General Considerations 



15 



brought to the surface by this process. Upwell- 

 ing occurs along the western coasts of continents 

 in intermediate and low latitudes, wherever the 

 wind-driven circulation removes surface water 

 from the coast. This water is replaced by deeper 

 water moving upward. Such coastal upwelling 

 has been found to be of the order of 1 to 3 

 meters per day. Upwelling also occurs in mid- 

 ocean where there are surface current diver- 

 gences, most notably along the equator in the 

 eastern and central Pacific. 



In regions of surface convergence, where 

 sinking waters may extend to the oceanic depths, 

 or may spread out at intermediate levels, ac- 

 cording to their density. In tropical and tem- 

 perate latitudes such sinking is confined to the 

 upper few hundred meters, but at high latitudes 

 the waters may reach great depths. Indeed, it is 

 in the convergence regions of high latitudes 

 that much of the intermediate and deep water 

 of the oceans are formed. 



In regions where increase of surface density 

 by evaporation, freezing out of ice, or cooling, 

 causes the surface waters to sink and be replaced 

 by the formerly deeper water. Deep thermal 

 convection occurs in high latitudes and extends 

 in some areas to the bottom ; for example, Ant- 

 arctic bottom water is formed in the Weddell 

 Sea by the cooling and sinking of the surface 

 waters, and the Atlantic deep water is formed 

 in a similar manner east of Greenland. Haline 

 convection takes place in regions where evapora- 

 tion exceeds precipitation or where freezing 

 prevails over melting. The latter in high lati- 

 tudes increases the intensity of the thermal 

 convection. Haline convection in winter is re- 

 sponsible for the characteristics of the deep 

 water of the Mediterranean Sea. This water 

 flows out into the North Atlantic at depths of 

 1,000 to 1,500 meters, and can easily be identi- 

 fied even on the western side of the ocean. 



The exchange between the surface layer of 

 the ocean and the deeper layers may be either 

 continuous or discontinuous. Some idea of the 

 rate of exchange can be obtained from various 

 estimates of the "age" or average residence time 

 of the water in the deeper layers. These es- 

 timates, which differ widely depending on the 

 data and assumptions used, have been sum- 

 marized by Wooster and Ketchum (Chapter 4 

 of this report) and by Craig (Chapter 3) . 



Three estimates for the water in the inter- 

 mediate layer of the Atlantic Ocean give resi- 



dence times between 7 and 140 years. Estimates 

 for the water below 2,000 meters vary from 50 

 to 1,000 years. An estimated upper limit based 

 on the measured heat flow through the sea floor 

 under the Pacific Ocean indicates that the Pacific 

 deep water is replenished in less than 1,000 

 years. The deep water in the Pacific may be 

 older than in the Atlantic because of the larger 

 volume of the Pacific. 



EXCHANGE FROM CONFINED BASINS 



The few data available for estimating the age 

 of water in confined basins have been considered 

 by Wooster and Ketchum (Chapter 4). These 

 indicate that the mean residence time of water 

 in the Mediterranean Sea is about 75 years. In 

 the Caribbean Sea the mean age cannot be less 

 than 6 years and, in the deeper part, may be 

 as much as 140 years. The deep waters of the 

 Black Sea apparently remain isolated for very 

 long periods. Transport considerations lead to 

 an estimated age of at least 2,500 years, while, 

 from consideration of phosphorus accumulation, 

 the age has been estimated at 5,600 years. 



VI. Biological Processes and Radioactive 

 Materials 



Uptake and accumulation of elements in organ- 

 isms 



Organisms take up from their environment 

 and their food and incorporate into their bodies 

 those elements required for their maintenance, 

 growth, and reproduction. The proportion of 

 various elements required by the organisms are 

 different than the proportions in the environ- 

 ment, and this results in concentrations of some 

 elements in the biosphere. 



The energy that drives the whole life cycle 

 is the energy of sunlight. This energy is bound 

 chemically in organic compounds by the photo- 

 synthesis of plants, and is passed along, through 

 the food chain, in the food of all the organisms 

 beyond the plants. The flux of energy, and 

 hence the flux of carbon, through the various 

 trophic levels measures the productivity of the 

 organisms at each level. Since the efficiency at 

 each stage of the chain is low (of the order of 

 10 per cent to 30 per cent) the flux decreases 

 at each step. The standing crop, or biomass, 

 of organisms at the different levels, or, in other 

 words, the amount of carbon present in the or- 

 ganisms at each level, may be greater or less 



