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Studies of the Antarctic and Present-Day Concepts of Global Glaciation

Published online by Cambridge University Press:  20 January 2017

O.P. Chizhov  and
V.M. Kotlyakov
O.P. Chizhov
Affiliation:
Institute of Geography, Academy of Sciences of the U.S.S.R., Staromonetny per.29, Moscow 109017, U.S.S.R.
V.M. Kotlyakov
Affiliation:
Institute of Geography, Academy of Sciences of the U.S.S.R., Staromonetny per.29, Moscow 109017, U.S.S.R.
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Abstract

Antarctic researches of the last decades have provided new facts which have made scientists change their ideas. The researches have shown a great difference between the history of the Antarctic and the northern hemisphere glaciation. The former is very old and stable, while the latter was ephemeral. An analogous one to the Antarctic ice sheet in the northern hemisphere is the Greenland inland ice. Both ice sheets are of continental-insular type; the Pleistocene ice sheets were of continental type proper. Icethickness measurements showed the topography of the bed under Antarctic ice and revealed a special type of ice sheet lying partly on the sea-floor: the West Antarctic continental-marine ice sheet. Such ice sheets could exist in the northern hemisphere too. They were especially unstable and their formation and destruction could serve as a triggering mechanism in alternating glacials and interglacials. The main concept is: all the great climatic variations during the Pleistocene were glacioclimatic ones where the feedback mechanism was the most important factor. But there are many details which are uncertain. The Antarctic and Greenland ice sheets are the key regions to research so that problems of glacioclimatology could be solved.

Type
Research Article
Information
Annals of Glaciology , Volume 3 , 1982 , pp. 50 - 53
Copyright
Copyright © International Glaciological Society 1982

Introduction

Antarctic researches since 1956–57, when the IGY programme was put into being, have given many new facts which made scientists change their points of view long ago established. It has been considered that the origin of the Antarctic ice sheet was more or less synchronous to the Pleistocene ones in the northern hemisphere and was called forth by the same causes and mainly by some changes of global climate.

Antarctic researches show that the age of the Antarctic ice sheet is much older than any ice sheet in the northern hemisphere. The glaciers in Antarctic mountains appeared about 38 million years ago (Reference KennettKennett 1977). For at least the last 10 million years the Antarctic ice sheet has existed and its size was always more or less the same as it is now (Reference LeMasurier and AdieLeMasurier 1970, Reference Rutford, Craddock, White, Armstrong and AdieRutford and others 1970, Reference Hayes and FrakesHayes and others 1973, Reference Kvasov and VerbitskyKvasov and Verbitsky 1981). Antarctic climate was created by the ice and snow of the continent, and, more than that, the Antarctic ice sheet formation changed the climate not only of the southern hemisphere but also the whole globe as well (Reference BudykoBudyko 1961, Reference Budyko1964, Reference Avsyuk and KotlyakovAvsyuk and Kotlyakov 1964, Reference Shumskiy, Krenke, Zotikov and OdishawShumskiy and others 1964). It is generally accepted now that all the large ice sheets change the climate around them. Snow and ice are products of climate, but when formed influence climate in return. There is some feedback mechanism (Reference VoeikoffVoeikoff 1884). It seems trivial now, but many scientists worked for about a century to make such a concept generally accepted. The study of Antarctic glaciation and climate for the last 25 years brought an important contribution to this concept.

Development of ideas

The first man who set forth the idea of feedback mechanisms between snow and ice on the one hand and climate on the other was Reference VoeikoffVoeikoff (1884); the second step was by Reference BrooksBrooks (1926); and the first author who proposed the theory of glacioclimatic autofluctuations was Reference GernetGernet (1930), see also Reference Chizhov and GernetChizhov 1969, Reference Chizhov1976). A theory very similar to his was quite independently published by Reference StokesStokes (1955), and soon after him Reference Ewing and DonnEwing and Donn (1956, Reference Ewing and Donn1958) developed their version of the same idea. Since this time, the theory has become known and widely discussed and criticized in literature. The latter authors revised their work and published some other papers (Reference Donn and EwingDonn and Ewing 1966, Reference Donn and Ewing1968). But the Gernet-Stokes theory (which has to be called by the names of those who were the first to set it forth) explained the chain of events qualitatively; it was not confirmed by calculations or mathematical modelling, and in addition, there were some mistakes in it as it was proposed firstly by Gernet and later by Ewing and Donn. The main mistake they made was in the significance they ascribed to the Arctic Ocean, which, they postulated, was alternately free of ice and ice-covered, causing the glaciation and deglaciation of the surrounding continents. As it is known now after deep-sea cores from the Arctic Ocean were studied, the Arctic Ocean was not free of ice. It had been ice-covered for about 3 or 4 million years (Reference ClarkClark 1971). In Stokes' description, more generalized, the theory sounds better.

