196 



COOKE 



2360 



g 2300 



o 

 E 

 o 

 o 



E 



2240 



2180 



204 to 540 atm 



Increase from 

 204 to 272 atm- 



62.5 125 187.5 250 312.5 375 437.5 



TIME, min 



Fig. 4 The effect of pressure rise on steady state. 



and the process can be repeated as many times as the upper pressure boundary 

 (334.1 atm) will allow. In this case the size of the pressure differential 

 determines the rate of removal of the previously precipitated phase, because the 

 solubility of the phase increases markedly with pressure. The greater the pressure 

 jump, the more rapidly the previously stable phase is dissolved and removed. It is 

 quite important to comment at this stage that the analytical calcite itself has not 

 undergone any dissolution. At no time while the pressure is below 334.1 atm 

 does the system fail to come back to steady state after a pressure change. If any 

 net dissolution were proceeding, a steady state could not be achieved. What if 

 the pressure is raised above 3 34.1 atm? When the pressure is raised somewhat 

 above this value, there is a rapid loss of newly unstable material which is 

 followed by a return to a total C0 2 value in the effluent seawater which is 

 clearly above background and thus due to the dissolution of the analytical 

 calcite. Thus, above 334.1 atm in this particular system, steady state can no 

 longer be achieved because the composition of the basement calcite is fixed at 

 100 ppM Mg 2+ , and a more stable, or less soluble, calcite, containing less than 

 100 ppM Mg 2+ , cannot precipitate upon the original surface because the original 

 surface must dissolve. These observations will be discussed further in the 

 following paragraphs. 



Analytical calcite showed dissolution only above a pressure of 3 34.1 atm in 

 Sargasso Sea water, but it was able to achieve steady state with this water at any 

 lower pressure. When the crossover pressure is approached from above in the 

 same system and total C0 2 is measured at selected pressures above 3 34.1 atm, 

 the system behaves as shown in Fig. 5. This experiment was performed many 

 times over a period of 15 months and gave the same results as long as the same 

 seawater and calcite were used. A line of best fit was drawn through the points 



