Holocene Palaeoenvironmental History of the Central Sahara: Palaeoecology of Africa Vol. 29, An International Yearbook of Landscape Evolution and Palaeoenvironments ... of Africa and the Surrounding Islands)

HOLOCENE PALAEOENVIRONMENTAL HISTORY OF THE CENTRAL SAHARA Palaeoecology of Africa International Yearbook of Landscap...

Author: Roland Baumhauer | Jürgen Runge

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HOLOCENE PALAEOENVIRONMENTAL HISTORY OF THE CENTRAL SAHARA

Palaeoecology of Africa International Yearbook of Landscape Evolution and Palaeoenvironments Volume 29

Editor in Chief J. Runge, Frankfurt, Germany

Editorial board

G. Botha, Pietermaritzburg, South Africa K.W. Butzer, Austin, Texas, USA E. Cornellissen, Tervuren, Belgium F. Gasse, Aix-en-Provence, France P. Giresse, Perpignan, France S. Kröpelin, Köln, Germany T. Huffmann, Johannesburg, South Africa E. Latrubesse, La Plata, Argentina J. Maley, Montpellier, France J.-P. Mund, München, Germany D. Olago, Nairobi, Kenya F. Runge, Altendiez, Germany L. Scott, Bloemfontein, South Africa I. Stengel, Pretoria, South Africa F.A. Street-Perrott, Oxford, UK M.R. Talbot, Bergen, Norway

Holocene Palaeoenvironmental History of the Central Sahara

Editors

Roland Baumhauer Department of Geography, Physical Geograpy, Julius-Maximilians University, Würzburg, Germany

Jürgen Runge Centre for Interdisciplinary Research on Africa (CIRA/ZIAF), Johann Wolfgang Goethe University, Frankfurt am Main, Germany

Front cover: Dead tree at the Plateau du Mangueni, NE-Niger. It symbolizes the changes in climate and landscape in Central Sahara caused by aridisation since the Early Holocene. Photograph by Jan Krause, Department of Earth Sciences, Physical Geography, Freie Universität Berlin, Germany. Financially Supported by DFG (German Research Foundation)

CRC Press/Balkema is an imprint of the Taylor & Francis Group, an informa business © 2009 Taylor & Francis Group, London, UK Typeset by Vikatan Publishing Solutions (P) Ltd., Chennai, India Printed and bound in Great Britain by TJ International Ltd., Padstow, Cornwall All rights reserved. No part of this publication or the information contained herein may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, by photocopying, recording or otherwise, without written prior permission from the publisher. Although all care is taken to ensure integrity and the quality of this publication and the information herein, no responsibility is assumed by the publishers nor the author for any damage to the property or persons as a result of operation or use of this publication and/or the information contained herein. Published by:

CRC Press/Balkema P.O. Box 447, 2300 AK Leiden, The Netherlands e-mail: [email protected] www.crcpress.com – www.taylorandfrancis.co.uk – www.balkema.nl

Library of Congress Cataloging-in-Publication Data Holocene palaeoenvironmental history of the Central Sahara / editors, Roland Baumhauer, Jürgen Runge. p. cm. Includes bibliographical references and index. ISBN 978-0-415-48256-1 (hardcover : alk. paper) 1. Palaeoecology -- Sahara. 2. Palaeoecology -- Holocene. 3. Palaeoecology -- Pleistocene. 4. Desert ecology -- Sahara -- History. I. Baumhauer, Roland. II. Runge, Jürgen, 1962- III. Title. QE720.2.S24H65 2009 560’.1793--dc22 2008045244 ISBN: 978-0-415-48256-1 (Hbk) ISBN: 978-0-203-87489-9 (e-book)

Contents

FOREWORD —Jürgen Runge

vii

IN MEMORIAM JOHANNA ALIDA COETZEE (1921–2007) —Klaus Heine

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PREFACE AND INTRODUCTION —Roland Baumhauer

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CONTRIBUTORS

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CHAPTER 1 GEOMORPHOLOGICAL AND PALAEOENVIRONMENTAL RESEARCH IN THE SOUTH-CENTRAL SAHARA IN REVIEW —Roland Baumhauer, Peter Felix-Henningsen & Brigitta Schütt CHAPTER 2 COMPARISON OF PROXY-BASED PALAEOENVIRONMENTAL RECONSTRUCTIONS AND HINDCAST MODELLED ANNUAL PRECIPITATION—A REVIEW OF HOLOCENE PALAEOENVIRONMENTAL RESEARCH IN THE CENTRAL SAHARA —Brigitta Schütt & Jan Krause CHAPTER 3 HOLOCENE PALAEOENVIRONMENTAL CHANGES IN CENTRAL SAHARA INFERRED FROM SEGGEDIM SCARP FOOT DEPRESSION (NE-NIGER) —Roland Baumhauer, Jens Brauneck, Barbara Sponholz, Erhard Schulz, Oumarou Faran Maiga, Ibrahim Sani & Simon Pomel CHAPTER 4 THE DESERT IN THE SAHARA. TRANSITIONS AND BOUNDARIES —Erhard Schulz, Abdelhakim Abichou, Aboubacar Adamou, Aziz Ballouche & Issa Ousseïni CHAPTER 5 PALAEO-CLIMATIC EVIDENCE OF SOIL DEVELOPMENT ON SAHELIAN ANCIENT DUNES OF DIFFERENT AGE IN NIGER, CHAD AND MAURITANIA —Peter Felix-Henningsen, Peter Kornatz & Einar Eberhardt

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23

39

63

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Contents

CHAPTER 6 ARE THERE VALUABLE PEDOLOGICAL PALAEOENVIRONMENTAL INDICATORS IN NORTHERN CHAD? —Ludger Herrmann, Mohamed Mounkaila & Frieder Graef

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CHAPTER 7 NEW DISCOVERY OF ROCK FULGURITES IN THE CENTRAL SAHARA —Barbara Sponholz

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CHAPTER 8 FLUVIAL GEOMORPHOLOGY AND PALAEOHYDROLOGY OF A SMALL TRIBUTARY OF THE PLATEAU DE MANGUENI, NE NIGER —Jan Krause & Brigitta Schütt

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CHAPTER 9 PALAEOECOLOGY OF THE GIANT CATFISH (ARIUS GIGAS, ARIIDAE) IN HOLOCENE SAHARAN AND TROPICAL WEST AFRICAN WATERS —Hélène Jousse & Wim Van Neer

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CHAPTER 10 NEOLITHIC DOMESTICATION AND PASTORALISM IN CENTRAL SAHARA: THE CATTLE NECROPOLIS OF MANKHOR (TADRART ALGÉRIENNE) —Michel Tauveron, Karl Heinz Striedter & Nadjib Ferhat

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CHAPTER 11 THE MICROSTRATIGRAPHY AND MICROMORPHOLOGY OF A HOLOCENE PALAEOLAKE IN SOUTHERN TUNISIA —Abdelhakim Abichou

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CHAPTER 12 DIFFERENT DIMENSIONS OF RECENT VEGETATION DYNAMICS OF NORTH AND WEST AFRICA —Brian Beckers & Brigitta Schütt

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CHAPTER 13 LANDMINES, DRUGS AND JUSTICE. THE RECENT HISTORY OF TWO SAHARAN MOUNTAINS (ADRAR DES IFORAS/MALI AND AIR MTS./NIGER) —Issa Ousseïni, Aboubacar Adamou & Erhard Schulz

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CHAPTER 14 THE SAHELIAN AND SAHARAN DUNE SYSTEMS OF NIGER. A COMPARISON OF THEIR GRANULOMETRIC CHARACTERISTICS —Ibrahim Mamane Sani & Issa Ousseïni

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REGIONAL/LOCATION INDEX

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SUBJECT INDEX

249

Foreword

Volume 29 of the series ‘Palaeoecology of Africa’ on the ‘Holocene Palaeoenvironmental History of the Central Sahara’ continues the publishing of interdisciplinary scientific papers on landscape evolution and on former environments of the African continent, e.g. changes in climate and in vegetation cover interconnected to all kinds of environmental dynamics from the Cainozoic up to the present, including since the Late Quaternary the growing influence of humans in many of the study areas. Since its re-edition in 2007/2008 with a modified editorial board—attended by the ‘Frankfurt Centre for Interdisciplinary research on Africa’ (CIRA/ZIAF, www.ziaf.de)— and a completely new and up-to-date layout, assisted by the publishers group Taylor & Francis, the series has already begun to be again recognized by the scientific community. However, Palaeoecology of Africa is not as yet been accredited by international citation indices, what is regarded as a certain drawback for the series. As it is essential to change this in the future, the editor is already working on this often quite long and tedious process in being considered by citation indices. Young academics and upcoming university professionals are especially encouraged and invited to consider this traditional journal—which has been on the “market of science journals” since 1966—as a media to communicate, and as an opportunity for meetings in minds to exchange global knowledge and ideas across scientific and cultural borders (cf. Heine, 2007 in PoA 28). In the past (1970s to 1990s) quite regularly the scientific contributions of the Biennial SASQUA (South African Society for Quaternary Research) conferences were published within Palaeoecology of Africa. As a vision for future volumes the editor likes to propose that some follow-up editions of the series could have again a focus on SASQUA activities (“back to the roots”). The up coming SASQUA meeting is scheduled for September 2009 in Knysna, South Africa. Fourteen papers are gathered together in this volume focusing on the most recent (Holocene) dynamics of the Central Sahara. They are mainly the outcome of the since 2005 DFG funded LIMNOSAHARA project (www.limnosahara.de) of the Universities of Würzburg, Giessen, FU Berlin (Germany) and Niamey (Niger). Many thanks go to all colleagues for submitting their papers to Palaeoecology of Africa. Special thanks go to Roland Baumhauer for supporting the editing process. Formatting of the papers to the PoA layout was again reliably done by Erik Hock to whom I am most grateful. Ursula Olbrich revised numerous figures and assisted by carrying out cartographic work on the book. The Taylor & Francis team in Leiden (The Netherlands) with senior editor Janjaap Bloom supported in a professional manner the editorial work. Finally, I like to thank the Deutsche Forschungsgemeinschaft (DFG) for assisting in printing this volume. Jürgen Runge Bangui and Frankfurt August 2008

