Due to the release of %I to the atmosphere from nuclear bomb ... exploiting the bomb-produced ..... explosions because neutron fluxes from each bomb may.
Nuclear Instruments North-Holland
Atmospheric
and Methods
in Physics
Research
B52 (1990) 483-488
transport of bomb-produced
483
36C1
H.-A. Synal ‘), J. Beer 2), G. Bonani ‘), M. Suter 3, and W. Wiilfli ‘) ” lnsrirut ftirMirrelenergiephysik, ETH-Hiinggerberg, CH-8093 Ziirich, Switzerland 2’ Enoironmen:al Physics, ETH-Ziirich, Insrifure for Aquatic Sciences and Warer Pollution Control, c/o EA WAG, CH-8600 Diibendorj Swilzerland .‘I Paul Scherrer Institur, c/o Insrirut fir Mirrelenergiephysik, ETH-Hiinggerberg, CH-8093 Ziirich, Swrtzerland
%I measurements have been made in an arctic ice core drilled near the Dye-3 site (6S011’N, 43O5O’W). The samples analyzed cover the period between 1945 and 1985 with annual resolution. Due to the release of %I to the atmosphere from nuclear bomb tests, the data shown a peak in the late fifties with %I fallout rates about three orders of magnitude higher than expected from cosmic ray production. The time resolution is now precise enough to resolve the structure of the descending part of the fallout pattern. From the fallout rates obtained, a stratospheric residence time for bomb-produced “Cl could be derived. A detailed interpretation of the data is done with a four-box atmospheric transport model. The large and well-defined “?I bomb pulse provides an excellent tracer for ground water studies.
1. Mmduction
Radioisotopes, such as “Be and ‘%ZI, which are continuously produced by interactions of high-energy cosmic ray particles in the atmosphere, have become important tracers of atmospheric transport processes. Both radioisotopes are produced in comparable spallation reactions. It is assumed that they become attached to aerosol particles shortly after their production. Their mean residence time in the stratosphere is expected to be 1-2 years. Once they are transported to the troposphere, the particles are either washed out by precipitation or are removed by dry deposition with a mean residence time of the order of weeks [I]. Due to the nature of stratosphere-troposphere air exchange the fallout of long-lived radioisotopes shows a pronounced latitude dependence with highest fallout rates at latitudes between 30° and 60”. The outlined transport process has not been investigated for ?I, but it seems that the gaseous Cl reservoir (e.g. HCI, Clz) must be also considered for the atmospheric chlorine transport [2]. So far it is not known whether a similar “Be and ‘%I deposition to polar ice can be assumed. This would be a basic condition to use the production independent “Be/%1 ratio as a dating tool. “Be/“Cl ratio measurements in ice from Camp Century covering the Maunder minimum period (16451715 A.D.) (3,4] and the transition from Holocene to Wisconsin (10000 B.P.) [3] show deviations from constancy. This indicates possible differences in the transport mechanism of these isotopes. In addition, absolute values of “Be/% ratios in polar ice deviate significantly from predicted values 0168-583X/90/$03.50
based on production rates. These facts shown the need for more detailed studies of %I transport. Arctic and Antarctic ice sheets contain a record of trace impurities in precipitation which extends over many thousands of years. These archives also contain man-made radionuclides which have been released into the atmosphere. In the case of 36CI, large amounts were produced and released to the stratosphere during atmospheric tests of nuclear weapons. At the time of the tests, this anthropogenic contribution of 36Cl was orders of magnitude greater than the total amount of “Cl due to cosmic ray production [3,5]. For this reason and due to the fact that release of bomb-produced .‘%I1 into the atmosphere took place at “well known” times, it has been possible to study atmospheric transport of 3”CI by exploiting the bomb-produced ‘%Zl input to the atmosphere. First measurements of bomb-produced 3”CI fallout in polar ice core samples indicate a ‘“Cl stratospheric residence time between 3-4 years [3,5], which is significantly higher than the ‘“Be stratospheric residence time. The %I residence time had been derived from the decrease of the observed fallout rates. But the time resolution of the first measurements was not sufficient to resolve a visible structure of the fallout pattern. This structure may occur due to additional “?I input to the stratosphere from bomb tests, which have been conducted in the atmosphere by France and China after the nuclear test ban treaty. For this reason the residence time of ‘%I determined by the early measurements might be too large.
