38 BROECKER 



slight excess of major cations over the major anions. The excess (or alkalinity) is 

 balanced by bicarbonate and carbonate. When a silicate rock that contains 

 sodium, magnesium, calcium, or potassium dissolves and reaches the ocean, the 

 anions balancing the cations will be bicarbonate or carbonate (instead of SiO]!} 

 or OH ). The ions in the rock, mainly silica, go into solution in neutral form and 

 do not balance any charge. There can be OH ions balancing the charge, but OH 

 combines rather rapidly with atmospheric C0 2 and makes bicarbonate; so it 

 ends up that charge balance is accomplished by bicarbonate ion. The C0 2 from 

 the air combines with rocks to release these cations; they are balanced in rivers 

 by HCO3 . The silica goes down the rivers into the ocean in neutral form. In the 

 ocean there must, on a fairly short time scale, be a means of removing these 

 cations because at the rate at which these excess cations are entering the sea 

 their excess (i.e., the oceans's alkalinity) would double every hundred thousand 

 vears. Therefore on a hundred thousand vears' time scale, the ocean must be 

 getting rid of these extra cations. Many people believe that there is some sort of 

 reverse weathering process in operation; these ingredients are reacting some- 

 where in the pores of sediments to form minerals which, although not identical, 

 would have the same bulk oxide composition as those which were being 

 weathered. The critical thing here is that, if the CO; gas content of the entire 

 ocean rises, then the CO? content of the atmosphere will rise and will in turn 

 increase the rates of weathering and reduce the rates of precipitation into 

 sediment pore waters. On the other hand, if the CO; content were lowered, it 

 would reduce weathering rates and increase precipitation rates (Table 1). We 

 could express this in terms of pH if so desired, but that would just be using 

 another variable to say the same thing. What 1 suggest, then, is that the 

 dissolved content of the ocean on a hundred thousand years' time scale must 

 come to that value such that the gain and loss of the major cations are balanced 

 (Table 2). If this is so, then we have fixed the chemical composition of the 

 ocean. The carbonate ion is fixed by the economics of the element carbon; the 

 C0 2 is fixed by the economics of the excess cations. Chemical equilibria within 

 the ocean then fix the amount of bicarbonate: C0 2 + CO3 + H 2 ^ 2 HCO3. 

 Once we have fixed the amount of bicarbonate, we have fixed the pH: 

 CO3 ^ if + HCO3. Once we have fixed the amount of C0 2 in water in contact 

 with the air, we have fixed the C0 2 content of the air. The carbonate alkalinity 

 of ocean water is also fixed because it is the sum of the charges balanced bv 

 HCO3 ana " CO3 . The total dissolved carbon content is the sum of the three 



— O — 



carbon species (C0 2 , HCO3, CO3 ). If we fix carbonate ion and dissolved C0 2 

 contents, then we have fixed the carbon chemistry of the ocean— atmosphere 

 system. 



If the system operates in this way, then it is very sensitive to all sorts of 

 environmental changes. If we were to double the rate of oceanic mixing, then we 

 would bring up twice as much phosphorus, nitrogen, and silica, and life would 

 respond, as we have seen in our lakes, to meet the challenge and fix all this 

 material into plants. That would make twice as much debris falling down, the 