In Memoriam

Johanna Alida Coetzee (1921–2007) Klaus Heine University of Regensburg, Institute of Geography, Regensburg, Germany It is with deep regret that I have to report that Professor Joey Coetzee died in Somerset West on April 28, 2007. Quaternary and Palynological Research has suffered a great loss. We mourn a colleague of national and international distinction, a brilliant and honest scientist, who advanced the field of palynology in Southern and Eastern Africa. She has published palaeoenvironmentally relevant groundbreaking and pioneering investigations on African pollen of Cainozoic age. Joey Coetzee was educated at Jeppe High School for Girls in Johannesburg and then University of Witwatersrand. She earned her master’s degree in 1946 and subsequently worked at the Universities of Wits and Natal for a short time. In August 1946, she accepted a position as assistant in the Department of Botany at the University of the Orange Free State. Inspired by Eduard van Zinderen Bakker’s pollen analytical investigations in East Africa, she embarked in testing the widely held hypothesis that pluvial phases correlate with glacial phases during the Last Glacial Maximum. Joey Coetzee devoted several years of intense work on these problems as a senior member of the research team of Eduard van Zinderen Bakker Sr. who headed the Department of Botany at the University of the Orange Free State and the Palynological Research Unit of the South African Council for Scientific and Industrial Research. For some time during 1953, Joey Coetzee worked in the Palynological Laboratory, Stockholm-Bromma, and also visited the colleagues in Bergen, Velp, Utrecht, and Cambridge. In the 1950s she worked on the pollen morphology of Southern African species and completed a collection and description of several thousand pollen grains and spores. It was the real base for any palynological study in Southern Africa. Furthermore, she analysed the air-borne pollen collected weekly for three years at seven different stations in South Africa and Namibia. The data of these pollen traps gave valuable information on the pollen spectra of the main vegetation types (e.g. Karoo) in their natural condition. In 1959 Joey Coetzee made a trip to East Africa, collected many pollen samples at the East African Herbarium at Nairobi and studied fossil pollen material from Uganda. In the following years she started her investigations of cores collected on Mount Kenya. All these and many other pioneering studies of Joey Coetzee marked the progress in the achievement of palynology and of understanding the past. In 1964 Joey Coetzee had to leave her study to concentrate on her East African pollen analytical research. The core from Sacred Lake was finished and showed remarkable results (Coetzee, 1964). Joey Coetzee’s findings led to the approval of the name Mount Kenya Hypothermal for the pollen zone which indicates a synchronous drop in temperature in many parts of Africa during the Upper Pleistocene. The general features of her diagram were a sound proof of the world-wide changes which occurred in the climate of the earth. Later, in her D.Sc.-Thesis (Coetzee, 1967) Joey Coetzee showed that the hypothesis ‘glacial phases = pluvial phases in tropical Africa’ was not true in Eastern and Southern Africa. This work, recently featured as a classic work in the journal Progress in Physical Geography, and probably the most cited palynological paper from the African continent, illustrates ‘the very essence of Quaternary palaeoenvironmental reconstruction and, indeed of physical

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geography’ (Meadows, 2007). Joey Coetzee rapidly acquired a reputation as an eminent researcher of palaeoenvironmental studies in Africa. She was among the first scientists to recognize the significance of changes in temperatures during the Last Glacial Maximum in tropical mountain areas of Africa. It was 1973 when I first met Joey Coetzee. The location was the INQUA conference in Christchurch, New Zealand, in the shadow of the glacially shaped Southern Alps. Together with Eduard van Zinderen Bakker Sr. she presented a paper on Global Temperature Changes and the African Quaternary Environment (van Zinderen Bakker and Coetzee, 1973). After their talk I discussed with Joey the results: During the Quaternary, Africa has been subjected to a wide range of variations in humidity which for a long time have been considered to have been in-phase over the entire continent. The pattern of rainfall distribution and the evaporation rate, however, depend on global and local temperature conditions. The longterm variations in the earth’s energy budget are therefore the primary cause of changes in the Quaternary environment. Correlations of radiometrically calibrated temperature curves are consequently of basic importance for the understanding of Quaternary chronology. Vegetation changes which occurred in the interior of Southern Africa during Late Glacial times show that detailed correlations existed between lower temperature and higher humidity versus higher temperature and lower humidity. This result indicates that climatic settings in this region differed fundamentally from that of tropical Africa (Coetzee, 1967) and that ‘pluvial conditions’ cannot be used for correlation purposes. In those days, my own chronological investigations of glacial deposits of the Mexican volcanoes showed that glacial climatic evidence is suitable for stratigraphic correlations neither (Heine, 1974) and thus my records from the New World tropics added to Joey Coetzee’s observations. Since that time, Joey and I were friends and colleagues. I met Joey many times in Southern Africa during conferences of the Southern African Society for the Quaternary (SASQUA), in her home at Bloemfontein and, after her retirement from the university, at Somerset West. Several times we spent weeks together with colleagues from South Africa and Germany in the field, especially in the Kalahari and the Namib Desert (Figure 1). And we met in Europe, in the Austrian Alps, and in my home. One of the most debated topics in Quaternary science which Joey and I discussed was methodological problems arising from the reconstruction of Late Quaternary palaeoenvironments by interpreting pollen data. The ‘palynological records’ of the Younger Dryas climatic oscillation, a millennium-long cooling event approx. 12.700–11.500 cal yr B.P. that interrupted the transition from the last glacial to present interglacial (Holocene) period, were subject to much debate in Southern Africa, Northern and Southern South America and New Zealand. Two conflicting hypotheses, both based on palynological sequences, have been proposed to document either this cooling event in the Southern Hemisphere (e.g. Heusser and Rabassa, 1987) or to question it (e.g. Singer et al., 1998). Joey’s fundamental knowledge of the methodology of pollen research advanced my understanding of Quaternary vegetation changes and the palaeoclimatic implications, not only in Eastern and Southern Africa, but also in other regions of the earth. Despite the fact that Joey Coetzee anticipated finding evidence of the Younger Dryas in East Africa and described and correlated particularly marked temperature fluctuations in South Africa (Aliwal North site) during the Late Glacial with the European older Dryas, Allerød and Younger Dryas, she listened to and respected my diverse views about the existence of the Younger Dryas cooling in the Southern Hemisphere. Another topic we argued about many times was the ‘Pluvial Theory’, established by Richard Forster Flint (1957) who suggested that the major periods of Northern Hemisphere ice advance were associated with phases of more humid climates in tropical Africa. Joey Coetzee’s reconstruction of the LGM temperature depression of between 5,1 and 8,8 °C and more arid conditions relative to the present day in tropical Africa (Coetzee, 1967;

In Memoriam

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Figure 1. Rest during field work at Sossus Vlei, Namib Desert, Namibia. From left to right: Joey Coetzee, Louis Scott (in the back), Eduard van Zinderen Bakker Sr., Almut Heine. Photo: K. Heine, June 9, 1981.

van Zinderen Bakker and Coetzee, 1972) questions the ‘Pluvial Theory’ and corroborates my observations from tropical Mexico. What came through during our discussions was Joey’s enthusiasm, and the exhilaration and intellectual stimulation of working on a collaborative interdisciplinary research field. In the 1970s and 1980s, Joey Coetzee began research work to explore links between the Tertiary pollen records and the unique fynbos vegetation in the Cape region (Coetzee, 1978a, b, 1993; Coetzee and Rogers, 1982; Coetzee and Muller, 1984; Coetzee and Praglowski, 1984, 1988). She found that fynbos replaced palm-dominated subtropical to tropical woodlands that alternated with conifer forests during the Neogene and linked the process to global cooling and Antarctic glacial history (Scott, 2007). Joey Coetzee edited Palaeoecology of Africa and of the surrounding islands and Antarctica, volume 10 to 17 (1978–1987) in collaboration with E.M. van Zinderen Bakker Sr. and volume 18 (1988). While volume 1 (1966) of Palaeoecology of Africa was devoted to reports which have been published during the years 1950–1963 under the title of ‘Palynology in Africa’ and deal with the initial research done on palynology in South Africa, Volume 2 (1967) showed that pollen analytical research could be linked with so many related disciplines that the entire field was better covered by the title ‘Palaeoecology’. The volumes 1 and 2 show the achievements in African palynology, Joey Coetzee’s very great importance and her contributions to the rapid development of palynological fundamentals during the years 1950–1965. Volume 3 (1967) was entirely devoted to the D.Sc.-Thesis by Joey Coetzee on pollen analysis in East and South Africa. It was this volume of Palaeoecology of Africa that made the series of the ‘small yellow books’ (Nicole

