192 COOKE 



describe research directed at one aspect of the general problem of the change of 

 calcite solubility with variation in pressure and seawater composition. 



A general description of the solubility of calcite in seawater should explain 

 both the removal of carbonate minerals from fresh sediment at great depths and 

 the sharp reduction in the carbonate composition of water near the 4000-m 

 depth. The basic process is dissolution, but the reasons for its discontinuous 

 effect are complex. If we assume that dissolution proceeds because the chemical 

 potentials of the components of carbonate minerals in the sea are less in solution 

 than in the solid state, then dissolution should be a continuous process with 

 increase of pressure. However, dissolution is observed not to increase linearly 

 with depth. Solubility of calcite as a function of seawater composition alone has 

 been investigated by many workers, notably by Weyl 5 and Pytkowicz 6-8 in a 

 number of articles between 1964 and the present. The behavior of foraminiferal 

 tests in seawater has been investigated by Berger. 9-12 The effect of pressure has 

 been examined by Pytkowicz, 7 Cooke, 13 and Peterson. 14 Most relevant to the 

 experiments described herein are Peterson's 1 1966 article and Berger's in 

 1967. These articles indicated that the effect of pressure, perhaps alone, might 

 be sufficient to maintain the lysocline, a level in the ocean below which 

 dissolution must proceed and above which dissolution does not proceed. 



EXPERIMENTATION 



Construction of the pressure apparatus shown schematically in Fig. 1 was 

 complete in the fall of 1969. This device allows seawater of a predetermined pH 

 and salinity to flow at a controlled and measured rate through a column of 

 finely divided analytical-grade calcite. 



The kinetics of the dissolution or precipitation of calcium carbonate are 

 relatively slow; if the calcite column length is too short or the seawater flow rate 

 too rapid, the system will not be able to achieve steady state and the observed 

 gain or loss of total carbon dioxide in the seawater that has passed through the 

 reactor will be a function of flow rate and solubility and not of solubility alone. 

 As insurance that observations were being made of calcite solubility only and 

 not of kinetic behavior as well, the experiments were run with the seawater flow 

 rate approximately 25% of the rate necessary to exceed the dissolution kinetics 

 of the system. 



The analytical system is shown schematically in Fig. 2. Seawater passing 

 through the calcite column at 0.90 ml/min leaves the high-pressure system 

 through a valve and enters a manifold, after which it is pumped by precision 

 pump (Buchler) to an acid-filled decomposition flask at 0.77 ml/min. The 

 balance of the flow escapes from the manifold through an RGI flowmeter, and 

 adjustment according to the flowmeter readings allows a regulated flow of 

 seawater to pass through the reactor. The precision pump used for the steady 

 supply of reacted seawater to the decomposition flask showed a variation of less 



