Geology Reference
In-Depth Information
Some scientists think that this deep,
warm oceanic water released meth-
ane from seafl oor methane hydrates,
contributing a greenhouse gas to the
atmosphere and either causing or
contributing to the temperature in-
crease at this time.
Subtropical conditions persisted
into the Eocene in North America,
probably the warmest of all the Ce-
nozoic epochs. Fossil plants in the
Eocene John Day Beds in Oregon
include ferns, fi gs, and laurels, all of
which today live in much more hu-
mid regions, as in parts of Mexico
and Central America. Yellowstone
National Park in Wyoming has a
temperate climate now, with warm,
dry summers and cold, snowy win-
ters, certainly not an area where you
would expect avocado, magnolia, and
laurel trees to grow. Yet their presence
there during the Eocene indicates
that the area then had a considerably
warmer climate than it does now.
A major climatic change took
place at the end of the Eocene, when
mean annual temperatures dropped
as much as 7° C in about 3 million
years (Figure 23.18b). Since the Oli-
gocene, mean annual temperatures
have varied somewhat worldwide,
but overall, they have not changed
much in the middle latitudes except
during the Pleistocene Epoch.
A general decrease in precipita-
tion during the last 25 million years
took place in the midcontinent region
of North America. As the climate be-
came drier, the vast forests of the
Oligocene gave way fi rst to
savannah
conditions (grasslands with scattered
trees) and finally to
steppe
environ-
ments (short-grass prairie of the desert margin). Many her-
bivorous mammals quickly adapted to these new conditions
by developing chewing teeth suitable for a diet of grass.
ICE
Flathead
Lake
Scablands
a
Lake Missoula
American
Falls Lake
PACIFIC
OCEAN
Pyramid
Lake
Lake
Lahontan
Lake
Russell
Great
Salt
Lake
Lake
Bonneville
Lake
Manley
Modern lake
Pleistocene lake
Flood route
0
30
0 km
◗
Figure 23.16
Pleistocene Lakes in the Western United States Lake Missoula
was a proglacial lake, but the others were pluvial lakes.
In a recently discovered Paleocene fl ora in Colorado with
about 100 species of trees, nearly 70 percent of the leaves are
smooth margined, and many have drip-tips. The nature of
the leaves, coupled with the diversity of plants, is much like
that found in today's rain forests. In fact, the Early Oligocene
fossil plants at Florissant Fossil Beds National Monument in-
dicate that a warm, wet climate persisted then.
Seafl oor sediments and geochemical evidence indicate
that about 55 million years ago an abrupt warming trend
took place. During this
Paleocene-Eocene Thermal Maxi-
mum
, large-scale oceanic circulation was disrupted so that
heat transfer from equatorial regions to the poles diminished
or ceased. As a result, deep oceanic water became warmer,
resulting in extinctions of many deepwater foraminifera.
The fi rst members of many of the living orders of birds, in-
cluding owls, hawks, ducks, penguins, and vultures, evolved
during the Early Paleogene. Beginning during the Miocene
Epoch, a marked increase in the variety of songbirds took
place, and by 5 to 10 million years ago many of the exist-
ing genera of birds were present. Birds adapted to numerous
habitats and continued to diversify during the Pleistocene,
but since then, their diversity has decreased slightly.
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