- Email: [email protected]
SIN-AMS facility
Nuclear Instruments and Methods in Physics Research B29 (1987) 211-215 North-Holland. Amsterdam
=Cl STUDIES
AT THE ETH/SIN-AMS
M. SUTER ‘) J. BEER’*2’ G. BONANI H.A. SYNAi” and W. WbLFLI I)
211
FACILITY ‘), H .J . HOFMANN
‘), D. MICHEL 2), H. OESCHGER
2),
‘) Institut fdrMittelenergiephysik, ETH-Hijnggerberg. CH-8093 Zurich, Switzerland 2, Physikalisches Institut der Universitiir Bern, CH-3012 Bern, Switzerland
36Cl has been measured in polar ice from Dye 3 (65O ll’N, 43” 50’W) and Camp Century (77O ll’N, 82O 08’W) in Greenland. In an exploratory study 4 series consisting of about 20 samples each have been selected covering approximately the following time periods: 12000-1OOCKl BP (transition Wisconsin-Holocene), 1530-1730 A.D. (Maunder minimum), 1942-1977 A.D. (nuclear bomb pulse) and 1978 (seasonal variations). The results are compared with “Be data of the same cores which were measured previously. The variations of the 1oBe/36Cl ratio are generally larger than expected from the experimental errors indicating that this ratio is not suitable for dating of old ice.
1. Introduction During the last two years the AMS facility in Zurich has been significantly improved for the detection of heavier ions. 36C1 can now be measured on samples containing more than 3 mg Cl with high reproducibility and a sensitivity of 1.5 X 10-14, which so far only has been reached with larger accelerators which allow a better suppression of the 36S background [I]. With a half-life of 301000 years 36C1 is a good candidate for dating old ground water, old polar ice and extraterrestrial material. At ETH a 36C1 measuring program has been started to explore the potential of 36C1 dating for polar ice and ground water studies. We are presenting here an overview of our ice core studies. The most direct way to study cosmogenically produced 36C1 and its variations caused by changes of the production rates and the atmospheric transport processes is to look at polar ice cores, which contain a continuous record of precipitations over long-time periods. “Be concentration profiles in ice cores have already been studied in detail. They show significant short- and long-term variations during the last 100000 years [2-41. Detailed records of 36C1 are vf special interest for two reasons: Firstly for any studies using the cosmogenic 36C1 as a tracer or a dating tool, knowledge of the input function is essential. Secondly, dating of old polar ice is an important but also a very difficult task. Especially for ice older than 10000 yr no precise dating technique is available. The widely used ice flow models are based on two parameters (annual layer thickness and depth of vertical strain rate changes), which are both not well known [5]. For younger ice it is possible to establish a time scale based on al80 mea0168-583X/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
surements. The seasonal variations of S”O allow year by year counting similar to the tree ring dating technique. In addition, special time marks caused by volcanic eruptions can be used to adjust the time scale. Also the i4C dating method has been applied for dating ice of about 10000 yr [6]. It is well known that the production rate and the transport of the long lived isotopes “Be and 36C1 are subject to large variations and that a single isotope is therefore not suitable for dating. There was, however, the hope that these fluctuations could be eliminated by using the 36C1/10 Be ratio which decreases with time with an apparent half-life of 370000 years. This ratio would provide an ideal tool for dating ice older than 50000 years. Earlier measurements on a few individual ice and snow samples showed significant variations in the ‘“Be/36C1 ratio [7]. For a better understanding of the ‘“Be/36C1 ratio detailed profiles of 36C1 and “Be are needed. 36C1 profile measurements have been performed at the University of Rochester. In 1982 samples from a shallow core from Dye 3, Greenland, covering the period 1950-1975 have been analyzed. The results show a large pulse of 36C1 due to nuclear weapons tests [8]. Recently a new set of ice core samples has been measured covering the time interval 1550-1850 A.D. A comparison with “Be data obtained at ETH show large variations of the ‘“Be/36C1 ratio of up to a factor of 5. These results indicate that individual samples covering only a few years cannot be used for dating [9]. We present new 36C1 data covering partly the same time period. In addition some samples from the transition between glacial and post glacial time have been measured.
III(b). GLACIOLOGY/CLIMATOLOGY
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M. Suter et al, / 36Cl studies at ETH/SIN
2. Experimental For the sample preparation the following procedure is used: Ice samples of l-2 kg weight are first cleaned by rinsing them with deionised water. After melting in a microwave oven and taking an aliquot, 0.5 mg 9Be and 3 mg Cl carrier is added, The aliquot is used later to determine the Cl concentration of the sample which is needed to calculate the correct 36Cl/C1 ratio. The volume is reduced by evaporation to about 20 ml. AgCl is precipitated by adding AgNO,. The precipitation is dissolved with NH, and filtered through a millipore filter (0.4 pm). Then the Cl is again precipitated in the form of AgCl. The filtering process is repeated until the filter looks clean. After the final precipitation the AgCl is dried in an oven. A similar procedure is described by Conard et al. [17]. A short time before the measurement the AgCl is pressed into the target holder, which is made of tantalum [l].
