Radiocarbon in the Environment

22 Radiocarbon in the Environment A. W. FAIRHALL and J. A. YOUNG

Downloaded by UNIV OF CINCINNATI on May 30, 2016 | http://pubs.acs.org Publication Date: January 1, 1970 | doi: 10.1021/ba-1970-0093.ch022

University of Washington, Seattle, Wash. 98105

Prior to 1950 the worldwide inventory of C was 2.2 X 10 atoms. Nuclear tests added an additional 6 X 10 C atoms to the atmosphere. This had a dramatic effect: for a time the C concentrations of the troposphere at mid -latitudes of the northern hemisphere were double their pre -1950 levels. The decline of the C levels of the atmosphere, owing to atmospheric mixing and exchange of CO with the sea, has given valuable insight into both of these proc esses. The C levels in the oceans are increasing as a result of this exchange. This increase should prove valuable as a tracer in oceanography. Other potential applications of the increase of C in the atmosphere and the sea are discussed. 14

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"Defore the development and testing of nuclear weapons the atmosphere, sea, and biosphere contained an estimated 2.16 Χ 10 atoms (cf. Table I) or 51 metric tons of C . This radioactive isotope of carbon, which decays with a half-life of 5730 years, is produced by the nuclear reaction N ( n , p ) C . For natural C the neutrons are produced by the action of cosmic ray primaries on atoms i n the upper atmosphere. Natu ral C therefore originates mostly i n the stratosphere. B y some mecha nism which is not understood the nascent C atoms eventually are oxidized to C 0 . The 315-p.p.m. of the atmosphere which is ordinary C 0 dilutes this C 0 by a factor of the order of 10 . Thus, atmos pheric C 0 has always been radioactive to the extent of about 13.5 ± 1 disintegrations per minute (d.p.m.) per gram of carbon (10, 22, 29, 34, 52), although deviations from this value amounting to several percent are known to have occurred during the past several thousand years. 30

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The nascent C mixes into the lower atmosphere where it eventually enters the sea and biosphere. Thus, a l l living things contain radioactive carbon to about the same extent. Upon their death any carbon which survives chemical decomposition gradually loses its radioactivity with the 1 4

401 Freiling; Radionuclides in the Environment Advances in Chemistry; American Chemical Society: Washington, DC, 1970.

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half-life of 5730 years. This forms the basis of the well known radio carbon dating method (34). W i t h the advent of nuclear weapons, particularly thermonuclear devices (hydrogen bombs), additional C was added to the atmosphere. This C , which we shall refer to as "excess C , " was produced by neutrons which escaped from the fireball interacting with nitrogen atoms of the atmosphere i n the same manner as the neutrons from cosmic rays. Since the bulk of this C was probably produced by a few very high energy devices exploded high i n the atmosphere, most of the excess C was likewise deposited i n the stratosphere. 1 4

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In assessing the environmental effects of the excess C it is con venient to measure the excess with respect to the natural levels of C which prevailed in the terrestrial biosphere prior to about 1950. By con vention, the natural level of C i n the terrestrial biosphere is represented by a sample of oxalic acid distributed by the National Bureau of Stand ards, weighted by the factor 0.95 to bring its specific radioactivity into agreement with measurements on 19th century wood (11). For purposes of computing C inventories we have arbitrarily adopted the value 13.5 d.p.m./gram carbon, corresponding to 5.85 Χ 10 C atoms/gram carbon, as the specific activity of the terrestrial biosphere, unperturbed by nuclear testing or by the combustion of fossil fuels during recent decades. 1 4

