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Geomorphic archives of Late Quaternary climatic fluctuations at both borders of the Western Sahara

André Weisrock
p. 139-141

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1Although a great number of excellent studies has already provided the best dated and most comprehensive evidence of late Quaternary environmental fluctuations in the tropical deserts, this volume presents five new local or regional geomorphic investigations in the Western Sahara and at its borders. How can we improve our knowledge of the continental chronology, of the duration and the origin of the Late Quaternary alternating dry and humid phases? This exciting research topic requires developing multidisciplinary studies, which are now in progress at both the northern and southern borders of the Western Sahara.

2As a first example, the present increase in dryness is shown in Mauritania by M. Mainguet et al. New landforms, as seif and transverse dunes that occur today, stem from sand depletion at the expenses of the main old fixed ergs. More sand storms are observed at Nouakchott, with a large increase in sand burial in the deposition areas.

3The travertine deposits of the northern border of Moroccan Sahara are used by A. Weisrock et al. for highlighting the wet phases of the Middle and Late Quaternary. The travertine formation episodes are identified by U-series and TIMS dating. In four sites of the southern High Atlas and Anti-Atlas piedmonts, the phases of travertine construction, which are thought to be “pluvial” periods, appeared during MIS 11, 9, 3 and 2. A large discontinuity in the results, between MIS 9-8 and MIS 3, corresponds to strong incision in the High Atlas, and are more detailed since MIS 5 at Wadi Noun, 29° N. The best correlations in the morphological evolution of the travertine constructions are reached at MIS 3, with evidence for abundant and regular flows. Each studied site presents however hydrological or morphological particularities, which add local effects to the climatic significance. Due to analytical errors, the duration of the building phases are not well known for the Middle Quaternary. For the Late Quaternary, the long-term 50 to 30 ka BP and the more precise 16 – 15 ka BP periods are the most representative, likely linked to the cooling of the subtropical eastern Atlantic. Most of the dating results are not yet precise enough to establish sound correlations at the scale of 1,000 years with abrupt climatic changes such as the Heinrich events.

4At the Southern border of the Sahara, in the Dogon district, Mali, L. Lespez et al. use the alluvial records of the Yame valley as responses to environmental changes in West Africa between 50 and 4 ka BP. In addition to archeological investigations, the research is supported by forty-one OSL and eighty-eight 14C datings, micromorphology of soils, palynology and anthracology. This multidisciplinary study allows precise results in fluvial stratigraphy, fluvial style and rate of alluviation. During the 50-30 ka BP-old period, the sediment was deposited in a large floodplain by high suspended-flood flows; a palaeochannel with gravelly sands shows a chronostratigraphic coincidence with the Heinrich 4 event at 40 ka BP. After a dry period favouring aeolian processes, a major hydrologic change is dated at 15 ka and corresponds to the first reactivation of the African monsoon north of the 10° N. The Early Holocene is characterized by detritic alluvial formations of a braided river system, with interbedded layers of greyish silt, which correspond to permanent but small lakes. Both indicate a northward shift of the summer monsoon after 11.5 ka BP. The Mid-Holocene shows a hiatus between 8.8 and 7.4 ka BP, due probably to drought. From 7.4 to 4 ka BP, three distinct sedimentary sequences show gravel, sand and silt beds with organic matter, and can be related to the African Humid Period.

5As already pointed out by P. Rognon and M. Williams (1977), both margins of the Western Sahara were wetter than now around 40 ka and this situation occurred again at ca. 15 ka. Whatever the ultimate causes for weak Saharan anticyclones might be at these periods, the geomorphic evidence for climatic phases are clear. On the contrary, the Holocene trends are not yet comparable at the two margins.

6Opposite to the Yame valley case study, the deposits of the Holocene lakes of the Great Western Erg of Algeria, described by Y. Callot and M. Fontugne, indicate a rainfall maximum between 9 and 7 ka cal BP in the central part of the Erg. At the borders of the Erg, lakes have subsisted till 5 ka cal BP, with an external alimentation. These observations can be best correlated with those of A. Bkhairi and M.-R. Karray regarding the Holocene terraces of Kasserine (Central Tunisia), that indicate two main climatic phases of sedimentation during the Capsian interval, between 9 and 6 ka BP, and during the Neolithic Optimum, before ca. 4.5 ka BP. The authors pointed out the emerging anthropogenic influence, illustrated by the formation of the Roman terrace between 2 and 1,4 ka BP. It is clear that only the Neolithic period can be regarded as a new wet period at both margins and in the central part of the desert during the Holocene.

7This special issue has been prepared thanks to contributions given at the André Weisrock Jubilee in Nancy University, June 2007, organised by S. Occhietti and D. Harmand. In addition, we are greatly indebted to M. Thorp (University of Dublin) and Y. Battiau-Queney, who have entirely corrected the English, to the reviewers and to J.-C. Thouret and J. Raffy for their helpful remarks.

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Bibliographie

Rognon P., Williams M.A.J. (1977) – Late Quaternary climatic changes in Australia and North Africa: a preliminary interpretation. Palaeogeography, Palaeoclimatology, Palaeoecology, 21, 285-327.

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Annexe

Fig. 1 - Upper pleistocene travertine dam in a now arid landscape, at Imi Ou Assif, morocco, 29°45'N.

Fig. 1 - Upper pleistocene travertine dam in a now arid landscape, at Imi Ou Assif, morocco, 29°45'N.

Photo. A. Weisrock, 2007, May.

Fig. 2 - 15,4 +/- 0,3 ka BP laminated travertine at Imouzzer Ida Ou Tanane waterfall, Morocco,. 30°45'N.

Fig. 2 - 15,4 +/- 0,3 ka BP laminated travertine at Imouzzer Ida Ou Tanane waterfall, Morocco,. 30°45'N.

Photos. C. Falguères, 2007, May.

  

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Table des illustrations

Titre Fig. 1 - Upper pleistocene travertine dam in a now arid landscape, at Imi Ou Assif, morocco, 29°45'N.
Crédits Photo. A. Weisrock, 2007, May.
URL http://geomorphologie.revues.org/docannexe/image/6633/img-1.jpg
Fichier image/jpeg, 224k
Titre Fig. 2 - 15,4 +/- 0,3 ka BP laminated travertine at Imouzzer Ida Ou Tanane waterfall, Morocco,. 30°45'N.
Légende Photos. C. Falguères, 2007, May.
URL http://geomorphologie.revues.org/docannexe/image/6633/img-2.jpg
Fichier image/jpeg, 190k
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Référence papier

André Weisrock, « Geomorphic archives of Late Quaternary climatic fluctuations at both borders of the Western Sahara », Géomorphologie : relief, processus, environnement, vol. 14 - n° 3 | 2008, 139-141.

Référence électronique

André Weisrock, « Geomorphic archives of Late Quaternary climatic fluctuations at both borders of the Western Sahara », Géomorphologie : relief, processus, environnement [En ligne], vol. 14 - n° 3 | 2008, mis en ligne le 08 décembre 2008, consulté le 21 octobre 2017. URL : http://geomorphologie.revues.org/6633 ; DOI : 10.4000/geomorphologie.6633

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Auteur

André Weisrock

andre.weisrock@wanadoo.fr

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