3. The bomb pulse in an ice core from Dye 3, Greenland In the fifties significant amounts of 36C1 were produced during marine nuclear weapons tests and were then transported into the stratosphere. This artificiallyproduced 36C1 dominates in precipitation and dry fallout during the last four decades. For 36C1 tracer studies it is important to have a detailed knowledge of this anthropogenic 36Cl input and its concentrations in precipitation as a function of time. Polar ice cores are the best suited archives for this kind of study. In 1982 a first series of 36C1 samples from Dye 3 station in Greenland was compared with model calculations based on data of location and neutron yields of the bomb tests. Although a relatively good quantitative agreement was found, the measured residence time was larger than expected [10,8]. At ETH 36C1 samples have been measured from another shallow core drilled in summer 1983 at Dye 3 station. Each sample averages a time interval of about two years. The series covers the time period from 1942-1977. The new data are presented in fig. 1. For comparison the results obtained at Rochester are also plotted. The two data sets show a good agreement considering that the individual samples are not covering the same time intervals and that the samples are from different cores. After 1970 the new data show in the average a lower 36C1 concentration. This leads to a steeper slope resulting in a shorter atmospheric residence time compared to the Rochester data. From the new data a mean residence time (i.e. the time it takes to decrease the concentration to l/e of its original value) of about 3 years can be derived, which is about a factor of 2 larger than that observed for 90Sr and 13’Cs [ll]. A possible explanation of this difference is that some of
I
I
1960
1950 Date
1970
of deposItton
Fig. 1. The 36C1concentration profile from a shallow ice core drilled in 1983 at Dye 3 station in southern Greenland. The results show a large pulse of 36C1 due to nuclear weapons tests. The data
are compared with earlier 36C1 measurements formed at Rochester [S].
per-
the weapons tests performed after 1960 introduced additional 36C1 into the atmosphere [8]. But it seems also feasible that there is a real difference in the residence time of 36C1 and other long lived radioisotopes. Sr, Cs and Be become attached to aerosols within a short time after their production, whereas Cl is also present in gaseous form (HCl) and takes part in photochemical reactions in the upper atmosphere [12]. The new data set also includes 4 data points covering the time before 1950 allowing us to determine the 36C1 level to be about 0.8 x lo3 atoms/g. prebomb previously [2], a Together with “Be data published value for the ‘“Be/36Cl ratio of about 14 is determined. The extrapolation of the experimental data of the 36C1 into recent time shows that the weapons tests still influence the 36C1 in precipitation at present (1987).
4. Seasonal variations of WI in polar ice From studies on polar ice and from measurements on rainwater, it is known that the radioisotope con-
hf.Suter etal./ 36CI~tudies atETH/SIN
213
tally produced 36C1. The “Be, the 36C1 and the ratios of the two radioisotopes are given in fig. 2 as a function of depth. The data show variations of more than a factor of 4 for the “Be and the 36Cl concentrations. The ratio of the two radioisotopes exhibits significantly smaller fluctuations indicating a correlation between the two isotopes. However, compared to experimental errors, the ratio cannot be considered as constant.
5. %CI in polar ice during the Maunder minimum From historical sun spot records it is known that between i645 and 1715 A.D. almost no sun spots were observed (Maunder Minimum) [14]. 14C measurements in tree rings show an enhancement of a few percent in the atmospheric 14C concentration during this period [15]. The “Be and 14C variations have been compared for the period 1200-1800 A.D. using a carbon cycle model to account for the different geochemical behaviour of the two isotopes. The good agreement between the two records strongly indicates that both iso-
4
460
480
DEPTH
500
520
[cm]
Fig. 2. Seasonal variations of “Be, 36C1 and ‘“Be/36Cl measured from samples from a pit at Dye 3 for the year 1978. The
I
I
I
I
I
3 2 I
dashed line gives the mean value of the ‘“Be/36C1 ratios. 0
centrations in precipitation show variations of more than a factor of 2 over the year. Usually the maximum fallout in the northern hemisphere is in June and July [ll]. These seasonal variations depend strongly on the exchange of air masses between stratosphere and troposphere. The simultaneous measurement of lo Be and 36C1 could give information on differences in the transport of the two radioisotopes. In a pilot study 20 36C1 samples covering the year 1978 have been analyzed. The samples originate from a pit dug near Dye 3 station, with a depth of 5.4 m corresponding to about 5 years. “Be and many other para meters have been measured previously on these samples and a detailed discussion of the results will be given elsewhere [13]. From the bomb pulse shown in fig. 1 we know that the 36C1 concentration in 1978 is about 3 times above the natural level, e.g. the measured 36C1 originates predominantly from the nuclear weapons tests. We have to be aware that this 36C1 has been produced by neutron activation of natural NaCl and has probably been introduced in this chemical form into the stratosphere. Its behaviour and the transport might be different from cosmogeni-
3 2 I 0 25 20 15 IO 5 0 1500
I
I
1
1550
1600
1650
YEAR
[AD]
Fig. 3. Profiles of “Be,
36C1 and 10Be/36C1 measured on samples from the Camp Century deep core. The mean value for the 1oBe/36C1 ratio is about 9 (dashed line). III(b). GLACIOLOGY/CLIMATOLOGY
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M. Surer el al. / “Cl
6fl
intervals of about 5 years, the new data are from 50 cm sections covering only about 1.5 year intervals each. In spite of this the general trend in both data sets is the same: Enhanced “Be and 36C1 concentrations around 1700 A.D and fluctuations in the ‘“Be/36C1 ratio of up to a factor of 4..
