Chapter 11 Tracer Studies of the Sea and Atmosphere 105 



carbon. Anderson (1953) who has made the displayed by the terrestrial plants and the atmos- 



most recent and detailed considerations of the phere. 



production rate, finds a rate of 2.6 carbon 14 The evaluation of the exchange time of COj 



atoms per cm^ and per sec. between atmosphere and sea from data on the 



The carbon 14 atoms are oxidized to CO2 and natural distribution of C^*, is discussed in See- 

 thus enter the normal geochemical and biologi- tion IV of this paper. 



cal cycles of carbon via the atmosphere. The The most recent and accurate measurement of 



distribution through the atmosphere and the the absolute radiocarbon concentration is that 



terrestrial plants is rapid, and the steady state of Suess (1955), based on comparisons with an 



radiocarbon concentration in these reservoirs is absolute standard obtained from the National 



taken as the basis for the so-called "modern" Bureau of Standards. Suess finds a concentra- 



specific activity of carbon, namely about 15 dis- tion of 1.238 x IQ-^^ atoms of O* per atom of 



integrations per minute per gram of carbon. carbon for average 19th century wood, corrected 



On the other hand, the transfer of carbon for decay to the present date and corrected for 



from the atmosphere to the sea is slow enough, isotopic fractionation. Based on this value and 



compared to the half-life, to produce a signifi- a half -life of 5568 years, we give below the 



cant difference between the predicted and ob- amounts of C" in metric tons, and the activi- 



served activity of carbon in the surface layers ties in megacuries present in the major reser- 



of the oceans. Carbon, as one of the lighter voirs on the earth (Craig, 1957 (a), calcu- 



elements, is subjected to natural fractionation lated from his Table 1). The figures for the 



of its isotopes in the various reactions it under- atmosphere and terrestrial living matter are nor- 



goes in its biogeochemical cycle (cf. section on malized for isotopic fractionation, while the or- 



stable isotope variations, below). The steady ganic and inorganic carbon in the ocean was 



state isotopic separation of the stable isotopes assumed to have an average age of 600 years 



C12 and C13 produces a C" concentration in relative to corrected 19th century wood, or 200 



surface ocean water bicarbonate and shell car- years relative to surface ocean bicarbonate (see 



bonate which is about 2.5 per cent higher than Section IV, this paper) . 



the C^^ concentration in terrestrial plants. It is Total C* Total activity 



thus known that the C^* concentration in ocean Reservoir metric tons megacuries 



bicarbonate and carbonate should be about 5 per TMrkT' IwTng ' matter' + ^'^"^ 



cent higher than the concentration in land humus 2.2 11.0 



plants, namely about 15.75 disintegrations per Ocean: Total organic matter. 3.8 17.6 



minute per gram of carbon (Craig, 1954). In ^^^^'''- Total inorganic car- 



, / ° ^ \ 1 bon 49.8 228.6 



actual tact, however, measurements show that 



the specific activity of bicarbonate and carbon- Totals 56.8 261.6 



ate from the ocean is about the same as the spe- ^^^ ^^^^^ ^^^j^j^^ ^f radiocarbon present on 

 cific activity of land plants. Thus the atmos- ^^^ ^^^^^^ ^^^^ corresponds to some 260 mega- 

 pheric C" activity has been increased 5 per ^^^^^^^ practically all of which is in the ocean, 

 cent by slow exchange of CO, between atmos- Using Anderson's figure for the production rate, 

 phere and sea, resulting in an "apparent age," ^ited above, and the decay constant of 3.945 x 

 relative to wood standards, of 400 years for the ^q-^^ ^^^^-^^ the calculated total inventory of 

 bicarbonate and carbonate shells in the surface radiocarbon on the earth is 78.4 tons, which 

 layers of the ocean (Craig, 1954, 1957 (a)), differs from the figure of 56.8 metric tons, ob- 

 Some 10 measurements have now been made on tained in Table II, by about 28 per cent. How- 

 marine plants, animals, and sea-water from the ever, the production rate, as estimated from cos- 

 Atlantic (Suess, 1954) and from the New Zea- rnic ray data and the counting of atmospheric 

 land area (Rafter, 1955) which indicate that neutrons, is uncertain to at least 20 per cent be- 

 the radiocarbon "age" of surface marine car- cause of the uncertainty in the reactor flux from 

 bonate is about 400 years; it is thus clear that which the neutron counters are calibrated. More 

 radiocarbon age determinations made on deep recent estimates of the production rate are lower 

 ocean waters must all be referred to this base- than the figure cited above and all that can be 

 line, rather than to the modern specific activity said about the agreement between the calculated 



