Page 134 - Earth's Climate Past and Future

110 PART II • Tectonic-Scale Climate Change

FIGURE 6-15 Sediments
suspended in rivers The annual yield
of suspended sediments is highest in
two regions: the Himalayas of
southeast Asia and the Andes of South
America. (Adapted from D. E. Walling
and B. W. Webb, “Patterns of Sediment
Yield,” in Background to Paleohydrology, ed.
K. J. Gregory [New York: Wiley, 1983].)

Suspended
sediment
2
(tons/km /year)
> 750
100–750
0–100
Deserts and ice

The best record of rates of erosion lies in sediments addition, some sediments deposited on the seafloor
deposited in ocean basins by rivers. By far the largest are eroded and redeposited, and this reworking skews
mass of young sediment in the ocean today is found on compilations of sediment deposition rates through
the Indian Ocean seafloor south of the Himalaya. time toward younger ages. Rapid deposition of huge
Deposition of this pile of sediment began near 40 Myr amounts of Himalayan sediment thus supports the
ago, increased near 25 Myr ago, and accelerated rapidly
near 10 Myr ago (Figure 6-16).
Climate model experiments indicate that this influx
Relative sediment influx
of sediment to the Indian Ocean is a result of two fac- 0 Larger
tors: (1) creation of steep terrain along the southern
Himalayan margin of the Tibetan Plateau and (2) the
fact that a plateau the size of Tibet in effect creates its
own weather, including the powerful South Asian mon-
soon (Figure 6-17). 10
Monsoons result from different rates of heating
of continents and oceans due to the different heat capac-
ities of land and water (companion Web site, pp. 9–11,
17–18). In summer, the continents (including the surface Myr ago 20
of Tibet) warm up more rapidly than the oceans and
heat the overlying air. The warm air rises and pulls in
moist ocean air. The high Himalayas on the southern
margins of the plateau form an obstacle to the incoming
30
ocean air, forcing it to rise and its water vapor to con-
dense in cooler temperatures at high altitudes. As a
result, these steep slopes become a natural focal point
for strong summer monsoon rains, which release latent
heat and fuel even more powerful monsoons. The pow- 40
erful south Asian monsoon came into existence in part as FIGURE 6-16 Himalayan sediments in the Indian Ocean The
a direct result of the rise of the Tibetan Plateau. rate of influx of sediments from the Himalayas and Tibet to the
Unfortunately, ocean sediments cannot give us a deep Indian Ocean has increased almost tenfold since 40 Myr
definitive estimate of global rates of physical weathering ago. (Adapted from D. K. Rea, “Delivery of Himalayan Sediment
in the past. Much of the sediment eroded from coastal to the Northern Indian Ocean and Its Relation to Global
mountain ranges and deposited in the nearby ocean is Climate, Sea Level, Uplift, and Seawater Strontium,” in Synthesis
soon subducted into nearby trenches. The large amount of Results from Scientific Drilling of the Indian Ocean, ed. R. A. Duncan
of sediment lost in this way cannot be quantified. In et al. [Washington, DC: American Geophysical Union, 1992].)