Sahara Pump Theory

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The Sahara Pump Theory is one which is used to explain the various phases by which African flora and African fauna have left that continent to penetrate the Middle East and possibly, thereafter, the rest of the world. African pluvial^[1] periods are associated with a "wet Sahara" phase, in which large lakes and many rivers are found.

An example of the Saharan pump has occurred since the Late Glacial Maximum (LGM). During this period, the Sahara desert was, if anything, more extensive than it is now, as tropical forests were of reduced extent. During this period, the cooler temperatures reduced the strength of the Hadley Cell where rising tropical air of the Inter-Tropical Convergence Zone(ITCZ) brings rain to the tropics, while dry descending air at about 20 degrees north, flows back to the equator, and brings desert conditions to this region. This phase is associated with high rates of wind-blown mineral dust, found in marine cores that come from the north tropical Atlantic. Around 12,500 BCE, the amount of dust in the cores in the Bølling/Allerød phase suddenly plummets shows a period of much wetter conditions in the Sahara, indicating a Dansgaard/Oeschger (DO) event (a sudden warming followed by a slower cooling of the climate). The moister Saharan conditions had commenced about 12.5 BCE, with the extension of the Inter-Tropical Convergence Zone (ITCZ) northward in the northern hemisphere summer, bringing moist wet conditions and a savannah climate to the Sahara, which apart from a short dry spell associated with the Younger Dryas, peaked during the Atlantic optimum climatic phase, at 4,000 BCE when mid-latitude temperatures seem to have been between 2-3 degrees warmer than in the recent past. Analysis of Nile River deposited sediments in the delta also shows this period had a higher proportion of sediments coming from the Blue Nile, suggesting higher rainfall also in the Ethiopian Highlands. This was caused principally by a stronger monsoonal circulation throughout the sub-tropical regions, effecting India, Arabia and the Sahara. The sudden subsequent movement of the ITCZ southwards with a Heinrich (H) event (a sudden cooling followed by a slower warming), linked to changes with the ENSO El Nino cycle, led to a rapid drying out of the Saharan and Arabian regions, which quickly became desert. This is linked to a marked decline in the scale of the Nile floods between 2,700 and 2,100 BCE. ^[2]

Fauna common in the Sahara during the wet phase, found at Tassili in the central Sahara

During these periods of the wet Sahara, the Sahara and Arabia become a Savannah grassland and African flora and fauna become common. During the following inter-pluvial dry arid period, the Sahara reverts to desert conditions usually as a result of the retreat of the West African Monsoon southwards. Evaporation exceeds precipitation, level of water in lakes like Lake Chad falls, and rivers become dry wadis. Flora and fauna previously widespread retreats northwards to the Atlas Mountains, southwards into West Africa, or eastwards into the Nile Valley and thence either south east to the Ethiopian Highlands and Kenya or northwestwards acros the Sinai into Asia. This separates populations of some of the species in areas with different climates, forcing them to adapt, possibly giving rise to allopatric speciation.

The Saharan pump has been used to date four waves of human emigration ^[3] from Africa, namely:

Homo erectus (ssp. ergaster) into Southeast and East Asia
Homo heidelbergensis into the Middle East and Western Europe
Homo sapiens sapiens "Out of Africa theory"
The spread of Semitic languages.

[edit] References

^ Van Zinderen Barker E. M. (1962-04-14). "A Late-Glacial and Post-Glacial Climatic Correlation between East Africa and Europe". Nature 194: 201 - 203. DOI:10.1038/194201a0.
^ Burroughs, William J. (2007) "Climate Change in Prehistory: the end of the reign of chaos" (Cambridge University Press)
^ Stokes Stephen, "Chronology, Adaptation and Environment of the Middle Palaeolithic in Northern Africa" http://www.human-evol.cam.ac.uk/Projects/nafrefched/efched.htm

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