6. =Cl during the Holocene-Wisconsin
00
6
12
4
IO
\
8
I% 0
6 4 2 0’
1100
siudies at ETH/SIN
I
I
1120
1140
I
1160
I 1180
DEPTH [cm] from Camp Fig. 4. Profiles of “Be, 36C1 and “Be/“Cl Century from the depth interval of 1100-1180 m. This interval corresponds to the end of the transition between glacial and post glacial times. The mean value of the ‘“Be/36C1 ratio is about 6.5 (dashed line).
topes mainly reflect production variations due to solar modulations of the galactic cosmic ray flux [16]. Since “Be and 36C1 are produced by spallation reactions, are expected similar changes of the 36Cl concentrations during periods of different solar activity. Calculations [17] show that during the Maunder minimum the ‘“Be/36C1 production ratio should not change by more than 10%. In fact the first 36C1 measurements (performed at Rochester) [l&19] show an enhanced 36C1 concentration during this period. At the same time the mean value of the 36C1 concentration is significantly higher than expected from calculations [20,21]. These data have been compared with “Be data measured at Zurich [9]. Although our 36C1 and “Be results are obtained from the same core (Camp Century) covering the same period the results (fig. 3) are not directly comparable to the data reported by the Rochester group. The reason is that the sampling length for the two sets is different. Whereas the samples analyzed at Rochester are from ice core sections of about 1.5 m length representing time
transition
The transition from Holocene to Wisconsin (10000 yr BP) is especially interesting because all parameters investigated so far in ice cores show strong variations during this period. From earlier “Be measurements it is known that the “Be concentration was higher by a factor of about 2 at the end of Wisconsin. The good correlation between the “Be record and the S180 profile led to the interpretation that the higher “Be concentrations are mainly the effect of lower accumulation rates [2,22]. Due to the thinning of the annual ice layers the time span covered by a single sample (1.5 m) is significantly larger (60-80 yr) than in the core of the Maunder minimum period. Any short-term variations of the transport processes should therefore be smoothed out. The results are plotted in fig. 4. 36C1 shows a concentration increase similar to the one for “Be and it is not apparent in the ‘“Be/36C1 ratio. The fluctuations of the ratio, however, are still large and comparable to those observed during the Maunder minimum.
7. Summary
and conclusions
The final ETH 36C1 measurements on polar ice samples revealed interesting and partly unexpected results: (1) As has been shown before [8] the 36C1 concentration at Dye 3 increased by about a factor of 500 due to nuclear weapons tests between 1950 and 1963. From the exponential decrease of the 36C1 concentration a mean residence time of about 3 yr is derived, which is significantly longer than that observed for other radioisotopes (“Be, ?Sr, 137Cs). This pulse can be used as a time mark (maximum: 1959/60) for dating or as an input function for tracing young groundwater systems. (2) The mean 36C1 concentration in prebomb ice samples is significantly (3-5 times) higher than expected from several calculations [20-22,171. Possible explanations are: Too low results of the calculations, additional non-cosmic-ray-induced sources of 36Cl, or different fallout patterns of 36C1 and “Be. (3) The 36C1 concentrations are generally higher during the Maunder minimum period (1645-1715 A.D.) indicating that 36C1, like “Be, reflects solar modulation effects. (4) At the boundary from Holocene to Wisconsin (11000 BP) the 36C1 increases by about a factor of 2. A
215
hf. Suter et al. / “?I studies at ETH/SIN
similar increase for “Be has been attributed mainly to a change in the precipitation rate. (5) In spite of the similarities between *OBe and 36C1 regarding solar modulation and accumulation rates, the 1oBe/36C1 ratio of the analyzed samples varies by factor of 2-5. Interestingly there seems to be no difference in this variability for samples representing weekly, annual or decadal averages of precipitation. Considering that a 10% change of the lo Be/36 Cl ratio corresponds to an age difference of 57 000 yr, dating by the lo Be/ 36C1 ratio does not seem feasible. Although we do not have a consistent explanation for all the observed facts, there are some points which should be considered: (1) the chemical behaviour of 36C1 and ‘OBe are very different. Changes of the ‘“Be/36C1 ratio resulting from different atmospheric transport and mixing mechanisms are therefore not surprising, especially when dealing with time scales of a few years. For longer time scales, however, more constant ratios would be expected but this is not seen in the experimental data. (2) The higher 36Cl concentrations than expected from calculations and the nonconstancy of the ‘“Be/36C1 ratio could also be explained by an additional noncosmogenic source [9]. This, however, contrasts the observed similar trends of 36C1 and “Be during the Maunder minimum period which indicates that the solar modulation of the production rate is about the same for both isotopes. As a consequence of the above arguments we favour the explanation that the observed variations of the 1oBe/36C1 ratio are p rimarily caused by atmospheric transport effects. If confirmed by additional data the ‘“Be/36C1 ratio may turn out to be a useful tool not for dating ice but for studying the past atmospheric transport and mixing processes. We thank C.C. Langway and B. Kapua for their help during the ice sampling. This work was financially supported by the Swiss National Science Foundation.
