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Discovering the Ice Ages
TPO19-3
：
Discovering the Ice Ages
In the middle of the nineteenth century, Louis Agassiz, one of the first scientists
to
study
glaciers,
immigrated
to
the
United
States
from
Switzerland
and
became
a
professor at Harvard University, where he continued his studies in geology and other
sciences. For his research, Agassiz visited many places in the northern parts of Europe
and
North
America,
from
the
mountains
of
Scandinavia
and
New
England
to
the
rolling hills of the American Midwest. In all these diverse regions, Agassiz saw signs
of
glacial
erosion
and
sedimentation.
In
flat
plains
country,
he
saw
moraines
(accumulations
of
earth
and
loose
rock
that
form
at
the
edges
of
glaciers)
that
reminded him of the terminal moraines found at the end of valley glaciers in the Alps.
The
heterogeneous
material
of
the
drift
(sand,
clay,
and
rocks
deposited
there)
convinced him of its glacial origin.
The areas covered by this material were so vast that the ice that deposited it must
have
been
a
continental
glacier
larger
than
Greenland
or
Antarctica.
Eventually,
Agassiz
and
others
convinced
geologists
and
the
general
public
that
a
great
continental glaciation had extended the polar ice caps far into regions that now enjoy
temperate climates. For the first time, people began to talk about ice ages. It was also
apparent that the glaciation occurred in the relatively recent past because the drift was
soft,
like
freshly
deposited
sediment.
We
now
know
the
age
of
the
glaciation
accurately from
radiometric dating of the carbon-14 in
logs buried in
the drift. The
drift
of
the
last
glaciation
was
deposited
during
one
of
the
most
recent
epochs
of
geologic
time,
the
Pleistocene,
which
lasted
from
1.8
million
to
10,000
years
ago.
Along
the
east
coast
of
the
United
States,
the
southernmost
advance
of
this
ice
is
recorded by the enormous sand and drift deposits of the terminal moraines that form
Long Island and Cape Cod.
It soon became clear that there were multiple glacial ages during the Pleistocene,
with
warmer
interglacial
intervals
between
them.
As
geologists
mapped
glacial
deposits
in
the
late
nineteenth
century,
they
became
aware
that
there
were
several
layers
of
drift,
the
lower
ones
corresponding
to
earlier
ice
ages.
Between
the
older
layers
of
glacial
material
were
well-developed
soils
containing
fossils
of
warm-climate
plants.
These
soils
were
evidence
that
the
glaciers
retreated
as
the
climate
warmed.
By
the
early
part
of
the
twentieth
century,
scientists
believed
that
four
distinct
glaciations
had
affected
North
America
and
Europe
during
the
Pleistocene epoch.
This
idea
was
modified
in
the
late
twentieth
century,
when
geologists
and
oceanographers
examining
oceanic
sediment
found
fossil
evidence
of
warming
and
cooling
of
the
oceans.
Ocean
sediments
presented
a
much
more
complete
geologic
record of the Pleistocene than continental glacial deposits did. The fossils buried in
Pleistocene
and
earlier
ocean
sediments
were
of
foraminifera
—
small,
single- celled
marine
organisms
that
secrete
shells
of
calcium
carbonate,
or
calcite.
These
shells
differ
in
their
proportion
of
ordinary
oxygen
(oxygen-16)
and
the
heavy
oxygen
isotope (oxygen-18). The ratio of oxygen-16 to
oxygen-18 found in
the calcite of
a
foraminifer's
shell
depends
on
the
temperature
of
the
water
in
which
the
organism
lived. Different ratios in the shells preserved in various layers of sediment reveal the
temperature changes in the oceans during the Pleistocene epoch.
Isotopic analysis of shells allowed geologists to measure another glacial effect.
They could trace the growth and shrinkage of continental glaciers, even in parts of the
ocean
where
there
may
have
been
no
great
change
in
temperature
—
around
the
equator, for example. The oxygen isotope ratio of the ocean changes as a great deal of
water is withdrawn from it by evaporation and is precipitated as snow to form glacial
ice.
During
glaciations,
the
lighter
oxygen-16
has
a
greater
tendency
to
evaporate
from
the
ocean
surface
than
the
heavier
oxygen-18
does.
Thus,
more
of
the
heavy
isotope
is
left
behind
in
the
ocean
and
absorbed
by
marine
organisms.
From
this
analysis of marine sediments, geologists have learned that there were many shorter,
more
regular
cycles
of
glaciation
and
deglaciation
than
geologists
had
recognized
from the glacial drift of the continents alone.
TPO19-3
译文：发现冰河时代
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