Page 102 - Earth's Climate Past and Future

78 PART II • Tectonic-Scale Climate Change

Uplift that created the supercontinent Pangaea also formed a
moderate-size plateau in east-central Europe, as well as
high mountain ranges in the eastern North America (the
Appalachians) and in northwestern Africa.
Steep Mass Mountain Slope The uplift weathering hypothesis focuses mainly
slopes wasting glaciers precipitation on plateaus created by occasional collisions of conti-
nents rather than on ever-present mountain belts. As
Table 4–3 indicates, times of continental collisions
that created plateaus match times of glaciations over
Increased the last 325 Myr. Like the BLAG hypothesis, the uplift
rock
fragmentation weathering hypothesis is consistent with the icehouse-
greenhouse-icehouse climatic sequence. But if recently
discoveries prove correct, neither the uplift weathering
hypothesis nor the BLAG hypothesis nor the polar
Increased
weathering position of Gondwana entirely is a complete explana-
and tion for the short glaciation in the Sahara near 440 Myr
CO removal ago (see Box 4-1).
2

4-11 Case Study: Weathering in the Amazon Basin
Global cooling
One way to evaluate the effect of uplift on chemical
FIGURE 4-23 Uplift weathering hypothesis Active tectonic weathering is to examine the drainage basin of the Ama-
uplift produces several tectonic and climatic effects that cause zon River of South America (Figure 4-24). This basin can
strong weathering of freshly fragmented rock. This process be divided into two major units: (1) the low-lying Amazon
removes CO from the atmosphere and cools global climate.
2 Basin, where easterly trade winds blowing in from the
Atlantic Ocean bring frequent precipitation to the rain
forests of Brazil, and (2) the high-elevation eastern slopes
weathering hypothesis. The first process, subduction of the Andes Mountains, which collect most of the rest of
of ocean crust underneath continental margins, is an the incoming precipitation carried by the trade winds.
integral part of plate movements and a process that is Scientists have determined the regional effects of
continually active in many regions on Earth. Because chemical weathering in this drainage basin by sampling
subduction occurs relatively steadily over time, the total the amount of chemically weathered ions flowing down
amount of high mountain terrain on Earth is likely to to the Amazon River in dissolved form. They found that
be relatively constant through time, even though the the upper tributaries of the Amazon emerging from
locations and heights of individual mountain ranges the foothills of the Andes carry almost 80% of the total
vary considerably. dissolved chemical load discharged by the river when
The second process that creates high terrain is the it enters the Atlantic. Despite its vast size, the lower
collision of continents, and these events are far less com- Amazon Basin adds only the remaining 20% of this
mon. Collision between India and Asia over the last 55 total. Most of the chemical weathering in the Amazon
Myr created the Tibetan Plateau of southern Asia, but drainage basin occurs in the Andes, at rates per unit area
no plateau-like feature remotely close in size to Tibet that are a factor of 40 higher than those in the lowlands.
existed on Earth between 240 and 55 Myr ago because How could this be so? This evidence seems espe-
no major continental collision of this kind occurred dur- cially at odds with what the eye actually sees in the two
ing that interval. Earlier, between 325 and 240 Myr ago, regions. In the lower Amazon rain forest, highly weath-
the collisions between Gondwana and other continents ered clays that are the products of intense chemical

TABLE 4-3 Evaluation of the Uplift Weathering (CO Removal) Hypothesis
2
Time (Myr ago) Ice sheets present? Continents colliding? Hypothesis supported?
325-240 Yes Yes Yes
240-35 No No Yes
35-0 Yes Yes Yes