phase is equal to the chemical potential of the same component in the 

 other phase, the ultimate analysis of the system will lie in determining its phase 

 relations. 



Chemical-reaction boundaries are only part of the problem of explaining 

 calcareous sediment distribution. The distribution is a function of biological 

 activity at the surface, oceanic circulation and local turbulence at all depths and 

 the time period during which postdepositional changes can occur, and all the 

 other factors discussed in this paper. This combination of physical and chemical 

 effects will determine the amount of C0 2 as carbonate minerals reaching and 

 remaining in the sediments, and the amount dissolving and returning to the 

 oceanic carbonate system. 



REFERENCES 



1. G. Arrhenius, Pelagic Sediments, in The Sea, Vol. Ill, pp. 655-718, Wiley-Interscience, 

 Inc., New York, 1963. 



2. M. N. Bramlette, American Association for the Advancement of Science, Publication 

 No. 67, p. 345, 1961. 



3. K. Turekian, Some Aspects of the Geochemistry of Marine Sediments, in Chemical 

 Oceanography, pp. 81-125, Academic Press, Inc., London, 1965. 



4. P. E. Cloud, Jr., Carbonate Precipitation and Dissolution in the Marine Environment, in 

 Chemical Oceanography , pp. 127-156, Academic Press, Inc., London, 1965. 



5. P. K. Weyl, The Solution Behavior of Carbonate Materials in Seawater, Stud. Trop. 

 Oceanogr., 5: 178-228(1967). 



6. R. M. Pytkowicz, and D. N. Connors, High Pressure Solubility of Calcium Carbonate in 

 Seawater, Science, 144: 840-841 (1964). 



7. R. M. Pytkowicz, Chemical Solution of Calcium Carbonate in Seawater, Amer. Zool, 9: 

 673-679(1969). 



8. R. M. Pytkowicz, A. Disteche, and S. Disteche, Calcium Carbonate Solubility in 

 Seawater at in situ Pressures, Earth Planet. Sci. Lett., 2: 430-432 (1967). 



