UCAR Office of Education and Outreach
Dr. Randy M. Russell
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UCAR Office of Education and OutreachDr. Randy M. Russell |
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Ice cores extracted from polar ice sheets and from glaciers worldwide are one type of paleoclimate proxy record - a source of data that tells us about past climates.
Ice cores from glaciers and the polar ice caps are probably the most comprehensive type of proxy record of past climates. Physical and chemical analysis of ice cores provides information on temperature, precipitation, atmospheric aerosols (such as dust and volcanic ash), and even levels of solar activity. Ice cores can provide data with a resolution as fine as yearly, and some records span periods of hundreds of thousands of years.
Continuous climate records embedded in ice form in areas where year-round cold temperatures prevent fresh accumulations of snowfall from melting in the summertime. The North and South Polar Ice Caps are obviously good locations for such accumulations, but high-altitude glaciers, even in low-latitude regions, can also be a source of ice cores. As layer upon layer of new snow builds up, the older layers on the bottom gradually get compressed. This compressed snow first transforms into a grainy material called "firn" that has a texture akin to granulated sugar. As the firn is buried deeper beneath subsequent layers, pressure eventually condenses it into solid ice (at a depth of about 100 meters in Antarctic samples, for instance). This ice layer can become quite deep over time; the 25 million year old East Antarctic Ice Sheet is more than 4.5 kilometers thick in places!
As the firn turns to ice, the air pockets between the grains become cut off from each other and produce bubbles in the ice. The bubbles serves as miniscule air samples from past eras, which scientists can use to study the state of Earth's atmosphere in ancient times. One of the most important paleoclimate indicators in such samples is the concentration of greenhouse gases (such as carbon dioxide and methane).
Scientists examine an ice core sample; inset shows layering (left). Closeup view of layers within a different sample; arrows indicate lighter summer layers (right).
Credits: Ken
Abbott, Office of Public Relations, University of Colorado, Boulder
(left). Anthony
Gow, United States Army Corps of Engineers, Cold Regions
Research and Engineering
Laboratory (right).
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The uppermost portions of an ice core exhibit a layered structure that shows yearly variation, providing an annual resolution like that in tree rings, corals, or varved lake sediments. Extreme pressures crush the deeper ice layers together so tightly that annual variations cannot be distinguished, though climate variations with lower resolutions can still be discerned. Typically, in ice samples from polar regions (primarily Greenland and Antarctica), the upper layers of ice have alternating light and dark layers. Light bands correspond to the relatively fresh, clean snows that fall in the summer when warmer conditions bring more moisture (and thus more precipitation) to these high-latitude locales. Dark bands mark the polar winter season, when little new snow falls on these frigid deserts and blowing snow is mixed with dust carried in from afar, discoloring the white snow. In many samples, this alternating light and dark layering is visible to the naked eye; in others, the layers can only be found by looking through polarized filters or via chemical analysis. Thick summer layers can indicate periods during which precipitation was heavier than usual, while especially dark winter layers can tell the tale of dry spells with regional or global dust storms.
The Greenland Ice
Sheet Project 2 drill site, located on the Greenland ice sheet at 72.6° N
and 38.5° W
at an elevation of 3,207 meters.
Credits: Michael
Morrison, GISP2 SMO, University of New Hampshire (left). Mark Twickler, GISP2
SMO, University of New Hampshire (right).
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Most ice cores are collected from the vast ice sheets covering Greenland and Antarctica, which provide records spanning the longest time periods. Scientists have drilled down through more than 3 kilometers of ice in Greenland to the bedrock beneath; the samples thus extracted represent about 130,000 years of history dating back to the last interglacial warm spell (during which Greenland was ice-free). Although deeper ice layers at a specific spot correspond to older time periods, the depth of ice at different sites doesn't always directly relate to the age of a record; different snowfall rates in different locales can lay down thicker or thinner layers over time. Although the ice in Antarctica is up to 4.5 km deep in spots, the oldest record recovered to date from Antarctica comes from a 3,270 meter deep drill hole that provides a history of climate dating back about 740,000 years. Ice from the bottom of a drill hole is much, much more compressed by pressure than is ice closer to the surface. At the South Pole firn turns to ice about 122 meters down and represents the climate of 100 years ago, while the bottom 100 meters of ice from the oldest Antarctic site represents the oldest 100,000 years of that record.
What type of information is archived in ice, and what does this data tell us about past climates? As was mentioned above, gas bubbles in the ice indicate the relative abundances of various atmospheric gases. Greenhouse gases like carbon dioxide and methane give us an insight into atmospheric heating. The thickness of an ice layer provides clues about local precipitation; a thick layer corresponds to greater snowfall. Layer thickness also may tell us something about global temperatures. Higher snowfall rates in polar regions generally correspond to more moisture in the global atmosphere, which usually arises during periods when the overall global temperature is high, causing increased evaporation from the oceans. Thus, counterintuitively, more snow tends to accumulate at the poles during global warm spells.