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Petit-Maire, Durban Febr. 1, 1989) well-known among African palaeoenvironmentalists. In the 1970s and 1980s, Palaeoecology of Africa became a forum for research results of scientists from various countries studying the palynology, palaeontology, biogeography, glacial geology, volcanology, geomorphology, oceanography, limnology, (palaeo-) climatology, archaeology, anthropology as well as theories on ice ages. In the publication process, Joey Coetzee distinguished herself as a personality with high principles and an intense interest in scientific communication and interchange of ideas. Joey Coetzee served on many committees in connection with Quaternary science and palynology. She was one who always gave credit to others rather than to herself. Through her far-ranging interests and achievements, she has left an enduring imprint on African palynological science. A world-class scientist, a woman of great integrity and an inveterate friend of animals, she was above all a nice fellow who just enjoyed nature in all its facets wherever she stayed. REFERENCES Coetzee, J.A., 1964, Evidence for a considerable depression of the vegetation belts during the Upper Pleistocene on the East African mountains. Nature, 204, pp. 564–566. Coetzee, J.A., 1967, Pollen analytical studies in East and Southern Africa. Palaeoecology of Africa, 3, pp. 1–146. Coetzee, J.A., 1978a, Climatic and biological changes in South-Western Africa during the Late Cainozoic. Palaeoecology of Africa, 10, pp. 13–29. Coetzee, J.A., 1978b, Late Cainozoic palaeoenvironments of Southern Africa. In: E.M. van Zinderen Bakker (ed.), Antarctic glacial history and world palaeoenvironments. A.A. Balkema, Rotterdam, pp. 110–127. Coetzee, J.A., 1993, African flora since the Terminal Jurassic. In: P. Goldblatt (ed.), Biological Relationships between Africa and South America. Yale University Press, New Haven and London, pp. 37–61. Coetzee, J.A. and Muller, J., 1984, The phytogeographic significance of some extinct Gondwana pollen types from the Tertiary of the Southwestern Cape (South Africa). Annals of the Missouri Botanical Garden, 71, pp. 1088–1099. Coetzee, J.A. and Rogers, J., 1982, Palynology and lithological evidence for the Miocene palaeoenvironment in the Saldanha region (South Africa). Palaeogeography, Palaeoclimatology, Palaeoecology, 39, pp. 71–85. Coetzee, J.A. and Praglowski, J., 1984, Pollen evidence for the occurrence of casuarina and myrica in the Tertiary of South Africa. Grana, 23, pp. 23–41. Coetzee, J.A. and Praglowski, J., 1988, Winteraceae pollen from the Miocene of the Southwestern Cape (South Africa). Relationship to modern taxa and phytogeographical significance. Grana, 27, pp. 27–37. Flint, R.F., 1957, Glacial and Pleistocene Geology. John Wiley, New York, pp. 1–553. Heine, K., 1974, Bemerkungen zu neueren chronostratigraphischen Daten zum Verhältnis glazialer und pluvialer Klimabedingungen. Erdkunde, 28, pp. 303–312. Heusser, C.J. and Rabassa, J., 1987, Cold climatic episode of Younger Dryas age in Tierra del Fuego. Nature, 328, pp. 609–611. Meadows, M., 2007 and Coetzee, J.A., 1967, Pollen analytical studies in East and Southern Africa. Palaeoecology of Africa 3, pp. 1–146. (Classics in physical geography revisited). Progress in Physical Geography 31, pp. 313–317. Scott, L., 2007, Professor Joey Coetzee 1921–2007. Review of Palaeobotany and Palynology, 147, pp. 1–2.

In Memoriam

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Singer, C., Shulmeister, J. and McLea, B., 1998, Evidence against a significant Younger Dryas cooling event in New Zealand. Science, 281, pp. 812–814. van Zinderen Bakker, E.M. and Coetzee, J.A., 1972, A re-appraisal of Late-Quaternary climatic evidence from Tropical Africa. Palaeoecology of Africa, 7, pp. 151–181. van Zinderen Bakker, E.M. and Coetzee, J.A., 1973, Global temperature changes and the African Quaternary environment. 9th INQUA Congress, Christchurch, New Zealand, 2–10 Dec. 1973, Abstracts, pp. 1–385.

Schematic vegetation map of Northern and Western Africa (for text see chapter 4, pp. 63–89).

Preface and Introduction

The environmental setting within the Central Sahara was subject to considerable changes during Late Quaternary, mainly driven by major global climate variations, although human impact increased constantly since Early Holocene. Such global events can be reconstructed with the help of reliefs, sediments and palaeosoils and their specific morphological, chemical and mineralogical properties. The 14 papers gathered together in this volume are mainly the outcome of the interdisciplinary German research project LIMNOSAHARA (www.limnosahara.de) financed by the German Research Foundation (DFG). The investigations were carried out by a collaboration of multiple disciplines, reaching from physical geography, palaeopedology and palaeolimnology to palynology and prehistory. The project’s focus is to ascertain new and established data on climate variations and associated palaeoenvironmental changes within the Central Sahara and to systematically collate and correlate them to results obtained from the Afro-Asian dry land belt and adjacent areas. The joint analysis of Late Quaternary landscape development and present environmental conditions in the Central Sahara will result in the modelling of Late Pleistocene and Holocene palaeoenvironments, emphasising various aspects. This will be achieved by transferring the highly localized information obtained from palaeolake sediments to the region by means of spatially high-resolution information about the morphodynamic processes currently shaping the landscape and the factors controlling them. The first—introductory—chapter gives an overview of the geomorphological and palaeoecological research in south Central Sahara on the basis of a literature review. Subsequently different proxy-data sources are introduced and discussed to conclude on the former dynamics of the palaeoenvironment of the central part of Sahara. The following chapter turns towards two aspects: on the one hand it summarizes and discusses more than 50 original publications on climate proxy- and on model-data from the Central Sahara; on the other hand, it compares the findings from the proxies with numerically modelled data by Kutzbach and Guetter’s CCM0 estimations for African precipitation over time slices at 3, 6 and 9 ka. The presented combination of “empirical” versus “modelled” data is interesting as nowadays the discussion often is dominated by the model approach exclusively. In chapter 3 extensive work on former lake/sebhka-sediments and fossil soils in the Seggedim region gave further evidence of Holocene palaeoenvironmental changes in an up to now not well explored region of NE Niger. In the paper a new 15 meter long core gained in 2005 is analysed and discussed by an interdisciplinary approach using geomorphological, sedimentological and geochemical investigations. A trend to more humid conditions at the beginning of the Holocene changing later on—while getting drier—into a sebkha environment with alternating salt and sand layers was evidenced by the findings. The fourth paper by Erhard Schulz and colleagues from Niger, Tunisia and France is dealing in the way of a review on different approaches of how to define and of how to characterize transitions, limits or boundaries of the Saharan desert. The authors mainly outline these by the vegetation and floristic content of the landscapes which are described in detail. The contribution illustrates the sensitivity of ecological margins due to former and future changes in climate.

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Figure 1. Location map of study sites.

The subsequent chapters five and six are concerned with (palaeo)pedological investigations. The paper of Peter Felix-Henningsen and members of his working group (Peter Kornatz, Einar Eberhardt) examines different soils developed on nowadays inactive dune systems in Mauretania, Niger and Chad as a proxy data source for palaeoclimatic interpretation. Besides conventional pedological analysis on the soil profiles also OSL dating was carried out. The overall results are mostly confirming the findings by other


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papers dealing with the course of Holocene environmental conditions in the Sahara. The second pedological paper gives an overview of a pedologically directed transect in Northern Chad in general search for suitable palaeoenvironmental indicators. Based on extended earlier research on fulgurites that were formed by lightning strike to silicate rocks, the author introduces in chapter 7 two new observations on rock fulgurites from the Central Sahara. In contrast to dune sand fulgurites that indicate monsoonal thunderstorms up to 18°N in the Sahara during the Mid-Holocene, the observed rock fulgurites are not necessarily correlated to the palaeoenvironmental conditions as they could have been formed by lightning events in the course of ephemeric thunderstorm events. On the basis of former geomorphological field work undertaken in the 1980 by the Würzburg working group, the so-called “Seeterrassental”—a catchment with only 31,5 km2 in size—in the Mangueni Plateau of NE Niger has been revisited by Jan Krause and Britta Schütt (chapter 8) using modern differential GPS measurements and SRTM data to reconstruct Holocene flow velocity and discharge, also applying different hydraulic equations on three identified terrace levels. The palaeoecological and ichthyological paper of Hélène Jousse and Wim Van Neer (chapter 9) studies the recent and former distribution of the Giant Catfish (Arius gigas Ariidae) in Westafrica, especially in the Niger basin. On the basis of the examination of a rare skeleton specimen from the British Museum anatomy of this species is studied and documented. The Holocene and the recent decline of this fish population is discussed from different perspectives considering former climate changes and also the influence of humans (subsistance fishing) in the region. In chapter 10 (Michel Tauveron and Karl Heinz Striedter) numerous findings of cattle bones and one almost complete cattle skeleton which had not been clearly identified as Bos taurus or Bos primigenious in the South-Eastern Algerian Sahara are suggesting palaeoenvironmental interpretations for the Holocene. Abdelhakim Abichou shows in his contribution (chapter 11) micromorphological investigations and cartographic surveys. In connection with some radiocarbon data the results allow the reconstruction of Holocene palaeoenvironmental conditions in the sebka Erg el Makhzen in Southern Tunisia. Chapter 12 is not mainly palaeoenvironmentally adjusted. Brian Beckers and Britta Schütt examine by an actualistic approach vegetation dynamics by using NDVI data and GBCP rainfall estimates on a huge transect running from North to West Africa. It underlines the recent sensitivity of the Central Saharan landscape that might have been in a way similar to the palaeoenvironmental conditions. Aside of variations in rainfall also human induced effects on vegetation dynamics are considered. Finally, the authors of chapter 13 (Issa Ousseïni, Aboubacar Adamou and Erhard Schulz) describe and discuss recent geoecological and environmental modifications in the Adrar des Iforas (Mali) and the Air (Niger) mountains that took and still take place against the backdrop of a civil war (the so-called “rebellion of Tuareg”). Therefore, this contribution is a combination of socio-economic and geopolitical with environmental and landscape issues. It is tried to show how environmental factors and dynamics of the environment can be linked to economic and political trends. Several very complex graphs/sketch maps represent time slices from 1910 to 2007 that are driving at a better understanding of this interdisciplinary approach. Another contribution (chapter 14) from the colleagues of the University of Niamey (Ibrahim Mamane Sani and Issa Ousseïni) makes an approach to understand the granulometric characteristics of the dune systems of Niger. Many thanks go to all colleagues for submitting their papers. Thanks are also due to the German Research Foundation (DFG) for its financial support to realize this publication. Special thanks go to Jürgen Runge for publishing in PoA. Roland Baumhauer Würzburg July 2008

