554 CYCLES OF ORGANIC AND INORGANIC SUBSTANCES 



million rotations, which would correspond to a rather thorough 

 mixing of the liquid phase. 



Of course, only the uppermost layer of sediment is in good 

 contact with the liquid ; the lower layers of sediment are practically 

 cut off from contact with the sea water. On the other hand, each 

 particle of a marine sediment has once been in the upper layer, 

 and has even been suspended in sea water, and has thus had some 

 chance to approach equilibrium with it.* Moreover, it seems that 

 a considerable part of the present sediments have been repeatedly 

 in contact with sea water. Excepting a few areas with primary 

 rocks, it seems that most present-day weathering and erosion 

 works on old sediments which will thus eventually be re-equili- 

 brated with the ocean. 



Another objection is that the temperature is remarkably uneven 

 in the ocean, in comparison with laboratory experiments: it varies 

 from around -|-30°C close to the surface in tropical waters to less 

 than 0°C in other parts. This is especially important for equilibria 

 with solid phases; equilibria between species in solution usually 

 adjust themselves more easily. 



In a first approximation one might perhaps use some "efficient 

 solubility product," valid for the temperature where a deposit is 

 formed, usually perhaps somewhere close to 0-5°C. We may re- 

 member the "law of the cold wall": if a condensable vapor is 

 brought into an empty container with uneven temperature 

 distribution, it will condense on the coolest wall, and the partial 

 pressure in the whole vessel will correspond to saturation at the 

 temperature of the coldest wall. 



The equilibria are also influenced by the pressure, which varies 

 from 1 to 1100 atm in the ocean, the average being perhaps 200 

 atm. In general, the solubility product of a salt will increase with 

 pressure because the molar volume is less in solution than in the 

 solid (see next section). At first glance, one might then perhaps 

 believe that, in a steady column of sea water of uniform tempera- 

 ture, a solid like CaCOs would dissolve at the lower end and 

 precipitate at the higher end. This, however, would have been a 



* Dr. Shishkina (1959) reported that the interstitial water in some deep-sea 

 sediments does not change its composition over periods of 100,000 years, which 

 may indicate that the precipitate is practically at equilibrium. 



