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influxes of ice-rafted debris from melting icebergs during the late Pleistocene, has
shown a strong correlation between these events (known as Heinrich events, after
their first recognition by Heinrich,
1988
) and peaks in dust flux (Jullien et al.,
2007
).
The phases of exceptional Saharan dustiness and aridity probably reflect southward
displacement of the Intertropical Convergence Zone (see
Chapter 2
), presumably
reflecting changes in North Atlantic oceanic and atmospheric circulation patterns
during Heinrich events.
12.9 Early Holocene pluvial climates in tropical deserts
We have seen that in the late nineteenth and early twentieth centuries, European and
American geologists discovered evidence of formerly higher lake levels in the deserts
of Asia, North America and Africa. These lakes were at first regarded as pluvial lakes
formed during glacial times, which is in general still true of the desert regions of
North America. The notion of glacial pluvial climates became solidly entrenched and
the so-called pluvial chronology was even used to provide a relative chronology for
prehistoric sites in East Africa, with each so-called pluvial being equated with one
of the alleged glaciations identified in the European Alps. The far-travelled coarse
alluvial deposits of great rivers like the Nile were also interpreted as having originated
during glacial pluvial climates.
Although there were dissenting voices, most notably those of Cooke (
1958
)and
Flint (
1959a
;
1959b
), it was not until the high strandlines of the East African Rift
lakes (Butzer et al.,
1972
) and the abundant remnants of former lakes scattered across
the Sahara (Faure,
1966
;Faure,
1969
) were directly dated in the late 1960s and
early 1970s that the glacial pluvial concept was finally abandoned for the arid and
semi-arid tropics. The African high lake levels were found to be 11,000-9,000 rather
than 21,000 calendar years old, that is of early Holocene rather than of Last Glacial
Maximum age (21
2 ka). Not only were the early Holocene climates of the tropical
deserts wetter than they are today, with highest lake levels around 11-9 ka, but
similar climatic conditions were true of the last interglacial around 125 ka. The desert
environments no doubt oscillated between these two extremes, with the interglacials
slightly warmer and very much wetter than today and the glacial maxima colder and
mostly drier. However, not all arid phases in the tropical deserts coincide with glacial
maxima, nor do all humid phases coincide with interglacial times. For instance, Lake
Chad (Servant,
1973
; Servant and Servant,
1980
) in the southern Sahara and Lake
Abhe (Gasse,
1975
) in the Afar Desert of Ethiopia were both very high for at least
10,000 years before 21 ka, when they fell rapidly. They were then intermittently
dry (Lake Chad) or constantly dry (Lake Abhe) until 15 ka, rising rapidly thereafter
to reach peak levels at 11-9 ka. After about 4.5 ka, both lakes have remained low
apart from occasional brief transgressions. The
±
30 ka to 21 ka phase of high lake
levels could be regarded as a humid glacial phase, and the 21 ka to 15 ka regression
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