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W e note in passing that the total natural C inventory of 2.16 X 10 atoms which is based on this value (cf. Table I) corresponds to a C decay rate of 1.63 disintegrations/sec./cm. of the earth, considerably below the estimated production rate of C atoms averaged over the last 10 solar cycles (111 years) of 2.50 ± 0.50 atoms/sec./cm. (35). F r o m a geophysical point of view it would be very surprising if the decay rate and the production rate of C were out of balance as seriously as the dif ference between the above two numbers would suggest. It is difficult to reconcile this discrepancy by errors i n computing the C inventory since the bulk of the C is in the sea, where the C concentration relative to the terrestrial biosphere is known fairly well. Since the C inventory is directly proportional to the value assumed for the concentration of C in the terrestrial biosphere, a specific activity of 20.6 d.p.m./gram carbon would bring the decay rate into balance with the present-day production rate. However, it is doubtful that the several direct measurements (10, 22, 29, 34, 52) of the specific activity could have underestimated this quantity by the — 3 5 % required to achieve concordance. The source of the discrepancy is therefore unknown unless the present-day produc tion rate is indeed significantly higher than the average production rate over the last 8000 years, the mean life of C . 1 4

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Freiling; Radionuclides in the Environment Advances in Chemistry; American Chemical Society: Washington, DC, 1970.

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The C concentration i n samples of atmospheric COo or COo ex tracted from sea water is also conveniently measured relative to the oxalic acid standard with due allowance for isotope fractionation effects. Experimentally, the quantities which are measured are the net radio activity, A (counting rate of the sample after subtracting the background of the counting apparatus), per unit weight of carbon, of the sample and of the N B S standard; and the ratio C / C of isotopic abundances of C to C in the sample relative to a standard. Most laboratories use proportional counting of either C 0 , C H , or C H to measure A . Samples w i l l typically contain from 1 to 5 grams of carbon. The overall precision of the measurements is around 0.5%. The experimental quantities are related to three others by the following equations (12): 1 4

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In these equations ( C / C ) refers to a sample of P D B belemnite, and 8 C is a measure of the isotope fractionation of the sample; 8 C is a measure of the difference i n radioactivity between the sample and the N B S standard, uncorrected for isotope fractionation; A C is the corre sponding quantity corrected for isotope fractionation. Both δ 0 and A C are in units of per m i l (°/), but the excess C is so large, up to 1000°/oo, that it is convenient to express this excess i n percent. W e shall therefore define excess C " to be 0.1 A C . This procedure gives a correct measure of the perturbation of nuclear testing on the atmosphere and terrestrial biosphere, but in the sea it should be born in mind that pre-1950 levels of C were generally significantly less than those of the atmosphere or terrestrial biosphere. Pre-1950 surface ocean water is generally taken to have had a A C value of —50°/oo (19), and i n the deep sea A C values as low as — 3 2 0 % have been observed (5). Thus, our definition of excess C when applied to sea water underestimates the perturbation of nuclear testing by at least 5 percentage points. The absence of data on the sea i n the early 1950's prevents a direct comparison of present C levels in sea water with pre-atomic era levels. However, the measurements which were made on the sea in the late 1950's (5, 6,10,13,14,18,23, 49) are probably indicative of pre-bomb levels of C except for those samples from near the sea surface in the northern hemisphere where excess C was already being detected. 1 3

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The nuclear testing which took place i n the 1950's added an esti mated 25 Χ 1 0 C atoms to the atmosphere (27). Although this repre sents only a small increment to the total C inventory of the world, i t produced a significant perturbation on the prevailing C levels of the atmosphere since the latter contains only a small fraction, ca. 2 % , of the total C reservoir. Thus, by 1961 atmospheric C 0 i n the northern troposphere was 2 5 % above the pre-1950 level of C . 2 71 4

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The 1958-1961 moratorium on testing of nuclear weapons i n the atmosphere was broken by the U.S.S.R. i n the fall of 1961. The U . S. resumed testing i n M a y 1962, and both nations conducted numerous tests until the end of 1962. Except for minor amounts from the smallscale tests by the Chinese and French there has been no significant pro duction of C from nuclear testing i n the atmosphere since 1962. 1 4

Table I shows our estimate of the situation which prevailed i n the various carbon reservoirs i n the pre-nuclear era and at the end of 1962. The 1961-1962 tests contributed an additional ^ 35 Χ 10 C atoms into the atmosphere, principally the stratosphere. These atoms therefore 27 1 4

Table I.

