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The Age of the Sahara Desert

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Science  10 Feb 2006:
Vol. 311, Issue 5762, pp. 821
DOI: 10.1126/science.1120161

Abstract

In the Sahara region, the age of onset of the desert condition has been uncertain until now. Here we report on the discovery of 7,000,000-year-old eolian dune deposits from the northern Chad Basin. This geological archive is the oldest known evidence for desert occurrence in the Sahara.

After the mid-Holocene humid period (∼6000 years ago), arid conditions developed throughout North Africa, culminating in the formation of the Sahara, which is the largest warm-climate desert on Earth (∼9,000,000 km2). However, earlier desert recurrences in the region are also documented. Direct evidence for eolian deposition is given by thermoluminescence dating for the Late Pleistocene; e.g., in Mauritania [25 to 15 thousand years ago (ka)] (1) or in Tunisia (86 ka) (2). The latter is currently considered as the oldest terrestrial record for desert conditions in the Sahara (2), even if firm evidence exists for a pre-Quaternary Great Western Sand Sea in Algeria (3). Some earlier arid episodes (Miocene-Pliocene) were also suggested by marine records off West Africa (4); but until now, no contemporary in situ eolian deposits were known in the Sahara region. In the northern Chad Basin, we recently identified and dated widespread outcrops of eolian dune deposits that are distributed over an area more than 2000 km2. Our results testify that the onset of recurrent desert conditions in the Sahara started at least 7 million years ago (57).

In interdune areas of the Djurab sand sea (Northern Chad, 16° to 16°20′N, 17° to 19°E) (Fig. 1A), pre-Quaternary successions show sandstone horizons alternating with lacustrine mudstones and diatomites. In the Central Djurab (16°15′N, 17°30′E), sandstone outcrops made up of poorly cemented quartz arenites are characterized by very well rounded sand grains with frosted surfaces. Large-scale thinning-up (2 to 0.2 m thick) successions of highly dipping (angle of 30° ± 2°) cross-strata are ubiquitous (Fig. 1B). In horizontal surfaces, present-day deflation highlights well-developed festoons that are up to 20 m wide. Cross-strata comprise alternating coarse-grained and fine-grained laminae, displaying a well-sorted bimodal grain size distribution. Texture and sedimentary structures indicate that these sandstones were deposited by migrating eolian sand dunes with typical grain flow and grain fall laminae preserved in foresets. Consistent dips in cross-strata reflect dominant paleowind orientation from the east-northeast to the west-southwest (Fig. 1C). Wind ripples, with similarly oriented axis, are occasionally preserved at the base of some foresets, recording secondary airflow along the dune lee faces. In the Toros Menalla region, these eolian sandstones are conformably overlain by a horizon bearing abundant vertebrates fossils, including Sahelanthropus tchadensis, the earliest known Hominid (5, 7). In this horizon, named the Anthracotheriid Unit, biostratigraphic correlation of the mammalian fauna indicates an age of 7 Ma (57).

Fig. 1.

(A) Location of the study area (red dot). (B) One example of the eolian sandstones in the Djurab area (16°13′42″N, 17°32′23″E) showing three cross-stratified sets (white lines) conformably overlain by the Upper Miocene fossils-rich perilacustrine sandstones (contact: red dotted line). The hammer is shown for scale (45 cm). (C) Paleowinds as shown in a rose diagram including 39 measured dune foresets [same locality as in (B)]. The black arrow indicates the mean resultant direction (234°N, 95% confidence interval = ±16°).

Additionally, eolian horizons are also recorded in the eastern Djurab (16°19′N, 18°41′E). Here, 0.5-m-high and 2-m-long basal dune foresets, dipping toward the west-southwest, are well developed. As for the previous locations, mammalian faunas within a conformably overlying sandstone indicate an age of 5 to 5.5 Ma (8). Moreover, for slightly younger layers (4 to 4.5 Ma), episodic arid conditions are also testified in nearby areas (16°20′N, 18°58′E) by eolian deposits and by fossil termite nests attributed to arid-environment–limited species (9, 10).

The northern Chad Basin eolian deposits provide the earliest (7 Ma, Upper Miocene) in situ record for arid climate and eolian sand accumulation in the Sahara. East-northeast–west-southwest winds (trade winds) prevailed in this region then, as they do today. This geological record extends considerably the demonstrated age for the onset of desert conditions in the Sahara. Repeated eolian-lacustrine sequences are strong evidence for arid-humid climate changes, suggesting that desert conditions existed repetitively rather than continuously, as in the Quaternary. Maley (11) showed that the length of a similar sequence in the Late Quaternary of Chad Basin was ∼20 thousand years and could so be associated with Milankovitch cycles. In addition, this terrestrial record validates distal marine proxies of pre-Quaternary Saharan dry episodes. In the future, paleoclimate models should integrate the age of strong arid recurrences over the Sahara at the scale of several millions of years. The creation of an eolian rock record is considered to be a very selective process (12), as confirmed by the rarity of preserved eolian deposits in North Africa. Chad Basin appears, therefore, as a unique place in the Sahara where, even if truncated, ancient eolian deposits are preserved, supplying thus the current lack of terrestrial records for the Miocene-Pliocene arid climate episodes in the Sahara.

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