Anatomy of an Ice Core
Locked in ice cores are clues to how climate evolved over thousands
of years
Ice is hot
Drilled from mountaintop glaciers and the world's great ice sheets, ice
cores provide scientists with unparalleled views of the planet's past.
Falling snow contains dust from volcanic eruptions and dissolved
chemicals from land and sea. As snow accumulates, these fingerprints of
past environments stay locked in layer upon layer of ice.
Face of a changing planet
Volcanic eruptions, dust storms, air pollution and El Niņo weather
patterns all leave signatures in the ice, enabling scientists to
understand how climate and other environmental factors interact.
University of Maine scientists study changes in atmospheric circulation,
temperature, sea ice extent, volcanics, environmental change, solar
variability, nutrients feeding marine ecosystems and other areas.
Why ice cores?
Instruments have reliably measured climate for the past century. To
understand the natural variability of climate requires knowledge of the
environment prior to instrumentation. Ice cores enable high- resolution
reconstructions of climate going back as far as 450,000 years. They are
essential tools in the research of UMaine's Climate Change Institute,
directed by scientist Paul Mayewski.
A frozen clock
The problem: how to tell one year from the next. The answer: look for a
clock in the ice, something that reaches a peak during the same time of
the year, every year. The levels of many chemicals transported by wind
rise and fall with the seasons. For example, sulfate is produced in
summer by sea life such as phytoplankton in the Southern Ocean; in
winter, sea salt is transported inland when marine storms intensify.
Deposited in the ice, these chemicals leave records of past climates.
Think life is tough where you work?
Scientists from UMaine's Climate Change Institute who retrieve ice cores
from Antarctica, Greenland, Asia and North America endure some of the
highest, coldest and windiest places on Earth. They use drills that bore
deep into the ice to extract cores up to 5 inches in diameter. In the
field, measurements and visual observations of the cores' layers are
recorded. Then the ice cores are packaged in plastic sleeves and shipped
in insulated boxes. It's important to avoid chipping or melting on the
long trip home. Cores come to UMaine or to the National Ice Core Lab in
Colorado. On campus, the ice cores are kept in a freezer at -27 degrees
Celsius.
Dust to dust bands
Dust on both ends of this ice core extracted from the Dry Valleys of
Antarctica indicates increased dust transport during the windy season.
The ice between the two dust bands is equal to one year of
precipitation. Invisible to the eye is the chemical composition of the
ice core: sodium and chloride that come from the ocean, magnesium and
calcium from continental sources (rocks/dust). Some visible dust found
in Antarctic cores is volcanic in origin. Each eruption has its own
chemical signature of dust and glass shards. It can take up to a year
for volcanic dust to be deposited in the ice, and there are times when
dust doesn't get to Antarctica. Much depends on the atmospheric
circulation and violence of the eruption.
The years melt away
Unlocking an ice core's secrets is a complex process. The ice must be
melted so that chemical measurements can be made. An ice core is placed
vertically on a heated aluminum or nickel block cylinder or "melt head"
in order to collect water samples in small vials. A core one meter long
takes about an hour to melt and yields up to 50 vial samples. The salts
from a human fingerprint can significantly alter the chemistry of the
ice, which is frequently in the parts per billion range. Scientists and
technicians always wear gloves to prevent contaminating the ice cores.
Historical evidence
The atmospheric evidence found in ice cores often correlates with
important dates in history. For instance in Arctic ice cores, scientists
have clearly "seen" the advent of the Industrial Revolution at the
beginning of the 20th century because of the rise in atmospheric acids
from burning fossil fuels. Other events that have left evidence in the
ice include the Dust Bowl of the 1930s and the Chernobyl nuclear
accident in 1986. Some of the most powerful tools for dating include
measurements of radioactivity levels from nuclear bomb testing in the
1950s and 1960s, and measurements of sulfate from large volcanic
eruptions, most notably Tambora in 1815.
by Nick Houtman
September-October, 2004
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