10Be measurements on polar ice: Comparison of Arctic and Antarctic records

Nuclear Instruments and Methods North-Holland, Amsterdam

in Physics

Research

“Be MEASUREMENTS ON POLAR ICE: COMPARISON OF ARCTIC AND ANTARCTIC

J. BEER, G. BONANI, H.J. HOFMANN, Insiitut fir Mittelenergiephysik,

H. OESCHGER Physics Institute,

ETH-Hoenggerberg,

203

B29 (1987) 203-206

RECORDS

M. SUTER, A. SYNAL and W. WGLFLI CH-8093 Ziirich, Switzerland

and U. SIEGENTHALER University of Bern, CH-3012 Bern, Switzerland

R.C. FINKEL LawrenceLivermore

National Laboratory,

94550 Livermore,

CA, USA

A new semicontinuous lo Be record from Byrd station, Antarctica, is presented and compared with the Camp Century record from Greenland. Although the comparison of the Holocene parts of the two profiles is seriously hampered by lack of a reliable time scale for the Byrd core, there is evidence for common features. The establishment of the existence of global features in toBe records is of great importance for the interpretation of “Be concentrations in terms of production rate changes due to helio- and geomagnetic modulation. In addition “Be time marks could be used to synchronize the climatic records of the Northern and Southern hemispheres.

1. Introduction During the last ten years, following the invention of the accelerator mass spectrometry technique, a steadily increasing number of “Be measurements has been carried out on various terrestrial archives [l]. Besides dating, which in the case of ice has proven to be far from straightforward [2,3], one of the most important potential applications of “Be is to reconstruct the history of solar activity and geomagnetic field intensity. Both magnetic fields carried by solar wind as well as those associated with the geomagnetic dipole moment modulate the cosmic ray flux impinging on the atmosphere and thereby influence the production rates of radioisotopes. As a consequence the “Be concentration measured in natural archives such as polar ice sheets contains information on production rates in the past. Production variations by factors up to 1.6 for solar and up to 3 for geomagnetic modulation are to be expected [4,5]. There is, however, a serious problem which complicates the interpretation of measured “Be variations in terms of solar and geomagnetic modulation: the “Be concentration in ice is not only determined by the production rate but also by transport and deposition processes. On a short time scale of days, variations of up to a factor of ten can be observed (unpublished data). About 2/3 of the “Be is produced in the stratosphere which, considering the mean residence time of more than one year, can be assumed to be well mixed. 0168-583X/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

About l/3 is produced in the troposphere where the residence time is relatively short (l-3 weeks), determined mostly by precipitation frequency. The exchange between stratosphere and troposphere usually takes place in the mid-latitudes during the first part of the year and is also subject to strong fluctuations. The “Be concentration measured at a certain location may therefore be strongly influenced by temporal and regional variations of the atmospheric transport processes. This leads to fundamental questions in interpreting “Be profiles: how representative for global changes is a “Be record from one specific location? Are the observed concentration variations caused by local, regional or global effects, and how can we distinguish between the different causes? There are two approaches to answer these questions. The first is to compare “Be with other records such as i4C and “0. Since 14C is produced in a similar way but shows a completely different geochemical behaviour, a comparison of these two isotopes helps to distinguish between production and transport variations. In fact a detailed “Be study of the Milcent [6] and the Camp Century (Beer et al., in preparation) ice cores revealed short term variations which could unambiguously be related to the so-called “Suess wiggles” in the i4C tree ring record, indicating the common effect of solar modulation on both isotopes. On the other hand, there is good agreement between “Be and #so records during the Wisconsin, which strongly supports the explanation III(b). GLACIOLOGY/CLIMATOLOGY

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that these “Be fluctuations are mainly due to climatic effects. The second approach, which is the main subject of this paper, is to compare “Be records from different sites. Since the most detailed continuous “Be record so far available was obtained from the Camp Century ice core in northern Greenland, we have chosen the Byrd core from Antarctica as a counterpart.

on polar ice

carried out using the accelerator mass spectrometer of the ETH Zurich [9]. The accuracy of the measurements is 3-lo%, the data are calibrated using our internal “Be ratio of 101.4 X standard with an assumed “Be/‘Be lo-‘* [lo].

