90 BROECKEK [CHAP. 4 



l37 Cs seem to be the best suited for tracer studies. 90 Sr (half-life 28 years) and 

 137 Cs (half-life 30 years) have the great advantage that, prior to bomb tests, 

 they were not present in the oceans, making corrections for natural background 

 unnecessary. Also the availability of extensive data as to the time and space 

 distribution of these isotopes in terrestrial soils provides a means of estimating 

 the quantity of fallout added to the oceans as a function of geographic location 

 and time. One potential difficulty is the possibility that 90 Sr and 137 Cs are in- 

 corporated into particles which move independently of the water. Although the 

 uniform oceanic distribution of dissolved calcium (chemically similar to 90 Sr) 

 and of dissolved potassium (chemically similar to 137 Cs) suggest that such 

 transport is not important, more complete data on the depth variation in the 

 concentrations of stable Sr and Cs in the oceans would provide a more reliable 

 evaluation. In the absence of these data, the ratio of 90 Sr to 137 Cs at various 

 levels in the ocean should provide a partial check. As 90 Sr and 137 Cs are dis- 

 similar chemically they should be separated to some extent during uptake by 

 particulate matter, whereas if movement with the water is dominant little 

 variation in the ratio would be expected. 



For bomb -produced tritium the input function is less well defined. Since 3 H 

 is not retained by the soils, as is the case with 90 Sr and 137 Cs, no permanent 

 record of the quantity added to a given area is available. Estimates based on 

 tritium concentrations in rainfall are not nearly so reliable because of the 

 limited areal and time coverage. Although the amount of bomb tritium added 

 to the surface-water systems probably exceeds that present from natural 

 sources by a factor of at least three, the uncertainty in the natural background 

 values will make interpretation difficult in many cases. 



Although the quantity of 14 C added to the carbon cycle by bomb testing is 

 small (approximately 1% of the natural inventory), it will nevertheless prove 

 very useful as a tracer. As a result of 14 C production by tests carried out before 

 the moratorium in the fall of 1958, the 14 C concentration in surface ocean water 

 may rise as much as 10% above pre-bomb levels. As the natural background is 

 known to about ±1% in many areas and since measurements can be made to 

 + 0.5% or better, the bomb 14 C should be measurable in surface and near- 

 surface masses. 



3. Steady-State Distribution of 14 C 



Since at this time only natural radiocarbon data are available in sufficient 

 quantity to warrant large-scale model calculations, the mixing rates in ocean 

 circulation models proposed to date will be evaluated on the basis of radio- 

 carbon data alone. Where other data are available they will be used as a 

 cross-check. 



The first question which arises in the consideration of oceanic mixing pro- 

 cesses is whether the ocean can be approximated as a steady-state system 

 (patterns and rates of mixing remain essentially constant with time). Un- 

 fortunately no definite answer can be given. However, since the complications 