0 1990 - Elsevier Science Publishers B.V. (North-Holland)
IV(a). ARCHAEOLOGY
H.-A.
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2. Measurements In order to get more detailed information on 36C1 fallout rates since the first nuclear explosions in 1945, 36Cl has been analyzed in an ice core drilled close to the Dye-3 site (65’11’N, 43°50’W). The time scale of the ice core was defined by measuring a H,O, profile, which shows a pronounced seasonal variation. The core was cut into pieces cont~ning the accumulated precipitation between two winters. Ice density and total accumulation rates were measured, The preparation of samples suitable for AMS measurement was performed at
of bomb-produced
%3
University of Beme. Forty samples covering the period from 1945 to 1985 with annual resolution were measured at the ETH/PSI AMS facility. The experimental setup for ‘kl measurements is described elsewhere [6,7]. From the measured 36Cl/Cl isotope ratios, 36Cl concentrations and fallout rates were calculated using ice density, accumulation rates and sample mass. The results are shown in table 1. A comparison of ‘%I1 concentration between the first me~urements and our new data is shown in fig. 1. All three ice cores had been drilled near the Dye-3 side. The data sets show a reasonable agreement considering that the samples come
Table 1 Results of MCI measurements from a Dye-3 ice core. The 36C1/Cl isotope ratios have been obtained after adding 3.8 mg chlorine carrier to each sample. The measurements had been performed at the ETH/PSI AMS facility. ETH label
Year
CL0186 CL01 87 CL0188 CL0189 CL0190 CL0191 CL0193 CL1094 CL1095 CL0196 CL0197 CL0198 CL0199 CL0200 CL0201 CL0202 CL0203 CL0204 CL0205 CL0206 CL0207 CL0208 CL0209 CL0210 CL0211 CL0212 CL0213 CL0214 CL0215 CL0216 CL0217 CL0218 CL021 9 CL0220 CL0221 CL0222 CL0223 CL0225
1985 1984 1983 1982 1981 1980 1978 1977 1976 1974 1973 1972 1971 1970 1969 1968 1967 1966 1965 1964 1963 1962 1961 1960 1959 1958 1957 1956 1955 1954 1953 1952 1951 19so 1949 1948 1947 1945
36cl/cl (X10_1s)
%Cf/kg ( x 106)
36Cl/cmz yr (X10’)
PI
38.1 66.3 117 109 114 109 92 139 135 320 494 338 325 874 975 1350 3110 1820 3570 4460 4870 8120 12100 20300 18900 18200 28600 17 100 13100 5800 871 426 143 49.8 44.0 53.1 35.9 79.6
1.49 2.97 5.04 2.71 2.83 3.81 2.63 4.20 4.27 14.9 11.3 8.42 9.93 32.54 28.1 45.0 73.6 67.1 127 76.4 164 198 293 362 339 470 706 347 303 111 19.8 11.3 3.3 1.24 1.21 1.43 0.90 1.64
44.4 110 226 176 167 170 162 243 211 749 701 796 485 1290 1510 1990 4170 2580 5410 6330 7 280 11900 20100 30700 27 800 26000 39000 25OOo 18600 8210 1220 612 204 72.9 66.1 74.0 84.2 120
25.6 17.8 11.9 12.6 12.6 13.0 12.7 10.7 11.6 6.4 6.4 6.1 6.6 4.8 4.5 4.0 1.7 3.8 3.4 3.9 3.9 3.8 3.7 3.7 3.7 3.7 3.7 3.7 3.7 3.8 4.9 6.5 10.1 18.4 18.5 16.0 15.7 12.9
Error
Accumulation fate
Ice density
Sample mass
IWyrl
k/cm2 1
Ikl
0.83 1.00 1.15 1.35 1.30 0.95 1.20 1.10 0.90 0.85 1.00 1.50 0.75 0.60 0.80 0.65 0.80 0.35 0.60 1.15 0.60 0.80 0.90 1.10 1.05 0.70 0.70 0.90 0.75 0.90 0.75 0.65 0.70 0.70 0.69 0.60 1.10 0.85
0.35 0.37 0.39 0.42 0.43 0.47 0.31 0.53 0.55 0.59 0.61 0.63 0.63 0.66 0.67 0.68 0.69 0.70 0.71 0.72 0.74 0.75 0.76 0.77 0.78 0.79 0.79 0.80 0.81 0.82 0.82 0.83 0.83 0.84 0.84 0.85 0.83 0.86
1.657 1.444 1.500 2.392 2.377 1.846 2.246 2.449 2.049 2.249 2.776 4.280 2.112 1.733 2.243 2.216 2.658 1.730 1.818 3.769 1.917 2.649 2.664 3.631 3.394 2.496 2.620 3.186 2.773 3.370 2.843 2.424 2.672 2.393 2.346 2.363 4.010 3.128
H.-A. Synal et al. / Atmospheric e-B+
This
transport of bomb-produced
‘b
485
experiment
t.olH5 ‘l”~l~l~~~~‘i’~l~i’l~it’~ll~‘iii~’li~~’llll’il” 1970 1960 1940 1950
1980
1990
Year Fig, 1. A
comparison between measured %l concentration in ice cores from Dye-3.
