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In order to understand Earth's ice ages, scientists must be able to piece together the past and determine exactly when in the geologic time tat.

This drill bit is tough enough and sharp enough to slice through meter after meter of solid ice. The bit is fitted in the core sleeve, a hollow tube into which the ice passes after being cut. (Lonnie Thompson, Byrd Polar Research Center, Ohio State University, NOAA Paleoclimatology Paleo Slide Set)

table these events occurred. The principal techniques they use are the analysis of ice cores and the study of oxygen isotopes.

In Antarctica, there is a 40-day time window in January and February (the Southern Hemisphere's summer) when ice core samples are generally acquired. Bubbles caught in the ice cores are representative of Earth's atmosphere at that time. The analysis of the bubbles trapped in the ice cores, as well as monitoring the movement of the ice sheets, allows scientists to look back in time to understand just how Earth looked and what processes were occurring millions of years ago.

Ice cores are a cylinder of ice four to five inches (10-13 cm) in diameter that are drilled out of the ice with a sharp drill bit. This coring device is sharp enough to slice through foot after foot of solid ice. The bit is fitted into a core sleeve, a hollow tube in which the ice is kept after it is cut to protect it.

Ice cores are one of the most accurate records climatologists have available with which to study the past. For example, a 1.9-mile (3-km)-long ice core removed from Antarctica preserved snowfall records for the past 740,000 years. According to the online site ScienceDaily researchers were able to determine past atmospheric temperatures as well as what the concentrations of gases and particles were in the atmosphere through time. Scientists were thus able to determine that Earth had gone through at least eight previous ice ages and interglacial periods. Dr. Eric Wolff of the British Antarctic Survey notes, "It's very exciting to see ice that fell as snow three-quarters of a million tat.

This drill bit is tough enough and sharp enough to slice through meter after meter of solid ice. The bit is fitted in the core sleeve, a hollow tube into which the ice passes after being cut. (Lonnie Thompson, Byrd Polar Research Center, Ohio State University, NOAA Paleoclimatology Paleo Slide Set)

Ice-core research is currently conducted in both Antarctica and Greenland, where climatologists study the evidence and attempt to put together the Earth's past. (University of Wisconsin-Madison Physical Sciences Laboratory, Ice Coring and Drilling Services and National Science Foundation)

years ago." Because the ice was deposited layer after layer on a yearly basis, interpreting the ice core is similar to interpreting the rings on a tree: Owing to the freezing-thawing differences between summer and winter, summer snow is less dense than winter snow. The resolution (detail) in the ice core is usually good enough to determine exact dates of occurrences. The principal method climatologists use to reconstruct Earth's past temperatures, which allows them to piece together ice ages in the past, is through the analysis of the isotopes of the oxygen and hydrogen atoms that make up the water molecules that initially fell on the ice sheet as snow. Isotopes are atoms that differ from one another in atomic weight. The number of protons and electrons are the same in all atoms of a particular element, but the number of corresponding neutrons can differ. For the element oxygen, there are eight protons, but there can be eight, nine, or 10 neutrons, which means it has three isotopes.

The standard oxygen atom in a water molecule is oxygen-16 (16O) with two hydrogen atoms. There does exist, however, a "heavy water" combination of oxygen and hydrogen that acts the same chemically as ordinary water. The difference in the latter is that it is composed of oxy-gen-18 (18O). Because the water component is slightly heavier, it does not evaporate as quickly as the "lighter water." Therefore, when water vapor does begin condensing, the heavier water is the first to join the water droplets or ice crystals that fall as rain or snow. During this pro-

Ice-core research is currently conducted in both Antarctica and Greenland, where climatologists study the evidence and attempt to put together the Earth's past. (University of Wisconsin-Madison Physical Sciences Laboratory, Ice Coring and Drilling Services and National Science Foundation)

More ice 2

300 400 500 Thousands of Years Ago

Less ice

G Infobase Publishing

300 400 500 Thousands of Years Ago

This graph shows the changing ice volume on Earth over the last 750,000 years. The different isotopes of oxygen found in foramin-ifera were analyzed to obtain the graph. The Gs designate the times when glaciers may have reached as far south as the midwestern United States.

cess, as the air becomes colder and colder, there is less and less heavy water in its water vapor.

Because of this physical property, when the number of oxygen-18 atoms in the ice or snow is compared with the number of oxygen-16 atoms in it, it can be used as a measuring stick of how cold the air was when the snow initially fell. The fewer the oxygen-18 atoms, the colder the air was. The ice core can then be dated to determine how long ago these temperatures occurred. The graph above shows the occurrence of eight major ice ages based on the 18O/16O records.

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