sign in sign up
<<
Home , Antarctic, Antarctica, Byrd Station, Plateau Station, Vostok Station

The isotope analysis of cores from station (), to the depth of 950 m

N. I. Barkov, E. S. Korotkevich, F. G. Gordienko and V. M. Kotlyakov

Abstract. Interpretation of the results of oxygen isotope analysis of ice cores obtained from the borehole at is carried out. The borehole reached the depth of 950 m. To interpret the age of the samples the model of ice deformation with non-uniform strain rates by Nye was used. The series of samples represented 46 500 . It is shown that the isotope profile of the Vostok borehole is mainly formed under the influence of a climatic agent and is not complicated by the proper effect of glacial cover. At the beginning of the Holocene, climatic warming occurred in Central Antarctica between 15 000 and 11 000 years ago; isotope shift made up 5%0, which is in keeping with a temperature increase of about 5°C. Four periods are distinguished during the period between 46 500 and 15 00 years ago. They are separated by three warmer periods. Mean 8(I80) makes up -58.6°/oo in cold periods and -56.8%o in 'warm' periods. During the last 11 000 years mean S(lsO) equals -53.8%o- The main result of the study is that for the last 50 000 years the basic temperature changes occurred synchronously in polar areas of both hemispheres.

Analyse isotopique des carottes de glace obtenues à la station Vostok (Antarctique) jusqu'à une profondeur de 950 m Résumé. On présente les résultats de l'analyse de la composition isotopique (oxygène) des carottes de glace obtenues à la station Vostok jusqu'à une profondeur de 950 m. Pour obtenir l'âge des échantillons, on utilise le modèle de déformation de la glace de Nye (taux de déformation non uniforme). La série d'échantillons représente 46 500 ans. On montre que le profil isotopique obtenu est principalement dû au facteur climatique et n'est pas perturbé par l'effet propre de la couverture de glace. Au début de l'Holocène le réchauffement climatique dans l'Antarctique central s'est produit entre 15 000 et 10 000 ans avant la période actuelle; la variation isotopique est de 5°/oo, ce qui suggère un réchauffement de l'ordre de 5°C. De — 46 500 à - 15 000 ans on distingue quatre périodes froides. Elles sont séparées par trois 18 périodes plus chaudes. Les S( 0) moyens sont de l'ordre de -58.6%0 dans les périodes froides et de -56.8°/oo dans les périodes 'chaudes'. Durant les 11 000 dernières années, la moyenne est de — 53.8%o- La principale conclusion de cette étude est que pour les derniers 50 000 ans les principaux changements de température se sont produits simultanément dans les régions polaires des deux hémisphères.

