Marine Records of African
Paleoclimate
Reference:
deMenocal, P.B. African climate change
and faunal evolution during the Pliocene-Pleistocene.Earth
and Plant. Sci. Lett, Frontiers, 6976, 1-22, 2004 (PDF).
deMenocal, P. B., Plio-Pleistocene African Climate, Science,
270, 53-59, 1995 (PDF)
 |
Eolian Dust Plume off NW Africa (SEAWIFS Image). Eastern Atlantic deep-sea sediments
receive nearly 500 million tons of wind-borne eolian dust from
subtropical Africa each year. Mineral dust abundance is directly
related to aridity in West African dust source areas [Propero
and Nees, 1986].
Eolian dust is transported over the subtropical
east Atlantic by both summer and winter winds and constitutes
a significant fraction of deep-sea sediments here. Significant
East African dust plumes deliver eolian material to the Gulf
of Aden and Arabian sea.
Pliocene-Pleistocene variations in African
aridity have been reconstructed from deep-sea sediment records
off West and East Africa [deMenocal, 1995; deMenocal and Bloemendal,
1995; Tiedemann et al., 1989; Clemens et al., 1996]
|
 |
| Ocean Drilling Program sites off
West and East African used to reconstruct subtropical African
climate during the Pliocene-Pleistocene. Terrigenous (eolian)
sediment deposition at Sites 662, 663, and 664 are influenced
by atmospheric deposition from the winter African trade winds;
deposition at Sites 659 and 661 are influenced by transport from
the summer (monsoon) wind field. Sites 721/722 and 231 reflect
transport by the Findlater Jet associated with the summer Asian
monsoon. The eolian fraction records (below) at Sites 661, 662,
663, 664, and 721/722 were produced using a chemical leach sequence
which isolates the mineral, terrigenous fraction free of biogenic
carbonates, opal, and organic carbon. Sites 659, 662, and 721/722
are constrained by oxygen isotopic stratigraphies. |
Marine sediment records of
African Paleoclimate Variability
Marine sediments accumulating off the western
and eastern margins of subtropical North Africa have provided
some of the most compelling evidence for progressive, step-like
increases in African aridity during the late Neogene.
The sediments of ODP Leg 117 (Arabian Sea) Sites 721 and 722 have
been the focus of much research into the late Cenozoic evolution
of the Indian monsoon and regional climate. Initial work at Site
722 demonstrated that large changes in regional aridity were linked
to the Pleistocene succession of glacial-interglacial cycles [Clemens
and Prell, 1991; Clemens and Prell, 1990]. Eolian dust
fluxes to Site 722 during glacial maxima were three to five times
higher than observed for interglacial periods; these low-latitude
aridity cycles were observed to be in-phase with the oxygen isotopic
record of high-latitude glacial-interglacial cycles.
Analysis of the full Pliocene-Pleistocene
interval (last ca. 5 Ma) at Sites 721 and 722 has produced new
perspectives on the evolution of regional climate associated with
the initial onset and subsequent growth of high-latitude glacial
cycles after ca. 2.8 Ma (Fig. 3). Study of eolian dust variations
at Sites 721 and 722, as well as analysis of six other sites in
the Gulf of Aden and off subtropical West Africa have demonstrated
that the onset of large amplitude regional aridity cycles were
closely linked to the development of high-latitude glacial cycles
[Bloemendal and deMenocal, 1989; Clemens et al.,
1991; Clemens and Prell, 1991; Clemens et al., 1996;
Clemens and Prell, 1990; deMenocal, 1995; deMenocal
and Bloemendal, 1995; deMenocal and Brown, 1999; deMenocal
et al., 1999; deMenocal and Rind, 1993; deMenocal
and Rind, 1996; deMenocal et al., 1993; Tiedemann
et al., 1994; Tiedemann et al., 1989].
Several features of the Pliocene-Pleistocene
evolution of subtropical African aridity variations are common
to all of these sites [deMenocal, 1995]. Prior to 2.8 Ma
subtropical African aridity varied at the 2319 kyr period
associated with low-latitude radiation forcing of monsoonal climate,
whereas after 2.8 Ma African aridity followed the longer 41 kyr
and then 100 kyr periods associated with glacial-interglacial
cycles of the late Pliocene and Pleistocene, documenting the post-2.8
Ma regulation of this low-latitude climate system by high-latitude
glacial climates (Fig. 3). Distinct shifts in African eolian variability
were observed at 2.8 Ma, 1.7 Ma, and 1.0 Ma, each tied to coeval
shifts in high-latitude climate. This sensitivity of African climate
to high-latitude glacial boundary conditions has been documented
using general circulation model experiments [Clemens et al.,
1991; deMenocal and Rind, 1993; Kutzbach and Guetter,
1986; Prell and Kutzbach, 1987]. Based on analysis of the
terrigenous (eolian) grain size record over the last 3.5 Ma at
Site 722, [Clemens et al., 1996] documented discrete shifts
in the intensity and phase of the Indian monsoon at 2.6 Ma, 1.7
Ma, and 1.2 Ma, and 0.6 Ma, further emphasizing the importance
of high- and low-latitude climate linkages throughout the Pliocene
and Pleistocene.
Late Neogene variations in African vegetation
have been reconstructed from analyses of fossil pollen preserved
in marine sediments off West and East Africa. The most detailed
study was conducted at ODP Site 658 off Cap Blanc, West Africa
[Dupont and Hoogheimstra, 1989; Dupont and Leroy,
1995]. The data document the steplike increases in West African
aridity coincident with the changes in African dust export above
(Fig. 4). Importantly, the data document the close correspondence
between high-latitude glacial conditions and African aridity,
as well as the progressive aridification of subtropical West African
after ca. 2.8 Ma [Dupont and Hoogheimstra, 1989; Dupont
and Leroy, 1995].
References
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deMenocal, P. B., Plio-Pleistocene African Climate, Science,
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