from ~fferent ice cores and cover a different accumulation period with different time resolution. Fallout rates (fig. 3) show a clear peak in the late fifties with a maximum 500 to 1000 times higher than expected from cosmogenic production. On the descending slope of the fallout pattern a structure is resolved with large variations in measured fallout rates. After 1985 the fallout came ahnost down to the natural level. This means that most of the bomb-produced atmospheric 36Cl content has now been removed from the atmosphere.
3. Model negations In order to obtain a more detailed interpretation, a box exchange model of the atmosphere was used. Atmosphere box models have been very successful in describing the transport and fallout pattern of other radioisotopes which are also produced in nuclear weapon tests, such as t4C [8] and 9”sr [9]. With a modified “OSr transport model 36C1 fallout rates had been calculated by Bentley et al. [lo]. During the time of maximum 36C1 fallout rates, these calculations show a good agreement with experimental data, but the slope of the fallout pattern could not be explained. For the interpretation of our data we use the four-box model as outlined in fig. 2. Since we are considering northern hemisphere deposition (Greenland) and since most nuclear tests were conducted in the northern hemisphere or near the
equator, we did not divide the atmosphere into northern and southern hemisphere boxes. On the other hand, we did divide the atmosphere into local (Nt, N2) and global boxes (N,, N4). The local boxes represent the region where the tests were conducted and where bomb-produced 36Cl (input function Q(t)) was injected into the local stratophere (Ni) and troposphere (N2). The conglobal
local
1I Qlf) ‘&
stratosphere
troposphere
deposition
Fig. 2. Four-box exchange model to describe the atmospheric transport of 36C1. IV(a).
ARCHAEOLOGY
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H.-A. Synal et al. / Atmospheric rransporr of bomb-produced ‘“Cl
nection between local and global atmosphere is made via stratospheric mixing followed by stratospheretroposphere exchange. The transport of air between the stratosphere and the troposphere is given by k,,, which is derived from the mean residence time of 36C1 in the stratosphere. The removal rate from the atmosphere (k) is given by the mean residence time of 36C1 in the troposphere. Cosmogenic input of 36C1 is controlled by the input functions B, and E,. For these calculations cosmogenic production in the troposphere can be neglected. Moreover, a steady-state system is assumed so that all air transfers from one box to another must be compensated by a reverse air flow. It is assumed that there is no connection between local and global tropospheric boxes. This assumption is reasonable because of the long transport times from the test sites to Greenland and the short tropospheric residence times of ?l. For this four-box model a set of four coupled linear differential equation can be derived. dN, -=B1+Q(t)+k,,Nz+rssN3-(ks,+kss+X)N,, dt
with a %Sr atmospheric transport model [9], it is assumed that stratospheric air is well mixed. A reasonable time constant which describes the mixing of air between the local and the global stratospheric box is two months. The residence time for “Cl in the troposphere is short. For calculation this time was set to two weeks. Since this time is short compared to the stratospheric residence time, the fallout pattern is nearly unaffected by varying this parameter. The most sensitive parameter is the stratospheric residence time. This parameter is derived from the experimental data. Between 1960 and 1964 the fallout pattern shows only an exponential decay of the fallout rates. During this period no large bombs tests had been conducted near the ocean and negligible ‘%l input to the atmosphere can be expected. Moreover the highest global atmospheric 36C1 content can be assumed before 1960. For these reasons the slope of the fallout pattern at the time can be fitted with an exponential function. The obtained decay constant corresponds to a mean residence time of 36Cl in the stratosphere of 2.0 f 0.3 years.