Recently, the autofluctuating nature of glacialinterglacial variations was confirmed by Reference Sergin and SerginSergin and Sergin (1978) and Reference SerginSergin (1979) who carried out a study of the origins and thermodynamic mechanism of the large-scale glacial-interglacial variations by constructing a simplified thermodynamic model of the glaciers-ocean-atmosphere global system. They considered an aperiodic (non-fluctuating) regime in the southern hemisphere and a fluctuating one in the northern hemisphere. The period and amplitude of fluctuations in the model ire of the same order of 50 magnitude as those known from palaeoreconstructions.

The glacioclimatic history of the Antarctic makes us admit the theory described above, because any external cause, if it could explain fluctuations, would have acted in both hemispheres. But the Antarctic glaciation, after it had formed, was and is stable. It means that we must look for other internal causes that make glaciers fluctuate.

Types of large ice sheets

There is a type of large ice sheet that we have now. It is the continental-insular type. There are two of them: the Antarctic and Greenland ice sheets. The areas they cover are measured in millions of square kilometres. Other ice caps are much less; their areas are not more than 20 to 30 000 square kilometres each. There are no ice caps of intermediate size existing now.

There is rather a good correlation between the area A and the thickness H of all ice caps and ice sheets (Fig. l). It can be expressed by an equation:

where K is a constant coefficient. The power of H is one that corresponds to the elliptical profile of the ice-cap surface (Reference ChizhovChizhov 1976: 142–145, Reference KhodakovKhodakov 1978: 148–149). It is seen in Figure 1 that there is a gap between the two contemporary large ice sheets (Antarctic and Greenland) and the other lesser ice caps. But when Pleistocene ice sheets increased their sizes they passed through all the intermediate ones. This means that such sizes can exist only in the process of increasing and decreasing the large ice sheets.

Fig.1. Area of ice caps and ice sheets A versus maximum height Hm of their surface (line I) or average thickness H̄ of ice (line II) in logarithmic scale. 1: Antarctic; 2: Greenland; 3 to 16: other ice caps of lesser area.

The ice sheets of the continental-insular type are stable ones. The Antarctic ice sheet is not less than 10 million years old; the age of the Greenland ice sheet is about 2 to 4 million years. But the time of existence of the Pleistocene ice sheets (Scandinavian, Laurentide, and others) was something like 10 to 20 000 years; they were ephemeral. They were the ice sheets of the continental type proper. While the main mode of ablation of the continental-insular ice sheets is calving, the mode of ablation of these Pleistocene ice sheets was melting over the wide ablation zone along their southern margins. The first author who understood and explained the stability of the Antarctic ice sheet by its insular position was Reference GernetE S Gernet (1930).

There is, and was in the past, the third type of large ice sheet, such as the West Antarctic: the continental-marine type lying partly on the sea-floor. Ice sheets like this could exist in the northern hemisphere too (Reference MercerMercer 1970). There is evidence that such an ice sheet was formed during the last glacial epoch over the Barents Sea continental shelf (Reference Schytt, Hoppe, Blake and GrosswaldSchytt and others 1968), and that the pan-Arctic ice sheet of the continental-marine type could cover other northern shelves and the central Arctic (Reference Hughes and DentonHughes and others 1977). Ice sheets of the continental-marine type were especially unstable, and their formation and destruction could be a triggering mechanism to advances and retreats during alternating glacials and interglacials. The origin and disappearance of continental -marine ice sheets can be connected with sea-ice formation, and many problems arise here (Reference OlaussonOlausson 1972, Reference Olausson1975, Reference ZakharovZakharov 1978, Reference Zakharov1980).

External influences

The fluctuations of insolation because of changing elements of the Earth's orbit attract the most attention among the external causes influencing alternation of glacials and interglacials. Their significance has been proved (Reference Hays, Imbrie and ShackletonHays and others 1976). But the fact is that, for the long pre-Pleistocene time, orbital influences were negligible and became noticeable only after the Pleistocene ice sheets of the continental type proper formed (Reference WoldstedtWoldstedt 1958). The influence of variations in the Earth's orbit is weak but is increased by glaciation. Reference WeertmanWeertman (1976) has shown that Milankovitch insolation variations do work, but by means of ice spreading in glaciers. It is very important by itself because an ice sheet, after reaching some critical size, began to spread without any climatic change (Reference TronovTronov 1951), Reference WeertmanWeertman 1961). Modelling by Reference Sergin and SerginSergin and Sergin (1978) has shown that Milankovitch insolation variations change the period and amplitude of glacial-interglacial fluctuations.