Distributions of Natural C and of Excess C in the Several Carbon Reservoirs at the End of 1962 1 4

Total Carbon, grams

Reservoir Atmospheric C 0 Terrestrial biosphere Humus M a r i n e biosphere (living)

6.8 3.1 1.1 3

Χ Χ Χ Χ

Oceanic detritus Dissolved organic i n sea

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Inorganic carbon i n sea top 100 meters below 100 meters

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Pre-1950 >*C Concentration, atoms/gram C 6.07 5.85 *C

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Value deduced for the Northern Hemisphere only. The larger ratio of ocean to land areas in the Southern Hemisphere may be expected to give a shorter residence time for that hemisphere. This value is therefore an upper limit for the atmosphere as a whole. Including also uptake by the terrestrial biosphere. The mean residence time for exchange with the sea alone is expected to be larger than this value by about 25%. a

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The mean residence time of carbon in the mixed layer of the sea before transfer into the deep sea is of considerable interest, for as has already been pointed out, the rate of this transfer w i l l eventually govern the levels of excess C i n the atmosphere. There have been several estimates of this residence time. Craig (19) concluded that it was most probably not more than 10 years, and in one of his calculations he deduced a value of 4 years. Broecker et al. (14) concluded it was 5 years in the Atlantic Ocean and 8 years i n the Pacific Ocean. N y d a l (45) found that for the North Atlantic it was around 3 years or less. The profiles of Figure 6, and a few others which are not shown, all show a significant penetration of excess C below the mixed surface layer, pointing to a short residence time, of the order of 2 years, i n the mixed layer of the sea before transfer below the thermocline into the deep sea. C o n sidering the size of the oceans these data are very meager, and no firm conclusions can be drawn from them. However, continued measurements of C i n the sea should help to establish a firmer estimate of this quantity. 1 4


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Besides the box model approach, the data of Figures 1, 7, and 8 have also been analyzed using an eddy diffusion model to account for the observed changes occurring i n the troposphere (40, 56). Taking the exchange of C 0 between the atmosphere and the sea to be proportional to the square of the w i n d speed over the sea surface, Young and Fairhall (56) were able to give a reasonable explanation of the observations using this model. This approach predicts that the oceans of the southern hemisphere w i l l be the principal sink for excess C . The rapid increase i n the levels of excess C i n surface ocean water at latitude near 40° S confirms this expectation. 2

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Besides these applications to geophysical problems, the excess C has interesting possibilities i n other areas of research. The large increase in C i n tropospheric air which took place i n 1963 means that so far as C is concerned plant material which grew later than 1962 is significantly different from plant material which grew before 1963. Using this fact Berger and L i b b y (4) have shown that the body organs, such as the heart, liver, and brain, turn over their carbon on a time scale of the order of weeks. It would be of interest if similar studies could be carried out on D N A extract to see if the same is true of the molecules carrying the genetic code. 1 4

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Another example is illustrated in Figure 9 (17). The outer, sapwood rings of a western redcedar, Thuja plicata Donn, which grew about 50 miles south of Seattle, show an increase in C which lags slightly behind the levels of C which prevailed in the atmosphere. This lag is attributed to a holdover of food supply from the previous year; a sizeable 1 4

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Figure 9.

**C concentration in tree rings and in heartwood extractives of western redcedar

holdover from the previous year, on the order of 8 0 - 9 0 % , is required to fit the observations. More likely the holdover is less than this each year but it extends over several years. The C content of the dark com ponent, extracted with acetone, which is present i n the outermost heartwood rings was also examined. These data are also shown i n Figure 9. The cellular structure of the outermost heartwood ring was laid down i n 1949, well before excess C perturbed the atmosphere significandy. The insoluble residue from the acetone extraction shows no excess C . H o w ever, the extractives, which amount to ca. 6% by weight of the dry heartwood, show significant amounts of excess C . I n the heartwood ring laid down i n 1949 it measures nearly 6 5 % above normal, comparable with the excess C i n the outermost sapwood ring. This illustrates what botanists already know: that the extractives are formed as metabolites i n the outermost growth ring and are deposited i n the dead cells at the periphery of the heartwood. It appears that further C studies could elucidate the chemistry of wood formation and provide interesting infor mation on the utilization of stored food supply by trees. 1 4

