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Thin cores of ice, thousands of meters deep, have been drilled in the
ice sheets of Greenland and Antarctica. They are preserved in special
cold-storage rooms for study. Glacier ice is formed as each year's snow
is compacted under the weight of the snows of later years. A slightly
darker layer that contains dust blown onto the ice sheet during summer,
when not much new snow falls, marks each year's new ice. The winter layer
consists of cleaner and lighter-colored ice. The layers are only millimeters
to centimeters thick. Counting the yearly layers can date them. The oxygen
in the water molecules also holds a key to past climate. Scientists are
able to use the oxygen atoms in the glacial ice as a proxy for air temperature
above the glacier.
Ice sheets on the continents have grown and then shrunk again four times
in the past half million years. Several climate proxies make that very
clear. Deposits of sediment left by these glaciers are present over large
areas of North America and Eurasia. Proxies for global temperature show
gradual cooling as the ice sheets form, and then very rapid warming as
the ice sheets melt back. The brief periods of warm temperatures between
glaciations are called interglacials. Past interglacials have lasted only
about twenty thousand years. Humankind developed civilization only within
the very last interglacial-and you are still in it!
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Three deep core sections show distinct annual
bands produced by the deposition of dust during the dry season
(dry season dust layers are represented by triangles). While
annual bands provide accurate relative dating (the age of
each ice band is known to be a year apart from directly adjacent
bands), paleoclimatologists also search for absolute dates
within a core chronology. Electrical conductivity measurements
(ECM), particle concentrations, and the ratio of heavy to
light oxygen molecules are other seasonally-variable core
parameters that can be used along with visual stratigraphy
in dating ice cores.
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