Contributors

Abdelhakim Abichou Département de Géographie, CGMED Tunis, Faculté des Lettres et des Sciences Humaines de Tunis, BP 1123, Tunis, Tunisie. Email: [email protected]. Aboubacar Adamou Département de Géographie, Faculté des Lettres et des Sciences Humaines, Université Abdou Moumouni, BP 435, Niamey, République du Niger. Email: [email protected]. Aziz Ballouche Laboratoire “Paysages & Biodiversité”, Université Angers, F-49045 Angers Cedex 1, France. Email: [email protected]. Roland Baumhauer Department of Geography, Physical Geography, Julius-Maximilians University, Am Hubland, D-97074 Würzburg, Germany. Email: [email protected]. Brian Beckers Department of Earth Sciences, Physical Geography, Freie Universität, Berlin, Malteserstr. 74-100, D-12249 Berlin, Germany. Email: [email protected]. Jens Brauneck Department of Geography, Physical Geography, Julius-Maximilians University, Am Hubland, D-97074 Würzburg, Germany. Email: [email protected]. Einar Eberhardt Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, D-30655 Hannover, Germany. Email: [email protected]. Peter Felix-Henningsen Institute of Soil Science and Soil Conservation, Justus Liebig University Giessen, Heinrich-Buff-Ring 26, D-35390 Giessen, Germany. Email: Peter.Felix-H@umwelt. uni-giessen.de. Frieder Graef Institute of Soil Science and Land Evaluation (310), University of Hohenheim, D-70593 Stuttgart, Germany. Email: [email protected]. Klaus Heine Institute of Geography, University of Regensburg, Universitätsstr. 31, D-93053 Regensburg, Germany. Email: [email protected]. Ludger Herrmann Institute of Soil Science and Land Evaluation (310), University of Hohenheim, D-70593 Stuttgart, Germany. Email: [email protected]. Hélène Jousse Naturhistorisches Museum Wien, Säugetiersammlung, Burgring 7, A-1010 Wien, Austria. Email: [email protected].

xxii Contributors

Peter Kornatz Institute of Soil Science and Soil Conservation, Justus Liebig University Giessen, Heinrich-Buff-Ring 26, D-35390 Giessen, Germany. Email: peter.kornatz@umwelt. uni-giessen.de. Jan Krause Department of Earth Sciences, Physical Geography, Freie Universität, Berlin, Malteserstr. 74-100, D-12249 Berlin, Germany. Email: [email protected]. Oumarou Faran Maiga Département de Géographie, Faculté des Lettres et des Sciences Humaines, Université Abdou Moumouni, BP 435, Niamey, République du Niger. Email: [email protected]. Mohamed Mounkaila Institute of Soil Science and land Evaluation (310), University of Hohenheim, D-70593 Stuttgart, Germany. Email: [email protected]. Issa Ousseïni Département de Géographie, Faculté des Lettres et des Sciences Humaines, Université Abdou Moumouni, BP 435, Niamey, République du Niger. Email: [email protected]. Simon Pomel DYMSET/CNRS, University of Bordeaux III, F-33405 Talence Cedex, France. Email: [email protected]. Ibrahim Mamane Sani Département de Géographie, Faculté des Lettres et des Sciences Humaines, Université Abdou Moumouni, BP 435, Niamey, République du Niger. Email: [email protected]. Erhard Schulz Department of Geography, Physical Geography, Julius-Maximilians University, Am Hubland, D-97074 Würzburg, Germany. Email: [email protected]. Brigitta Schütt Department of Earth Sciences, Physical Geography, Freie Universität, Berlin, Malteserstr. 74-100, D-12249 Berlin, Germany. Email: [email protected]. Barbara Sponholz Department of Geography, Physical Geography, Julius-Maximilians University, Am Hubland, D-97074 Würzburg, Germany. Email: [email protected]. Karl Heinz Striedter Frobenius Institute, Johann Wolfgang Goethe University, Grüneburgplatz 1, D-60323 Frankfurt, Germany. Email: [email protected]. Michel Tauveron Independent Archaeologist, F-19600 Lissac sur Couze, France. Wim Van Neer Royal Belgian Institute of Natural Sciences, Vautierstraat 29, B-1000 Brussels, Belgium and Katholieke Universiteit Leuven, Laboratory of Animal Biodiversity and Systematics, Ch. Deberiotstraat 32, B-3000 Leuven, Belgium. Email: willem.vanneer@ bio.kuleuven.be.

CHAPTER 1

Geomorphological and palaeoenvironmental research in the South-Central Sahara in review Roland Baumhauer Department of Geography, Physical Geography, Julius-Maximilians University, Würzburg, Germany Peter Felix-Henningsen Institute of Soil Science and Soil Conservation, Justus Liebig University, Giessen, Germany Brigitta Schütt Department of Earth Sciences, Physical Geography, Freie Universität, Berlin, Germany ABSTRACT: The main interest of the interdisciplinary “Limnosahara” research project of the German Research Foundation (DFG) composed of Physical Geographers, Palaeopedologists, Palaeobotanists and Archaeologists from the Universities of Würzburg, Berlin, Giessen, Frankfurt and Niamey, is to elucidate the Holocene palaeoenvironmental history of the Central Sahara. This introductory chapter gives an overview of the geomorphological and palaeoecological research in South-Central Sahara on the basis of a literature review. Subsequently different proxy-data sources are introduced and discussed to conclude on the former dynamics of the palaeoenvironment of the central part of Sahara.

1.1 THE SOUTH-CENTRAL SAHARA—GEOGRAPHICAL SETTING The Southern Central Sahara, between 17° and 23°N and 11° and 15°E, belongs to the northern part of the hydrological Chad Basin. It is a typical part of the plateau and plains landscapes of the Central Sahara, framed in the West and East by the Saharan mountain regions of Aïr and Tibesti. To the North the study region extends to the southern fringe of the Murzuq Basin; to the South it reaches as far as the Sahara-Sahel boundary, marked by the northern limit of immobile ancient sand dunes. The central and largest part of the study region is taken up by vast sand plains, the largest being the Ténéré, grading southwards into the sand seas Erg de Ténéré, Erg de Fachi-Bilma and the Grand Erg de Bilma, all characterized by closely spaced NE–SW oriented longitudinal dunes. The sand plains and ergs are interrupted by N–S-oriented scarplands, partly broken up into isolated massifs or plateau remnants, a common feature of their western forelands being elongated endorheïc depressions. They are like stepping stones between the most conspicuous landforms in the North of the region—the

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Roland Baumhauer et al.

Figure 1. Location map of NE-Niger.

plateaus of Djado and Mangueni at the southern fringe of the Murzuq Basin—and the fossil dune region of Tin Toumma and In-Madama to the South, already belonging to the geomorphological Lake Chad Basin. The average elevation of the entire area is 400 to 500 m asl; the escarpments may rise above the forelands by heights ranging from a few tens of metres to several hundred metres (Figure 1). The meteorological and climatological data situation of the region is extremely poor in terms of space and time. There are only few weather stations, and their data are often not available for political reasons. The plains of the Central Sahara are particularly sparsely equipped with only ten weather stations, almost all of which are located along its northern limit. For the South-Central Sahara, the only more or less complete record is that from the station at Bilma (18°41' N, 12°55'E, 335 m asl) from 1922 onwards. For the military post of Madama to the North (21°51' N, 13°45' E, 546 m asl), rainfall data are only available from 1939 to 1943. Because of the lack of proxy data, meteorological publications on the present climate and climatic history of the region are largely model-based and often rely too much on the transfer of data from deep-sea cores to be a suitable basis for further studies in a

Geomorphological and palaeoenvironmental research in the South-Central Sahara in review

