Sahara pump theory

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Carvings of fauna common in the Sahara during the wet phase, found at Tassili in the central Sahara

The Sahara pump theory is a hypothesis that explains how flora and fauna migrated between Eurasia and Africa via a Levantine land bridge. The theory observes that African pluvial periods are associated with a "wet Sahara" phase, during which larger lakes and more rivers exist.[1] This causes changes in the type of animals found in the area. Regardless of the aridity of the greater Sahara, migration along the river corridor was halted when, during a desert phase 1.8-0.8 million years ago (mya), the Nile ceased to flow completely[2] and possibly flowed only temporarily in other periods[3] due to Nubian Swell uplift.

Mechanism

During periods of a wet or "Green Sahara", the Sahara and Arabia become a savanna grassland and African flora and fauna become common. Following inter-pluvial arid periods, the Sahara area then reverts to desert conditions, usually as a result of the retreat of the West African Monsoon southwards. Evaporation exceeds precipitation, the level of water in lakes like Lake Chad falls, and rivers become dry wadis. Flora and fauna previously widespread as a result retreat 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 north-east across 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.

Plio-Pleistocene

The Plio-Pleistocene migrations to Africa included the Caprinae in two waves at 3.2 Ma and 2.7-2.5 Ma; Nyctereutes at 2.5 Ma, and Equus at 2.3 Ma. Hippotragus migrated at 2.6 Ma from Africa to the Siwaliks. Asian bovids moved to Europe and to and from Africa. The primate Theropithecus experienced contraction and its fossils are found only in Europe and Asia, while Homo and Macaca settled wide ranges.[4]

185,000–20,000 years ago

Between about 133 and 122 thousand years ago (kya), the southern parts of the Saharan-Arabian Desert experienced a wet period with increased monsoonal precipitation, around 200–100 mm/year. This allowed Eurasian biota to travel to Africa and vice versa.[5] The growth of speleothems (which requires rainwater) was detected in Hol-Zakh, Ashalim, Even-Sid, Ma'ale-ha-Meyshar, Ktora Cracks, Nagev Tzavoa Cave. In Qafzeh and Es Skuhl caves, where at that time precipitation was 600–1000 mm/year, the remains of Qafzeh-Skhul type anatomically modern humans are dated from this period, but human occupation seems to end in the later arid period.

The Red Sea coastal route was extremely arid before 140 and after 115 ka. Slightly wetter conditions appear at 90–87 kya, but it still was just one tenth the rainfall around 125 kya. Speleothems are detected only in Even-Sid-2.[5]

In the southern Negev Desert speleothems did not grow between 185–140 kya (MIS 6), 110-90 (MIS 5.4-5.2), nor after 85 kya nor during most of the interglacial period (MIS 5.1), the glacial period and Holocene. This suggests that the southern Negev was arid to hyper-arid in these periods.[5]

The coastal route around the western Mediterranean may have been open at times during the last glacial; speleothems grew in Hol-Zakh and in Nagev Tzavoa Caves. Comparison of speleothem formation with calcite horizons suggests that the wet periods were limited to only tens or hundreds of years.[5]

From 60–30 kya there were extremely dry conditions in many parts of Africa.[6]

Last Glacial Maximum

An example of the Saharan pump has occurred after the Last Glacial Maximum (LGM). During the Last Glacial Maximum the Sahara desert was more extensive than it is now with the extent of the tropical forests being greatly reduced.[7] During this period, the lower temperatures reduced the strength of the Hadley Cell whereby 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 BC, the amount of dust in the cores in the Bølling/Allerød phase suddenly plummets and 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 begun about 12,500 BC, with the extension of the ITCZ northward in the northern hemisphere summer, bringing moist wet conditions and a savanna climate to the Sahara, which (apart from a short dry spell associated with the Younger Dryas) peaked during the Holocene thermal maximum climatic phase at 4000 BC when mid-latitude temperatures seem to have been between 2 and 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, affecting India, Arabia and the Sahara.[citation needed] Lake Victoria only recently became the source of the White Nile and dried out almost completely around 15 kya [8]

The sudden subsequent movement of the ITCZ southwards with a Heinrich event (a sudden cooling followed by a slower warming), linked to changes with the El Niño-Southern Oscillation 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 2700 and 2100 BC. [9]

Human migration

The Saharan pump has been used to date four waves of human migration from Africa, namely:[10]

The formation of the modern Sahara, as a result of the 5.9 kiloyear event is also considered to be a part of the same mechanism in operation.

See also

References

  1. ^ Van Zinderen Bakker 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.
  2. ^ http://gsa.confex.com/gsa/2008SC/finalprogram/abstract_136189.htm
  3. ^ Williams, Martin A.J.; Talbot, Michael R. (2009). "Late Quaternary Environments in the Nile Basin". 89: 61. doi:10.1007/978-1-4020-9726-3_4. {{cite journal}}: Cite journal requires |journal= (help)
  4. ^ Hughesm, Jk (2008). "Theropithecus and 'Out of Africa' dispersal in the Plio-Pleistocene". Journal of Human Evolution. 54 (1): 43–77. doi:10.1016/j.jhevol.2007.06.004. ISSN 0047-2484. PMID 17868778. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
  5. ^ a b c d Vaks, Anton; Bar-Matthews, Miryam; Ayalon, Avner; Matthews, Alan; Halicz, Ludwik; Frumkin, Amos (2007). "Desert speleothems reveal climatic window for African exodus of early modern humans" (PDF). Geology. 35: 831. doi:10.1130/G23794A.1. {{cite journal}}: Unknown parameter |unused_data= ignored (help)
  6. ^ Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1073/pnas.0510792103, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1073/pnas.0510792103 instead.
  7. ^ Adams, Jonathan. "Africa during the last 150,000 years". Environmental Sciences Division, ORNL Oak Ridge National Laboratory.
  8. ^ Stager, J. C.; Johnson, T. C. (2008). "The late Pleistocene desiccation of Lake Victoria and the origin of its endemic biota". Hydrobiologia. 596: 5. doi:10.1007/s10750-007-9158-2.
  9. ^ Burroughs, William J. (2007) "Climate Change in Prehistory: the end of the reign of chaos" (Cambridge University Press)
  10. ^ Stephen, Stokes. "Chronology, Adaptation and Environment of the Middle Palaeolithic in Northern Africa". Human Evolution, Cambridge University.
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