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Glaciation and the Pleistocene

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Title: Glaciation and the Pleistocene


1
Glaciation and the Pleistocene
2
Over the last two million years, severe climatic
changes had tremendous influence on life on
earth. These events were recent enough that a
number of techniques that they can be studied
using techniques that are not available for more
ancient times.
Pleiostocene pack rat midden in eastern Nevada
3
Over its history, the earth has undergone many
periods of extensive glaciation. Often, this
resulted when plate movements positioned large
continental blocks over the poles. Much of the
Mesozoic and early Cenozoic, however, were
characterized by mild climates. Therefore, the
glacial and interglacial cycles of the
Pleistocene represented a dramatic shift.
The Wisconsin maximum
4
During the Pleistocene, the earth experienced
numerous glacial-interglacial cycles. The
glaciers were enormous, often 2-3 km thick. At
their maximum extent, they covered up to a third
of the earths land surface. At the maximum extent
of the last glacial period, ice sheets extended
to about 45º N. latitude
During the last glacial maximum (about 18,000
years B.P.) the Gulf Stream helped keep the North
Atlantic relatively warm. It also cooled
southern Europe and Africa as it flowed southward.
5
The last glacial maximum was largely (not
entirely) a North American phenomenon.
6
During these glacial maxima, prevailing winds
shifted. Moist air penetrated into the interior
of most continents, causing wet (glacio-pluvial)
conditions regions that are now arid. The same
climate patterns led now moist tropical regions
to be drier during glacial maxima. Glaciation was
largely a northern hemisphere phenomenon, simply
because of the distribution of land masses.
7
Move through the next few slides to examine the
glacial recession in North America since the
Wisconsin glaciation, the last glacial maximum of
about 18,000 B.P. (before present).
  • Watch for
  • The retreat of the Laurentide ice sheet, leaving
    the Great Lakes behind.
  • The opening of unglaciated areas between the
    Laurentide and the Cordilleran ice sheets.
  • The creation of massive Lake Agassiz, which will
    ultimately burst through the ice dam and empty
    into the forming Hudson Bay.

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The dumping of Lake Agassiz is thought by some
paleoclimatologists to have triggered the
Younger Dryas.
The present-day Minnesota River Valley was a
drainage basin for Lake Agassiz.
14
What caused the ice ages? Scientists once
believed that they had resulted from changes in
the output of solar radiation from the sun. This
does not appear to be the case. Instead, the
Pleistocene ice ages seem to have resulted from
changes in the Earths orbit which, in turn,
resulted in changes in incident solar
radiation. These changes are known as Milanovitch
cycles.
15
  • Milankovitch cycles are periodic changes in
  • the eccentricity of the earths orbit, i.e., how
    elliptical the orbit is. This occurs with a
    period of about 100,000 years.
  • obliquity, i.e. the tilt of the earths axis to
    the plane of the ecliptic. This occurs with a
    period of about 41,000 years, over which time the
    tilt changes from about 22.1? to about 24.5?.
    The current tilt is about 23.5?.
  • The precession of the point of the poles. This
    occurs with a period of about 22,000 years.

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Its also believed that, as ice developed, the
increased albedo of the earth led to further
cooling.
20
Whats the Evidence?
By analyzing the oxygen contained in the CaCO3
shells of preserved marine invertebrates, we can
determine the amount of 16O and 18O present in
the atmosphere at the time of the shell
formation. The lighter 16O evaporates more
rapidly during warmer periods. With calibration,
this ratio can be used to esimate temperatures.
21
Past and (predicted) future changes in the
Milankovitch cycles. Periods of the cycles
remain relatively constant, but their amplitude
varies considerably.
22
Estimated average global air temperature over the
past 850,000 years (inferred from oxygen isotope)
measurements from ice cores. Note that we are
currently in a very warm interglacial period. In
general, interglacials tend to be brief in
duration. The earth has undergone at least twenty
major glacial periods over the past 2 million
years.
If youre wondering how these determinations can
be made, heres a link to a site that describes
the use of oxygen ratios in paleoclimatology.
23
Records of global ocean temperatures over the
last 140,000 years indicate that the last two
shifts from interglacials to glacial maxima took
place of just a few thousand years.
24
Air temperatures from Eastern Europe over the
last 10,000 years.
25
and the variation in mid-latitude air
temperatures from the Northern Hemisphere over
the last thousand years. Again, note that the
changes can be rapid.
Global temperatures have varied significantly
over the last millenium. After the relatively
warm Middle Ages, temperatures cooled rapidly and
we experienced the Little Ice Ages. A recent
warming trend began about 150 years ago, and
another cooling trend began around 1940.
26
This cooling trend seems to have been halted and
reversed over the last twenty years, apparently
by anthropogenically-induced warming.
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During the Pleistocene, climates were influenced
far south of the glaciers. This figure indicates
that many unglaciated regions of North and South
America were from 4 to 8?C. cooler in the
Pleistocene than they are today.
29
The Laurentide ice sheet which existed from
18,000-9,000 B.P. caused changes in prevailing
wind patterns. This had great influence on local
climates.
While, globally, average ocean temperatures just
dropped a couple of degrees, the temperature of
the North Atlantic dropped as much as 10?C.
30
As winds moved down the very tall, steep faces of
the glaciers, they warmed adiabatically. This
moderated conditions in regions adjacent to the
glaciers.
31
Sea Level Change
32
The lowering of sea level during the Pleistocene
led to the formation of Beringia, which connected
North America and Asia. These eustatic changes
were global fluctuations in sea level that
resulted from the freezing of massive quantities
of water.
33
A better view of Beringia.
34
Lower sea level also resulted in the connection
of many of the islands of Indonesia with the
mainland of Asia and Australia,
respectively. Wallaces Line, a major
biogeographic division, reflects the separation
between those glacial land masses.
35
Two different methods have been used to estimate
ancient sea levels, oxygen istope data and the
depth of fossil coral reefs. Estimates derived
from these two estimates agree reasonably well.
36
An examination of sea level changes throughout
the Pleistocene reveal how rapidly (in a
geological sense) the transition from glacial to
interglacial conditions can occur.
37
The coastline of the southeastern U.S. has also
changed dramatically since the last glacial
maximum.
38
During the most recent glacial retreat, sea level
rose rapidly, creating a shallow sea that covered
the Saint Lawrence River Valley, Lake Champlain,
and the Ottawa River.
39
A depiction of animals at the edge of the
Champlain Sea. Many marine fossils have been
found there, including a number of large marine
mammals.
40
This explains the disjunct distribution of plants
such as the seaside spurge, Euphorbia
polygonifolia.
41
Biogeographic Responses to Glaciation
A disjunct distribution
42
  • Biogeographic changes were triggered by
    three environmental changes that occurred as a
    result of the glacial/interglacial cycle.
  • Changes in the location, extent, and
    configuration of prime habitats.
  • Changes in the nature of climatic and
    environmental zones.
  • Formation and removal of dispersal routes.

