Rapid environmental changes during Upper Holocene in the Northern margins of the Sahara desert
by
Benazzouz, Mohamed T.
University Mentouri Constantine, Algeria

The existence of a transatlasic aeolian system through the eastern Saharan Atlas indicates massive migrations of mobile sands from the High Plains in the North, to the Sahara desert to the South and confirms that in Algeria, there is not a progression of desertification from the Sahara toward the North. Fig. 1: The transatlasic aeolian system: dunes on relief, northern slope of the Atlas mountains in the Zibans Range The age of these sand displacements is ancient and dates back to the upper Pleistocene and the Holocene. A new chronology of upper Holocene, supported by 12 new 14C dates indicates a succession of arid periods and humid periods. This new chronology characterises an evolutionary stage since 4 000 years BP marked by the rapidity of palaeoenvironments instabilities during the upper Holocene period through the Zibans Range. The originality of landscapes of the Algerian eastern Saharan Atlas takes place here in the presence of dune fields within the chain, on reliefs and on all sides of the Saharan Atlas. The search of the origin and the age of these dunes and of their progression, will give us a better lighting for the understanding of mechanisms of the desertification that affects the Hodna basin and the Zibans Range in north Algeria. The discovery of a transatlasic aeolian system through the eastern Saharan Atlas shows massive migrations of sands (Fig. 2) from the High Plains in the North, to the Sahara in the South and confirms that there is no progress of desertification in Algeria from the Sahara toward the North (Benazzouz, 2000). The reconstitution of an evolutionary stage after 4000 years BP confirms the speed of palaeoenvironment instabilities through the eastern Algerian Saharan Atlas during upper Holocene. KEY-WORDS: environmental changes,, desertification, aeolian morphogenesis, upper Holocene, northern Sahara. Fig. 2: Sand falls as falling dunes, covering jebel Kenfoud at the West of Bou Saada showing the movement of sand from the Zahrez aeolian system to the Hodna aeolian system. 1 - A new chronology of aeolian morphogenesis through the Zibans Range and the Hodna basin during the upper Holocene A first group of five dates situated chronologically the morphogenic events in the period between 3 300 years BP to 2 260 years BP, whereas the different formulations of morphoclimatic evolution worked out by Ballais (1995) for the Zibans, do not include dates corresponding to this interval of the prehistoric upper Holocene. This period is unknown because of the absence of aeolian forms or deposits in this area but also the absence of published dates. 1-1- The evidence of a humid phase occurred at 3 300 - 3 000 yr BP It is about the stake in evidence of a humid phase that occurred at c. 3 300 - 3 000 yr BP through the Hodna and the Zibans Range. In two different sites, one could have dated two palaeosols, the first is formed at 3 100 yr BP in the sand ramp of the jebel Saifoun (Fig. 3) at the west part of the Zibans and the second, located at the East is occurred at 3 300 yr BP in the terrace of oued Selga Seghir, situated at the southern limit of the plain of El Outaya. In fact, the beginning of the aridification is well admitted toward 4 500 - 4 000 yr BP, and the last date (4 000 yr BP) generally situates in time the extension of aridity in North Africa (Bolle, 1996), but with some contradictions. In the Zibans, it was a favourable period to pedogenesis attested by the palaeosol in the sand ramp of jebel Saifoun which occurred at 3 100 ± 100 years BP and of the accumulation of the terrace of the oued Selga-Seghir at 3 300 ± 70 years BP. However this humid phase which we just detected in the Zibans and dated at 3 300 - 3 000 yr BP is important, because it confirms our hypotheses (Ballais and Benazzouz, 1994, p. 68) concerning a strongly fluviatile morphogenesis phase affecting alluvium depositions of the main lower terrace in the valley of oued Chéria-Mézeraa (Némemcha). Fig.3: Stratigraphic log of jebel Saifoun sand ramp 1-2- A dry phase toward 2 900 - 2 500 yr BP A second group of three radiocarbon dates on the fossil windblown sands produces an identical age: 2 840 ± 40 years BP, in fossil erg of jebel Sahbana, dated by radiocarbon on fragments of ostrich eggs 2 790 ± 95 years BP: in fossil sand ramp of jebel Fozna, dated by radiocarbon on land gasteropoda (Helix). 2 660 ± 45 years BP: in fossil erg of Fennd Baroud, dated by radiocarbon on fragments of ostrich eggs. The interest of these direct chronological criteria reveal first the possibility of identification of the progressive drying of the climate at the prehistoric upper Holocene occurred at 2 900 - 2 500 yr BP, then, and confirm here, the reconstitution of the Holocene morphoclimatic evolution of the eastern Maghreb (Ballais, 1995, p. 332) with proposing some complements. This dry phase is dominated by deflation but especially by the mobilization of windblown sands that announces the aridification of the climate. 1-3- The originality of the humid phase at 2 400 - 2 200 yr BP in the Zibans Range The existence of a humid climatic phase is characterised by: the deposit of sands at Helicella streamed between dunes of Grand Erg Oriental at Ksar Rhilane Tunisia (Petit Maire et al. 1991) by the accumulation of gyttja in the chott Rharsa in Tunisia (Ballais, 1992), by the fine alluviums along the Némemchas Range (Ballais and Benazzouz, 1994). The field geoindicators of this phase in the Zibans, have been localized and dated in the site of Foum Zgag at the East of M'Doukal (Benazzouz, 2000), in the upper part of the sand ramp that is gullied strongly and covered by a glacis with pebbles and gravels. This spreading glacis is well developed through the Zibans and the Hodna and marks the interruption of the previous arid period. While, the end of this humid period seems more precocious here, with the deposition of reddish sand occurred at 2290 ± 60 years BP (Gif. 9 871) which trenched marshy formations of oued el Ogla and could indicate a drier environment, premises of a period dominated by incision downstream and developed by 2200 years BP to about 1650 years BP, until II-III century AD, where no deposit has been recognized (Ballais 1995, p. 335). Dating results, indicate here 2450 ± 40 years BP (Gif. 9 474) and corresponds to the minimum age of this humid phase identified in the Zibans and the Hodna (Benazzouz, 2000) and belongs to the historical late Holocene characterised by the deposition of the lower low terrace body and its dissection as well as by desertification which particularly process at the XXth century by the remobilisation of aeolian sands. 