sulfides is known to occur in the Black Sea, estuaries, fjords, and other 

 restricted environments, but these areas are probably too small to affect 

 the SO4 - concentration of seawater. Because of the restricted areas in 

 which Reactions 2-4 can occur in the ocean, it is likely that SO^^ is 

 increasing in concentration in seawater. 



Reaction 5. A higher degree of evaporation is required to precipitate 

 halite than gypsum. Thus, the question asked under Reaction 2 is even 

 more significant. In that no other mechanism for chemical removal of 

 sodium and chloride is known (except possibly Reaction 12), we must 

 conclude that these elements are increasing in concentration in seawater. 



Reactions 6 and 7. Reactions of some aluminum silicates have the 

 potential of removing SiOi, HCO:!^ and cations from seawater or near- 

 surface interstitial waters. Although aluminum silicates are present in 

 all oceanic environments, there is little evidence that reactions are 

 occurring. Because the exact nature of the aluminum siHcates entering 

 the sea from rivers is unknown, predictions of the kinetics or the loca- 

 tion of these reactions are impossible. 



Reaction 8. Exchange reactions of metals on clays have been thor- 

 oughly studied. They do not appear to be of sufficient magnitude to 

 regulate the concentration of ions such as K+ and Mg^^ in seawater. 



Reaction 9. The precipitation of sepiolite should occur in seawater 

 or in near-surface interstitial waters. There is little evidence that it 

 does. The regulation of Mg+" in seawater by any combination of Reac- 

 tions 6-9 is still a puzzle (Drever, in press). 



Reaction 10. Anhydrous silica (quartz, trydimite, or chrystobalite) 

 does not appear to react with seawater although it is commonly under- 

 saturated with respect to these phases. Some deep Pacific waters are 

 supersaturated with respect to quartz, yet no reactions appear to be 

 occurring there (Wollast, in press). 



Reaction II. Biogenic opal (SiOi-nHsO) is precipitated in enormous 

 quantities in surface waters of the ocean as diatom and radiolarian 

 tests. All seawater and near-surface interstitial water is undersaturated 

 with this phase, and at most only a few percent of this opal reaches the 

 sea floor. It is not known where or how this resolution occurs. Wollast 

 (in press), and Hurd (1972a,b) have suggested some mechanisms. 



Reaction 12. The hydration of basaltic glass on the sea floor and 

 subsequent reaction of the hydration products with seawater may re- 

 move cations from seawater. Little is known about these reactions. 



Many questions regarding mechanisms and locations for the regula- 

 tion of major constituent concentrations in seawater are mentioned 

 above. Although reactions that are probably vahd can be written, the 

 locations of these reactions in the ocean system are essentially unknown. 

 A summary of our ignorance on these matters is given in Table 3. 



10 



