Page 90 - Earth's Climate Past and Future

66 PART II • Tectonic-Scale Climate Change

500 over the South Pole by 125 Myr ago, yet no ice sheet
South Pole developed. Antarctica remained directly over the pole
450 position but largely ice-free from 125 Myr ago until near 35 Myr
ago, when ice reappeared. Here again we face the mys-
tery encountered earlier: How could a landmass cen-
tered on a pole remain ice-free (or largely so) for almost
430 Arabia 100 Myr?
South America Clearly the polar position hypothesis cannot fully
Africa
India explain the sequence of glaciated and unglaciated
380 intervals over the last 500 Myr. Yet the hypothesis is
successful to this extent: ice sheets developed only
on landmasses that were at polar or near-polar posi-
300
tions, consistent with their polar locations today.
330
This correlation (see Table 4-1) confirms that over
Antarctica the last 500 Myr continents had to occupy polar posi-
250
Direction Australia tions for large-scale glaciation to occur. But the same
of ice flow record also tells us that the presence of continents in a
polar position does not guarantee that ice sheets will
form.
FIGURE 4-8 Gondwana glaciation and the South Pole
Changes in the position of the south magnetic pole in relation
to the continent of Gondwana are largely the result of the
slow movement of Gondwana. Glaciations occurred in the
northern Sahara about 440 Myr ago and in southern
Gondwana (South Africa, Antarctica, India, South America,
and Australia) 325–240 Myr ago. The (shallow) water shown
between the modern continental outlines was land during
Pangaean times. (Adapted from T. J. Crowley et al.,
“Gondwanaland’s Seasonal Cycle,” Nature 329 [1987]: 803–7,
based on P. Morel and E. Irving, “Tentative Paleocontinental Maps
for the Early Phanerozoic and Proterozoic,” Journal of Geology 86
[1978]: 535–61].)

From 325 to 240 Myr ago, Gondwana continued its
slow journey across the South Pole, and a huge region
centered on the south-central part of the continent was
glaciated (see Figure 4-8). The ice sheets were centered
on modern Antarctica and South Africa, and they spread
out into adjoining regions of South America, Australia,
and India. Because of the correspondence between the
area of Gondwana that was glaciated and its position at
or near the south magnetic pole, this long interval of
glaciation is fully consistent with the polar position
hypothesis.
By 240 Myr ago, Gondwana had moved northward
and the glaciation had ended. The absence of ice after
that time agrees with the positioning of the land away
from the South Pole. By that time, the northern part
of Gondwana had begun to merge with the northern
continents and form the even larger supercontinent
Pangaea. FIGURE 4-9 Glacial striations Sediment rubble carried in
After 180 Myr ago Pangaea began to break up. Its the bottom layers of ice sheets about 440 Myr ago gouged
southernmost part, which included the modern conti- striations in North African bedrock similar to those in modern
nents of Antarctica, India, and Australia, moved back ice in Alaska, shown here. (Carr Clifton.)