ice age
General Overview of the Ice Ages
Climate Change and Oxygen Isotopes
The details of such climate changes were first seen in deep-sea
sediments containing the shells of small planktonic organisms called
foraminifers. This can be done because when foraminifers are alive,
they fix within themselves a ratio of two types of atoms of oxygen. The "normal" oxygen isotope, which is by far the most abundant, has eight protons and eight neutrons in its nucleus;
it is called "oxygen-16." The "heavy" oxygen isotope, called
�oxygen-18,� has two more neutrons in the nucleus, but has the same
number of protons and electrons.
Oxygen-16 is found in higher concentrations in snow and ice, while
oxygen-18 is enriched in the ocean. Therefore, whenever more water is
extracted to make more ice the ocean leaves its �isotopic fingerprint�
in the oxygen. This enrichment effect is, in turn, seen in the carbonate shells of the foraminifers (made of CaCO3),
because the carbonate precipitates out of the seawater, and the oxygen
used to build the carbonate crystals reflects the composition of the
seawater. Through this method of analyzing oxygen isotopes
in foraminifers, scientists have been able to determine when the Earth
has produced more glaciers, and hence determined the times when ice ages
have occurred.
Ice Ages and Ice Cores
The carbon dioxide content of the atmosphere
for the last 4 glacial cycles is known from drilling into the ice in
Antarctica, where ancient air has been trapped and now can be extracted.
These results show that carbon dioxide follows the change in sea level
rather closely: when carbon dioxide increases, sea level rises and vice versa. A similar relationship is seen for methane.
Most likely, changes in trace gases help drive sea level up and down,
and the changes in sea level in turn change the content of trace gases
in the atmosphere in a feedback loop. Some of these climatic changes
seem to be extremely rapid.
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