This paper contains the results of isotope analyses of ice samples from the borehole at Vostok station, drilled in 1972-1973. The drilling reached the depth of 950 m. For the entire borehole a good core, 12 cm in diameter, was obtained. The measurements were carried out in 1972 and 1975 with laboratory equipment of the Moscow University. The samples, delivered to the laboratory in solid state, were analysed for lsO-variations with mass spectrometers MI-1305 (in 1972) and MI-1309 (in 1975). The samples were melted at room temperature immediately before the analyses. For the isotope analyses the methods by Cohn and Urey (1938) were used with some changes. All 180-values are expressed in form of deviations S(lsO) in °/oo relative to 'standard mean water' (SMOW). The measured accuracy for not fewer than three measurements on each sample reached ± 3%0. The selection of samples from the Vostok borehole, their transport to Leningrad and delivery to the laboratory was fulfilled by the Soviet Expedition under the guidance of N. I. Barkov and E. S. Korotkevich. Laboratory studies and interpretation of the results were carried out under the leadership of V. M. Kotlyakov and F. G. Gordienko. 382 The isotope analysis of ice cores from Vostok 383 The results of the isotope analysis of the upper 507 m of the Vostok borehole core have already been published (Barkov et ah, 1974, 1975). In the first series of small samples measuring 5-50 cm vertically were used. They correspond to 2-20 years of ice accumulation, but very often the samples contained sediments of 3-4 years. In the second series of experiments each sample was 0.5-5 m long, i.e. it covered 20-200 years, which produced reliable mean data and excluded accidental results. The samples of the second series were obtained from another borehole drilled at Vostok station near the first one ; they contained the core from the depth of 500 m as well. A number of analyses from the upper 500 m were repeated because the quality of samples from the second borehole was higher. The results differed insignificantly from the earlier ones presented in the above-mentioned papers. The accumulation rate, necessary to calculate the age of the ice, was obtained from data on stake observations for the last 10 years, and also from results of isotope analyses of ice samples, 2-5 mm thick. The latter were obtained in entire sequences from five horizons. During preparation of the samples utmost care was taken to prevent isotopic exchange between the samples and ambient water vapour. Some scatter of the data might be caused by inadequate methods of sample selection from the core. The layers, only a few millimetres thick, were scraped along the whole core, then they were melted and mixed. It could be that this scraping was not ideal and the amount of ice from different areas varied slightly. According to the series of samples taken from the horizons at depths of 48, 82 and 160 m, accumulation made up 2.9 cm of ice per for the last 5000 years. Surveys by one of the authors at Vostok station showed, that in this area accumulation is missing once in 5-6 years. If we take into account such omissions, the mean thickness of an annual layer equals 2.4 cm of ice or 2.2 g/cm2 per year, which is similar to the present-day accumulation rate of atmospheric . In order to interpret the age of the' data, we used the model of ice deformation with non-uniform strain rates proposed by Nye (Dansgaard and Johnsen, 1969). Two variants of conditions on the bed were considered : freezing of the ice to the bedrock or exceeding the temperature of the pressure melting point. They have already been analysed by Dansgaard et al. (1971) while interpreting the data on the Camp Century borehole. Nye's model presupposes constancy in time of parameters influencing ice dynamics. Of course this assumption is rather rough and will require more refined models for the determination of the age of the ice. We used the above-mentioned model mainly because it had been applied to an interpretation of isotopic data from the Camp Century borehole. The use of the same methods to determine the age of the ice permits the data to be compared in a more consistent way. With the model used, the age of the core from Vostok station (T) can be expressed in a first approximation by

T _ (2H - h)r lp2ff - h 2ajj 2y — h where aK is the thickness of the primary layer of annual accumulation; H is the thickness of the ice cover (3450 m, the upper 100 m make a firn layer); h = 400 m is the height above the bed above which the horizontal component of the strain rate becomes constant; y is the height of the level of the core sample above the bed ; and 384 N. i. Barkov eta/. T is the time unit, here equal to one year. The error of this age determination is estimated to be +5 per cent. It is well known that the shape of isotope profiles determined by data from deep boreholes in the ice depends on climatic changes (primarily on temperature conditions) and on variability of the absolute height of the surface of glacier covers, which is not constant in time. The absolute height of the and ice sheets was, evidently, greater during than it is now. In the area of Camp Century the glacial cover was 1200-1300 m higher than the present one. This is also confirmed by results of the analysis of ice samples from this borehole for gas content (Raynaud and Lorius, 1973). In the area of the ice cap surface was 400-600 m higher during the Wisconsin stage than it is now. The bigger height of the surface caused lower temperature of deposited on the ice cap, which, in its turn, brought about the formation of glacier ice of lighter isotopic composition. This reason may be called the 'glaciologicaP agent in formation of isotope profiles of glacial cover in contrast to proper climatic factors. What is the contribution of the glaciological agent to the formation of the isotope profile of the Vostok borehole? Reconstructions of the Antarctic for the stages of the Wisconsin and the maximum glaciations (see Voronov, 1960) show considerable fluctuations of its height in West Antarctica and peripheral areas of up to 1500-2000 m. Proceeding from the general laws of ice deformation and possible variability of the shape and thickness of the accompanying its larger dimensions, we may conclude, that the height of the surface in the area of Vostok station, which is not far from the ice divide of the East Antarctic dome, grew up by 100 m in the Wisconsin and changed by not more than 300 m in the period of the Quaternary maximum. Consequently, the isotope profile of the Vostok borehole is essentially influenced by climatic factors only and, thus, testifies purely climatic coolings or warmings, and is not complicated by the effect of the height of the sheet. The data from the Vostok borehole are of great palaeoglaciological importance. Figure 1 presents the results of oxygen isotope analysis of the core down to 950 m. The isotope profile of the Vostok borehole is plotted according to the data of 300 ice samples. According to the age model they cover 46 500 years BP. Figure 1 shows dramatic temperature changes occurring in Antarctica more or less at the same time as in Greenland (Dansgaard et al, 1971, figure 9). According to the data from the Vostok borehole, climatic warming, which occurred between the and the Holocene, started 15 000 years ago and became stabilized 11 000 years ago, on a level close to the present one. At Vostok station the isotope shift, which took place between 15 000 and 11 000 years ago, equals 5%o and is caused by climatic factors only. The corresponding shift at Camp Century made up 12%o- Perhaps almost the whole difference of 7°/00 reflects the contribution of the glaciological agent—the higher position of the glacier surface in the west of Greenland during the Wisconsin stage. According to the data from 15 Antarctic stations a relation between mean values of S(180) in deposited atmospheric precipitations and mean annual values of temperature (t) of the air was established (Gordienko and Barkov, 1973). This interdependence is approximately S(180) = (0.84; - 7.5)%o If this is applied to the past, the difference between the upper (11 000 years ago) and the lower (15 000-24 500 years ago) sections of the Vostok station profile shows, that in Central Antarctica the temperature was 5° lower during the The isotope analysis of ice cores from Vostok 385 &("0) -

