4 LIVINGSTONE 



Takahashi's estimate of total annual photosynthesis at sea is the classic one 

 of Riley, which is now known to have been too high. The gross photosynthesis 

 of the sea is more nearly 50 than 500 times the river input, but there is no 

 reason to expect it to bear any particular relation to any particular input, 

 atmospheric or riverine. The carbon dioxide involved comes mostly from 

 internal recycling, in large part recycling within the surface mixed layer, 

 although there are important inputs by turbulent transfer from the deep water 

 and in places from upwelling as well. There is, in principle, no reason why a 

 marine photosynthetic system should not be completely cyclic with respect to 

 carbon, and no reason at all to believe that the real ocean requires an annual 

 diffusive input equal to the amount of carbon used in photosynthesis. 



The diffusive flux from the atmosphere is enormous, but it is offset by an 

 opposing flux of comparable size. Through most of geologic time, the sea has 

 been a source, not a sink, for atmospheric carbon dioxide. The calcium carried 

 down the rivers to the sea has not accumulated there but has been precipitated 

 as carbonate, releasing carbon dioxide equivalent to the carbonate precipitated. 



A sudden and massive injection of carbon dioxide into the atmosphere 

 would, of course, reverse the net flow across the sea surface and dissolve oceanic 

 carbonates to establish a new steady state. If all the industrial increase in 

 atmospheric carbon dioxide had occurred in a single year, there is no question 

 that it would reverse the diffusive flow. If the atmospheric carbon dioxide were 

 equilibrating quickly with the enormous mass of oceanic carbon, even the slow 

 addition that has taken place since 1860 would reverse the net diffusive 

 exchange. Actually, however, this is approximately the situation only in the 

 polar oceans. Elsewhere the equilibration takes place initially with only the 

 shallow mixed layer containing a mass of carbon comparable with the mass in 

 the atmosphere. The exchange with deep-ocean water is a very much slower 

 process, so that any increase in the partial pressure of carbon dioxide in the 

 atmosphere will very quickly engender a corresponding increase in back partial 

 pressure from the sea. With the atmospheric carbon dioxide increasing at a rate 

 of 0.5% per annum, we must be very close to the point of reversing the net 

 diffusive exchange across the sea surface. Given the uncertainties in some of the 

 data, we may even have passed it. It is by no means self-evident, however, that 

 the sea has become a net sink of carbon dioxide already, although a 

 progressive slowing down in the rate at which it normally delivers carbon to the 

 atmosphere has been enormously effective in buffering the atmosphere against 

 the effects of industrial combustion, and the enormous diffusive exchange with 

 seawater has been a major control of atmospheric-carbon-isotope geochemistry. 



Encyclopedia accounts are biased in the direction of simplicity, and 

 probably the carbon geochemists are really not as enamored of simple 

 quasi-equilibrium models as their accounts for nonspecialists suggest. As an 

 interested and not completely naive observer, however, I would be happy to see 

 more obvious concern with the complex steady-state fluxes through green plants 

 and humus, with carbonic acid in soil water and its reactions with rocks, 



