FACTORS CONTROLLING CO, IN OCEANS AND ATMOSPHERE 



37 



River input 



100 moles 

 rrf 2 10" 3 years 



[Ca 



2 + - 



[CO^ 



ksp- 



_9 _T 



CaCO., production, 500 moles m 10 years 

 Super saturation 



Calcite compensation J — 1 \ level 

 CaCO, 



Undersaturation 



CaC0 3 dissolved, 



400 moles rrf 2 1 0" 3 years 



preserved, 

 100 moles 

 m" 2 10" 3 years 



Y\%. 4 The behavior of carbon in the sea. Carbonate-ion concentration at 

 compensation level determined by carbon economics and calcium content of 

 the sea. 



carbon deficit that could not be maintained for long. The ocean solves this 

 problem with a feedback mechanism which adjusts its chemistry such that 

 roughly 80% of the ocean floor is bathed in waters corrosive to CaC0 3 (i.e., 

 undersaturated); only 20% of the ocean floor is bathed in water that is 

 supersaturated. The so-called calcium carbonate compensation level that 

 separates sediments low in calcium carbonate from sediments rich in calcium 

 carbonate can move up and down such that this overproduction is always taken 

 back. We now have the major mechanism for carbon balance in the ocean. 

 Carbon balance is controlled by bringing the carbonate-ion concentration to 

 such a value that an appropriate fraction of the ocean floor is bathed in 

 undersaturated water. If the ocean were to start disposing of too much of the 

 calcium carbonate produced, the carbonate-ion content of the ocean would be 

 lowered. The compensation level would then rise. Now the question is, "Now do 

 we know it is not the calcium content that changes?" On the time scale I am 

 talking about, this would not be possible. Calcium has a residence time in the 

 ocean of at least a million years, whereas the carbonate-ion adjustment time is 

 roughly 10 thousand years. So, for any short transient, it will be the 

 carbonate-ion content that adjusts and not the calcium content. The shape of 

 the COl" vs. depth curve is fixed by the life and mixing cycles; the absolute 

 value is fixed by the ocean's need to take back the right amount of calcium 

 carbonate in order to keep its budget in balance. Now this alone does not 

 determine the carbon content of the ocean because carbonate ion makes up only 

 about 8% of the total dissolved carbon in the ocean. Most of the carbon is in the 

 form of bicarbonate ion. 



We have to fix one other parameter in this complex system. To do so we 

 must consider the alkalinity of the ocean. We know that in the ocean there is a 



