The second important aspect of model calculations is the "feedback" 

 of the chemical distributions as a check on the physical models. Even 

 the simplest chemical systems such as dissolved oxygen and nutrients 

 are characterized by much more complicated and variable distributions 

 than those observed for temperature, salinity, and density. Incorporation 

 of such components into the simple physical transport models provides 

 a more stringent constraint for the model itself. 



A third important aspect of the appUcation of chemical variations 

 to physical models is the increasingly important use of natural and 

 artificial radioisotopes for to incorporate real time into the cirulation 

 models. The major radioisotopes presently useful for general circulation 

 models are carbon- 14, radium-226, and potentially silicon-32. These pro- 

 vide time scales of the order of 8000 years, 2000 years and 700 years for 

 mixing and circulation studies. Each is involved in chemical and particu- 

 late interaction processes in the sea. Both stable carbon and radiocarbon 

 are supplied to the deep sea by the oxidation of organic material and 

 the dissolution of calcium carbonate contained in the particulate flux. 

 The actual application of the radiocarbon "clock" to mixing and circula- 

 tion processes demands a most careful and detailed study of variations 

 in total dissolved inorganic carbon, dissolved oxygen, alkalinity, and 

 the stable carbon isotope carbon- 13, together with the measurement 

 of specific activity variations of radiocarbon in the sea. Similar con- 

 siderations apply to radium-226, which is involved in the particulate 

 interaction problem and is introduced into the sea by diffusion from 

 bottom sediments, and to silicon-32, which is involved in the stable 

 silicon cycle in the sea. 



As important class of constituents for model studies is the group of 

 "stable-conservative" tracers, which difi'er in their relationships to 

 salinity throughout the sea but are not altered by in-sitii chemical pro- 

 cesses. These constituents, which include some dissolved gases, the 

 stable isotopic water molecules HDO and H2 0'^, and certain trace 

 elements, receive their initial variability at th sea surface through 

 air-sea interaction processes. Some of these tracers, such as hehum- 

 3 and helium-4, are injected into the sea at the bottom and in certain 

 specific areas such as the crests of ocean rises. These provide especially 

 valuable localized tracers for regional circulation and mixing studies 

 in deep and bottom water. 



At the present time, several types of analytical and numerical models 

 of varying complexity have been applied to the distribution of chemical 

 and isotopic species. One-dimensional diffusion and advection models 

 with in-situ production and consumption terms have been used for the 

 carbon- 12, carbon- 13, carbon- 14, oxygen and nutrient systems in the 

 Pacific with encouraging results. Simple vertical diflFusion models 

 with constant and variable diffusivities have been applied to Radon 



28 