[31 J. Beer, M. Andree,
H. Oeschger, U. Siegenthaler, G. Bonani, H.J. Hofmann, E. Morenzoni, M. Nessi, M. Suter, W. Wolfli, R. Finkel and C. Langway, Nucl. Instr. and Meth. B5 (1984) 380. 141 G.M. Raisbeck, F. Yiou, D. Bourles, C. Lorius, J. Jouzel and N.I. Barkov, Nature 326 (1987) 273. H.B. Clausen, W. Dansgaard, N. [51 C.U. Hammer, Gundestrup, S.J. Johnson and N. Reeh, J. Glaciology 20
(1987) 3. [61 M. Andree, E. Moor, J. Beer, H. Oeschger, B. Stauffer, G. Bonani, H.J. Hofmann, E. Morenzoni, M. Nessi, M. Suter and W. Wiilfli, Nucl. Instr. and Meth. BS (1984) 385. [71 K. Nishiizumi,D. Elmore, P. Kubik, G. Bonani, M. Suter, W. Wiilfli and J.R. Arnold, Lunar Planet. Sci. 17 (1986) 621. [8] D. Elmore, L.E. Tubbs, D. Newman, X.Z. Ma, R. Finkel,
[9]
[lo] [ll] [12] [13]
[14] [15] [16]
[17] [18]
[19] [20] [21] [22]
References [23]
111 H.A. Synal, J. Beer, G. Bonani, H.J. Hofmann, M. Suter and W. Wolfli, these Proceedings (AMS ‘87) Nucl. Instr. and Meth. B29 (1987) 146. PI J. Beer, M. Andree, H. Oeschger, B. Stauffer, R. Baher,
K. Nishiizumi, J. Beer, H. Oeschger and M. Andree, Nature 300 (1982) 735. D. Ehnore, N.J. Conard, P.W. Kubik, H.E. Gove, M. WahIen, J. Beer and M. Suter, these Proceedings (AMS ‘87) Nucl. Instr. and Meth. B29 (1987) 207. H.W. Bentley, F.M. Davis, S. Grifford, D. Elmore, L.E. Tubbs and H.E. Gove, Nature 300 (1982) 737. E.R. Reiter, Atmospheric Transport Processes, Part IV, Radioactive Tracers, US Department of Energy (1978). R.J. Cicerone, Rev. Geophys. and Space Phys. 19 (1981) 123. J. Beer, A. Neftel, H. Oeschger, U. Schotterer, J. Schwander, U. Siegenthaler, G. Bonani, M. Suter, W. Wolfli, R.C. Finkel, H.B. Clausen, C.C. Langway, Jr., P. Jacobs, D. Klockow and H. Gaeggeler, in preparation. J.A. Eddy, Science 192 (1976) 1189. M. Stuiver and P.D. Quay, Science 207 (1980) 11. J. Beer, U. Siegenthsler, H. Oeschger, M. Andree, G. Bonani, M. Suter, W. Wolfli, R. Finkel and C. Langway, Proc. 18th Int. Cosmic Ray Conf., Bangalore vol. 9 (1983) p. 317. A. Blinov, private communication. N.J. Conard, D. Elmore, P.W. Kubik, H.E. Gove, L.E. Tubbs, B.A. Chnmyk and M. Wahlen, Radiocarbon 28 (1986) 556. N.J. Conard, Masters Thesis Rochester University (1986). D. LaI and B. Peters, Handbuch der Physik, vol. 46/2 (Springer, Berlin, 1967) p. 551. K. O’Brien, J. Geophys. Res. 84 (1979) 423. F. Yiou, G.M. Raisbeck, D. Bourles, C. Lorius and N.I. Barkov, Nature 316 (1985) 616. H. Oeschger, J. Houtermans, H. LoosIi and M. Wahlen, Radiocarbon Variations and Absolute Chronology, 12th Nobel Symp., ed. I.U. Olsson, (Interscience, New York, 1969) p. 471.
G.Bonani, C. Staller, M. Suter, W. Wolfli and R.C. Finkel, Radiocarbon 25 (1983) 269.
