Page 130 - Earth's Climate Past and Future

106 PART II • Tectonic-Scale Climate Change

episodes of volcanism have been radiometrically dated, occurs on the margins of the Pacific Ocean, where sedi-
and the estimated volume of volcanic rock produced can ments are CaCO -poor because of strong dissolution on
3
be added to the amount produced by spreading and the seafloor by corrosive deep waters (Chapter 2). Much
subduction (see Figure 6-12). This adjustment does not of the global carbonate total is now being deposited on
change the basic picture. Inferred rates of volcanism the Atlantic seafloor where dissolution is less intense. If a
and CO input still increase during the last 15 Myr, and future change in the plate tectonic regime were to initiate
2
the modern rate of CO addition appears almost com- subduction in the Atlantic Ocean, an enormous amount
2
parable to that 40 Myr ago, even though most of the of carbonate would be available to be carried down into
greenhouse-to-icehouse cooling occurred during this trenches for later melting and eventual release to the
time interval. atmospheres through volcanoes. As a result, atmospheric
CO values would increase even in the absence of changes
2
in spreading rates. At this point, this idea has not yet been
IN SUMMARY, the evidence indicates that the tested.
spreading rate hypothesis may have explained global
cooling before 15 Myr ago, and particularly before
30 or 40 Myr ago. But it predicts a warming during 6-7 Evaluation of the Uplift Weathering Hypothesis
the last 15 Myr, when a major cooling has actually
occurred. To demonstrate that the uplift weathering hypoth-
esis explains global cooling during the last 50 Myr,
three main requirements must be met: (1) the amount
An alternative possibility has been considered but of high-elevation terrain in existence today must be
not yet formulated into a full hypothesis. The concept is unusually large in comparison with earlier intervals;
that the amount of carbon carried down into ocean (2) this high terrain must be causing unusual amounts
trenches may have varied because of changes in the type of rock fragmentation; and (3) the exposure of fresh
of sediments being subducted, even in the absence debris must be causing unusually high rates of chemical
of changes in spreading rates. Today, most subduction weathering.

Rockies
Colorado
Plateau
Tibet
East African
Plateau

Andes

Antarctic ice sheet

FIGURE 6-13 Earth’s high topography Earth today has only a few regions where broad areas
of land stand more than 1 km high (shown in brown, blue, and white). Except for the high ice
domes on Antarctica and Greenland, the highest bedrock surfaces are the Tibetan Plateau and
other high terrain in southern Asia, the Andes of South America, the Rocky Mountains and
Colorado Plateau of North America, and the volcanic plateaus of eastern and southern Africa.
(Courtesy of Peter Schloss, National Geophysical Data Center, Boulder, CO.)