3. Results in the Byrd ice core In fig. 1 the loBe concentration is shown as a function of depth. The curve can be divided into 3 main parts: a zone of decreasing concentration between 180 and 500 m with some distinct narrow peaks; a zone of slight decreasing concentration between 500 and 1100 m with three broad peaks; and, below 1100 m, a zone of higher values with strong fluctuations at the end of the last glaciation. The prevalence of higher “Be concentrations during glacial times agrees with earlier data [ll-131. The high concentrations coincide with low 6180 values (fig. 2) which suggests that the higher “Be concentrations were caused by lower precipitation rates. Indeed a more detailed comparison of the “Be and the S’*O [8] curves in fig. 2 reveals a good correlation between “Be and S’*O for the glacial period. During the Holocene, however, the trends of “Be and al80 are just the opposite and above 900 m there is no clear correlation between the “fine structure” of the two curves. This indicates that these Holocene ‘cBe variations are not primarily the result of climatic effects. In order to compare the “Be profiles from Camp Century and Byrd, time scales are needed. In the case of

2. Experimental The Byrd core (80° S, 120° W) was drilled by CRREL and reached bedrock at a depth of 2164 m in 1968 [7]. The present-day accumulation rate is about 14 cm of water-equivalent. The mean annual temperature is - 28O C so that melting of ice can be excluded at least in recent year. In contrast to Camp Century, the ice rheology at Byrd station is rather complicated due to the topography of the bedrock and the more variable Holcene climate in the Antarctic than in the Arctic [8]. Unfortunately the core quality is poor for the depth interval 400-900 m. In order to obtain a semicontinuous profile, a thin slice was cut from every second core tube resulting in samples of l-2 kg of ice corresponding to a depth )I interval of about 10 m. The ice samples were cleaned by rinsing them with deionised water. After melting in a microwave oven 0.5 mg 9Be carrier was added. Then the volume was reduced to about 10 ml by evaporation. Be(OH), was precipitated by adding NH,OH and converted into Be0 at 900°C. All measurements were

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3

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1-

0 0

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I 400

I 600

I I000

I 600 DEPTH

I 1200

1 1400

16

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Fig. 1. “Be concentration of the Byrd core in units of lo4 atoms/g versus depth.

J. Beer et al. / “Be measurements on polar ice

I

I

205

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s s L

BE-10

-30

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and al80

Camp Century a time scale was constructed using two time marks, the Maunder Minimum period (1645-1715 A.D.) and the transition to the the Younger Dryas (11000 BP). By means of an ice flow model [14] the intermediate depth range was converted into time assuming a constant accumulation rate. Comparison between lo Be and l4 C revealed an excellent synchroneity of all the major wiggles during the last 5000 yr. In the

I

I

1000

1500

2000

Ovl) [8] of the Byrd core versus depth.

case of the Byrd core, the situation is more difficult. There is an indication of a dip in the 61s0 profile at 1130 m depth, at the end of the Glacial-Holocene transition, which can be interpreted as Younger Dryas cold phase with an age of about 11000 years [15]. However, due to the complex rheological regime it is difficult to accurately model the age-depth relation of the Byrd core. We therefore used the ice flow model

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I

I

BYRD

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’

0.5 CAMP

8000

CENTURY I

I

I

6000

4000

2000

YEAR Fig. 3. “Be

concentrations

I

I

0

2000

(f3c/A~)

normalized to the respective mean value, of the Byrd and the Camp Century (180-1050 m) after applying the “wiggle matching” procedure. III(b).

cores for the Holocene

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J. Beer et al. / “Be measurements