III(b). GLACIOLOGY/CLIMATOLOGY
=Cl STUDIES
AT THE ETH/SIN-AMS
M. SUTER ‘) J. BEER’*2’ G. BONANI H.A. SYNAi” and W. WbLFLI I)
211
FACILITY ‘), H .J . HOFMANN
‘), D. MICHEL 2), H. OESCHGER
2),
‘) Institut fdrMittelenergiephysik, ETH-Hijnggerberg. CH-8093 Zurich, Switzerland 2, Physikalisches Institut der Universitiir Bern, CH-3012 Bern, Switzerland
36Cl has been measured in polar ice from Dye 3 (65O ll’N, 43” 50’W) and Camp Century (77O ll’N, 82O 08’W) in Greenland. In an exploratory study 4 series consisting of about 20 samples each have been selected covering approximately the following time periods: 12000-1OOCKl BP (transition Wisconsin-Holocene), 1530-1730 A.D. (Maunder minimum), 1942-1977 A.D. (nuclear bomb pulse) and 1978 (seasonal variations). The results are compared with “Be data of the same cores which were measured previously. The variations of the 1oBe/36Cl ratio are generally larger than expected from the experimental errors indicating that this ratio is not suitable for dating of old ice.
1. Introduction During the last two years the AMS facility in Zurich has been significantly improved for the detection of heavier ions. 36C1 can now be measured on samples containing more than 3 mg Cl with high reproducibility and a sensitivity of 1.5 X 10-14, which so far only has been reached with larger accelerators which allow a better suppression of the 36S background [I]. With a half-life of 301000 years 36C1 is a good candidate for dating old ground water, old polar ice and extraterrestrial material. At ETH a 36C1 measuring program has been started to explore the potential of 36C1 dating for polar ice and ground water studies. We are presenting here an overview of our ice core studies. The most direct way to study cosmogenically produced 36C1 and its variations caused by changes of the production rates and the atmospheric transport processes is to look at polar ice cores, which contain a continuous record of precipitations over long-time periods. “Be concentration profiles in ice cores have already been studied in detail. They show significant short- and long-term variations during the last 100000 years [2-41. Detailed records of 36C1 are vf special interest for two reasons: Firstly for any studies using the cosmogenic 36C1 as a tracer or a dating tool, knowledge of the input function is essential. Secondly, dating of old polar ice is an important but also a very difficult task. Especially for ice older than 10000 yr no precise dating technique is available. The widely used ice flow models are based on two parameters (annual layer thickness and depth of vertical strain rate changes), which are both not well known [5]. For younger ice it is possible to establish a time scale based on al80 mea0168-583X/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
surements. The seasonal variations of S”O allow year by year counting similar to the tree ring dating technique. In addition, special time marks caused by volcanic eruptions can be used to adjust the time scale. Also the i4C dating method has been applied for dating ice of about 10000 yr [6]. It is well known that the production rate and the transport of the long lived isotopes “Be and 36C1 are subject to large variations and that a single isotope is therefore not suitable for dating. There was, however, the hope that these fluctuations could be eliminated by using the 36C1/10 Be ratio which decreases with time with an apparent half-life of 370000 years. This ratio would provide an ideal tool for dating ice older than 50000 years. Earlier measurements on a few individual ice and snow samples showed significant variations in the ‘“Be/36C1 ratio [7]. For a better understanding of the ‘“Be/36C1 ratio detailed profiles of 36C1 and “Be are needed. 36C1 profile measurements have been performed at the University of Rochester. In 1982 samples from a shallow core from Dye 3, Greenland, covering the period 1950-1975 have been analyzed. The results show a large pulse of 36C1 due to nuclear weapons tests [8]. Recently a new set of ice core samples has been measured covering the time interval 1550-1850 A.D. A comparison with “Be data obtained at ETH show large variations of the ‘“Be/36C1 ratio of up to a factor of 5. These results indicate that individual samples covering only a few years cannot be used for dating [9]. We present new 36C1 data covering partly the same time period. In addition some samples from the transition between glacial and post glacial time have been measured.
III(b). GLACIOLOGY/CLIMATOLOGY
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M. Suter et al, / 36Cl studies at ETH/SIN
2. Experimental For the sample preparation the following procedure is used: Ice samples of l-2 kg weight are first cleaned by rinsing them with deionised water. After melting in a microwave oven and taking an aliquot, 0.5 mg 9Be and 3 mg Cl carrier is added, The aliquot is used later to determine the Cl concentration of the sample which is needed to calculate the correct 36Cl/C1 ratio. The volume is reduced by evaporation to about 20 ml. AgCl is precipitated by adding AgNO,. The precipitation is dissolved with NH, and filtered through a millipore filter (0.4 pm). Then the Cl is again precipitated in the form of AgCl. The filtering process is repeated until the filter looks clean. After the final precipitation the AgCl is dried in an oven. A similar procedure is described by Conard et al. [17]. A short time before the measurement the AgCl is pressed into the target holder, which is made of tantalum [l].