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continental region such as this (e.g. Flohn and Nicholson, 1980; Kutzbach, 1980; Adams and Tetzlaff, 1984; Macayeal, 1993; Cubasch et al., 1997; Montoya et al., 1998; Stauffer et al., 1998; Crowly, 1999; von Storch et al., 2000). Ultimately, the factors governing the climate of the Central Sahara are its latitudinal position within the trade-wind belt and its extreme continentality. Although the region is affected by tropical monsoonal air from the Equatorial Atlantic and, in the North, by the Mediterranean circulation, neither influence modifies the extreme aridity of the region. At a low cloud cover throughout the year and summer maximum temperatures of above 40 °C, average annual precipitation is barely above 20 mm. 1.2 GEOLOGICAL SITUATION The post-Palaeozoic development of Eastern Niger to the south rim of the Plateau du Djado was described by Faure (1966); together with the work by Pirard (1964) on the hydrogeology of Eastern Niger, this study is still the foundation of all geological work in the region. The Southern Central Sahara consists of a wide-ranging system of tectonic basins and broad uplifts. Mainly in the transition areas, gently inclined Palaeozoic, Mesozoic and Tertiary sediments, unconformably overlying an etchplain cutting across the Precambrian metamorphic and crystalline basement, have been eroded to form the typical plateau and scarpland terrains of the Central Sahara. The core of the South-Central Sahara belongs to the geological basin of Bilma; to the North, the plateaus at the southern Murzuq rim are part of the geological Murzuk Basin, while to the South, the Massif d’Agadem is part of the geological Chad Basin (cf. Faure, 1966). The geological basin of Bilma occupies an area of 400 × 300 km in the centre of the Nigrian part of the hydrological Chad Basin. Its longer axis is delineated by the Bilma Escarpment. The western half of the basin, delimited by outcropping basement to the North, South and West, has been filled by up to 1.000 m of Cretaceous to Palaeocene continental and marine sediments, without any underlying Palaeozoic strata (Faure, 1966). Hardly any geological and geomorphological information has been published about the eastern parts of the Bilma Basin. To the West the Bilma Basin is separated from the Aïr Mountains by the grabens of Achegour and Adrar Madet, to the Southwest, by the Téfidet-Lake Chad graben system from the Termit Basin. To the North, there is just a gradual transition to the Murzuq Basin. Within the system of Central-Saharan basins and uplifts the geological Murzuq Basin, approximately 1.000 × 600 km large, has a central position, geologically and morphologically being a depression with elevated rims. The gently inclined strata form an almost uninterrupted fringe of outward-facing escarpments around the basin (Grunert, 1983). Because of the basin structure, the ages of the sediments exposed at the surface decrease from the rim to the centre (Klitzsch, 1970, 1971). Along the southern fringe, the surficial rocks are mainly marine sediments of the Upper Carboniferous and the continental series of the Mesozoic, up to the Lower Carboniferous (Plauchut et al., 1960). As mentioned, the southernmost parts of the Central Sahara belong to the geological Chad Basin. 1.3 GEOMORPHOLOGICAL AND QUATERNARY EXPLORATION More recent geological studies of the South-Central Sahara have mainly been undertaken by the “Würzburg Africa Research Group” headed by Horst Hagedorn. From 1977 to 1991, ten expeditions to the region were carried out, with varying participants. From 1992 to 2000 all of the Nigrian part of the South-Central Sahara was inaccessible because of

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Figure 2. Emi Fezzan, Butte of the northern Plateau de Tchigai.

civil-war-like conflicts. Except for a few selective studies on the prehistory of the region, carried out on the Djado Plateau from a base in Algeria (Striedter et al., 1995; Striedter, 1997, 1999), a whole decade passed without field work in this part of the Sahara. Only a small number of studies exist on the Late Quaternary evolution of the SouthCentral Sahara. Those by Faure (1966), Servant (1973), Servant-Vildary (1978) and Maley (1980) have their regional focus in the Chad Basin; the Saharan space is dealt with only marginally and very selectively. The north easternmost parts of Niger, in particular from east of the plateaus of Djado and Mangueni to the Chadian and Libyan borders, is still largely unknown from this perspective, except for some reports by French colonial officers (e.g. Capit. Freydenberg, 1907), and some geological reconnaissance work (Dalloni, 1948; Kilian, 1937; Kilian and Furon, 1934; Pirard, 1964). 1.3.1 Tertiary to Mid-Pleistocene landform history According to the stratigraphy described by Faure (1966) and Klitzsch (1970, 1971), postsedimentary landform development in the Central Sahara began after the deposition and diagenesis of the Messak/Nubian Sandstone of the rim of the Murzuq Basin and of the Bilma and Emi Bao formation of the Bilma Basin. The shaping of the present landscape should already have started in pre-Upper Cretaceous times. From the study of the major landforms of North-Eastern Fezzan and groundwater studies of the Central Sahara, Klitzsch (1974) and Klitzsch et al. (1976) place the Messak Sandstone in the Jurassic and also assume that an escarpment landscape had already developed in pre-Late Upper Cretaceous times. In contrast, Faure (1966) for the Bilma Basin and Busche (1982) for the western and southern rim of the Murzuq Basin and the northern parts of the Bilma Basin assume an Upper Cretaceous age of the Bilma and Emi Bao formation and the Messak Sandstone. Up to the onset of the Cainozoic, the study region was part of a depositional surface rimmed by the exposed crystalline uplifts of Gargaf-Hoggar to the North and Northwest, and the Tibesti-Syrte uplift to the East and Northeast (Busche, 1982). To the South, during the Palaeocene and Eocene, the sea reached Western Niger, Mali and North-Western Nigeria (Greigert and Pougnet, 1967). The marine transgression from the South did not go beyond the present foreland of the Tibesti Mountains (Klitzsch, 1970). For the continental realm between the seas, Erhard (1956) and Elouard (1959) identified a time of lateritic deep weathering under a humid-tropical climate, based on their study of the Continental Terminal, the southern correlative sediment of extended etchplanation to the North. According to Faure (1966), tectonic uplift in Eastern Niger since the Upper Eocene and the Oligocene caused the large-scale removal of the lateritic weathering mantle there. For the southern and western rim of the Murzuq Basin, Busche (1982), in his comprehensive study on the development of the region, together with the Djado Plateau and northern “Kaouar”


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(summarized by Baumhauer, 1986; Baumhauer et al., 1989; Skworonek, 1988; Busche, 1998) proposed a downwearing by etchplanation of the Messak Sandstone by at least 100 m. To him, the humid to wet-and-dry tropical conditions favouring etchplanation—of the African Surface of King 1967, cf. also Gellert (1971)—persisted to the Miocene, as Eocene sediments have also been etchplanated and intensive chemical weathering can be traced to at least that time, with decreasing intensity even to the end of the Tertiary (Busche, 1982; 1998). In a persistingly humid environment intensive silicification, following the almost complete removal of the lateritic weathering mantle, affected the level landscape close to a high groundwater level. Busche (1982, 1998) regards such silicified surfaces as a special type of planation landscape preserved on the plateaus of the Central Sahara, and also as a time marker: the end of silcrete formation, i.e. its beginning dissection, coincided with a time of increasing crustal movements, expressed in the doming and onset of volcanism first in what was to become the Hoggar, and then also in the Tibesti Mountain regions. Following silcrete formation there began a time of intensive surficial and subsurface silicate karstification, it appears to have continued to the Early Pleistocene (cf., among others, Busche, 1982; Busche, 1998; Busche and Sponholz, 1992; Hagedorn and Sponholz, 1990; Sponholz, 1992). The tectonic movements increasing since the Miocene uplifted most of the Cretaceous continental sediments and also, South of 17°N, the parts of Eastern Niger blanketed by the Continental Terminal, whereas the Chad Basin to the East began to subside. This led to a general gentle tilting of the strata to the East, together with continued downwearing by planation of parts of the rising terrains by an average of 100 m, the grand creusement postérieur au Continental terminal, followed, during the Early Pleistocene, by another phase of erosion during which also the Pliocene fluvial sediments (quartz gravel beds incorporating fragments of Continental terminal) were dissected in the North, and erosion to the South increased from 50 to 150 m (Faure, 1966). At the same time, in the region of subsidence west and northwest of present Lake Chad, up to 1.000 m of sediment were deposited of the Groupe du Tchad (Pirard, 1967), and, in the centre of the geological Chad Basin, about 100 m of the Bahr el Gazal series (Servant, 1983). According to Busche (1982, 1998) uplift still under the conditions of tropical chemical weathering, and a continuation of silicate karstification. The combined processes, up to the end of the Tertiary, resulted in the formation of a landscape of erosion scarps rimming dissected plateaus and a lower level of intra-plateau basins and etchplains. Regional differences of uplift and thus of different efficiency of etchplanation caused scarps to become a few hundred metres high, like the Messak Mellet now forming the western geomorphological rim of the Murzuq Basin, only a few tens of metres, like the southernmost parts of the Dissilak escarpment forming the western rim of the Djado Plateau, or, intermediate, the Bilma Escarpment. The latter is further characterized by the fact that selective planation, following intensive sandstone karstification, could keep pace with the rate of uplift, so that much of the original surface was transformed into a pattern of plateaus and basins (Busche, 1982; Busche, 1998). The fluvial and aeolian transformation of the landforms took place under semi-humid to semi-arid climates of the Early Pleistocene: also the lowermost etchplain level became fluvially dissected, those parts of the Late Tertiary etchplain abutting against the escarpments and rims of intra-plateau basins became more steeply inclined pediments, escarpment and inselberg profiles became steepened, with the development of a free face, and the endorheïc scarpfoot depressions (see above) came into existence (Busche, 1982, 1998) towards the end of the Early Pleistocene or somewhat later, during an extremely pronounced arid phase (Busche and Stengel, 1993; Busche, 1998). As described by Grunert (1983), in a detailed study focusing on the ancient landslides formed along the heterolithic parts of escarpment slopes of the western and southern rim of the Murzuq Basin, the slides followed a period of fluvial dissection and slope