S('eO) -62 -58 ~54 -50%° FIGURE 1. Oxygen-18 values of the from Vostok station, Antarctica, plotted against the depth below the actual surface and against the time (T) since the deposition of the ice. The broken-line indicates the absence of a continuous core.

Wisconsin stage than during the next stage including the present day. This is the real extent of warming in the centre of the at the end of the Wisconsin. Judging by the isotope data from Vostok station, in Central Antarctica (see Fig. 1) the Holocene climatic optimum took place 7500-5500 years ago— this agrees in time with the Greenland data (Dansgaard et al., 1970, figure 3), where the corresponding warming dates back as far as 8000-4500 years. Two short-period warm peaks, which differ a bit in age, can easily be seen in the isotope profile from Vostok station between the late Wisconsin and the Holocene. They should be connected with Boiling and Allerod . 3500 years ago a cooling took place, which lasted 400 years and which was established according to the higher quality data from the second borehole. This cold spell, however, cannot be found in the Camp Century core. As far as the period from 15 000 to 46 500 years ago is concerned, we can observe the alternation of warmer and colder periods against the general background of the cold Wisconsin. According to our data three 'warm' periods, dating back 27 000-24 500, 36 000-32 500, and 41 000-39 000 years can be 386 N. I. Barkov eta/. distinguished. They alternate with colder stages. Two of them (24 500-15 000 and 32 500-27 000 years ago) date back to the late Wisconsin, and two other stages (39 000-36 000 and 46 500-41 000 years ago) refer to the Middle Wisconsin. Mean S(180)-values for the distinguished 'cold' and 'warm' periods are given in Table 1.

TABLE 1

'Cold' periods Years BP 46 500-41 000 39 000-36 000 32 500-27 000 24 500-15 000 S(lsO) [»/„„] -58.7 -58.7 -58.1 -58.8 ' Warm' periods Years BP 41 000-39 000 36 000-32 500 27 000-24 500 18 S( 0) [»/„0] -56.6 -57.5 -56.0

We should mention for the sake of comparison that according to the already published data on the upper part of the Vostok isotope profile, covering the 18 sequence of the last 11 000 years, the mean value of 8( 0) is -53.8%0. This value corresponds approximately to the present-day temperature conditions. A comparison of the two profiles from Vostok station and from Camp Century shows, that, in general, the curves are similar. This means that for the last 50 000 years the basic temperature changes occurred synchronously in the polar areas of both hemispheres. It should be emphasized, however, that the ice deformation models cause some error in the determination of the age of the ice. Conditions at Vostok station are much more conservative than at Camp Century and, at least for the last 50 000 years, the errors in the age determination are smaller at Vostok. The errors are small enough, however, for both cores to allow a very general conclusion about the synchronism of the main temperature tendencies in both hemispheres.

