A palaeolimnological N-S transect through the Eastern Sahara
by
Philipp Hoelzmann
Max-Planck-Institute for Biogeochemistry, Jena, Germany

Palaeolimnological data indicate that the Sahara was considerably wetter than today and extensively vegetated during the early to mid-Holocene between ~9500 and ~5000 years B.P. Between 10° and 30°N northern Africa experienced a more than tenfold expansion of observed surface water when 8.3% of the area were occupied by lakes (3.5%) and wetland (4.8%) compared to only 0.5% for today. The highly continental position of the eastern Sahara provides an ideal area to investigate hydrological changes as robust phenomena. In the eastern Sahara groundwater levels started to rise about 9300 years B.P. and within a few decades the aquifers were loaded so that the palaeo-piezometric surface was as much as 25 m higher than it is today. A mosaic of freshwater lakes and swamps developed in the lowlands and the uplands generated up to 800 km long fluvial systems which superceded the endorheic drainage of the eastern Sahara. These palaeoenvironments functioned as migration paths for large mammals and permitted human occupation of large areas of the Sahara. Different stages of palaeolake evolution, ranging from non-existence of the lake through stable freshwater conditions and back to non-existence, were identified. These coincide with three different main phases of intensive neolithic settlement between c. 8000 to 4000 a BP which exhibit several distinguishable pottery phases. The dimensions of the palaeolakes were used to estimate maximum Holocene annual precipitation values. Sedimentological and geochemical investigation of other observed palaeo-lakes and swamps of Western Nubia and the eastern Sahara prove a monsoon-controlled climatic regime (south of 22°N) with a self-stabilizing regional precipitation regime that was dependend on water vapour from numerous lakes, swamps, river plains and a dense vegetation cover. Assessments of lake level or relative water depth (lake status) through time (15 ka B.P. to present) based on a consensus interpretation of the available physical, chemical and biological data from cores or exposed sections from the Eastern Sahara show that all lowland sites demonstrate a nearly synchronous and abrupt onset of the Holocene wet phase. However, sites related to mountains seem to register an earlier onset of more humid conditions due to an orographic effect which accelerated infilling of the aquifers. During the Holocene a gradient of increasing evaporation impact and/or decreasing rainfall in a northerly direction was established: while lacustrine sediments were formed in the South (<22°N) playa sediments were deposited between 10 to 22°N. In contrast to the rapid onset a more time-transgressive fading of wetter conditions, which started earlier in the North, is observed within the palaeolake records.

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Date received: March 26, 2001

Copyright © 2001 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # cagc-32.