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144 PART III • Orbital-Scale Climate Change

north. At times when summer insolation was much the ocean and because these ocean cores are hundreds
stronger than it is today, the strengthened summer and even thousands of kilometers away from the land,
monsoon expanded northward and eastward, bringing the only explanation for the presence of the diatoms is
much heavier rainfall to these high-elevation regions. wind transport. In arid and semiarid regions, winds
In effect, rainfall in the North African tropics exerts a scoop out (“deflate”) sediment from the beds of dried-
remote control on the salinity of the subtropical out lakes and blow the fine debris far away, some of it to
Mediterranean Sea via the Nile River. the nearby oceans.
Satellite sensors have detected the buried remnants The lake diatoms in these tropical Atlantic cores
of streams and rivers that once flowed across Sudan (see must have come from North Africa, which lies directly
Figure 8-7) but are now covered by sheets of sand blow- upwind in the prevailing northeasterly flow of winter
ing across the hyperarid southeastern Sahara Desert. trade winds. The intervals in the Atlantic cores contain-
The fact that these streams once flowed eastward and ing freshwater diatoms mark times in the past when
joined the Nile River indicates that lower-elevation North African lakes were drying out and their muddy
regions also contributed to the Nile’s stronger flow dur- lakebeds were becoming exposed to strong winter trade
ing major monsoons. winds.
The records in the Atlantic cores show that lake
diatoms were delivered in distinct pulses separated by
8-3 Freshwater Diatoms in the Tropical Atlantic
23,000 years. As was the case for the Mediterranean
Evidence that North African lakes fluctuate at the sapropels, this 23,000-year tempo in diatom influxes is a
23,000-year tempo of orbital precession can also be direct indication of a connection to the tropical mon-
found in sediment cores from the north tropical soon fluctuations in North Africa. In this case, however,
Atlantic Ocean (Figure 8-8). These sediments con- each diatom pulse occurs later than the summer insola-
tain layers with high concentrations of the opaline tion maxima by 5000 to 6000 years (Figure 8-9).
(SiO ·H O) shells of the freshwater diatom Aulacoseira This delay makes physical sense as part of the
2 2
granulata. Because these diatoms could not have lived in sequence of events during a typical monsoon cycle.

Winter
trade Monsoon lakes
winds drying out
FIGURE 8-8 Drying of monsoonal lakes
North African lakes filled by strong monsoon
rains later dried out and were exposed to
erosion by winds. Lake muds containing the
Windblown freshwater diatom Aulacoseira granulata (inset)
lake diatoms were carried by winds to the tropical
Atlantic. (Inset courtesy of Bjørg Stabell,
University of Oslo.)