by Academician Vladimir KOTLYAKOV, Director of the Institute of Geography, Russian Academy of Sciences
Late in 1957 a caravan of snowmobiles set out from the Antarctic settlement Mirny to break ground for a Soviet station in the eastern part of the Ice Continent. At that time hardly anyone could guess how fortunate the choice of the site would prove to be. We were bidding farewell to our comrades embarking on a journey toward the south geomagnetic pole of the Globe. And that journey of theirs was quite a success: already in December of that year the new station, Vostok, began its work. At first our men were concerned with geophysical and meteorological measurements. But what they found soon was that the site of the station was the coldest spot on earth, with the mercury dropping down to- 89.2 0 C(-127 0 F), the lowest air temperature recorded on our planet.
ICE ABOUT THE CLIMATE OF THE PAST
The Vostok Station was set up on an ice plateau 3,488 m above sea level. The ice cover there is 3,740 meters thick (the upper 120 m is under snow and firn, then comes solid ice). Judging by direct measurements and aerospace data, the ice sheet is moving at an annual rate of 3 meters.
The first ice cores (samples) there were taken in the late 1960s. Piercing the ice cover is the best way of learning about its internal structure. Thus we can learn something about the stratification of ice, its isotope composition, structure and temperature, and also about the quality and quantity of various impurities, or admixtures. What we need first is a hole.
In those years the Soviet Union, the United States, Western Europe and Japan developed appropriate drilling equipment. Sinking a borehole in ice involves specific difficulties, for one, those caused by the rapid healing of open deep holes due to the plasticity of ice. That is to say, the walls of such a borehole come together all too soon. To avoid that the hole has to be filled with non-freeze liquids of the same density as the ambient environment. By the end of the 1980s we had collected data on five deep holes- two of them sunk in Greenland, and three in Antarctica. In these regions surface glaciers do not thaw at all or else the thawing is too weak to interfere with their formative process.
One of the above five boreholes is at the station Vostok. This hole is significant in more ways than one. Among other things, the drilling work was kept up for many years, on and off, because of breakdowns, cable ruptures and loss of borehole tools. Every now and then new holes had to be made-by 1998 there were as many as five at the station.
Way back in the 1970s Soviet and French experts undertook the work of ice core data processing and interpretation, with the Americans joining up in 1989; such international cooperation proved of great benefit and brought results of global significance. *
* See: V. Kotlyakov, "Antarctica Reveals Its Secrets", Science in Russia, No. 1, 1997. - Ed.
In January 1998 the deepest borehole of the Vostok station stood at 3,623 meters. But then the work had to be stopped at 125 m from the ice sheet's sole for fear of contaminating a lake situated below. As to the ice mass, it furnished information on global climatic changes over the last 420 thousand years taking in four complete climatic cycles. The isotopic data on sea floor sediments characterizing fluctuations in the level of the World Ocean confirm the global nature of such changes.
A detailed continuous cross-section of the isotopic composition of ice according to deuterium (D) was obtained to a depth of 3,310 m. The values of бD (deviations of the content of this hydrogen isotope from its concentration in SMOW-standard mean oceanic water) varies from 420 to 480 percent, including comparatively brief periods of interglacial warmings (from 420 to 460 percent) and longer cold snaps (from 460 to 480 percent) when large areas of dry land in both hemispheres were under a thick glacial mantle.
The isotopic composition of ice is a function of many factors. This is primarily the air temperature on the upper boundary of the inversion layer (the surface cold layer of the atmosphere) in which atmospheric precipitation is formed. Therefore бD values can be easily transformed into those of air temperature. The age of ice is computed from a special model of ice diffluence (flow), and the accuracy of such computations according to the standard scale is not below +/- 15 thousand years. For most of the time intervals it is above 10 thousand, while for the last 110 thousand years it persists at a level of +/-5 thousand years.
A body of data obtained from the ice core extracted at the Vostok station includes the concentration of deuterium in the ice sheet, the concentration of dust (aerosols brought in from desertlands), sodium (sea aerosols) and of hothouse gases - carbon dioxide (CO 2 ) and methane (CH 4 ). The overall amplitude of glacial-interglacial changes of temperature is about 8 0 C for the inversion layer and about 12 0 C for temperature on the surface of the ice sheet.
Temperature variations estimated according to the presence of deuterium were found to be analogous for all the four climatic cycles, though the latter two cycles were shorter than the former two. Yet all these differences notwithstanding, the plots showed the selfsame saw-like pattern for warm interglacials (stages 11.3, 9.3, 7.5 and 5.5 corresponding to the intervals 400, 350-300, 250 and 100-150 thousand years ago); these warm interglacial periods followed in the wake of ever colder interstadial events which rapidly reverted to yet another interglacial.
Studying the aerosols, we found that at glacial maxima the atmospheric dust loading (expressed by the number of microparticles deposited on the ice shield surface) rose manifold: the concentration of sea aerosols in ice during cold epochs was three- and fourfold as high as in
the Holocene, while the amount of continental particles soared 20 to 30 fold. By the way, this phenomenon confirms the fact known to every glaciologist: the glacial climate is characterized by enhanced aridity (dryness) and by the intensive lifting and transfer of dust in the atmosphere.
