5.9 kiloyear event
The 5.9 kiloyear event was one of the most intense aridification events during the Holocene Epoch. It occurred around 3900 BC (5,900 years BP), ending the Neolithic Subpluvial and probably initiated the most recent desiccation of the Sahara desert. Thus, it also triggered worldwide migration to river valleys, such as from central North Africa to the Nile valley, which eventually led to the emergence of the first complex, highly organised, state-level societies in the 4th millennium BCE. It is associated with the last round of the Sahara pump theory.
A model by Claussen et al. (1999) suggested rapid desertification associated with vegetation-atmosphere interactions following a cooling event, Bond event 4. Bond et al. (1997) identified a North Atlantic cooling episode 5,900 years ago from ice-rafted debris, as well as other such now called Bond events that indicate the existence of a quasiperiodic cycle of Atlantic cooling events, which occur approximately every 1,470 years ± 500 years. For some reason, all of the earlier of these arid events (including the 8.2 kiloyear event) were followed by recovery, as attested by the wealth of evidence of humid conditions in the Sahara between 10,000 and 6,000 BP. However, it appears that the 5.9 kiloyear event was followed by a partial recovery at best, with accelerated desiccation in the millennium that followed. For example, Cremaschi (1998) describes evidence of rapid aridification in Tadrart Acacus of southwestern Libya, in the form of increased aeolian erosion, sand incursions and the collapse of the roofs of rock shelters. The 5.9 kiloyear event was also recorded as a cold event in the Erhai Lake (China) sediments.
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