Abrupt onset and termination of the African Humid Period: rapid climate responses to gradual insolation forcing

A detailed (ca. 100 yr resolution) and well-dated (18 AMS C-14 dates to 23 cal. ka BP) record of latest Pleistocene-Holocene variations in terrigenous (eolian) sediment deposition at ODP Site 658C off Cap Blanc, Mauritania do cuments very abrupt, large-scale changes in subtropical North African clima te. The terrigenous record exhibits a well-defined period of low influx bet ween 14.8 and 5.5 cal. ka BP associated with the African Humid Period, when the Sahara was nearly completely vegetated and supported numerous perennia l lakes; an arid interval corresponding to the Younger Dryas Chronozone pun ctuates this humid period. The African Humid Period has been attributed to a strengthening of the African monsoon due to gradual orbital increases in summer season insolation. However, the onset and termination of this humid period were very abrupt, occurring within decades to centuries. Both transi tions occurred when summer season insolation crossed a nearly identical thr eshold value, which was 4.2% greater than present. These abrupt climate res ponses to gradual insolation forcing require strongly non-linear feedback p rocesses, and current coupled climate model studies invoke vegetation and o cean temperature feedbacks as candidate mechanisms for the non-linear clima te sensitivity. The African monsoon climate system is thus a low-latitude c orollary to the bi-stable behavior of high-latitude deep ocean thermohaline circulation, which is similarly capable of rapid and large-amplitude clima te transitions. (C) 1999 Elsevier Science Ltd. All rights reserved.