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CHAPTER 8 • Insolation Control of Monsoons 145

June insolation (30˚N) Windblown the lakebeds were already fully exposed partway into the
_
+ diatoms drying trend.
0
Another indication of a link to the North African
summer monsoon comes from the amplitude of the
diatom peaks. Each 23,000-year diatom pulse has the
20,000
same relative strength as the immediately preceding sum-
mer insolation maximum (see Figure 8-9). This pattern
is consistent with a scenario in which stronger insolation
40,000
maxima drove stronger summer monsoon maxima, which
created bigger lakes, which provided larger sources of
Diatoms diatom-bearing sediments for subsequent transport to
60,000 blown the ocean.
Years ago to ocean

Lakes
80,000
empty 8-4 Upwelling in the Equatorial Atlantic
Atlantic sediments contain additional evidence consistent
Lakes
fill with the hypothesis that the North African monsoon
100,000 fluctuates at the 23,000-year tempo of orbital precession.
Cores in the eastern Atlantic just south of the equator
show that the structure of the upper water layers has
120,000 varied with a prominent 23,000-year rhythm. Part of
the reason for this response is that the North African
summer monsoon imposes an atmospheric circulation
140,000 pattern that overrides the local circulation.
Threshold value When the North African summer monsoon is rela-
FIGURE 8-9 Delayed diatom deposition in the Atlantic tively weak (as it is today), trade winds along the equa-
Diatoms from North African lakes were deposited in the tor have a strong east-to-west flow (Figure 8-10A). The
tropical Atlantic Ocean some 5000 years after the intervals of strongest trade winds occur in southern hemisphere
strongest monsoons, as the lakes dried out. (Adapted from W. winter (July and August) and blow from the South
F. Ruddiman, “Tropical Atlantic Terrigenous Fluxes Since 25,000 Atlantic toward the equator. Part of this flow crosses the
Years B.P.,” Marine Geology 136 [1997]: 189–207; based on E. M. equator, turns to the northwest, and enters North
Pokras and A. C. Mix, “Earth’s Precession Cycle and Quaternary Africa in the summer monsoon flow, but this part of the
Climatic Changes in Tropical Africa,” Nature 326 [1987]: 486–7.) flow is not strong when the monsoon is weak, as it is
today. Instead, strong trade winds blow mainly toward
the west and drive warm surface waters away from
Lakes in North Africa filled to maximum size during the equator (companion Web site, pp. 22–24). This
the summer insolation maxima that drove the strong upper-ocean flow causes a shallowing of the seasonal
monsoons. These high lake levels deposited lakebeds thermocline, a subsurface region of steep temperature
rich in diatoms, but the high water levels in the lakes for gradients between the warm surface waters and much
a time kept these sediments from being exposed to cooler temperatures below. As the thermocline shal-
winds, eroded, and blown to the oceans. lows, cooler waters rich in nutrients rise toward the sea
Then, as summer insolation began to decrease toward surface just south of the equator.
the next insolation minimum, the monsoon weakened, In contrast, at times when summer insolation was
summers became drier, and the lake levels began to drop. higher than it is today, the stronger summer monsoon
The fall in lake levels exposed the diatom-bearing silts flow altered this circulation pattern (Figure 8-10B). A
and clays to winter winds, which blew them out to the much larger portion of the southeast trade-wind flow
ocean. Once the lakes had dried out completely and most crossed the equator, turned to the northeast, and was
of the diatom-bearing sediments had been blown away, drawn into North Africa in the monsoon circulation.
transport of diatoms to the ocean slowed or stopped, even This strengthening of the monsoon flow into Africa
though the monsoon continued to weaken as summer weakened the westward trade-wind flow along the
insolation continued to fall toward the next minimum. equator, and the weaker trade winds reduced the
As a result, the diatom pulses sent to the ocean should lag upwelling of cold waters, leaving the surface waters
well behind the summer monsoon maxima because of the poorer in nutrients from below.
time needed to dry the lakes, but they should precede the Changes between these two circulation patterns
subsequent summer insolation minima because many of over time can be measured by examining variations in