3. The bomb pulse in an ice core from Dye 3, Greenland In the fifties significant amounts of 36C1 were produced during marine nuclear weapons tests and were then transported into the stratosphere. This artificiallyproduced 36C1 dominates in precipitation and dry fallout during the last four decades. For 36C1 tracer studies it is important to have a detailed knowledge of this anthropogenic 36Cl input and its concentrations in precipitation as a function of time. Polar ice cores are the best suited archives for this kind of study. In 1982 a first series of 36C1 samples from Dye 3 station in Greenland was compared with model calculations based on data of location and neutron yields of the bomb tests. Although a relatively good quantitative agreement was found, the measured residence time was larger than expected [10,8]. At ETH 36C1 samples have been measured from another shallow core drilled in summer 1983 at Dye 3 station. Each sample averages a time interval of about two years. The series covers the time period from 1942-1977. The new data are presented in fig. 1. For comparison the results obtained at Rochester are also plotted. The two data sets show a good agreement considering that the individual samples are not covering the same time intervals and that the samples are from different cores. After 1970 the new data show in the average a lower 36C1 concentration. This leads to a steeper slope resulting in a shorter atmospheric residence time compared to the Rochester data. From the new data a mean residence time (i.e. the time it takes to decrease the concentration to l/e of its original value) of about 3 years can be derived, which is about a factor of 2 larger than that observed for 90Sr and 13’Cs [ll]. A possible explanation of this difference is that some of
I
I
1960
1950 Date
1970
of deposItton
Fig. 1. The 36C1concentration profile from a shallow ice core drilled in 1983 at Dye 3 station in southern Greenland. The results show a large pulse of 36C1 due to nuclear weapons tests. The data
are compared with earlier 36C1 measurements formed at Rochester [S].
per-
the weapons tests performed after 1960 introduced additional 36C1 into the atmosphere [8]. But it seems also feasible that there is a real difference in the residence time of 36C1 and other long lived radioisotopes. Sr, Cs and Be become attached to aerosols within a short time after their production, whereas Cl is also present in gaseous form (HCl) and takes part in photochemical reactions in the upper atmosphere [12]. The new data set also includes 4 data points covering the time before 1950 allowing us to determine the 36C1 level to be about 0.8 x lo3 atoms/g. prebomb previously [2], a Together with “Be data published value for the ‘“Be/36Cl ratio of about 14 is determined. The extrapolation of the experimental data of the 36C1 into recent time shows that the weapons tests still influence the 36C1 in precipitation at present (1987).
4. Seasonal variations of WI in polar ice From studies on polar ice and from measurements on rainwater, it is known that the radioisotope con-
hf.Suter etal./ 36CI~tudies atETH/SIN
213
tally produced 36C1. The “Be, the 36C1 and the ratios of the two radioisotopes are given in fig. 2 as a function of depth. The data show variations of more than a factor of 4 for the “Be and the 36Cl concentrations. The ratio of the two radioisotopes exhibits significantly smaller fluctuations indicating a correlation between the two isotopes. However, compared to experimental errors, the ratio cannot be considered as constant.
5. %CI in polar ice during the Maunder minimum From historical sun spot records it is known that between i645 and 1715 A.D. almost no sun spots were observed (Maunder Minimum) [14]. 14C measurements in tree rings show an enhancement of a few percent in the atmospheric 14C concentration during this period [15]. The “Be and 14C variations have been compared for the period 1200-1800 A.D. using a carbon cycle model to account for the different geochemical behaviour of the two isotopes. The good agreement between the two records strongly indicates that both iso-
4
460
480
DEPTH
500
520
[cm]
Fig. 2. Seasonal variations of “Be, 36C1 and ‘“Be/36Cl measured from samples from a pit at Dye 3 for the year 1978. The
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I
3 2 I
dashed line gives the mean value of the ‘“Be/36C1 ratios. 0
centrations in precipitation show variations of more than a factor of 2 over the year. Usually the maximum fallout in the northern hemisphere is in June and July [ll]. These seasonal variations depend strongly on the exchange of air masses between stratosphere and troposphere. The simultaneous measurement of lo Be and 36C1 could give information on differences in the transport of the two radioisotopes. In a pilot study 20 36C1 samples covering the year 1978 have been analyzed. The samples originate from a pit dug near Dye 3 station, with a depth of 5.4 m corresponding to about 5 years. “Be and many other para meters have been measured previously on these samples and a detailed discussion of the results will be given elsewhere [13]. From the bomb pulse shown in fig. 1 we know that the 36C1 concentration in 1978 is about 3 times above the natural level, e.g. the measured 36C1 originates predominantly from the nuclear weapons tests. We have to be aware that this 36C1 has been produced by neutron activation of natural NaCl and has probably been introduced in this chemical form into the stratosphere. Its behaviour and the transport might be different from cosmogeni-
3 2 I 0 25 20 15 IO 5 0 1500
I
I
1
1550
1600
1650
YEAR
[AD]
Fig. 3. Profiles of “Be,
36C1 and 10Be/36C1 measured on samples from the Camp Century deep core. The mean value for the 1oBe/36C1 ratio is about 9 (dashed line). III(b). GLACIOLOGY/CLIMATOLOGY
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6fl
intervals of about 5 years, the new data are from 50 cm sections covering only about 1.5 year intervals each. In spite of this the general trend in both data sets is the same: Enhanced “Be and 36C1 concentrations around 1700 A.D and fluctuations in the ‘“Be/36C1 ratio of up to a factor of 4..
