solution and solid phases, but also the time scale or kinetics by which 

 the exchange occurs. 



In this consideration, an ion exchange has been identified as a process 

 separate from adsorption; however, in practice these two are closely 

 related, and the considerations of adsorption apply to a large extent to 

 ion-exchange processes. 



Precipitation and dissolution — The distribution of trace elements is 

 affected by their incorporation into solid phases such as calcium car- 

 bonate (Thompson and Bowen, 1969), opal (Chester, 1965), and various 

 metal hydroxides (Cronan and Tooms, 1969). The movement of these 

 solids through the oceans and their alteration, deposition, and/or solu- 

 tion provide mechanisms for the transport of trace elements. 



The incorporation of trace elements into calcium carbonate and opal 

 is probably biologically controlled, and the transport of trace elements 

 in these solid phases can be studied through analytical determinations 

 of their concentrations in calcareous and siliceous tests, coupled with 

 a knowledge of the rates of precipitation and solution of calcium car- 

 bonate and opal in the ocean. Our knowledge of trace-element concentra- 

 tions in calcareous and siliceous tests and of the rates of solution of these 

 tests is negligible. For instance, the chemical and biological factors 

 influencing the oceanic distribution of barium must be known in studies 

 involving the radioactive tracer radium-226, but the concentration of 

 barium in calcareous and siliceous tests is not well known. 



The properties, rates of reactions, and distribution of metal hydroxides, 

 such as iron and aluminum, in seawater and in interstitial waters, are 

 poorly known. These compounds deserve further study because of their 

 importance in the formation of ferro-manganese nodules (Mero, 1965) 

 and in the alteration of clays (Sillen, 1961). 



Gas bubbles — Gas, bubbles in the ocean have been considered in 

 connection with their eff'ect on the injection of air into seawater (Bieri, 

 1971; Craig and Weiss, 1971) and on gas exchange (Kanwisher, 1963; 

 Atkinson, 1972). In shallow water, gas bubbles may be produced by 

 biological processes such as oxygen production from photosynthesis 

 and hydrogen sulfide and methane generation from anaerobic decompo- 

 sition of organic matter. Extension of these studies, which have been 

 carried out largely through analyses of noble gas saturation anomalies, 

 will most likely continue to reveal unique information on the role of 

 gas bubbles in the ocean. 



It is generally assumed that bubble formation by /// situ production 

 is inhibited by hydrostatic pressure; however, it is still necessary to 

 consider conditions that can stabilize small gas bubbles under relatively 



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