FACTORS CONTROLLING C0 2 IN OCEANS AND ATMOSPHERE 33 



sediments is on the order of one hundred million years. This time constant is 

 dictated mainly by the great plate motions on the surface of the earth. The other 

 important time constant is the residence time of carbon in the 

 ocean— atmosphere system. How long does it remain in this system before being 

 removed? We know from the amount of carbon in the ocean— atmosphere 

 system and by rough estimates of how much is being added by rivers to the sea 

 that it remains for about one hundred thousand years. Once trapped in the 

 sediments, a carbon atom remains there 10 years, and, when finally released, it 

 remains in the ocean only 10 s years before being removed. If we run carbon 

 around such a cycle, we will come up with a distribution coefficient of one part 

 in the ocean— atmosphere reservoir to one thousand parts in the sediments. By 

 contrast, chloride, which is very soluble in the sea and has a very low probability 

 of removal, is roughly equally distributed between the sea and the sediments 

 with about a 10 -year residence time in sediments and roughly a 10 8 -year 

 residence time in the sea. 



What goes on during the 10 5 years that a carbon atom spends in the 

 ocean— atmosphere system? To understand this we must have some idea of the 

 carbon cycle within the ocean. We will ignore the atmosphere to a large extent 

 because I feel that for time scales of hundreds of years or more, the atmosphere 

 is largely at the mercy of the ocean. The ocean dictates to the atmosphere what 

 its C0 2 content should be. It is only when we have short-term transients (such as 

 the current consumption of fossil fuels) that the atmosphere plays an important 

 role. 



We can view ocean circulation in terms of a two-box model consisting of a 

 warm surface ocean and a cold deep ocean separated by the main thermo- 

 cline — that region where the temperature goes from about 20 C to an average 

 of about 2 C (Fig. 2). We find that across this boundary the largest chemical 

 differences occur. For instance, there is about 20% more carbon dissolved in 

 cold, deep water than in warm surface water. In deep Atlantic water, which is 

 one of the main suppliers for the Pacific Ocean, dissolved carbon content is 

 intermediate between the surface value and the deep Pacific value. This carbon 

 distribution in the ocean is created by the interaction between the main 

 circulation cycle and the biological cycle. The mixing cycle consists of a 

 northward flow of surface water through the Atlantic toward the polar regions 

 where it is cooled and sent into the deep sea. This water then travels around the 

 ocean, down the Atlantic, around Africa into the Indian Ocean, and into and up 

 the Pacific. As it goes, it upwells more or less uniformly. The deep current is 

 depleted by the upwelling that Stuiver spoke of in his advection— diffusion 

 model. The formation of deep water is localized in a small area, and upwelling 

 occurs over a broad area of the ocean. Interacting with this mixing cycle is a 

 particulate cycle. Plants manufacture various kinds of matter in the surface 

 water, and, although much of this debris is degraded by animals living in the 

 same water, perhaps 10 to 20% falls through the main thermocline into the deep 

 sea. This is why the deep sea is enriched in most chemical properties with respect 