6. =Cl during the Holocene-Wisconsin
00
6
12
4
IO
\
8
I% 0
6 4 2 0’
1100
siudies at ETH/SIN
I
I
1120
1140
I
1160
I 1180
DEPTH [cm] from Camp Fig. 4. Profiles of “Be, 36C1 and “Be/“Cl Century from the depth interval of 1100-1180 m. This interval corresponds to the end of the transition between glacial and post glacial times. The mean value of the ‘“Be/36C1 ratio is about 6.5 (dashed line).
topes mainly reflect production variations due to solar modulations of the galactic cosmic ray flux [16]. Since “Be and 36C1 are produced by spallation reactions, are expected similar changes of the 36Cl concentrations during periods of different solar activity. Calculations [17] show that during the Maunder minimum the ‘“Be/36C1 production ratio should not change by more than 10%. In fact the first 36C1 measurements (performed at Rochester) [l&19] show an enhanced 36C1 concentration during this period. At the same time the mean value of the 36C1 concentration is significantly higher than expected from calculations [20,21]. These data have been compared with “Be data measured at Zurich [9]. Although our 36C1 and “Be results are obtained from the same core (Camp Century) covering the same period the results (fig. 3) are not directly comparable to the data reported by the Rochester group. The reason is that the sampling length for the two sets is different. Whereas the samples analyzed at Rochester are from ice core sections of about 1.5 m length representing time
transition
The transition from Holocene to Wisconsin (10000 yr BP) is especially interesting because all parameters investigated so far in ice cores show strong variations during this period. From earlier “Be measurements it is known that the “Be concentration was higher by a factor of about 2 at the end of Wisconsin. The good correlation between the “Be record and the S180 profile led to the interpretation that the higher “Be concentrations are mainly the effect of lower accumulation rates [2,22]. Due to the thinning of the annual ice layers the time span covered by a single sample (1.5 m) is significantly larger (60-80 yr) than in the core of the Maunder minimum period. Any short-term variations of the transport processes should therefore be smoothed out. The results are plotted in fig. 4. 36C1 shows a concentration increase similar to the one for “Be and it is not apparent in the ‘“Be/36C1 ratio. The fluctuations of the ratio, however, are still large and comparable to those observed during the Maunder minimum.
7. Summary
and conclusions
The final ETH 36C1 measurements on polar ice samples revealed interesting and partly unexpected results: (1) As has been shown before [8] the 36C1 concentration at Dye 3 increased by about a factor of 500 due to nuclear weapons tests between 1950 and 1963. From the exponential decrease of the 36C1 concentration a mean residence time of about 3 yr is derived, which is significantly longer than that observed for other radioisotopes (“Be, ?Sr, 137Cs). This pulse can be used as a time mark (maximum: 1959/60) for dating or as an input function for tracing young groundwater systems. (2) The mean 36C1 concentration in prebomb ice samples is significantly (3-5 times) higher than expected from several calculations [20-22,171. Possible explanations are: Too low results of the calculations, additional non-cosmic-ray-induced sources of 36Cl, or different fallout patterns of 36C1 and “Be. (3) The 36C1 concentrations are generally higher during the Maunder minimum period (1645-1715 A.D.) indicating that 36C1, like “Be, reflects solar modulation effects. (4) At the boundary from Holocene to Wisconsin (11000 BP) the 36C1 increases by about a factor of 2. A
215
hf. Suter et al. / “?I studies at ETH/SIN
similar increase for “Be has been attributed mainly to a change in the precipitation rate. (5) In spite of the similarities between *OBe and 36C1 regarding solar modulation and accumulation rates, the 1oBe/36C1 ratio of the analyzed samples varies by factor of 2-5. Interestingly there seems to be no difference in this variability for samples representing weekly, annual or decadal averages of precipitation. Considering that a 10% change of the lo Be/36 Cl ratio corresponds to an age difference of 57 000 yr, dating by the lo Be/ 36C1 ratio does not seem feasible. Although we do not have a consistent explanation for all the observed facts, there are some points which should be considered: (1) the chemical behaviour of 36C1 and ‘OBe are very different. Changes of the ‘“Be/36C1 ratio resulting from different atmospheric transport and mixing mechanisms are therefore not surprising, especially when dealing with time scales of a few years. For longer time scales, however, more constant ratios would be expected but this is not seen in the experimental data. (2) The higher 36Cl concentrations than expected from calculations and the nonconstancy of the ‘“Be/36C1 ratio could also be explained by an additional noncosmogenic source [9]. This, however, contrasts the observed similar trends of 36C1 and “Be during the Maunder minimum period which indicates that the solar modulation of the production rate is about the same for both isotopes. As a consequence of the above arguments we favour the explanation that the observed variations of the 1oBe/36C1 ratio are p rimarily caused by atmospheric transport effects. If confirmed by additional data the ‘“Be/36C1 ratio may turn out to be a useful tool not for dating ice but for studying the past atmospheric transport and mixing processes. We thank C.C. Langway and B. Kapua for their help during the ice sampling. This work was financially supported by the Swiss National Science Foundation.