43
  • The responses of biotas can also be placed
    into three categories.
  • Some species were able to move with their optimal
    habitat as it changed location.
  • Some species remained in place and adapted to new
    conditions.
  • Some species underwent range reductions, and many
    ultimately became extinct.

44
Vegetation zones in Europe during the last
glacial maximum (Würm). The zones for most types
were shifted to the south by 10? to 20?
latitude. In some cases, east-west mountain
ranges blocked the southward range shift.
45
In contrast, the north-south mountain ranges
and major rivers of North America made it easy
for high latitude biomes to shift to the south.
The next few slides show the gradual northward
retreat of those biomes following the Wisconsin
maximum of 18,000 B.P.
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Pollen profiles in the Andes Mountains of South
America show how vegetation zones have shifted
upward since the last glacial maximum. Compare
the illustrations below, showing the distribution
of vegetation on Andean slopes at the time of the
glacial maximum and today. The increasingly warm
climate has forced the retreat of the
cold-adapted vegetation found higher on the
slopes. Note the upward shift of most vegetation
zones. Realize how this elevation shift relates
to the latitudinal shifts seen earlier.
51
And look at this. These graphs represent the
upper elevational limits of tropical forests (as
determined from pollen analysis) in mountains of
three different regions (East Africa, New Guinea,
and South America) over the last 33,000 years
. Note that in these widely separated regions,
the elevation of the tropical forest decreased
from about 28,000 B.P. to around 16,000 B.P.,
then began to increase. This represents the
cooler climate of the glacial period (during
which the forests shifted to lower elevation),
followed by the warming of the interglacial with
the concurrent upward shift of the forest.
52
We see dramatic latitudinal vegetation shifts in
southern hemisphere regions as well. Note the
dramatic changes in vegetation zones along the
eastern coast of Australia. This shift from a
dry-adapted woodland to rain forest illustrates
that the responses to glacial and interglacial
conditions are not uniform.
53
The overall change in vegetation types in North
America has been a dramatic decrease in tundra,
with increases in deciduous forests, northern
hardwoods, mixed forests, and coniferous forests.
54
Inland lodgepole pines have expanded their range
northward over the last 12,000 years. The
northern range boundaries (as indicated by pollen
records) at various dates before present are
indicated by the dots.
55
The northward range expansion of the white spruce
following the retreat of the Laurentide glacier
was facilitated by prevailing winds (shown by the
white arrows). After the range expansion had
reached the edge of the glacier, the northerly
winds aided in a rapid range expansion to the
north.
56
The next few slides illustrate the range shifts
of four species of rodents during the Holocene.
In each, the shaded area represents the present
range, while the dots indicate the location of
late Pleistocene fossils. The range shifts
differ from species to species. The direction
and length of the arrow indicate the magnitude
and direction of the range shift.
This one is the collared lemming (Dicrostonyx).
57
The range shift of the brown lemming (Lemmus) is
smaller in magnitude.
58
The eastern chipmunk (Tamias striatus) shifted to
the northeast.
59
The northern pocket gopher (Thomomys talpoides)
shifted to the west.
60
How can we explain differences in the rate of
range expansion of tree species following the
recession of the glaciers?
61
Differences in range shifts may have a number of
results. In these cases, species which
co-occurred during the most recent glacial
maximum have become disjunct today. In these
figures, current ranges of species are shown.
The dot represents a location where late
Pleistocene fossils of all are found together.
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Significant changes in elevational distribution
of vegetation have occurred since the
Pleistocene. This is in the mountainous region
of the American Southwest near southern Arizona.
Notice that the elevational range was lower
during the glacial periods of the Pleistocene.
66
The next two slides show the distribution of
vegetation zones in the southwestern United
States during the last glacial period compared
with those today. By moving back and forth
between the two slides, you can see that desert
regions have expanded greatly, while vegetation
zones adapted to cooler climates have declined.
Alpine habitat has disappeared, while the
coniferous forests that were widespread during
the glacial period have declined dramatically.
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Glacial Lake Agassiz covered much of present-day
central Canada about 9000 B.P. About 8000 B.P.,
