110 



Atomic Radiation and Oceanography and Fisheries 



in the vapor pressure data is such that the 

 value could hardly be less than about 20°. The 

 delta values for fresh waters show a general 

 correlation with latitude or distance from the 

 ocean; there is a general decrease in the heavy 

 isotope concentration as the latitude varies from 

 equatorial to polar, reflecting the continuous 

 loss of vapor from the poleward moving air 

 masses. 



Isotopic variations such as mentioned above 

 can be measured quite simply and precisely 

 with the mass spectrometer, and it is evident, 

 from the ranges of variation cited, that such 

 studies on meteoric waters can provide a wealth 

 of information concerning meteorological trans- 

 fer and mixing phenomena in the atmosphere. 

 The average water vapor of the earth has 

 roughly the composition 8D=— 10%, 80^ » = 

 — ll%o, but large variations, related to the 

 amount of liquid water which has condensed 

 out of the vapor, occur, and thus such studies 

 are directly adapted to problems of water vapor 

 transport over both the oceans and continents. 



The situation in the oceans themselves is 

 somewhat more complicated. The oxygen iso- 

 topic composition of ocean waters has been 

 studied by Epstein and Mayeda (1953), and 

 the deuterium variations in the same samples 

 by Friedman (1953) ; these writers also an- 

 alyzed nine fresh water samples and first eluci- 

 dated the D-O^^ relationship in natural waters. 

 The surface layers of the oceans are in general 

 enriched in the heavy isotopes relative to mean 

 ocean water because of the net storage of HoO^*' 

 in the stagnant and circulating fresh water and 

 vapor; the extent of this enrichment reflects 

 the hold up at the boundary of the mixed 

 surface layer, namely the thermocline. On the 

 other hand, the deeper layers of the ocean are 

 depleted in deuterium and oxygen 18, relative 

 to mean ocean water, because of the influx of 

 glacial melt water in polar latitudes, the glacial 

 waters having 8 values at the lightest ends of 

 the ranges cited in the preceding paragraphs. 

 Thus the oceans are isotopically upside down 

 with the heavy isotopes concentrated at the 

 surface, and the isotopic composition parameters 

 in general correlate with salinity. 



Epstein and Mayeda (op. cit.) showed that 

 the salinity-oxygen 18 variations in marine 

 waters were consistent with a model in which 

 the oceanic precipitation is progressively de- 

 pleted in the heavy isotopes as a function of 



the extent of precipitation from the local atmos- 

 pheric reservoir. Salinity, of course, is uniquely 

 related to the direct amount of fresh water 

 removed by evaporation or added by meltwater 

 dilution, but the relationship in the case of 

 isotopic composition is more complex. This is 

 because the isotopic composition of fresh water 

 precipitating over the oceans, or added by run- 

 off or melting of ice, is variable, depending on 

 the history of the air mass from which it was 

 precipitated. The correlation between isotopic 

 composition and salinity is therefore more or 

 less local, reflecting the particular relations ob- 

 taining on the average in the area. As a result, 

 the isotopic composition parameters, rather than 

 being simply transforms of salinity, and thus 

 not inherently very useful for the study of 

 transfer problems, become important parame- 

 ters for such studies because of the reflection 

 of areal conditions in a manner diff^erent from, 

 but related to, the salinity parameter. Examples 

 of this eflfect are discussed in Part IV, where 

 applications to transfer studies are treated. 



The isotopic composition of atmospheric 

 oxygen is an interesting case of adjustment 

 of a reservoir composition to steady state non- 

 equilibrium biogeochemical transfer processes. 

 Oxygen would exist in the atmosphere in the 

 absence of living plants because of photodisso- 

 ciation of water vapor in the atmosphere, with 

 subsequent escape of hydrogen from the earth. 

 However, oxygen is cycled through the bio- 

 sphere so rapidly that its isotopic composition, 

 rather than reflecting its mode of formation, 

 may be adjusted to a steady state balance be- 

 tween photosynthetic formation and respiratory 

 uptake. The oxygen produced in photosyn- 

 thesis is in isotopic equilibrium with the water 

 taken up by the plants and is very close in 

 isotopic composition to this water; however 

 the atmospheric oxygen is some 23%o enriched 

 in oxygen 18 relative to average ocean water. 

 Lane and Dole (1956) have measured the 

 preferential uptake of oxygen 16 by various 

 animals and land plants and concluded that the 

 net fractionation is such as to account quantita- 

 tively for the atmospheric oxygen composition. 

 Respiration in the oceans shows a much smaller 

 selective oxygen 16 uptake (Rakestraw et al., 

 1951; Dole et al., 1954) and the isotopic com- 

 position of oxygen dissolved in ocean water is 

 variable and dependent on the amount of oxy- 

 gen which has been taken up from the local 