[31 J. Beer, M. Andree,
H. Oeschger, U. Siegenthaler, G. Bonani, H.J. Hofmann, E. Morenzoni, M. Nessi, M. Suter, W. Wolfli, R. Finkel and C. Langway, Nucl. Instr. and Meth. B5 (1984) 380. 141 G.M. Raisbeck, F. Yiou, D. Bourles, C. Lorius, J. Jouzel and N.I. Barkov, Nature 326 (1987) 273. H.B. Clausen, W. Dansgaard, N. [51 C.U. Hammer, Gundestrup, S.J. Johnson and N. Reeh, J. Glaciology 20
(1987) 3. [61 M. Andree, E. Moor, J. Beer, H. Oeschger, B. Stauffer, G. Bonani, H.J. Hofmann, E. Morenzoni, M. Nessi, M. Suter and W. Wiilfli, Nucl. Instr. and Meth. BS (1984) 385. [71 K. Nishiizumi,D. Elmore, P. Kubik, G. Bonani, M. Suter, W. Wiilfli and J.R. Arnold, Lunar Planet. Sci. 17 (1986) 621. [8] D. Elmore, L.E. Tubbs, D. Newman, X.Z. Ma, R. Finkel,
[9]
[lo] [ll] [12] [13]
[14] [15] [16]
[17] [18]
[19] [20] [21] [22]
References [23]
111 H.A. Synal, J. Beer, G. Bonani, H.J. Hofmann, M. Suter and W. Wolfli, these Proceedings (AMS ‘87) Nucl. Instr. and Meth. B29 (1987) 146. PI J. Beer, M. Andree, H. Oeschger, B. Stauffer, R. Baher,
K. Nishiizumi, J. Beer, H. Oeschger and M. Andree, Nature 300 (1982) 735. D. Ehnore, N.J. Conard, P.W. Kubik, H.E. Gove, M. WahIen, J. Beer and M. Suter, these Proceedings (AMS ‘87) Nucl. Instr. and Meth. B29 (1987) 207. H.W. Bentley, F.M. Davis, S. Grifford, D. Elmore, L.E. Tubbs and H.E. Gove, Nature 300 (1982) 737. E.R. Reiter, Atmospheric Transport Processes, Part IV, Radioactive Tracers, US Department of Energy (1978). R.J. Cicerone, Rev. Geophys. and Space Phys. 19 (1981) 123. J. Beer, A. Neftel, H. Oeschger, U. Schotterer, J. Schwander, U. Siegenthaler, G. Bonani, M. Suter, W. Wolfli, R.C. Finkel, H.B. Clausen, C.C. Langway, Jr., P. Jacobs, D. Klockow and H. Gaeggeler, in preparation. J.A. Eddy, Science 192 (1976) 1189. M. Stuiver and P.D. Quay, Science 207 (1980) 11. J. Beer, U. Siegenthsler, H. Oeschger, M. Andree, G. Bonani, M. Suter, W. Wolfli, R. Finkel and C. Langway, Proc. 18th Int. Cosmic Ray Conf., Bangalore vol. 9 (1983) p. 317. A. Blinov, private communication. N.J. Conard, D. Elmore, P.W. Kubik, H.E. Gove, L.E. Tubbs, B.A. Chnmyk and M. Wahlen, Radiocarbon 28 (1986) 556. N.J. Conard, Masters Thesis Rochester University (1986). D. LaI and B. Peters, Handbuch der Physik, vol. 46/2 (Springer, Berlin, 1967) p. 551. K. O’Brien, J. Geophys. Res. 84 (1979) 423. F. Yiou, G.M. Raisbeck, D. Bourles, C. Lorius and N.I. Barkov, Nature 316 (1985) 616. H. Oeschger, J. Houtermans, H. LoosIi and M. Wahlen, Radiocarbon Variations and Absolute Chronology, 12th Nobel Symp., ed. I.U. Olsson, (Interscience, New York, 1969) p. 471.
G.Bonani, C. Staller, M. Suter, W. Wolfli and R.C. Finkel, Radiocarbon 25 (1983) 269.
III(b). GLACIOLOGY/CLIMATOLOGY