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A number of other interesting possibilities for utihzing the excess C in the atmosphere as a tracer of natural processes come easily to mind. Not much is known about the rate of turnover of humus i n the soil. Measurements of C in soil humus over the next several years, while the terrestrial biosphere continues to fix carbon with significant amounts of excess C , should help to determine the rate of turnover of carbon i n the reservoir of humus. Some work along these lines is already i n progress (41). 1 4

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In the marine biosphere organisms which live near the surface show significant levels of excess C . It would be of interest to know i f the organisms which live at great depth show similar amounts of excess C , implying a rapid downward propagation of the food chain. If the food chain propagates downward very slowly, deep-feeding organisms w i l l be slow to show evidence of excess C . Another problem of marine biology is the very large reservoir of dissolved organic matter i n the sea. N o t much is known about this material besides its distribution, which i n the deep sea is remarkably uniform (37). The mechanisms of its formation and destruction are unknown. It is not known whether it has accumulated over long periods of time, thousands of years perhaps, or whether it is a reservoir which turns over fairly rapidly. Measurements of C would be desirable to help answer some of these questions. 1 4


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Conclusion The large perturbation produced in the level of C in tropospheric COo i n 1963-65 has left its mark in the terrestrial biosphere and is producing measurable changes i n near-surface waters of the sea. It is clear that man has unwittingly initiated a geophysical experiment on a global scale. Provided that the present moratorium on atmospheric tests by the two major nuclear powers is continued, the 1962-62 nuclear tests w i l l have provided an unparalleled opportunity—we hope a unique opportunity—for gaining increased insight into many of the processes taking place i n our environment. 1 4

Literature Cited (1) Arnold, J. R., Anderson, E. C., Tellus 9, 28 (1957). (2) Banse, K., Progr. Oceanog. 2, 55 (1964). (3) Berger, R., Fergusson, G. J., Libby, W. F., Am. J. Sci. Radiocarbon Suppl. 7, 336 (1965); 8, 467 (1966). (4) Berger, R., Libby, W. F., Am. J. Sci. Radiocarbon Suppl. 9, 477 (1967). (5) Bien, G. S., Rakestraw, N. W., Suess, H. E., Limnology Oceanog. Suppl. 10, R25 (1965).



