Earth in Space, Vol. 8, No. 3, November 1995, p. 9. © 1995 American Geophysical Union. Permission is hereby granted to journalists to use this material so long as credit is given, and to teachers to use this material in classrooms.
The history of the world’s climate is recorded in the layers of sediment that accumulated over thousands of years in ice and rock. Paleoclimatologists are studying sediment encapsulated in deep Antarctic ice to answer a few perplexing questions about the conditions that prevailed during the ice ages.
by Vostok Project Members:
S. S. Abysov, M. Angelis, N. I. Barkov, J. M. Barnola, M. Bender, J. Chappellaz, V. K. Chistiakov, P. Duval, C. Genthon, J. Jouzel, V. M. Kotlyakov, B. B. Kudriashov, V. Y. Lipenkov, M. Legrand, C. Lorius, B. Malaize, P. Martinerie, V. I. Nikolayev, J. R. Petit, D. Raynaud, G. Raisbeck, C. Ritz, A. N. Salamatin, E. Saltzman, T. Sowers, M. Stievenard, R. N. Vostretsov, M. Wahlen, C. Waelbroeck, F. Yiou, P. Yiou
Ice cores drilled at Vostok Station, Antarctica, 10 years ago by Russia, France, and the United States (see figure 1) are providing a wealth of information about past climate and environmental changes over more than a full glacial-interglacial cycle. The ice cores show that East Antarctica was colder and drier during glacial periods than during the Holocene and that atmospheric circulation was more vigorous during glacial times. The ice cores also support evidence from studies that use deep-sea sediment to reconstruct changes in past sea level and oceanic temperature. These studies link Pleistocene climate change with the position of the Earth on its orbit and tie carbon dioxide and methane concentrations to temperature.
Fig. 1. Map of Antarctica. The deep drilling sites are indicated by black dots.
Vostok research station has operated year-round for more than 37 years. In the 1970s, researchers from the Soviet Union drilled a set of holes 500–952 m deep in the ice (see figure 2). These holes have been used to study the oxygen isotope composition of the ice, which showed that ice of the last glacial period was present below about 400 m depth. Then three more holes were drilled: in 1984, Hole 3G reached a final depth of 2202 m; in 1990, Hole 4G reached a final depth of 2546; and in 1993 Hole 5G reached a depth of 2755 m. The station was temporarily closed in January 1994, but it reopened last November and drilling continued during the winter of 1995. The core, the longest ever drilled, has now reached 3100 m. It is 50 m longer than the core from Greenland that previously held the record.
Fig. 2. The tall structure is the drilling tower at the Vostok Antarctic site.
Ice cores provide continuous information on key properties of paleoclimate including local temperature and precipitation rate, humidity, and wind speed. Ice cores also record changes in atmospheric composition. They can be used to measure trace gas concentrations, chemical impurities of terrestrial and marine origin, other trace compounds or isotopes, cosmogenic isotopes, extraterrestrial material, and aerosols of volcanic and anthropogenic origin. Ice cores from Vostok, Antarctica, were the first to cover a full glacial-interglacial cycle. And, despite recent drillings in central Greenland, they still carry the distinction of being the only ice cores that scientists are certain have remained undisturbed for the last interglacial and the penultimate glacial periods.
Interpreting Paleoclimate From Ice Cores
Two elements—deuterium and oxygen 18—are important because they can be used to reconstruct past temperature changes in polar regions. In Antarctica, a cooling of 1°C results in a decrease of 9 per mil deuterium. An accurate chronology is essential for interpreting ice core paleoclimate data. At Vostok, accumulation is too low for recognizable annual signals to form, so we developed a chronology combining an ice flow model and an accumulation model that accounts for the fact that accumulation was lower during colder periods and vice versa.
Because the accumulation rate is governed by saturation water vapor pressure, past accumulation may be estimated from the temperature record. Accumulation rates inferred in this way are supported by measurements of beryllium 10 (10Be), an isotope produced by the interaction of cosmic rays and the upper atmosphere, can be used to determine past snow accumulation in Vostok ice. Deposition of this cosmogenic isotope is assumed to be constant. The chronology of the ice at Vostok has been established down to 2546 m, which is dated at 220,000 years before present. The combined deuterium record from the 3G and 4G cores shows the last two glacial-interglacial transitions with atmospheric temperature changes of about 6°C. The last ice age is characterized by three minima separated by slightly warmer episodes called interstadials. The penultimate glacial is characterized by the same sequence of interstadial events and taken as a whole the last two glacial periods appear very similar.
The chronology of the Vostok ice core is also supported by a glaciological model. Southern Ocean temperature variations correlate with those at Vostok. Also, because photosynthesis transmits seawater variations to atmospheric O2, the variations in 18O of O2 in air trapped in the Vostok ice roughly coincide with variations in 18O of seawater reflected in the isotopic content of the forams in deep-sea sediments. There is also a correlation between the Vostok dust concentration and the record of mass accumulation rate in a core taken from the Indian Ocean.
