Greening the Sahara: stalagmites, isotopes and mankind’s route out of Africa

Author: Mike Rogerson.

Editor: Derek Gobbett.

Dr Rogerson presented a multidisciplinary study of the Recent History of the Sahara. The climatic changes of the Late Pleistocene and Recent are likely to be reflected in North Africa by wetter periods during European cold stages and by desertification during warm stages.  Rock drawings in the Sahara depicting animals such as Giraffe, although undated, indicate a wetter climate. Recent sediments on the floor of the Mediterranean show variation in the amounts of organic content and wind blown Saharan sand. 

In caves in the Eocene limestones of Cyrenaica, the growth of stalagmites, dated at 65 to 30 Ka by U-series dating, is not uniform but shows three growth phases corresponding to wetter periods which immediately predate more organic-rich sediments in the Mediterranean.  Oxygen isotope studies on fluid inclusions in the stalagmites show that the rain came mainly from the Atlantic, especially when stalagmite growth was rapid, and was not due to a northward shift of the monsoon, A large mass of tufa dated at 75Ka contained fossil remains of zebra, buffalo, tortoise and plants. The rate of tufa growth indicated a very wet climate 120Ka ago during the Ipswichian Interglacial.  There was no stalagmite growth during this period as the cave systems would have been completely flooded.

He then went on to outline the evidence for river systems flowing to the Mediterranean from the Tibetsi Mountains of southern Libya. Trace amounts of Neodymium from the Tibetsi, roots of large vascular plants, organic rich soils and freshwater snails can be traced across the Sahara. These ancient river systems delineate probable migration routes for animals and man “out of Africa”.