Pliocene

Upper Pliocene vegetation and climate in northwest and southern Africa.

Pollen from ODP Site 658 (off Cape Blanc, 21°N 19°W) recorded the climatic history and the migration of the boundary between the southern Sahara and Sahel for the period between 3.7 to 1.7 Ma (Leroy & Dupont 1994, 1997, Dupont & Leroy 1995, 1999). Throughout the record, cyclic variation of stages with large global ice volume (glacials) and stages with small global ice volume (interglacials) are registered. Generally, northwest African climate was arid during glacials and relatively humid during interglacials. Spectral analysis of pollen influx values and oxygen isotopes indicates that forcing of NW African climate by obliquity only started after 2.53 Ma, when glaciations became strong enough to have a global impact. The beginning of the trade winds as indicated by the pollen record started at 3.17 Ma. The strength of the trade winds was generally lower during the Pliocene than during the Pleistocene.

The aridity of the Sahara is ancient. Two major steps toward desertification, at 3.5-3.2 and 2.6 Ma, are described. From 3.7 to 3.5 Ma, the climate was relatively humid with the exception of some minor phases of aridity. Mangrove swamps grew at least 5° north of their modern northernmost position. From 3.5 to 3.2 Ma, an increase in aridity in the southern Sahara and Sahel during five dry periods paralleled glacial conditions. High percentages of liguliflorous composites indicate a unique composition of the savanna vegetation. After reestablishment of humid conditions at 3.25 Ma, the climate progressively became drier. Arid conditions culminated at 2.6 Ma. From then on, the pollen diagram displays time-transgressive maxima interpreted as repetitive southward shifts of the Saharan-Sahelian boundary.

On the Southern Hemisphere, Pliocene records from ODP Site 1082 offshore Walvis Bay (21°S 12°E) showed two important shifts in terrestrial climate variability corresponding to the upwelling history along the Namibian coast. Aridification at 2.2 Ma and expansion of winter rain climate between 3.1 and 2.2 Ma both suggest a link between the climate of southern Africa and the positions of oceanic and atmospheric systems over the South Atlantic and the Southern Ocean. A northward advance of the polar fronts can be connected with an increase in winter rainfall. Increased aridification in southern Africa resulting in less river discharge, more arid vegetation, and the northwards extend of desert and semi-desert after 2.20 Ma, is coeval with a shift in the Namibian Upwelling system from predominantly advection to increased upwelling (Dupont et al. 2005, Dupont 2006).

Both in northwest and in southern Africa climatic variability increased during the period of intensification of the Northern Hemisphere Glaciations (2.8-2.4 Ma). The impact is stronger in northwest Africa. The dominating influence of the low-pressure cell over the Sahara seems to increase climatic variability over northwest Africa, while influence of the high pressure cells over the Atlantic and Indian Oceans seems to temper the fluctuations in southern African climate.

Dupont, L.M., 2006. Late Pliocene vegetation and climate in Namibia (southern Africa) derived from palynology of ODP Site 1082. Geochemistry, Geophysics, Geosystems, 7(1). doi: 10.1029/2005GC001208.
Dupont, L.M., Leroy, S.A.G., 1995. Steps toward drier climatic conditions in Northwestern Africa during the Upper Pliocene. In: Vrba, E.S., Denton, G.H., Partridge, T.C., Burckle, L.H. (eds). Paleoclimate and Evolution, with emphasis on human origins. Yale University press, New haven, p. 289-298.
Dupont, L.M., Leroy, S.A.G., 1999. Climatic changes in the Late Pliocene of NW Africa from a pollen record on an astronomically tuned timescale. In: Wrenn, J.H., Suc, J.-P. and Leroy, S.A.G. (eds). The Pliocene: Time of Change. American Association of Stratigraphic Palynologists Foundation, p. 145-161.
Dupont, L.M., Donner, B., Vidal, L., Pérez, E.M., Wefer, G., 2005. Linking desert evolution and coastal upwelling: Pliocene climate change in Namibia. Geology 33: p. 461–464.
Leroy, S.A.G., Dupont, L.M., 1994. Development of vegetation and continental aridity in northwestern Africa during the Late Pliocene: the pollen record of ODP Site 658. Palaeogeography, Palaeoclimatology, Palaeoecology 109: p. 295-316.
Leroy, S.A.G., Dupont, L.M., 1997. Marine palynology of the ODP Site 658 (N-W Africa) and its contribution to the stratigraphy of Late Pliocene. Geobios 30: p. 351-359.

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