d N,
-=kk,,N,-(k+k,,+X)N,, dt
d N, -=B,+k,,N,+k,,N,-(r,,+k,,+h)N,, dt
dN, -=k,,N,-(k+k,,+X)N,. dt Exchange coefficients: k 51 = stratosphere 3 troposhere, k = troposhere = stratosphere, k: = stratosphere (N]) * stratosphere (N,), = stratosphere ( N3) * stratosphere (N,), ‘;r k = troposphere 3 archive. System parameters: N* = amount of 36C1-atoms in individual atmospheric boxes, A = 36C1 decay constant, Bat = cosmic %I input (N,, N3). Q(r) = bomb-produced 36C1 input to N,. Due to the complex function Q(r) giving the bomb-produced %l input to N,, this system is solved numerically using the Runge-Kutta formalism.
4. Made1 parameters For the model calculation the ratio of the sizes of the stratospheric box to the tropospheric box is set as 1:4, similar to the ratios of air masses actually contained in the stratosphere and troposhere. The ratio between local and global part of the atomsphere is 1:lO’. Model calculations show that the system is not very sensitive to this parameter. In particular the exponential decrease of fallout rates is unaffected. Based on the results obtained
5. Input function The main source of bomb-produced 36Cl is neutron activation of Cl- in the sea. An estimate of “Cl production in thermonuclear explosions taking place over the ocean can be made by assuming a total production of 2 kg ‘%Zl per megaton yield [S]. This estimate can only give a rough idea of “Cl production in nuclear test explosions because neutron fluxes from each bomb may vary over a wide range, depending strongly on the type of bomb. Moreover the abundance of the 3’C1 target nuclei may differ by orders of magnitudes depending on whether explosions over land or at high altitudes are considered. For these reasons a classification of all known bomb test into four groups has been made. Information on nuclear test explosions are taken from the FOA report published by Research Institute of National Defense, Sweden [ll]. The first group consists of all tests taking place on ships. The highest VI production is expected from those tests. Overall 34 tests of this class have been reported, mainly in 1954, 1956 and 1958. The second class contains all atmospheric test which were carried out on small islands. During the time between 1946 and 1962, 105 test explosions from the USA and Great Britain in this class are known. After the nuclear test ban treaty of 5-8-1963, about 40 French atmospheric tests were carried out between 1966 and 1974 at Mururoa. These tests also belong to class 2. All atmospheric test explosions over land surfaces belong to group three. During the time of high 36C1 release into the atmosphere in the fifties and early sixities the small contribution from land-based tests is not visible, despite
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H.-A. Synal et al. / Atmospheric transport of bomb-produced 36Cl l
-
8
=meaaured
falloutdata
Shuulated falloutdata
l.OE+E
l.OE+E
1940
1950
1960
1970
1980
1990
Year Fig. 3.The obtained 36C1fallout rates are compared with the results from model calculations. For the calculations a 36C1stratospheric recidence time of 2 years has been used. The arrows indicate the dates when 36C1release to the atmosphere has been considered.
the high total yield of test explosions
over land surfaces. But in the late seventies, when atmospheric 36Cl content approached natural levels, tests from this class gave a visible contribution. At least seven Chinese tests at Lop Nor have to be considered. All subsurface nuclear tests are grouped into the fourth class. No “Cl release to the atmosphere is expected from these tests. To reproduce the experimental data an assumption concerning the amount of 36C1released from all known test explosions had to be made. For this purpose the yield of each nuculear bomb test has been considered, but it is impossible to use a direct correlation between the yield of the test explosion and the total amount of 36Cl released to the atmosphere during an individual test. Overall about 78 kg ‘%Zl have been assumed, with maximum amounts of 15, 23 and 25 kg 36C1 in 1954, 1956 and 1958. A compilation of assumed input values is given in table 2. With these input values, Q(t), model calculations were carried out to obtain 36C1fallout rates.
the nuclear test ban treaty have to be considered in interpreting the observed fallout pattern. Although the basic trend in fallout rates can be fitted with a stratos heric residence tune of about 3.5 years, neglecting all 38 Cl input after 1958, it is impossible to obtain a comparable structure in calculated fallout rates. With the obtained “Cl stratospheric residence time of about 2 years a good agreement between measured fallout rates and model calculations can be achieved. Therefore this stratospheric residence time seems reasonable, although the steep fluctuations in the observed fallout data cannot be reproduced. On the other hand, it should be mentioned that besides a variable input fiction, temporal variations of fallout rates have to be considered. The seasonal variation cannot be resolved by our data, but irregular fallout rates mauy occur, caused by the variable intensity of statosphere-troposphere air exchange. This might explain the steep fluctuations in the observed fallout pattern.
