Page 170 - Earth's Climate Past and Future

146 PART III • Orbital-Scale Climate Change

Strong
Weak monsoon
monsoon winds
winds
Cool Warm
nutrient- nutrient-
rich water Equator poor water Equator

Strong SE trades Weak SE trades
A B

FIGURE 8-10 Effect of monsoons on southeast trade winds (A) When monsoonal circulation
over North Africa is weak, strong southeasterly trade winds in the eastern tropical Atlantic cause
cool, nutrient-rich waters to rise close to the surface. (B) When a strong monsoon circulation
over North Africa weakens the trade winds, tropical waters are warm and depleted in nutrients.

the relative amounts of planktic organisms that inhabit which is changes in the surface climate that provides the
near-surface waters and leave shells in the sediments precipitation that feeds the groundwater. As a result,
below. In equatorial Atlantic sediments, planktic for- changes in overlying air masses are an important con-
aminifera and coccoliths are the most common shelled trol on variations in the δ O signal of cave calcite
18
organisms (Chapter 2). Different species of these two (Appendix 1).
kinds of plankton prefer different environmental condi- Records from caves in southern China show δ O
18
tions near the sea surface—either warmer waters with variations so large that they can be explained only by
fewer nutrients or cooler waters rich in nutrients. Sedi-
ment cores from the Atlantic Ocean just south of the
equator show 23,000-year cycles of alternating abun-
dances in these two types of plankton, still another
indication of the effects of the North African summer
monsoon.

Orbital Monsoon Hypothesis: Regional
Assessment

The evidence from North Africa and surrounding seas
supports the orbital monsoon hypothesis, but mon- A July
soon circulations also exist on other continents. The Low High
south Asian monsoon is the most powerful monsoon High Low
of all because of the size of the land mass and the Precipitation
extent of the high topography of Tibet (Chapter 6). As
a result, a large area of low pressure and heavy mon-
soonal precipitation covers southern Asia in summer
(Figure 8-11).

8-5 Cave Speleothems in China and Brazil

Stalactites and stalagmites are constructed of calcite
(CaCO ) deposited by groundwater dripping in caves.
3
These deposits build up layer by layer over thousands to B January
Pressure
tens of thousands of years, and they can be very accu-
rately dated by radiometric analysis of small amounts of FIGURE 8-11 Seasonal patterns of pressure and
thorium and uranium (Chapter 2). In addition, the rela- precipitation In the summer season of each hemisphere,
tive amount of the isotopes of oxygen in the calcite lay- heating of the continents produces low pressures and heavy
ers varies though time because of several factors, one of monsoonal rainfall.