Geoscience Reference
In-Depth Information
years. h e warm interglacials are more like brief but repeating aberrations
that occur just at er the peak of each glacial cycle.
At the peak of the last glacial cycle, or the “Glacial Maximum,” about
20,000 years ago, two-mile-thick ice sheets covered the northern United
States, Canada, and northern Europe. h e ice was composed of water that
had evaporated of the oceans, lowering global sea level by some 360 feet.
h e glaciers that covered these continents are now gone, so the ice they held
cannot be studied. However, detailed information about the Last Glacial
Maximum, as well as the preceding cycles of ice and sea levels, are contained
in the landscapes that were once buried under these ice sheets as well as in
the permanent ice sheets of Greenland and Antarctica. Even sediments accu-
mulating beneath the oceans hold critical information about past glacial and
interglacial cycles.
Ice cores extracted by drilling through these high-latitude ice sheets contain
records of past climates spanning hundreds of thousands of years, including
information about the earth's temperature, variations in atmospheric gases
like carbon dioxide and methane, solar activity, and wind speeds. Likewise,
sediments have been accumulating beneath the major ocean basins for many
millions of years. h ese sediments contain a continuous record of past ocean
temperatures, sea-level change, ocean productivity, ocean circulation, and
more. When compared with the ice core records, they show that, during
glacial periods, sea levels dropped, and the oceans were cooler and more pro-
ductive. h e ice core and ocean sediment archives provide vital background
information about long-term climate change on a global scale.
h e Holocene period, which spans the past 11,000 years, is the most recent
warm or interglacial period. It is really just a continuation of the Pleistocene
era but is demarcated by Earth scientists as a separate period in recognition of
the abundant information that is available about it. In the rest of this chapter,
we describe the types of evidence used to piece together the climate history
of the North American West over millennia.
climate clues
Sediments are produced as the forces of climate and water erode the earth's
surface over time, yielding a treasure trove of information about past climate,
including the remains of plants and animals, both the hard parts and sot er
organic parts. h
ese sediments are deposited year at er year, for centuries and