Even the first examinations of the ice samples at the Vostok station revealed a full correlation between the concentration of CO 2 and CH 4 and temperature. Thus the hothouse gases are playing an important role kind of intensifying the astronomical effects determined by the position of the earth in orbit, and they could make a substantial contribution to the glacial- interglacial fluctuations of the climate.
Yet another thing is just as important. The main dependence between the hothouse gases and temperature variations held for all the glacial-interglacial cycles. During inter-glacials the concentration of CO 2 in ice rose from 180 to 280-300 ppmv, and that of methane - from 320-350 to 650-670 ppbv, i.e. nearly twice as much; remarkably, the growth of CO 2 concentration to interglacial values proceeded very rapidly, while the reverse transition - to low glacial values - was far more sluggish.
The curves plotted for the concentration of the hothouse gases over the past 420 thousand years show the following dependence: such concentrations of these gases (CO 2 and CH 4 ) as registered today (360 ppmv and 1,700 ppbv, respectively) had never occurred in the past. At the same time their level in the pre-industrial period (ca. 280 ppmv and 650 ppbv, respectively) was found to be characteristic of all the interglacials; and in the stages 5.5, 9.3 and 11.3 the concentrations of CO 2 and CH 4 were even higher than in the pre-industrial era (when maximum values were equal to 300 ppmv and 780 ppbv, respectively).
A deuterium curve plotted from the data obtained at the Vostok station shows that the Holocene, continuing for well over 11 thousand years now, proves to be the longest interglacial over the past 420 thousand years. Although different from it in their nature, stages 5.5 and 9.3 are similar to it in duration, form and amplitude. Each has a warm period about 4 thousand years long
followed by a relatively rapid cooling and then a slow fall of temperature.
So the entire body of data on the ice core extracted at the Vostok station tells us that over the past 420 thousand years the global climate has been fluctuating in an amplitude within the steady limits of this variability. Furthermore, the characteristics typical of the latter climatic cycle have been identified within earlier cycles too. In practical terms, a 100 thousand-year variability prevails in all the parameters studied, even though periodicities of 40 thousand and 20 thousand years have been identified as well.
In each of the four stages studied by us the termination of glacial epochs was accompanied by a definite pattern of changes in the ambient environment. At first the temperature and content of CO 2 and CH 4 kept steadily on the rise, while the concentration of aerosols went down. But the concentration of CH 4 rapidly increased with an increase in temperature, a fact attesting to a possible warming also in the higher latitudes of the Northern Hemisphere and to an intensive melting of the permafrost built up over many years.
Consequently, a long and steady Holocene is a unique feature of the climate over the past 420 thousand years, a factor that has had special significance for the development of human civilization. As to the elegant correlation of atmospheric CO 2 and CH 4 with temperature, alongside the soaring concentrations of these gases in the last few decades, it provides additional ground for discussing the future course of our planet's climate.
A GIANT LAKE UNDER THE ICE SHIELD
The location of the Vostok station has yet another vantage point: it is above a huge subglacial lake. In the 1960s Russian scientists furnished theoretical proof: if a glacier's thickness exceeds a critical value, the ice of its bed starts thawing. According to calculations carried out at the Mirny station, the actual thickness of ice as far as 350 km inland from the coast was exceeding this value. Thus almost all over Central Antarctica, despite the very low mean air temperature, there has been a continuous thawing of ice at the bed of glaciers.
In the 1970s the R. Scott Polar Institute of Great Britain carried out an extensive program of aerial radiosonde observations over the central part of East Antarctica. Here and there the aircraft flew over districts where major bodies of water
were registered under the ice sheet; such pools were dubbed subglacial lakes. Later on the shots made by the European earth satellite ERS-1 showed irregular relief features in the vicinity of the Vostok station. The latest antarctic maps indicate a characteristic flattened-out part of the ice cover under which there ought to be a lake. Here are its main parameters: location-76 0 30' S, 102-106 0 E; length - 230 km, width - 50 km; surface area - ca. 10,000 km 2 ; depth - down to 600 m; ice sheet above the lake - 3,700-4,200 m thick; the possible thickness of floor deposits - 100-200 m; the melting temperature of benthic ice at the ice-water interface - from - 2.4 to - 3.15 0 C.
Descending into the lake from the west, the ice starts afloat at 3,550 m, moving on the water surface all the way to the eastern shore where it converts into an ice shield again. Relief features at the edge of the lake are rather irregular, which fact points to complex processes attendant to ice floe and subsequent compactification. And so the huge mass of ice above the lake may be considered a kind of shelf glacier. All that is a natural phenomenon utterly novel to science, a geographical discovery of world significance.
The subglacial lake has been isolated from the outside world for a million years-perhaps even longer than that. Thermal conditions vary in different parts of this lake, and thus cause internal currents and active energy exchange. The melting ice releases the atmospheric air captured hundreds of thousands and millions of years ago. Under like conditions there ought to be some microfauna and microflora in the lake, a tantalizing opportunity for biological studies.
Comprehensive studies of this huge subglacial lake under the Vostok station are the chief task now. Research scientists are planning to "remote-sense" it by seismosounding and radar. Thereupon they could go through the remaining 125 m of ice above the lake's surface and reach its water-naturally, without taking any risk of polluting this unique body. And more. There is every ground for belief that Antarctica will soon supply us with data on the climate of the earth throughout the Pleistocene covering a million and some years. Thereby we shall get a better understanding of the problem of global climatic changes in this new, 21st century/
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