Aridity and humidity: two aspects of the late Miocene climate of North Africa and the Mediterranean

Abstract

The physiography of North Africa is not greatly changed from that of the Messinian. With the drawdown of the Mediterranean in the late Messinian the then existing river systems were incised into the landscape and emptied into Lake Cyrenaica, which occupied the substantially drained Eastern Mediterranean Basin. The record of incision provides a record of the magnitude of the Messinian river systems. An analysis of these river systems demonstrates that Messinian rainfall, a consequence of the Zeit Wet Phase, was greatest in east and south-central North Africa, in keeping with the derivation of the water mainly from the Indian Ocean. In central North Africa the Eosahabi River flowed from Messinian Lake Chad eroding the East Tibesti Valley and cutting a channel which is especially well preserved near the coast of the Gulf of Sirt. The penetrated sediments of Lake Cyrenaica and the marginal basins of the Messinian transgression are known as the Upper Evaporites and are generally fine clastics, gypsarenites and gypsum/anhydrite. They were rapidly deposited. Underlying them is the halite of the Lower Evaporites. This sequence of Upper and Lower Evaporites is much like the lower halite of the Tortonian South Gharib Formation and the overlying clastics and anhydrite of the Messinian Zeit Formation in the Gulf of Suez/Red Sea area. This type of sequence is termed an evaporitic couplet and, as has been demonstrated for the Gulf of Suez/Red Sea, is a consequence of a transition from a dry to a more humid climate in a restricted or semirestricted basin. This transition occurred at about 7.5 Ma in the Gulf of Suez/Red Sea and at about 5.8 Ma in the Mediterranean. The Zeit Wet Phase manifest in the above events developed in association with the initiation and development of the Asian monsoon and the drying of the Mediterranean. It can be understood in terms of the development of an evolving monsoon/desert system. The Asian monsoon was initiated 8–7 Ma in association with the uplift of Tibet; at this time the North African desert zone was displaced northwards to be over the Mediterranean and central and eastern North Africa became seasonally humid. A concept is advanced in which the early stage of the development of the Asian monsoon is seen as having two phases alternating at the precessional ca 21 ka cycle. One phase transferred moisture from the Indian Ocean mainly to southern Asia, the other phase transferred moisture mainly to North Africa. With the drawdown of the Mediterranean at about 5.8 Ma the Zeit Wet Phase intensified. A further northward movement of the humid and desert zones occurred at the time of drawdown. With the Messinian and early Pliocene transgressions the wet phase ameliorated but a substantial river system still crossed central North Africa. At about 4.6 Ma North Africa became drier probably in response to the developing dominance of the features of the monsoon that transferred moisture mainly to southern Asia, features of the monsoon that are well recognised today.