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(6) Bien, G. S., Rakestraw, N. W., Suess, H. E., Tellus 12, 436 (1960). (7) Bien, G., Suess, H., Radioactive Dating Methods Low Level Counting, I.A.E.A. Vienna, 105-115 (1967). (8) Bolin, B., Eriksson, E., "The Atmosphere and the Sea in Motion," pp. 130-142, Rockefeller Institute Press and Oxford University Press, New York, 1959 (9) Brodie, J. W., Burling, R. W., Nature 181, 107 (1958). (10) Broecker, W. S., Tucek, C. S., Olson, Ε. Α., Intern. J. Appl. Radiation Isotopes 7, 1 (1959). (11) Broecker, W. S., Olson, Ε. Α., Am. J. Sci. Radiocarbon Suppl. 1, 111 (1959). (12) Ibid.,3,176 (1961). (13) Broecker, W. S., Olson, Ε. Α., Science 132, 712 (1960). (14) Broecker, W. S., Gerard, R., Ewing, M., Heezen, B. C., J. Geophys. Res. 65, 2003 (1960). (15) Broecker, W. S., Walton, Α., Science 130, 309 (1959). (16) Buddemeier, R. W., Fairhall, A. W., Yang, I. C., Young, A. W., unpub lished data. (17) Buddemeier, R. W., Fairhall, A. W., unpublished data. (18) Burling, R. W., Garner, D. M., New Zealand Geol. Geophys. 2, 799 (1959). (19) Craig, H., Tellus 9, 1 (1957). (20) Fabian, P., Libby, W. F., Palmer, C. E., J. Geophys. Res. 73, 3611 (1968). (21) Feely, H. W., Seitz, H., Lagomarsino, R. J., Biscaye, P. E., Tellus 18, 316 (1966). (22) Fergusson, G. J., Nucleonics 13, 18 (1955). (23) Fergusson, G. J., Proc. Roy. Soc. A243, 561 (1958). (24) Fergusson, G. J., J. Geophys. Res. 68, 3733 (1963). (25) Garner, D. M., Nature 182, 466 (1958); New Zealand J.Geol.Geophys. 1, 577 (1958). (26) Gudiksen, P. H., Fairhall, A. W., Reed, R. J., J. Geophys. Res. 73, 4461 (1968). (27) Hagemann, F., Gray, J., Machta, L., Turkevich, Α., Science 130, 542 (1959). (28) Hagemann, F. T., Gray, J., Machta, L., U. S. At. Energy Comm. Health Safety Lab. Rept. HASL-159 (1965); HASL-166 (1966). (29) Hayes, F. N., Williams, D. L., Rogers, B., Phys. Rev. 92, 512 (1953). (30) Junge, C. E., J. Geophys. Res. 68, 3849 (1963). (31) Kigoshi, K., J. Radiation Res. 1, 111 (1960). (32) Kigoshi, K., Endo, K., Bull. Chem. Soc. Japan 34, 1740 (1961). (33) Lal, D., Rama, J. Geophys. Res. 71, 2865 (1966). (34) Libby, W. F., "Radiocarbon Dating," 2nd ed., University of Chicago Press, Chicago, 1955. (35) Lingenfelter, R. E., Rev. Geophys. 1, 35 (1963). (36) Menzel, D. W., Goering, J. J., Limnol. Oceanog. 11, 333 (1966). (37) Menzel, D. W., Deep-Sea Res. 14, 229 (1967). (38) Munnich, K. O., Vogel, J. C., Naturwiss. 45, 327 (1958). (39) Munnich, K. O., Vogel, J. C., Proc. Symp. Radioactive Dating, I.A.E.A. Vienna, 189-197 (1963). (40) Munnich, K. O., Roether, W., Radioactive Dating Methods Low Level Counting, I.A.E.A. Vienna, 93-104 (1967). (41) Nakhla, S. M., Delibrias, G., Radiometric Dating Methods Low Level Counting, I.A.E.A. Vienna, 169-176 (1967). (42) Nydal, R., Nature 200, 212 (1963). (43) Nydal, R., Tellus 18, 272 (1966).



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(44) Nydal, R., Radioactive Dating Methods Low Level Counting, I.A.E.A. Vienna, 119-128 (1967). (45) Nydal, R. J., Geophys. Res. 73, 3617 (1968). (46) Olsson, I. U., Karlen, I., Am. J. Sci. Radiocarbon Suppl. 7, 331 (1965). (47) Olsson, I. U., Karlen, I., Stenberg, Α., Tellus 18, 293 (1966). (48) Patterson, R. L., Blifford, I. H., Science 126, 26 (1957). (49) Rafter, Τ. Α., Fergusson, G. J., New Zealand J. Sci. Technol. B38, 871 (1957). (50) Rafter, Τ. Α., New ZealandJ.Sci. 8, 472 (1965). (51) Revelle, R., Suess, Η. E., Tellus 9, 18 (1957). (52) Suess, Η. E., Science 122, 415 (1955). (53) Tauber, H., Science 131, 921 (1960). (54) Ibid., 133, 461 (1961). (55) Willis, Ε. H., Nature 185, 552 (1960). (56) Young, J. Α., Fairhall, A. W., J. Geophys. Res. 73, 1185 (1968). RECEIVED February 24, 1969. Work supported by the U. S. Atomic Energy Commission under Contracts AT(45-1)-1776 and AT(45-1)-2091.


Radiocarbon in the Environment

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