Beyond their use as dating tools, ice cores convey specific geochemical information. Variations in 10Be concentrations are caused by factors other than accumulation changes. The existence of peaks in 10Be around 35 and 60 kyr B.P. have been attributed to increased production of 10Be. The 18O of O2 record also contains information about fractionation by biogeochemical and hydrologic processes. Similarities between Vostok and Southern Ocean temperatures indicate that the Vostok record is representative of a large geographical area, while agreement with the 18O of deep-sea core suggests that the broad features of this record are somewhat global.
Changes in terrestrial aerosols hold the key to past climate. More dust was present in glacial periods than during interglacials; this suggests that glacial periods were characterized by extensive deserts, intense surface winds in the desert source regions, and more efficient transport along the imaginary circular path that runs perpendicular to the equator through the poles. This idea of stronger circulation during glacial periods is reinforced by the fact that glacial values of marine aerosols are much higher than interglacial levels.
Another important aspect of change in the past atmosphere’s aerosol load is a secondary aerosol composed of nonseasalt sulfate and methanesulfonic acid (MSA), an oxidation product emitted by marine organisms. Although studies based on MSA measurements show that the link between climate and biogenic marine activity could be more complex than initially thought, both nonseasalt sulfate and MSA records indicate that the ocean-atmosphere sulfur cycle is extremely sensitive to climate change. Sulfate aerosols affect climate by “thickening” the atmosphere.
Air initially enclosed in Vostok ice provides our only record of variations in the atmospheric concentrations of CO2 and CH4 over a complete glacial-interglacial cycle. For both greenhouse gases, concentrations are higher during interglacial periods than during full glacial periods. Since preindustrial times, levels of CO2 and CH4 have increased sharply. A close correlation between these gas concentrations and the Vostok isotopic temperature has been confirmed by extending the record over part of the previous cycle. However, at the end of the last interglacial, the CO2 decrease significantly lags Antarctic cooling, while CO2 and Antarctic temperatures increase during the warmings of the glacial-interglacial transitions. Interestingly, at least during certain deglaciation periods, the trace gas increase precedes the onset of most melting of the northern ice sheets by several thousand years.
From a climatic viewpoint, CO2 and CH4 have played an important role. Together with the growth and decay of the Northern Hemisphere ice sheets, these greenhouse gases have amplified the initial orbital forcing, and they account for about half of the glacial-interglacial climate changes. This supports the idea that significant greenhouse warming will occur in the next century.
Ice sheet modeling is used to date ice cores and study long-term interaction between climate and the dynamics of large ice sheets. Information gained from studies of ice texture and fabric, ice rheology, and ice densification is crucial to this objective. Models predict that the ice sheet over Vostok will thin during cold periods. In agreement, the long-term trends of total air content in the ice show that during colder periods air pressure was higher. This supports the idea that elevation of the ice sheet was lower. Other studies confirm that microorganisms—including species that have disappeared elsewhere—could survive in the deep Antarctic ice for many thousands of years following anabiosis.
Sample Analysis: Preliminary Results
In France and the United States, analysis of samples of the last 200 m of core 5G are nearly complete and exciting results have emerged. The bottom part of the core (2755 m) corresponds to an age of 240 kyr B.P., reaching back to the penultimate interglacial period. The warmest part of this penultimate interglacial was likely as warm as today in Antarctica. One of the most remarkable result derived from this new record is the striking similarity of the last two climatic cycles, which is not documented in any other paleorecord.
- Chronology: Arranging events in their proper sequence.
- Cosmogenic Isotope: An isotope that can be used to study the age and origin of the Earth.
- Forams: A family of aquatic microorganisms that are important as age indicators, as rock-building agents, and in seafloor deposits.
- Fractionation: The separation of chemical elements in nature.
- Ice Age: A time of extensive glacial activity also called a glacial epoch (see definition for Pleistocene).
- Ice Sheet Modeling: Ice sheet models incorporate ice-flow laws that account for the mechanical and thermodynamical properties of the ice.
- Interstadial: Warmer substage of a glacial stage, marked by a temporary retreat of the ice.
- Isotope: A particular atom of an element that has the same number of electrons and protons as the other atoms of the element, but a different number of neutrons. The temperature at which an oxygen-bearing geologic material formed can be determined by studying the oxygen isotope it contains.
- Minima: Time or position of the greatest retreat of a glacier.
- Pleistocene: The “Great Ice Age,” during which glaciers and ice sheets covered the land masses 4 times over 2 million years ago. These massive ice advances were separated by longer warm interglacial periods.