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oversteepening under semi-arid conditions in the Plio-/Pleistocene transition period, possibly triggered by a renewed rise in precipitation. In comparison to present-day landsliding in humid regions, Grunert (1983) assumes at least 400 mm of winter rains, at lower temperatures and thus reduced evapotranspiration compared to today, for their formation. A common phenomenon throughout the Sahara, fossil landslides also exist along the escarpment rims of the Bilma Basin, but have not yet been studied there. For the landform history outlined above, mainly derived from geomorphological and palaeogeographical observations, it is assumed that climates were perennially humid to seasonally humid tropical, with deep-reaching chemical weathering especially during the earlier parts of the post-sedimentary development of the region. This is largely contradicted by Skowronek (1988) in a broad study of soils as indicators of climatically governed land formation of the Central Sahara. From palaeopedological and sedimentological studies he concludes that no humid-tropical soil was formed since the Early Tertiary, and that, in consequence, there was no climate since then which could have led to deep-reaching and intensive chemical weathering and soil formation. For the rim of the Murzuq Basin he assumes times of a soil-forming environment much more humid than today, but hardly beyond 200 mm of episodic rainfalls. Similarly, Barth and Blume (1975), from their study of the escarpments of the western rim of the Murzuq Basin, concluded that only marginally more humid conditions than today had existed throughout the Pleistocene, envisaging an arid-morphodynamic erosional history. 1.3.2 Late Pleistocene and Holocene development The first radiocarbon dates for the upper Late Pleistocene and Holocene of the plateau landscapes west and southwest of the Murzuq Basin were published by Busche et al. (1979), Busche (1982) and Grunert (1983). Largely on the basis of its morphostratigraphic position, they equalled a rhizome-rich terrace body grading into a diatomitic lake sediment in the Enneri Achelouma area, below the south rim of the Plateau du Mangueni, with the Middle Terrace of the Tibesti Mountains (mainly Jäkel 1979), although the dates from the plateau itself suggest a later onset of sedimentation. Dates from lake sediments/slack water sediments deposited in another tributary valley of Enneri Achelouma, grading into a reddish-brown sandy terrace at its mouth, indicated uninterrupted sedimentation from 8 to 7 ka BP. Another lake sediment from the foreland of Col d’Anai, about 250 km NNW of Achelouma, at the base of a reddish-brown, sandy terrace, was dated to 15,8 ka BP. The similarities among these terraces, and also with other sedimentary bodies of the region, led the authors to conclude that, despite the age differences, they are all part of the same Middle Terrace. Following the palaeoclimatic interpretation by Jäkel (1979) from the Tibesti (Enneri Bardagué), they assumed that the region around the Plateau du Mangueni, during the last major humid phase, between 16 and 7,4 ka BP, had come under the influence of a winter west-wind regime (from 16 to 8 ka BP according to Jäkel), and had thus experienced wet conditions earlier than the region to the South, where inroads of rain-bringing tropical air were thought to have started not before 13 ka. In conclusion, the northern and southern parts of the region had not always been subjected to the same climate (Busche et al., 1979). For Busche (1982, 1998) the deposition of the mostly fine-grained Middle Terrace is the major geomorphological event of the Late Pleistocene and Holocene. The several millennia of sedimentation are thought to have followed a long phase of pluvial-time soil formation during the Upper Quaternary, with largely stable conditions on the plateaus and slopes. With decreasing humidity the soil cover supplied the fines for the deposition on the valley floors of the region from about 16 ka to 7 ka BP. Schulz (1980), in studying the


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Holocene vegetation development of the region, concluded that during the time of Middle Terrace deposition it resembled the present vegetation, but covered much larger areas with a higher ground cover and a greater number of species. He assumes a gradual thinning during the Holocene, interrupted by climatic oscillations, but the pollen spectra studied do not allow a detailed temporal development to be read from the sediments. First results of prehistoric studies by Striedter (1992) and Ferhat and Striedter (1993) confirm the presence of Palaeolithic and Mesolithic traces, like those identified by Baumhauer et al. (1991) and Tillet (1983, 1989) for the Southern Djado Plateau, the Ténéré and its frame of plateaus (see below). The Neolithic finds from the central Djado region allow the identification of a culture group mainly defined by its pottery (Striedter, 1992, 1995), possibly closely related to the Ténéréen (cf. Roset, 1982, 1987). If confirmed, this would not only considerably enlarge the region inhabited by the Ténéréen culture group, but would also mean that the Djado region was inhabited by herders, as long as the climatic conditions permitted. Of particular interest are the petroglyphs of the central Djado Plateau. The engravings, in particular, compare well with those of the Ahaggar, Tassili n’Ajjer, Acacus and Fezzan (among others, Aumassip, 1993); they are characterized by a sequence of epochs and schools whose overlaps should permit the setup of a relative chronology (Striedter, 1992; Ferhat and Striedter, 1995; Striedter, 1997). Lake sediments Palaeolake sediments are a common occurrence in the South-Central Sahara of NorthEastern Niger. Their location and state of preservation are determined by the geomorphic structure of the region. Area-wise and in the shape of yardangs up to 10 m high, they are found in the dune depressions of the ergs framing the Ténéré to the South and East (cf. figure 3), as well as in the endorheïc depressions West of isolated plateaus or the plateau remnants of the eroded scarplands of the South-Central Sahara (Busche 1998). In the depressions at the foot of Emi Bao (Seggedim oasis) and along the Southern Bilma escarpment (Kawar), they had in places developed to a maximum depth of about 40 m. Within the sand plain of the Ténéré there are only thin deposits in shallow basins of various sizes. The few lake deposits studied so far occur in extremely favourable locations within the plateau landscapes of the western and southern fringe of the Murzuq Basin (Busche et al., 1979; Grunert, 1983; Striedter et al., 1995) and in the Southern Plateau du Tchigai (Busche, 1998). From the next intended study region East of the plateaus of Mangueni and Djado, there are first observations on widespread plains with a lacustrine cover and reports on the terminal basins of rivers as well as bedrock basins or playas filled with clays and diatomites (mainly Pirard, 1964; Capitaine Renaud, 1926; Striedter and Ferhat, personal communication). Publications on the stratigraphy and its palaeoclimatic and -environmental interpretation of the lacustrine deposits of the outgoing Pleistocene and Holocene of North-Eastern Niger were first presented by Servant (1973), who—on the basis of Faure (1965, 1966, 1969) and Faure et al. (1963)—tied his studies in with Cainozoic continental sedimentation and climate history. Servant-Vildary (1978) outlined the history of the lakes by means of their diatom flores, and Maley (1980) presented the climatic history of the region during the last 30.000 years based on palynological data. All these studies focus on the Chad Basin; of North-Eastern Niger, only the southern fringe—Southern Ténéré, Erg de Fachi-Bilma, Grand Erg de Bilma, or the region around Bilma—is dealt with selectively. Several field campaigns between 1981 and 1991 addressed the potential and problems of palaeoenvironmental reconstruction from palaeolakes of the Southern Central Sahara (Baumhauer, 1986, 1987, 1988, 1989, 1990; among others).

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Figure 3. Kafra scarpfoot depression in Central Ténéré with yardangs formed in diatomite.

Swamp-iron crusts In the former shallow water margins of Pleistocene and Holocene palaeolakes of the Ténéré, the Tchigai Plateau and the Erg de Bilma, gleyic palaeodune sands are generally overlain at the surface by low ridges or a horizontal mesh of swamp-iron rhizoconcretions. There are only few references to them from the Sahara of Eastern Niger (Maley, 1980; Baumhauer, 1993). Similar deposits from palaeolake margins of the Eastern Sahara were described by Kröpelin (1993). The origin and nature of these stratigraphically and palaeoclimatically significant crusts were studied by Felix-Henningsen (1997, 1998): the former roots and stems of swamp vegetation had been transformed into massive goethite casts, which had impregnated the palaeodune sand around them, often with an outer zone of lepidocrocite. Together with the iron oxide, there were concentrations of P, Ca, Mg and heavy metals. Their formation and morphological differentiation were linked to the gently inclined shorelines and shallows of the palaeolakes within the ancient dunes. Formerly increasing water depth shows in the transition from a massive layer of rhizoconcretions to indurated plant stems sticking out of the sand following deflation. Because of the gently rolling to often perfectly horizontal surface of the Ténéré, swamp iron concretions are generally frequent and may cover large continuous areas. Within the more accentuated relief of the Erg de Bilma there is a pattern of narrow shoreline fringes on the flanks of sand-dune depressions. The concretions originated from the oxidation and precipitation of reduced iron ions of the lake water and their concentration around the roots of reeds and macrophytes. High redox potentials can be excluded for the altogether shallow lake environments because of the low oxygen content of the warm water in the first place and further microbiotic