Graphsof temperature variation from present-day values (blue), atmospheric
carbon dioxide concentration (green), and dust (red) based on data from
ice
cores retrieved at the Vostok drilling site in Antarctica.
Credits: Petit
J.R., Jouzel J., Raynaud D., Barkov N.I.,Barnola J.M., Basile I., Bender
M., Chappellaz J., Davis J., Delaygue G., Delmotte M., Kotlyakov V.M.,
Legrand M., Lipenkov V., Lorius C., Pépin L., Ritz C., Saltzman
E., Stievenard M. (1999). Climate and Atmospheric History of the Past
420,000 years from the Vostok Ice Core, Antarctica. Nature 399: 429-436.
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Many types of aerosols (extremely fine particles), often carried to the polar regions from afar by winds, settle upon snow and become trapped within ice. These aerosols can include soot produced by burning (forest fires, slash-and-burn agriculture, industrial output from smokestacks, etc.), ash from volcanic eruptions, and dust from large-scale dust storms. Traces of unusual isotopes of certain elements also show up in ice core records. Chlorine-36 and iodine-129, produced by early atmospheric testing of nuclear weapons, have been found in ice cores from the Fremont Glacier in Wyoming. Beryllium-10 concentrations, linked to cosmic ray intensities, are an indirect indicator of solar activity.
The ratio of concentrations of two isotopes of oxygen in the water molecules in ice serves as a proxy indicator of global temperatures. Oxygen has two commonly occurring natural isotopes, the usual 16O (which makes up more than 99% of naturally occurring oxygen on Earth) and the less abundant 18O. The two extra neutrons in 18O cause water molecules containing this isotope to be heavier than normal water molecules. These heavier water molecules cannot escape from ocean water to become water vapor in the atmosphere via evaporation as readily as lighter water molecules. This tendency for preferential evaporation of 18O varies, however, as a result of the ocean temperature. Through a rather complex chain of events involving the global water cycle, this disparity between concentrations of oxygen-18 and oxygen-16 shows up in the snow that falls in polar regions, and thus in the ice formed from this snow. The net result is that the ratio of 18O to 16O in ice samples provides clues about global ocean temperatures and the extent of the polar ice caps at a given time in Earth's history.
Scientists must be careful in their interpretations of ice core data. For example, the gases trapped in bubbles in ice may do not have exactly the same age as the surrounding ice. For a few years after it falls, snow has air spaces between the snowflakes that are still exposed to the atmosphere, so gases can move between the snow and the atmosphere. As the deeper layers of snow are transformed into firn, the movement of air between these gaps and the atmosphere diminishes. However, this gas exchange does not cease entirely until the firn turns to ice and the pores become sealed bubbles. The gas in such bubbles therefore includes atmospheric "samples" from the years of the snow-to-firn-to-ice transition period, somewhat blurring the resolution of the ice core records in terms of gas samples. The pores tend to close off at depths of 50-150 meters in the Antarctic, but at much shallower depths in cores from mountain glaciers. In polar ice samples, the difference between the "ice age" and the "gas age" can be as large as about 150 years.
An ice core sample being removed from the drill tube.
Credits: Lonnie
Thompson, Byrd Polar Research Center, Ohio State University
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Ice cores are extracted using drills with ring-shaped bits on the ends of hollow tubes. Cores are typically removed in sections that are six meters long, so drilling to a depth of more than a kilometer requires many repetitions of the drilling and core removal process. Upon removal from the drilling rig, the long cores are generally cut into shorter 2-meter segments and their surfaces are carefully cleaned of fluids used in the drilling process. Deep ice is especially brittle, so the cores must be handled carefully and must obviously be stored in cold conditions. The cores are kept in a deep freeze as they are transported from their collection sites to one of a handful of archival storage and research facilities around the world. One such facility, the U.S. National Ice Core Laboratory which is maintained by the USGS and NSF near Denver, Colorado, stores over 14,000 meters of ice cores from 34 drill sites in Greenland, Antarctica, and high mountain glaciers in the Western United States at a temperature of minus 35° C. The cores are split lengthwise for study in a laboratory that is kept at a temperature of minus 22° C.
Scientists extrude
an ice core from the barrel of a drill tube, taking care not to
break the brittle ice. Butyl acetate, used in the drilling process,
is cleaned from the surface of the core before sawing the ice into
2 meter sections.
The
cloudy layers
visible in this 6 meter core section were formed when dust fell
onto the ice sheet and was entrained in the ice.
Credits: Mark Twickler, University of New
Hampshire.
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Last modified: 13 October 2010
Created: 6 October 2010
