FEATURE ARTICLE
The Eurasian Arctic During the Last Ice Age
A vast ice sheet once covered the Barents Sea. Its sudden disappearance 100 centuries ago provides a lesson about western Antarctica today
Anders Elverhøi, Martin Siegert, Julian Dowdeswell, John-Inge Svendsen
Primary Causes
To understand fully the glacial history of the Eurasian Arctic during the last ice age, one needs to appreciate why ice ages arise in general and how they can cause a continental sea to fill with ice that is more than a kilometer thick. The geological record indicates that huge ice sheets repeatedly formed and decayed in the Eurasian Arctic as a response to pronounced climatic oscillations throughout the past 2.7 million years. The previous interglacial interval, when the climate on Earth was comparable with the present, lasted from 128,000 to 115,000 years ago. It was followed by an ice age that suddenly ended 11,700 years ago. During this ice age as many as three periods of glacial advance and decay took place. The most recent ice sheet that spread across the shelf areas started to form some 30,000 years ago and reached its maximum extent some 10,000 years or so later.
In an ice age, huge volumes of water shift from the oceans to the polar ice sheets, which lowers sea level, at times by as much as 120 meters. The clearest record of this vast redistribution of water comes from the examination of the three naturally occurring oxygen isotopes (16O, 17O and 18O) in various geological materials. Why are these oxygen isotopes so telling? Water containing the lightest form of oxygen (16O) evaporates more rapidly than water with the heavier isotopes (17O and 18O). So water comprising "light oxygen" preferentially goes into the ice sheets, and during an ice age the water in the oceans becomes enriched in heavy oxygen. If, say, a marine organism forms a shell of calcium carbonate (CaCO3) at this time, it will contain a larger than average dollop of heavy oxygen. When that organism dies, its shell drops to the sea floor leaving a convenient record of the isotopic state of the ocean in the past.
Geologists have collected many long records of the ocean's shifting oxygen isotopes from the analysis of sediments recovered from the floor of the deep sea. They have also measured the isotopic composition of the ice that has accumulated in Antarctica. The oxygen-isotope signals from the ice and ocean sediments tell a remarkably similar story: The climate changes associated with ice ages repeat at frequencies of about 100,000, 40,000 and 20,000 years. Why does the climate oscillate at these three frequencies? The answer lies in the orbit of the Earth around the Sun.
The first orbital parameter to consider is eccentricity, the deviation from perfect circularity. The Earth's orbit changes from an elliptical to a circular path with a frequency of roughly 100,000 years. The second parameter of interest is the tilt of the Earth's axis, which oscillates between 22.2 and 24.5 degrees at a frequency of about 40,000 years. The third is the position of the Earth within its elliptical orbit during Northern Hemisphere summer, which changes at a frequency of approximately 20,000 years. These oscillations affect the amount of radiation received at the Earth's surface at various times of the year. If the three orbital parameters conspire to reduce the radiation to the Northern Hemisphere in the summer, glaciers and ice sheets expand, bringing on an ice age.
The relation between observed climate oscillations and the theoretical predictions about their periods is excellent. There is, however, a slight problem: The changes to the solar inputs associated with orbital variations are far too small to cause the climate changes required to grow an ice sheet. What is needed is a means by which subtle orbital effects can be amplified into drastic shifts in climate. Several such feedback mechanisms are possible; all probably contribute in some way to the waxing and waning of ice ages.
Perhaps the simplest mechanism to understand is ice-albedo feedback. The reflectivity of the Earth's surface (its albedo) controls the amount of solar radiation that bounces back from the Earth into space. If the albedo is high, more radiation reflects, and the Earth cools. If the albedo is low, the planet's surface absorbs more radiation, and the world warms. Snow and ice are, of course, very reflective. As snow fields and ice sheets expand in response to global cooling, the increase in surface albedo causes an increase in the reflection of solar radiation, resulting in a further reduction in air temperature.
Another feedback mechanism depends on atmospheric carbon dioxide (CO2), which affects climate because it enhances the greenhouse effect. For reasons not yet fully understood, during glacial times the concentration of atmospheric CO2 diminishes. Hence, a cooling that arises from other causes lowers CO2, which lessens the greenhouse effect, yielding further cooling.
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