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consumption for the decomposition of plant remains. Thus the formation of rhizoconcretions can only be due to the specific physiology of the swamp vegetation, namely the existence of an aerenchyme supplying the roots with oxygen from the air. The release of excess oxygen from the roots caused a high redox potential locally, leading to oxide precipitation around each of the roots. The movement of ions from the reduced sediments of the lake floor to the root space was mainly due to diffusion. Palaeoclimatically the existence of swamp ore deposits stands for prolonged periods of a constant lake level. Their presence at various heights above the lake bottoms, most evident on the flanks of the dune depressions is witness to the trend of shrinking palaeolakes with increasing aridity, interrupted by phases of lake-level stability (cf. Felix-Henningsen, 2000). Ancient sand dunes, swamp iron deposits, palaeolake deposits, as well as the palaeosoils dealt with in the following chapter, are sensitive indicators for changing environmental conditions of widespread occurrence. Their dating is expected to contribute significantly to our knowledge of the regional differentiation of the Central and South-Saharan past environments. Palaeosoils On the ancient dunes of Niger’s Central and Northern Sahel, an almost continuous cover of palaeosoils has been preserved, the youngest of which developed during the Early Holocene humid phase. For the Nigrian region, studies have been published on their nature and palaeoclimatic interpretation by Grunert (1988), Völkel (1988, 1989), Pfeiffer and Grunert (1989), Pfeiffer (1991), and Völkel and Grunert (1990). Studies from other parts of the Sahel have been presented by, among others, Felix-Henningsen (1983, 1984, 2000), who studied palaeosoils from the Southern Central Sahara to the Northern Sahel during an expedition in 1991. The quite thick palaeosoils remained generally well preserved after their formation in the Holocene; only parts of their upper horizons underwent either erosion or burial by colluvium and windblown sand. In the South and Central Sahara, too, palaeosoils and relict soils are still widespread on Quaternary fluvial sediments and on ancient Pleistocene and Holocene dunes. In deflation areas they may also be exposed at the surface and are overlain by windblown sand of varying thickness or by dunes. Völkel (1988, 1989) and Felix-Henningsen (1992, 2000) report that the palaeosoils have been preserved to different degrees in the various landscapes of the region, depending on their degree of development during the pluvials and geomorphic activity in the subsequent phases of aridity. In the Ténéré the original ancient-dune terrain was fairly levelled, and pluvial-time palaeosoils have been preserved over wider areas than in other regions of the South and Central Sahara. Neolithic tumuli on level ground indicate that the soil surface around them has been lowered by a few decimetres at most since their construction. In the more accentuated palaeodune landscape of the Erg de Bilma, deflation and surface wash have removed much of the soils from dune tops and steeper slopes. Preservation was better only in the lower parts of depressions. On the windward and leeward dunes and fluvio-aeolian sand ramps of the scarplands to the North and the Zouar uplands, the environment was more favourable to soil formation during the pluvials than in the adjacent plains. Less evapotranspiration in wind-protected positions allowed the earlier onset of soil formation at the beginning of a pluvial phase, and a prolongation at its end. Additional runoff from the rocky slopes above was probably also conducive to weathering and thus soil formation. Differences in thickness, colour and stability of soil profiles of palaeosoils of most likely the same age may thus be explained by their position either on the escarpment slopes or far away from them. Their protected leeward position was also conducive to their preservation during the subsequent

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arid phases. As yet no palaeopedological information is available for the region east of the plateaus of Djado and Mangueni. Studies to date suggest that palaeosoils exist from at least three pluvials in the study area: a Late Pleistocene pluvial (c. 40–20 ka BP), the Ghazalien (Servant, 1983), an Early Holocene pluvial (c. 14–7,5 ka BP), the Tchadien (Servant, 1983), and a Mid-Holocene pluvial (6,5–4 ka in the Western Sahara), the Nouakchottien (Michel, 1973). Each of the pluvials was preceded by an arid phase of dune-sand deposition and dune formation. During the oldest arid phase, at 40 ka BP, palaeodunes were deposited in the Erg Ancien, extending far into the Southern Sahel (Gavaud, 1965). The pedostratigraphic interpretation of palaeosoil relics spread over large areas is based on grouping those with a similar inventory of characteristics and a comparable stage of development. For those deviating in both respects, formation during another pluvial phase is inferred. As there have been repeated shifts of the monsoonal climatic regime to the North into the Sahara (cf. Warren, 1970), zonal differences in weathering intensity and profile development have to be expected for the soils of each pluvial. This should be taken into account when comparing palaeosoils from different meridional regions. Some statistical evidence, though based on only a small number of samples, was presented by Felix-Henningsen (2000). 1.4 DEVELOPMENT OF THE PALAEO-ENVIRONMENTS The palaeoenvironmental evidence obtained by Baumhauer from the Ténéré, the escarpment forelands and the ergs to the South is in good agreement on a millennium scale with the chronology of the Late Pleistocene and Holocene climate and landscape changes obtained for the Southern Sahara (cf. Baumhauer, 1984–2002; Gasse, 1988; Maley, 1980; Servant, 1973; Servant-Vildary, 1978). With regard to lake formation, however, there appear to have been sufficiently humid conditions from the end of the Pleistocene to the Mid-Holocene only, with a decreasing tendency to 6,5 to 5,5 ka BP and interrupted by several short arid phases, with the onset of fully arid conditions since 5 ka BP at the latest. Finely bedded lake sediments from the depression of Seggedim, at the foot of the northernmost part of the Bilma Escarpment (and thus the northernmost deposit studied in the South-Central Sahara so far), have yielded evidence that the decisive change in climate and landscape development to the present hyperarid conditions took place even earlier in the Holocene and ended by about 6,5 ka BP. By that time, the transition from the Early Holocene fresh-water environment to sebkha conditions had become definitive (Baumhauer et al., 2004). The very close contact between Sudanese and Saharan vegetation evident for the Early Holocene became non-existent by 6,5 ka BP. Palynological evidence since 7 ka BP indicates a contracted permanent vegetation with a species composition similar to the present one (Baumhauer et al., 2004). This suggests a significantly weaker climatic gradient up to the Mid-Holocene than today, due to the interference of the monsoonal and Atlantic (Mediterranean) cyclonic circulation elements, which gradually became weaker during the Mid-Holocene (Baumhauer et al., 2004). Support comes from the palaeopedological studies conducted by Felix-Henningsen (1992–2000), as well as from first, still highly selective palaeoecological studies in the Djado Plateau region referred to above (Striedter et al., 1995). Felix-Henningsen (1992, 2000) conducted detailed pedochemical, micromorphological and clay mineralogical studies on the origin of palaeosoils. He concluded that the palaeosoils formed on the ancient dunes of the region developed during semi-humid conditions. His macro- and micromorphological, soil-physical, geochemical and claymineralogical analyses allow a sequence of processes to be reconstructed. The formation


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of browned to partly rubefied horizons is characteristic of Pleistocene and Holocene chromi-cambic arenosols; they originated from silicate weathering as part of a pedogenetic sequence, preceded by desalinization, decarbonatization and acidification due to leaching by percolating soil water in a more humid climate. Newly formed clay minerals—though quite immobile in the soil-water solution—were very uniformly distributed within the Bv horizons, suggesting an originally quite uniform distribution of the weatherable minerals, best explained by the deposition of dust simultaneously with the sands. Electrostatic piggybacking of dust particles is a common phenomenon in aeolian sand transport (cf. Stengel, 1992). Dust deposited during the pluvial phases was probably also spread throughout the soil by percolation and bioturbation. For a review of the significance of dust sedimentation in the Sahel and Sahara cf. Völkel (1989, 1991), Stahr et al. (1994), Hermann et al. (1994), and Hermann (1996). The decreasing haematite content in the palaeosoils of the Early Holocene from South to North and West to East (Felix-Henningsen, 1992, 2000) suggests decreasing humidity to the North and East during the Early Holocene pluvial. Similarly the decreasing thickness and weathering intensity to the North suggests a reduction of time available for the soilforming processes by the later onset and earlier termination of the Early Holocene pluvial phase in relation to the shifting monsoon regime. Rubification, clay-mineral formation and aggregate stability show the same regional trend. A very small-scale, slope-related variation of the Fe-oxide minerals exists in the palaeosoils in transition to the Northern Sahel. In the formerly quite humid lower slopes and valley floors between the ancient dunes, only goethite was formed in the developing soil. The input of organic matter was possibly higher in those positions due to their higher vegetation density, which blocked the aging of the original ferrihydrite to haematite and was conducive to the neoformation of goethite. Similar slope conditions in the more central parts of the Southern Sahara could also have caused the simultaneous formation of goethitic and rubefied palaeosoils next to each other under pluvial conditions. These finds show the need for studying the small-scale or catena variation of soil properties in palaeopedological studies of the region, to make sure that specific local conditions are not mistaken as evidence for sweeping pedogeographical and palaeoclimatological interpretations. Palaeoclimatic evidence from numerous studies in the Western Sahara point to another humid phase between 4,5 and 2,5 ka BP, followed by the onset of the hyperarid conditions still prevailing today (among others: Michel, 1973; Petit-Maire, 1989, 1991, 1994; Petit-Maire and Kröpelin, 1991; Petit-Maire and Riser, 1983; Reichelt et al., 1992; Völkel, 1988; Mauz and Felix-Henningsen, 2005; Felix-Henningsen and Mauz, 2005). In contrast to evidence from the Western Sahara, studies by the Berlin Collaborative Research Centre SFB 69 in the Eastern Sahara identified only an Early- to Mid-Holocene pluvial, beginning around 9,5 ka BP and persisting, with decreasing humidity and interrupted by several arid phases, to around 4 ka BP—thus longer than in the South-Central Sahara— before the abrupt change to hyperaridity took place (among others: Guo et al., 2000; Pachur et al., 1987, 1990, 1991, 1996; Kröpelin and Soulie-Märsche, 1991; Pachur and Hoelzmann, 1991; Hoelzmann, 1992; Kroepelin, 1993, 1999; Pachur, 1997, 1999; Holzmann et al., 2000; Pachur and Altmann, 2006). Workers from all parts of the Sahara agree that, beyond the general changes of climate and the environment during the Holocene, there also existed a palaeoclimatic zonality with latitudinal shifts of the environmental belts, for the Southern Sahara Servant (1983), Servant-Vildary (1978) and Maley (1980), for the Western and West-Central Sahara the working group around Petit-Maire (among others: Petit-Maire, 1987, 1991, 1994; PetitMaire and Kröpelin, 1991; Petit-Maire and Riser, 1983; Schulz, 1991), and for the Eastern Sahara the group around Pachur (among others: Guo et al., 2000; Pachur and Hoelzmann, 1991; Hoelzmann, 1992; Kröpelin, 1993, 1999; Pachur, 1997, 1999; Hoelzmann et al.,