REFERENCES Barkov, N. I., Gordienko, F. G., Korotkevich, E. S. and Kotlyakov, V. N. (1974) Pervye resultaty izucheniya ledyanogo kerna iz scvazhiny so stantsii Vostok (Antarctida) isotopno- kislorodnym metodom (The first results of the study of ice cores from the bore hole at Vostok station (Antarctica) with the oxygen isotope method). Dokl. Acad. Sci. USSR 1U, no. 6, 1383-1386. Barkov, N. I., Gordienko, F. G., Korotkevich, E. S. and Kotlyakov, V. N. (1975) Isotopno- kislorodnye issledovaniia 500-metrovogo ledjanogo kerna iz skvazhiny stantsii Vostok (Oxygen isotope analysis of the 500 metre long core form the bore hole at Vostok station). Information Bull. Soviet Antarctic Expedition, no. 90, L., 39-49. Cohn, M. and Urey, H. (1938) Oxygen exchange reactions of organic components and water. /. Amer. Chem. Soc. 60, no. 3, 679-687. Dansgaard, W. and Johnsen, S. J. (1969) A flow model and a time scale for the ice core from Camp Century, Greenland. J. Glaciol. 8, no. 53, 215-222. Dansgaard, W., Johnsen, S. J., Clausen, H. B. and Langway, C. C. Jr (1970) Ice cores and . In Radiocarbon Variations and Absolute Chronology (Proceedings of the Twelfth Nobel Symposium, held at the Institute of Physics at Uppsala University, Sweden, August 1969) (edited by I. U. Olson), pp. 337-351: Wiley Interscience Division, Wiley, New York. Dansgaard, W., Johnsen, S. J., Clausen, H. B. and Langway, C. C. Jr (1971) Climatic record revealed by the Camp Century ice core. In The Late Glacial Ages (edited by K. K. Turekian), pp. 37-56: Yale University Press, New Haven and London. Gordienko, F. G. and Barkov, N. I. (1973) Variatsii soderzhanija "O v sovremennykh osadkah Antarctidy (Variations of lsO-content in present-day precipitations in Antarctica). Information Bull. Soviet Antarctic Expedition, no. 87, L., 40-43. Raynaud, D. and Lorius, C. (1973) Climatic implications of total gas content in ice at Camp Century. 243, no. 5405, 283-284. The isotope analysis of ice cores from Vostok 387 Voronov, P. S. (1960) Opyt restavratsii lednikovogo schita Antarctidy epokhy maksimalnogo oledenenija Zemli (An of restoration of the Antarctic Ice Sheet during the stage of maximum glaciation of the ). Information Bull. Soviet Antarctic Expedition, no. 23, L., 15-19.

DISCUSSION Orheim: Your curve of S(180)-variations over the past 46 000 years shows a much larger scatter than those published for other areas. Do you have an explanation?

Dansgaard: A possible explanation for the apparent scatter may be found in the fact that the accumulation rate upstream from Vostok is extremely low and varying. snow may be removed by drift from some areas to others.

Picciotto : What is the sample thickness on your scale ?

Kotlyakov The sample thickness is 0.5-5 m.

Picciotto : It also surprises me that you find these large isotope variations in your core. In the 40-60 m firn core from station, where the conditions are similar, considerably less pronounced short-term variations were found.

Kotlyakov: (written reply, submitted after the symposium) We suppose that the scatter in the absolute 8(180)-values from the Vostok borehole compared to the data of Dansgaard et al. (1977) is connected with the different lengths of the samples used. Our data were obtained from much longer and thus more representative samples. This is evident from the following comparison:

Our data Data from Dansgaard et al. (1977) 18 18 Depth [m] Length [cm] 8( O)[°/00] Depth [m] Length [cm] 3( O)[°/00] 134.5-139.6 510 -55.6 about 138 25 (-56) - (-57) 372.1-375.9 380 -60.7 about 374 60 -61 435.3-440.0 470 -58.5 about 438 40 -62 437.38-437.42 4 -59.5 472.8-476.2 340 -59.3 about 477 50 -61

Unfortunately we possess no data from the and would be glad to see them. And of course our standards and those used by Dansgaard still remain uncomparable. We hope that it will be possible to compare the standards in the near future.

REFERENCE TO DISCUSSION Dansgaard, W., Barkov, N. I. and Splettstoesser, J. (1977) Stable isotope variations in snow and ice at Vostok, Antarctica. In Isotopes and Impurities in Snow and Ice (Proceedings of the Grenoble Symposium, August-September 1975), pp. 204-209: IAHS Publ. no. 118.