Conclusion

The history of Cenozoic glaciation as it is known owing to Antarctic studies together with the achievements of marine geology seems clear in general. And it is simpler in the Antarctic than in the Arctic.

Shifting of the Antarctic continent into its polar position, which took place in Mesozoic time, did not immediately result in its glaciation. Rather even and warm climates predominated over the globe. The first glaciers appeared in the Antarctic mountains only in the Tertiary period, about 38 million years ago (Eocene-Oligocene boundary). Their appearance was connected with general uplift of the land and increased precipitation, which was the consequence of departing of Australia from the Antarctic and forming the strait between them. The formation of Drake Strait later resulted in the circumpolar current of west winds round the Antarctic continent. This current isolated it and provided for precipitation which gave enough accumulation on glaciers. Glaciers grew in the mountains, joined together, descended onto the neighbouring plains, and reached the sea, producing icebergs. About 20 million years ago, the East Antarctic ice sheet formed, later the West Antarctic one, and by 5 million years ago, the Antarctic ice cover reached its maximum size, which was more than at present (Reference KennettKennett 1977, Reference Kvasov and VerbitskyKvasov and Verbitzky 1981). At the same time, growing glaciation led to cooling of the air and the surrounding water owing to loss of heat from the melting of icebergs. Cooling waters sank down, where a cold deep layer was forming and spreading gradually towards the north up to the Equator and farther. The mass balance of the Antarctic ice sheet caused its quasistability. Ice margins calve when they reach the sea, balancing the accumulation of snow on the surface of the ice sheet. Spreading farther north is limited by the sea.

Global cooling led to the formation of glaciers in the northern hemisphere, first of all on the great near-polar island of Greenland, where the Atlantic current brought enough precipitation. Glacial and climatic history of Greenland is similar to that of Antarctica. Its glaciation was stimulated by the same causes and resulted in the same consequences as the glaciation of the Antarctic, i.e. further gradual cooling and ice formation over the neighbouring lands around the North Atlantic. Study of deep-sea sediments shows the appearance of sea ice in the Arctic Ocean about 3.5 million years ago (Reference ClarkClark 1971). At the same time the glaciers of Greenland could reach the sea. The age of the Greenland ice sheet is estimated as 3 million years or so. But the history of its glaciation is known less well than the history of Antarctica. The sequence of events in the Arctic does not seem as evident as in the Antarctic, where the first glacial event was the inception of glaciers in the Antarctic mountains. The sea ice could appear much later. But how did events go in the north polar area?

There are many discussions around the problem. Some authors think that the sea ice appeared first in the north polar area (Reference ZakharovZakharov 1978, Reference Zakharov1980), starting the chain of events known in the glacial history of the northern hemisphere. However, the appearance of sea ice in the deep central Arctic Basin before other glacial events is difficult to imagine (the salt water must be cooled below freezing point over its whole depth). It is evident that the freeze-up of seawater could begin first near the mouths of great rivers where a surface layer of light fresh water could form. But all the same, one must agree that at the snow-line in the mountains, the temperature would descend lower and snow cover would form earlier than at sea level. Everything convinces us that, in the northern hemisphere, glaciation formed first in mountain regions and, what is most important, among mountains in Greenland where it began from small mountain glaciers and then spread over the island as a large ice sheet.

A special and difficult problem is the further evolution of glaciation in the northern hemisphere which is quite different from Antarctic glaciation. The glaciation was fluctuating and its fluctuations had very great amplitudes. They began, as it is generally accepted, about 700 000 years ago when ice sheets of continental type proper reached their maximum sizes. Their southern margins spread up to 40 or 50°N in the Atlantic part of the northern hemisphere. Their changes were of autofluctuating character, but their number is not known exactly, and the mechanism of fluctuation is understood only in general. Many details remain which are difficult to explain.

Researches in the Antarctic have given much to the understanding of glacioclimatic evolution. But a good deal of studies have to be done for better understanding. The Greenland ice sheet and the Arctic Ocean are the key regions of study for the glaciologist, besides the Antarctic.

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Figure 0

Fig.1. Area of ice caps and ice sheets A versus maximum height Hm of their surface (line I) or average thickness H̄ of ice (line II) in logarithmic scale. 1: Antarctic; 2: Greenland; 3 to 16: other ice caps of lesser area.