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Paleoanthropology

Old desert, new knowledge

Until now, it was thought that the oldest sand dunes in the Sahara were only 86,000 years old. A team of Franco-Chadian scientists recently uncovered formations of fossil dunes proving the Sahara to have known its first desert-like conditions more than 7 million years ago.(1)

détail roche p 15

© P. Duringer-MPFT/CNRS-ULPS

Detail from a fossil dune. For sedimentologists, the aspect of the sand and quartz grains, the presence of wind wrinkles and oblique sediment deposits are all characteristics of a desert formation.


 

Since their discovery of Abel, the first Australopithecine west of the Rift Valley in 1995, and their 2001 discovery of Toumaï, the oldest known hominid, researchers from the Franco-Chadian Paleoanthropological Mission (MPFT) led by Michel Brunet,2 have been focusing their attention on the Toros-Menalla fossiliferous area of the Djurab desert in northern Chad, a region bearing abundant vertebrate remains.

 

Understanding the emergence of ancient hominids requires an accurate knowledge of their living environments. Working alongside paleontologists, geologists have been trying to retrace these different environments through the study of sedimentary and biosedimentary deposits including vertebrate but also invertebrate fauna. Through the analysis of fauna and deposits, scientists are thus able to determine if sandstone has been deposited by water (a river), by wind (dunes), or in a lake. Ancient deposits are then compared with the present environment, using the concept of actualism, meaning that the same physical laws governed in the past as they do at present. Each environment has a particular geo-biological signature: lithology (grain size, texture and color of the mineral content), sedimentary structures (the three-dimensional physical features of sedimentary rock), geometry of the deposits and paleontological content. In Northern Chad, the geologists discovered sandstone outcrops that were composed of fine poorly cemented white sands, made up of quartz grains that were well sorted, well rounded, matt, and frosted. They also showed typical alternations of grain-fall (gravity-driven depositions of individual particles) and grain-flow (collective movement of solid particles) laminations. This texture and structure enabled the scientists to identify the Toros-Menalla area as migrating fossil eolian dune formations. By comparing the evolutionary degree of the mammalian fauna from the Chadian outcrops with that of southern, northern, and eastern African sites, they were able to date them back to 7 million years ago.

 

With the help of prehistoric rock carvings showing giraffes, elephants, and crocodiles in regions that are dry today, as well as through geological and sedimentary findings, we now know that the Chad Basin has been under successive humid (mega lake Chad) and arid conditions since the late Miocene.

In the middle of the Holocene period, 5000 years ago, the mega lake Chad occupied a surface of 400,000 km2, as opposed to the 5000 km2 it accounts for today. The fact that the present drought is part of a long-standing cycle indicates that climate change has always been a part of the Earth's dynamic. “Even if some climate change is imputable to man, it is only a variation among the millions of variations the earth has known”, explains Philippe Duringer, from the Surface Geochemical Center in Strasbourg.3 “Every sedimentary deposit, however old, shows evidence of these climate variations. In the Sahara, it's even more obvious because the piles of sediment show an alternation between so-called green periods (humid climate) and yellow periods (arid to desert-like climate).” Research suggests the Sahara has experienced humid/dry alternations for at least ten, maybe fifteen, or even twenty million years.

The leap from 86,000 years to 7 million years is considerable, effectively a revolution in the understanding of how climate change shaped the Sahara desert, but unfortunately there are very little archives in situ. Most of the Sahara is suffering from erosion and many deposits have already vanished because although there are dunes, there is very little accumulation: Over time, the whole of northern Chad has hollowed out by 100 meters, simply from the wind's removal of fine dust and sand, known as eolian deflation. Scientists working on the Franco-Chadian project are confident that more discoveries are yet to be made, from older to more recent evidence of climatic changes.

 

Marion Girault-Rime 

Notes :

1. Schuster M, et al., “The Age of the Sahara Desert,” Science. 311 (5762): 821. 2006.
2. From the Geobiology, Biochronology and Human Paleontology laboratory (Laboratoire de Géobiologie, biochronologie et paléontologie humaine, CNRS / Université de Poitiers joint lab).
3. Centre de Géochimie de la surface (CNRS / Université de Strasbourg joint lab).

Contacts :

> Mathieu Schuster
Laboratoire Domaines océaniques, Plouzané.
schuster@univ-brest.fr

> Michel Brunet & Patrick Vignaud
Laboratoire Géobiologie, biochronologie et paléontologie humaine, Poitiers.
michel.brunet@univ-poitiers.fr
patrick.vignaud@univ-poitiers.fr

> Philippe Duringer
Centre de Géochimie de la surface, Strasbourg.
duringer@illite.u-strasbg.fr


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