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was due to the configuration of the continents at the
time.
During the
Silurian period
(443.7-416 m.y.a.),
which followed the glaciation, Gondwana continued
to migrate southward, and most of the Northern Hemi-
sphere was covered in ocean. Globally averaged tem-
peratures recovered to a high of
22
◦
C. Temperatures
remained high throughout the Silurian period due to the
strong greenhouse conditions, with CO
2
(g) persisting
at 3,000 to 4,000 ppmv. Sea levels increased due to the
melting of ice at the higher temperatures.
During the
Denovian period
(416-359 m.y.a.),
land-
plant coverage of the Earth surged
.The resulting
green plant photosynthesis caused a sudden reduction
in atmospheric CO
2
(g) that continued throughout the
period, causing an initial drop in temperature. Simulta-
neously, the spread of plants decreased the continental
albedo by 10 to 15 percent (Posey and Clapp, 1964).
The lower albedo caused temperatures to recover so that
they were similar at the end of the period to those at the
beginning,
∼
Figure 12.16.
Stromatolites in Hamelin Pool, Shark
Bay, Western Australia.
C
Marek Pilar/
Dreamstime.com.
Siberia
, and
Baltica
(present-day northern Europe)
near 540 m.y.a.
22
◦
C.
The
Carboniferous period
(359-299 m.y.a.) is a
period before the dinosaurs during which giant plants
and insects inhabited the continents and coal beds
formed in regions now known as eastern North America
and western Europe. Prior to the period, CO
2
(g) lev-
els were
∼
12.3.2.4. From 542 to 100 Million Years Ago
Between 542 and 100 m.y.a., which encompasses the
Paleozoic era
(542-251 m.y.a) and much of the
Meso-
zoic era
(251-65.5 m.y.a.), the Earth's average temper-
ature was relatively high, but with interruptions by two
important glaciations.
From 570 to 460 m.y.a., global CO
2
(g) mixing
ratios were thirteen to twenty times their current value,
sea levels were at an all-time high, and the North-
ern Hemisphere was covered with water above 30
◦
N
latitude due to the configuration of the continents,
which were concentrated in the Southern Hemisphere.
About 480 m.y.a., the high CO
2
(g) resultedinanaver-
age air temperature of
∼
1,500 ppmv and global temperatures were
15
◦
C,
respectively, today. At the beginning of the period, the
Earth dropped into the
Karoo Ice Age
,which lasted 100
million years (360-260 m.y.a.), with temperatures ulti-
mately decreasing to
20
◦
C, which compares with 393 ppmv and
∼
∼
12
◦
C. This ice age was caused
primarily by the restriction in circulation of polar and
tropical waters caused by the reorganization of land
masses so that they stretched from pole to pole and by
the existence of a polar land mass (at the South Pole)
that supported the accumulation of a large amount of
ice. The ice age was also enhanced by the continued
drop in CO
2
(g) due to a surge in plant growth during
the period.
The Karoo Ice Age persisted into the
Permian period
(299-251 m.y.a.), dissipating around 260 m.y.a. At that
time, the continents merged together to form one super-
continent,
Pangaea
(Figure 12.17a), which broke apart
around 200 to 170 m.y.a. (Figure 12.17b). The Pan-
gaean climate, on average, was drier and warmer than
today (Crowley and North, 1991). The warm temper-
atures allowed the dinosaurs to evolve starting 232 to
234 m.y.a. and thrive throughout the
Triassic
(251-
199.6 m.y.a.) and
Jurassic
(199.6-145.5 m.y.a.; Figure
12.17c) periods and beyond, until 65.5 m.y.a.
∼
22
◦
C, compared with
15
◦
C
∼
∼
today.
At the beginning of the
Ordovician period
(488.3-
443.7 m.y.a.), Gondwana was centered near the Equa-
tor, whereas Laurentia, Siberia, and Baltica were sep-
arate island continents. During the period, Gondwana
moved south, extending between the Equator and the
South Pole, causing the warm Earth to cool so that, by
460 m.y.a., temperatures were similar to those today.
Continued cooling caused an ice age to set in by 447
to 443 m.y.a. The average temperature during the ice
age was
12
◦
C, resulting in a mass extinction. The ice
age lasted only 0.5 to 1.5 million years. The glaciation
was precipitated by a drop in CO
2
(g) from 7,000 to
4,400 ppmv, still more than ten times its current level.
The strong cooling in the presence of high CO
2
(g)
∼
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