8 GOLDBERG [CHAF. 1 



several tenths of a per cent (as opposed to a sea-water concentration of 0.03 u.g/1.) 

 and may form solid solutions with the manganese oxides. 



The fractionation of isotopes by geochemical and biological processes has 

 proven especially useful for determining the nature of the mechanisms involved. 

 The isotopic compositions of sea-waters, for example, are strongly influenced 

 by their previous involvement in condensation, evaporation, freezing and 

 melting processes. Both hydrogen and oxygen have a pair of stable isotopes. 

 iJI (protium) and 2 H (deuterium), and 16 and 18 0, which are significantly 

 fractionated in a similar way in oceanic waters (Epstein and Mayeda. 1953; 

 Friedman, 1953). Craig (1961) has pointed out that the relationship 



Xj) = 0.6359X o - 1105.6 



is approximately obeyed by sea-waters, where Xd and A r o are the atom fractions 

 of deuterium and 18 in atomic parts per million. 



The water vapors over the surfaces of the ocean are depleted in the heavier 

 isotopes relative to the condensed waters. With subsequent precipitation from 

 the vapor phase, the first rain preferentially accumulates the waters containing 

 deuterium or 18 0, causing the vapor to be further depleted in these isotopes. 

 Thus surface waters of the oceans are enriched in the heavier isotopes, a pro- 

 cess which resembles a multiple-stage fractionation (Epstein and Mayeda, 

 op. cit.) 



Friedman found that in general equatorial surface waters are enriched in 

 deuterium relative to higher latitude waters and pointed out that the evapor- 

 ated lighter waters were deposited subsequently as rain nearer the polar 

 regions. Also, surface waters of the ocean tend to be enriched in the heavier 

 isotopes relative to deeper waters and such results are attributed to the polar 

 origins of the deeper waters. 



The isotopic composition of the dissolved oxygen gas has been determined 

 by Rakestraw et at. (1951), who found that the 18 0/ 16 ratio mirrored the total 

 dissolved oxygen with a maximum in the ratio as a function of depth closely 

 coinciding with the oxygen minimum. They attribute these results to a pre- 

 ferential metabolism of 16 over 18 by marine vegetation, plankton, bacteria 

 and other sea life. 



The isotopes of nitrogen and sulfur also show promise of extending our 

 knowledge of oceanic chemical reactions. Richards and Benson (1961), on the 

 basis of N2/A and 29 > 28N 2 / 28 . 28j^ 2 ratios, have postulated the existence of 

 nitrogen gas produced by the reduction of nitrate ions in the anaerobic environ- 

 ments, the Cariaco Trench, the Caribbean Sea and Dramsfjord, Norway. 

 This biogenic nitrogen attains values of 0.5 ml/1, in the more reducing zones of 

 these waters. Thode, MacNamara and Fleming (1953) have observed a frac- 

 tionation of the sulfur isotopes, 32 S and 34 S, in the precipitation of sulfides 

 and sulfates in the marine environment in which the sulfides have been 

 depleted and the sulfates enriched in the heavier isotope. 