1-4- The arid period occurred at 2 200 – 1 650 yr BP An arid period was identified between 2200 and 1650 14C yr BP and was dominated by incision of rivers where human’s role increased. The present day remobilisation of sands does not justify an aridification of the climate. 2 - Rapid environmental changes and morphogenic successions The Holocene period and in particular, the upper Holocene is characterized by the speed of the palaeoenvironments instability. The research of tendency from some rare palaeoenvironments criteria would be very useful to a better understanding of the present evolution in these regions of the Hodna and the Zibans. So, our demonstration takes in account, in the first place, the present period that is submitted to a phase of incision downstream. Then, we search in the Holocene chronological succession for similar Holocene morphological phenomenon and try to characterize the type of evolution that they probably followed. We have put currently in evidence a morphogenic succession similar to the succession in progress, and we can situated it at c. 3 500 - 2 500 yr BP, which succession was applied to the context of the site of the Foum ez Zgag sand ramp (Fig. 4) on the upper sequence of the sketch. Fig. 4: Representative sand ramp on northern slope of Saharan Atlas: Foum ez Zgag The analysis of this phase shows that this one starts by the stabilization of sands of the sand ramp and their fixing with the development of a glacis ; followed by an episode of trenching the formation of glacis and the sequence finished by an intense deflation with accumulation of windblown sands. Those morphogenic actions constitute a morphogenic succession that appears in the beginning by an increase of the humidity as intense and concentrated rains and finished in a dry ambiance of aridification of the climate. If one considers the present period, it seems to start by the accumulation of the historic terrace: 1350 years BP - 1470 years BP or 610 years BP (Ballais 1992) followed by the cutting of this historic terrace and since some decades, the present remobilisation of sands with a strong deflation. We have just proven that the present period writes down itself in a morphogenic succession that had taken place effectively by the past no far, we are able therefore to consider conditions of evolution of the present morphogenesis provided that to integrate consequences of the anthropogenic geosystems. The interest of morphogenic succession, stake in evidence in the Zibans confirms: The present remobilisation of windblown sands occurs on a normal morphogenic succession; The present remobilisation of sands does not take place in an aridification of the climate; The process of desertification appears recently in the XXth century. In a general way, one considers in North Africa, that until the Roman colonization, the environment evolves on natural climatic conditions. Since, the anthropogenic actions became more and more prevailing with the fluctuations of the soil occupation during the historic times (Benazzouz, 1996) that could be a decisive factor in the understanding of the development of the desertification Ballais J.L., 1992. Morphogénèse et géosystèmes Holocènes dans l'Atlas Saharien oriental et Bas-Sahara (Algérie, Tunisie), Wurzb. Geogr. Arb., Wursburg, 84, 1-16. Ballais J.L., 1995. Conquêtes et dégradation du milieu au Maghreb oriental pendant l’Antiquité et le Moyen Age, in L’Homme et la dégradation de l’Environnement, XV rencontres internationales d’Archéologie et d’Histoire d’Antibes, Editions APDCA, Juan les Pins, 1995. Ballais J.L. and Benazzouz M.T. 1987. Sédimentation quaternaire lacustre et éolienne dans le Bas-Sahara Algérien . 8th Regional Meeting of Sedimentology: 54-55, Tunis . Ballais J.L. and Benazzouz M.T. 1994. Données nouvelles sur la morphogénèse et les paléoenvironnements tardiglaciaires et holocènes dans la vallée de l'oued Chéria-Mezeraa (Némemcha, Algérie orientale), Rev.Méditerranée,t.80,n.3-4. 59-71. Benazzouz M.T., 1996. The causes of desertification in the northern Algerian Sahara. Geomorphic hazards, edited by O. Slymaker. John Wiley,183-195. Benazzouz M.T., 2000. Morphogénèse éolienne Holocène et actuelle dans l’Atlas Saharien oriental (Algérie): conséquences sur la désertification. Thèse Doctorat d’Etat, Université Mentouri Constantine, 350 p. Boye M., Marmier F., Nesson C. and Trecolle G., 1978. Les dépôts de la sebkha Mellala (Ouargla, Sahara algérien) . Rev. géom. dyn. 28, 2-3. 49-62. Estorges P., Aumassip G. and Dagorne A., 1969. El Haouita, un exemple de remblaiement fini-wurmien, Libyca, XVII, 53-92, Alger. Petit-Maire N., Burollet F., Ballais J.L., Fontugne M., Rosso J.C. and Lazaara A., 1991. Paléoclimats holocènes du sahara septentrional. Dépôts lacustres et terrasses alluviales en bordure du Grand Erg Oriental à l’extrême-Sud de la Tunisie. C.R.Acad. Sc. Paris, t. 312, Série II, 1661-1666. Ritchie J.C., 1984. Analyse pollinique de sédiments holocènes supérieurs des hauts plateaux du Maghreb oriental. Pollen et Spores, vol. XXVI, n° 3-4, 489-496.

Date received: November 26, 2003

Copyright © 2003 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 # camu-11.