Page 43 - Earth's Climate Past and Future

CHAPTER 2 • Climate Archives, Data, and Models 19

FIGURE 2-2 Lake cores Hundreds
of cores have been taken from small
lakes and analyzed for records of
changes in pollen (vegetation) and
lake level over the last several thousand
years. (National Paleoclimate Data
Center, NGDC, Boulder, CO.)

Pollen
Lake levels

in the deeper parts of lakes. Lake sediments that fill
depressions left behind by melting glaciers are espe-
cially important climate archives for the last 20,000
years (Figure 2–2).
Ice and wind are also powerful agents of sediment
erosion and transport in some regions. Ice sheets that
reach maximum size and then begin to retreat leave
behind long curving ridges called moraines. Moraines
contain a jumbled mix of unsorted debris carried by ice,
ranging from large boulders to very fine clay. When an
ice sheet readvances over moraines deposited earlier, it
erodes the earlier debris and incorporates it into the
newer deposits. Unraveling a climate history from this
kind of record is like trying to decipher repeated episodes
of writing that have been largely erased on a blackboard.
By contrast, the coarse debris carried to the ocean and
dropped by melting icebergs into the underlying sedi-
ments can survive in a more protected environment.
Strong winds weather rocks and form fine sediment
particles in regions with dry climates. Winds form sand
dunes that slowly migrate across desert areas, but con-
tinuous reworking of the sand particles complicates
efforts to use dunes as climate archives. Winds also pick
up smaller, silt-sized grains, lift them high in the air,
and transport them far from their original sources. In
regions where the winds weaken, the silt is deposited in
sequences called loess. Loess deposits are excellent cli-
mate repositories of the last 3 million years, especially
in China (Figure 2–3). Finer sediments carried off the
continents by winds and deposited in ocean sediments
are also useful indicators of climate. FIGURE 2-3 Windblown loess Strong winds have deposited
For the portion of geologic time younger than thick layers of silt-sized grains in southeast China during the
100 million years, climate scientists have access to an last 3 million years. The total thickness of these loess deposits
additional climate archive: sediments preserved in can reach several hundred meters. In many regions people
ocean basins. Deep-sea sediments spanning the last sev- have created homes in the loess cliffs. (Courtesy of Steven
eral million years cover almost two-thirds of Earth’s Porter, University of Washington.)