results only as a first approximation and tried then in a next step to improve the correlation between the two “Be curves using the method of “wiggle matching”. Basically we matched the three most prominent peaks; the two double peaks at 450 m and 550 m and the broad peak at 900 m depth. We then related the Byrd depth scale to the Camp Century depth scale by interpolating in a linear fashion between these peaks. The age of the upper end of the core was estimated by a combination of isotopic and chemical techniques [16]. The result for the Holocene part (180-1050 m) is shown in fig. 3. For the comparison, curves from the two cores have been normalized by dividing by the corresponding mean value. During the last 2000 yr the agreement between the two curves is relatively good as far as the trend and the number of peaks are concerned. However, there are some significant differences in the amplitudes of the peaks. The period 4000-O B.C. is especially problematic since there are only few features in the Byrd profile which can be used for the matching procedure. This period corresponds to the fractured part of the core. At the older end, between 6000 and 4000 B.C., the shapes of the two curves are similar. Although the “wiggle matching” procedure is somewhat artificial, it is difficult to believe that the overall agreement observed is only an artefact, especially when one considers the great differences between the two cores with regard to both time resolution and climatic regime. The clear indications of common features in both records therefore must reflect global events affecting “Be concentrations. A crucial test would be the construction of an independent time scale for the Byrd core. Unfortunately there is only little hope that the fractured part between 400 and 900 m can be dated by other means. Final confirmation of this conclusion must await the detection of the same features in other ice cores [17].

4. Conclusions In spite of its preliminary character, the comparison of the Camp Century and the Byrd “Be records provide some evidence for common features in the Arctic and the Antarctic cores. If confirmed this would mean that “Be concentrations do indeed reflect global events. A comparison of the observed relative amplitudes could provide information about differences of the atmospheric mixing and transport processes in the two regions. In addition “Be profiles could help to synchronize the climatic record of the two hemispheres, which

on polar ice

is very important for the reconstruction climate during the last 10000 years.

of the world’s

We are very grateful to C.C. Langway and B. Kapuza for their help during the sampling period and to MIGROS for providing us a cold storage room. This work was financially supported by the Swiss National Science Foundation.

References [l] Proc. 3rd Int. Symp. on Accelerator Mass Spectrometry, eds., W. Wi%li, H.A. Polach and H.H. Andersen, Nucl. Instr. and Meth. B5 (1984) 91-448. [2] M. Suter, J. Beer, G. Bonani, H.J. Hofmann, D. Michel, H. Oeschger, H.A. Spa1 and W. Wolfli, these Proceedings (AMS ‘87) Nucl. Instr. and Meth. B29 (1987) 211. [3] D. Elmore, 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. [4] D. LaI, submitted to EPSL. [5] A. Blinov, personal communication. [6] J. Beer, H. Oeschger, M. And&e, G. Bonani, H.J. Hofmama, M. Nessi, M. Suter, W. Woelfli, R.C. Finkel and C.C. Langway, Proc. 18th Int. Cosmic Ray Conf., Bangalore 9 (1983) 317. [7] A.J. Gow, H.T. Ueda and D.E. Garfied, Science 161 (1968) 1011. [8] S.J. Johnsen, W. Dansgaard, H.B. Clausen and C.C. Langway, Nature 235 (1972) 429. [9] M. Suter, R. Balzer, G. Bonani, H.J. Hofmann, E. Morenzoni, M. Nessi, W. Wolfli, M. And&e, J. Beer and H. Oeschger, Nucl. Intr. and Meth. BS (1984) 117. [lo] H.J. Hofmamr, J. Beer, G. Bonani, H.R. von Gunten, S. Raman, M. Suter, R.L. WaIker, W. Wolfli and D. Zimmermamr, these Proceedings (AMS ‘87) Nucl. Instr. and Meth. B29 (1987) 32. [ll] G.M. Raisbeck, F. Yiou, M. Fnmeau, J.M. Loiseaux, M. Lieuvin, J.C. Ravel and C. Lorius, Nature 292 (1981) 825. [12] G.M. Raisbeck, F. Yiou, D. Bourles, C. Lorius, J. Jouzel and N.I. Barkov, Nature 325 (1987) 273. [13] J. Beer, M. And&, H. Oeschger, B. Stauffer, Ch. StolIer, M. Suter, W. Wolfli and R.C. Finel, Radiocarbon 25 (1983) 269. [14] C.U. Hammer, H.B. Clausen, W. Dansgaard, S.J. Johnson and N. Reeh, J. Glacial. 20 (1978) 3. [15] C.U. Hammer, H.B. Clausen and H. Tauber, Radiocarbon 28 (1986) 284. [16] C.U. Hammer, personal communication. [17] F. Yiou and G.M. Raisbeck, presented at 4th Int. Symp. on Accelerator Mass Spectrometry, Niagara-on-the-Lake, Canada (27-30 April 1987) Abstract Book, p. 83.