the ice dam holding the waters behind it burst
with a catastrophic release of fresh water into
Hudson Bay.
70
Shyok Ice Lake is a modern-day example of a
post-glacial lake formed when retreating glaciers
served as dams and meltwater accumulated in
glacier-carved valleys. Shyok is located along
the western edge of the Himalayas.
71
Kettle lakes were formed when retreating glaciers
left ice blocks behind on the outwash
plain. Lakes formed in the outwash and in the
glacial till by the melting blocks of ice.
72
A glacial plunge pool lake is formed when
meltwater runs across the surface of the glacier
and then pours down its face. A circular lake is
carved at the foot of the glacial face.
73
Chapel Lake is one of several plunge pool lakes
created by post glacial rivers after the
Marquette advance of the most recent ice age. Its
greatest depth is 140 feet.
74
Pluvial lakes were found across the western
portion of North America during the Wisconsin
glacial maximum. During the glacio-pluvial
period, these areas experienced wetter conditions
than the present. Most of what is now desert was
then lakes and marshes. Massive Lake Bonneville
has, today, been reduced to Great Salt Lake. The
dissection of other pluvial lakes into smaller
bodies of water has led to many instances of
vicariant speciation. A well-studied example of
this occurred with the desert pupfish.
75
Haffer identfied six principal areas within the
Amazon basin that are characterized by high
endemicity. He proposed that these areas were
refugia. He thought that they had remained as
regions of rain forest during the glacial maxima,
while the regions around them became more arid.
Haffer and his colleagues felt that these regions
were isolated during the glacial maxima, and that
the isolation was long enough for significant
speciation to take place. They felt that these
speciation events explained, in part, the high
diversity of the Amazon basin.
76
Regions receiving high rainfall were thought to
have served as Pleistocene refugia. Patterns of
distribution of some groups, like the toucanets
illustrated above right, seemed to fall in line
with this hypothesis.
77
Nunataks are refugia that persisted within or
adjacent to the ice sheets. It appears that such
ice-free areas might have existed between the
Laurentide and Cordilleran ice sheets, and in a
area of present-day southern Wisconsin known as
the driftless refugium.
78
There may have also been ice-free areas in
mountainous regions along the Pacific
Coast. These sites may have served as refugia,
as well as acting as migration corridors during
full glacial conditions.
79
This diagram represents the number or endemics in
various regions of Alaska.
The high frequency of endemic plant species in
central Alaska (as many as 30 species in areas)
suggests that much of Beringia remained
unglaciated and served as a refugium for Arctic
plant species during the Pleistocene.
80
In North America, mass extinctions of terrestrial
vertebrates occurred during the late Pleistocene
and early Holocene. Among the species that
disappeared were mammoths, ground sloths,
sabertooth cats, and giant bison.
81
Also going extinct were the giant teratorns
vultures, shown here in comparison to modern
condors.
82
Paul Martin proposed that that the extinction of
North American megafauna was the result of
overkill by advancing human populations. This
figures indicates the advance of human population
in North America, beginning with the movement of
humans across Beringia to colonize modern-day
Alaska. They progressively moved to the south.
As they moved, species of large mammals became
extinct.
83
Quaternary extinctions were nonrandom. They
varied significantly in timing from location to
location, but always hit the largest taxa the
hardest.
84
Geonynornis newtoni went extinct about 50 mya.
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Africa is an exception, where the extinction of
mammalian megafauna did not follow the same
pattern. This may have been because the mammals
of Africa evolved with man, and were not
exposed as naïve prey to sophisticated
Pleistocene hunters.
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Extinction rates among moderately large
herbivores during the late Pleistocene.
93
Are there alternative explanations for the
Pleistocene extinctions? It does not appear that
the late Pleistocene extinctions can be related
directly to glaciation or any other catastrophic
geological event, since the disappearance of
North American megafauna did not take place until
long after the retreat of the Wisconsin glaciers.
94
Extinction rates among native and immigrant large
herbivores and carnivores in North America during
the late Wisconsin. Immigrants did better than
natives.
95
Selective extinctions of large megafaunal mammals
in Australia after the arrival of humans. Species
suffering extinction are shown in black. Those
in gray became extinct after European
colonization.
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