6. Results
7.Conclusions
The calculated 36C1fallout rates are shown in fig. 3 together with the experimental data. To reproduce the clear structure on the descending edge of the measured fallout curve, only small 36Cl contributions (=: lo-500 times smaller than at peak maximum) are necessary. This indicates that atmospheric tests carried out after
36C1fallout data are now available from an Artic ice core for a period since 1945. The annual resolution of the new experimental data clearly resolved the structure of the observed fallout pattern. A stratospheric residence time for bomb-produced 36C1 of 2 + 0.3 years could be derived from the data. Model calculations IV(a). ARCHAEOLOGY
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Table 2 Classification of all known atmospheric bomb tests. The assumed 36C1 input to stratosphere used for model calculations is given in the sixth column. Year
1945 1946 1948 1949 1951 1952 1953 1954 1955 1956 1957 1958 1960 1961 1962 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1976 1977 1978 1979 1980 1983
Atmosuheric tests total
Tests on barges
3 2 3 17 11 15 8 17 27 46 84 3 31 79 1 1 8 5 6 9 6 5 6 9 3 1 2 l? 2 l?
Tests on islands
Tests over land
Assumed 36C1 input [kg]
transport of bomb-produced
‘h
servoirs, e.g. in ground water. To adopt these data to other locations the latitude dependence of 36C1 fallout has to be considered. It should, however, be possible to obtain information on this latitude dependence by measuring the “Cl bomb peak at other locations in glacial ice.
3 2 3
1
4 3
4
2
6
20 6 32
22
33
3 3 5
0.05 0.01 1 13 7 15 2 17 1 40 30 3 31 46 1 1 5 2
0.45 0.80 0.05 15.0 8.0 23.0 0.05 25.0
2.0
0.20
3 1 2 l? 1 l?
0.30 0.05 0.25 0.20 0.20 0.03 0.02 0.05 0.05
show that additional 36C1 input to the atmosphere has to be considered to reproduce the observed fallout pattern. These data rovide an important tool to trace Yti bomb-produced Cl at other locations or in other re-
Acknowledgements We would like to thank Prof. H. Oeschger and his coworkers for providing the ice sample. In addition we are grateful to A. Blinov for his help with sample preparation. Part of this work was supported by the Swiss National Science Foundation.
References [l] D. Lal and B. Peters, Handbuch der Physik (Springer, Berlin, 1967) pp. 551-612. [2] M. Wahlen, B. Deck, N. Tanaka, J.S. Vogel, J. Souton, P.W. Kubik, P. Shamra and H.E. Gove, presented at this conference (5th Int. Conf. on Accelerator Mass Spectrometry, Paris, France, 1990). [3] M. Suter, J. Beer, C. Bonanj D. Michel, H. Oeschger, H.-A. Synal and W. Wolfh, Nucl. Instr. and Meth. B29 (1987) 211. [4] D. Elmore, N.J. Conard, P.W. Kubik, H.E. Gove, M. WahIen, J. Beer and M. Suter, Nucl. Instr. and Meth. B29 (1987) 207. [5] D. Elmore, L.E. Tubbs, D. Newman, X.Z. Ma, R.C. Finkel, K. Nishiizumi, J. Beer, H. Oeschger and M. Andree, Nature 300 (1982) 735. [6] H.-A. Synal, J. Beer, G. Bonani, J.H. Hofmann, M. Suter and W. Wolfh, Nucl. Instr. and Meth. B29 (1987) 146. [7] H.-A. SynaI, ETH Dissertation, no. 8987 (1989). [8] R. Nydal, J. Geophys. Res. 73 (1968) 3617. [9] P.W. Krey and B. Krajewski, J. Geophys. Res. 75 (1970) 2901. [lo] H.W. Bentley, F.M. Phillips, S.N. Davis, S. Grifford, D. Elmore, L.E. Tubbs and H.E. Gove, Nature 300 (1982) 737ff. [ll] I. Zander and R. Araskog, Fiirsvaret Forskningsanstalt Avdelning, Report 4 (1973).
