SECT. 1] THE OCEANS AS A CHEMICAL SYSTEM 21 



pressure effect upon the saturation value of barium, assuming a constant 

 sulfate concentration, results in a value 2.5-fold greater at depths around 

 5000 m than at the surface. Thus, if one uses a sulfate ion concentration in sea- 

 water of 28 millimolar and activity coefficients of divalent ions of 0.1, a theo- 

 retical value of 70 [j.g of barium per liter is computed for oceanic depths of 

 4000 to 5000 m. Although saturation values of barium are not approached in 

 surface ocean waters, they may be reached at greater depths. 



What types of reaction then govern the depth distribution of barium? The 

 gross similarity of the concentration profile of this element with those of the 

 nutrient species suggests a relationship of barium to the biochemical cycles in 

 the sea. The release of high concentrations of sulfate ions during the oxidation 

 of organic sulfur in biological materials can result in a subsequent formation 

 of barium sulfate within the decomposing biophase. As simultaneous sinking 

 and combustion occur continuously in this organic micro-environment, part of 

 the incorporated barium may be returned to sea-water, where this element is in 

 an undersaturated state. The net result of such a process would be the con- 

 veyance of barium from shallow to deep waters. Part of the barium may end up 

 in the sediments. In fact, concentrations of this element are markedly higher 

 in pelagic sediments below biologically productive oceanic areas than in bottom 

 samples below the more barren seas (Goldberg and Arrhenius, 1958; Goldberg, 

 1958). The high barium contents are associated with both siliceous and cal- 

 careous deposits, although the concentration of this element is not markedly 

 high in the siliceous or calcareous hard parts of organisms. The barium is 

 probably accumulated in the sediments through these chemical reactions 

 involving the organic debris. 



A second example of a vertical distribution reflecting chemical processes in 

 the ocean may be found in the depth profiles of 226 Ra, which show a similar 

 distribution to those of barium, i.e. an increase in the abundance of this isotope 

 going from the surface to less shoal waters. This radionuclide is a member of 

 the 238 U series and its immediate parent is 2 30Th (ionium) : 



4.5xl0 9 y 24d _ 6.7h T 250,000v _, 80,000 y „„„,, 



238XJ 1 234Xh > 2 34p a > ^XJ — ■ — > 2 30Th — — * 226 Ra 



a ft ft a a 



1600 y 



„ ].64xl0-'s 20.8111 ^ 10 111 3.1m _ 3.8(1 ' 



210pb < 214p < 214Bi < 214p D < 218p < 222R n 



a ft ft a a 



Koczy et al. (1957) noted that the 226 Ra is in excess by about six-fold over the 

 amount which could be supported by 230 Th. Ionium is removed quite rapidly 

 from sea-water after its formation (the residence time of thorium is about 

 350 years) to the solid phases on the sea-floor. 



To account for these increases, Koczy and co-workers suggested that the 

 radium diffuses into the sea-water subsequent to its birth from ionium in the 

 sediments and was subject to a vertical transport from the bottom due to eddy 

 diffusivity. The total amount of radium given off by the sediments was calcu- 

 lated to vary between 1.2 and 1.8 x 10~ 10 g of Ra/m 2 /year. Further, many of 