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2000; Pachur and Altmann, 2006), as well as Haynes et al. (1989) and Ritchie and Haynes (1983). From their palaeoecological studies Hoelzmann (1992), Kröpelin (1999), Neumann (1989), Pachur (1999) or Kuper and Kröpelin (2006) have inferred that ecosystem structures in the Eastern Sahara during the Early Holocene pluvial optimum resembled those of the savannas around 13°N of today, related to a northward shift of the Sahel boundary in East Africa by about 600 km. This is in good agreement with the known northward occurrence of the EarlyHolocene pluvial soils in Eastern Niger (Felix-Henningsen 1992 to 2000). A direct comparison of the soils is problematic, though, as only a few studies on the palaeosoils on ancient dunes are available for the Eastern Sahara, though with supporting evidence from environmental history studies in the Goz region of the Sudanese Sahel (Warren, 1970; Felix-Henningsen, 1983, 1984; Gläser, 1987). Palaeosoils from several pluvials have been identified there on ancient dune fields of different ages, characteristic enough to serve as stratigraphic markers. Another piece of evidence comes from the palynological interpretation of the sediment core of Seggedim (Baumhauer et al., see chapter 3): up to at least 20°N there existed a Mid-Holocene (Sahelian) savanna vegetation, with an influx of Saharan flora elements as early as 7 ka BP. This early apparent aridification is also attributed to a persistent human impact along the boundary between the Sudanian and Sahelian vegetation since MidHolocene times. In conjunction with progressive climate deterioration, this interference led to the typical Sahelian savanna vegetation, possibly sooner than would have happened under purely natural conditions (Baumhauer et al., 2004; Schulz, 1991; Wasylikowa, 1992). The studies conducted by the Collaborative Research Centre SFB 268 “Cultural development and language history of the West African savanna” are only marginally concerned with the Late Quaternary landscape development and present morphodynamics of the South-Central Saharan region. The high-resolution pollen diagrams from the Manga grassland (Salzmann, 2000; Salzmann and Waller, 1998) yield no evidence of humaninduced development of the Sahel, thus contradicting the findings from the Central Sahara. The authors admit, however, that the problem of finding indicators of human impact has not yet been solved (Salzmann, 2000). At the start of the SFB 268 studies, a very early human influence was assumed, mainly owing to the high pollen percentages of a secondary shrub and pollen of certain plants that exist as weeds today (Ballouche and Neumann, 1995). More recent evidence suggests that human impact dates back only to the third millennium B.C. and originally consisted in a combination of nomadic animal husbandry, food gathering and some local cultivation without permanent settlements (Neumann et al., 2001). The transition to a sedentary economy did not occur until around 3 ka BP. For the Southern Chad Basin the model presented by Servant (1973) was largely accepted, assuming an arid phase from 20 to 12 ka BP, a rapid lake-level rise around 9 ka BP, a maximum level around 6,5 ka BP, a short arid phase around 3,5 ka BP, and ever since a general shrinking of the lake. Pedological findings from Bama Ridge—older dune sands with chromic arenosols and younger ones with cambic arenosols (Thiemeyer, 2002)—suggest more than one maximum lake level, the youngest one occurring around 6,5 ka BP. Climate oscillations at the beginning of the Holocene, around 11,4 to 10,3 ka BP, have been derived from the analysis of clay deposits in the dune depressions. The situation in the Southern Chad Basin resembled that of Burkina Faso today: permanent settlement began around 3 ka, the people living on a combination of hunting, gathering and fishing, animal husbandry and the cultivation of domesticated Pennisetum millet (Gronenborn, 1998). Earlier permanent settlement had been prevented by frequent floods. The first settlers are assumed to have come from the North and Northwest. No information on an early human impact in the region has been found so far. However, if millet cultivation was introduced to the Sahel from the Sahara, as assumed by Neumann


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et al. (2001), the question arises whether the environment there during the third millennium B.C. was still Sahelian or already Saharan. To explore environmental changes in the northern part of the Chad Basin, P. FelixHenningsen and R. Baumhauer led a geo- and bioscientific expedition to the Central Sahara in March and April 1997, as part of the DFG-funded project “Late Quaternary landscape development and present sediment dynamics between Lake Chad and Tibesti.” During this expedition, the palaeopedology group from the University of Giessen studied the palaeosoils on ancient dunes and lake sediments along a transect from East of the lake to the oasis of Faya, just South of the Tibesti Mountains. An additional research focus was the alternation of palaeodune and -lake sediments in former terminal lakes and in the Bodélé Depression West of Faya. Field work and TL/OSL dating revealed that the sand dunes between the Northern Sahel and Faya are of Early to Mid-Holocene age, with progressively younger ages to the South, consistent with progressive aridity since the pluvial maximum. Stratigraphy and datings agreed with previous palaeolimnological results on the former extent and lake levels of Lake Chad. However, the expedition’s results showed that earlier studies had erroneously attributed the ancient dunes of the region to the Pleistocene. Soil formation mainly took place during the subsequent Mid-Holocene pluvial starting around 3,1 ka. It led to moderately developed cambic arenosols, overlain by humic colluvia on lower dune slopes. Soils that had developed in a terrestrial environment on low dune shields during the early pluvial became gleyic with the rising groundwater table and were eventually buried beneath lake sediments when flooded by the Palaeochad. Compared to the deeply weathered and rubefied chromi-cambic arenosols of the Pleistocene sand dunes of Eastern Niger, those in the Northern Chad region show a clearly lesser degree of soil development. This is because in Niger soil formation had already started with the onset of the Early Holocene pluvial. Additionally, due to the West-East gradient of continentality and thus humidity of the present climate, pedogenic processes are still at work in the soils of the Nigrian palaeodunes, whereas soil formation in the Northern Sahel zone of the Chad region to the East came to a complete standstill with the onset of Saharan hyperaridity. Deposition and illuviation of carbonates, salts and silica and their aeolian sand blanket show that the A-horizons and B-horizons are fossil. As well as the dune soils West of Faya and of the Bodélé buried by lake sediments, gleyic to wet gleyic soils with humic A-horizons were found intercalated with the lake sediments, also subaquatic soils of former shallow-water areas with carbonatic and oxidic rhizoconcretions. For lack of funds they have not yet been dated and thus cannot be placed in a stratigraphy. Even so, repeated alternations of dune sands with lake sediments and the presence of gleys and rhizoconcretions within the lake sediments point to abrupt oscillations between humid and arid phases. Whereas the presence of palaeosoils on the fossil dunes is evidence of increasing humidity, palaeosoils intercalated with the lake sediments reflect somewhat lower lake levels and thus decreasing humidity. Towards the end of the last lake phase of the region, gleys with all their typical characteristics developed on the largely diatomitic sediments. 1.5 PRESENT MORPHODYNAMICS The present morphodynamics of the Southern Central Sahara has been underrepresented in research studies, as the emphasis of most publications has been on the landform, environmental and, derived from it, the palaeoclimatic history of the region, except for a number of publications on the Tibesti Mountains by workers at the Bardai Research Station of Freie Universität Berlin in the late 1960s (e.g. Grunert, 1970; Jäkel, 1971, 1979; Molle, 1969, 1971; Pachur, 1966), or by Mainguet (e.g. 1968) for neighbouring areas.

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The publications by Busche et al. (1979), Busche (1982), to a larger extent Busche (1998) and Grunert (1983) concern the western and southern fringe of the Murzuq Basin. Insights into most recent, largely aeolian morphodynamics may be gained from a number of wide-range palaeoclimatological studies, for example by Mainguet (1995), Servant (1973), Sirocko et al. (1993), or the numerous studies of the PAGES—PEPIII: Past Climate Variability Through Europe and Africa project (for project-summary see Batterbee, et al., 2004); however, their respective models and superordinate conclusions have still to be verified. No specific studies have been conducted modern fluvial processes in the region, for the simple reason that during the rather short stays, at least of the Würzburg team, no rainfall events took place in the hyperarid environment. Observations have been made, though, on aeolian activity. On the corrosion side, there is much evidence of present processes, but most of the aeolian corrosion landforms date from older times of much higher wind speeds, or a higher availability of grinding sands, when, channelled around the Tibesti Mountains, complete sections of the scarplands of the region were worn down and their remains became streamlined. During most likely several such phases, one of them also with extremely corrosive winds from the South (-West), the region affected was extended much more to the North and South than the region presently subjected to the low-energy corrosion. In most parts of the region, the fossil nature of the corrosion landforms is testified to by their coating of decaying desert varnish. In soft saprolitic rocks and in wind-exposed positions. In the Ténéré region and on the plateaus of the Western Murzuk Basin, corrosion may nevertheless have worn down surfaces by a few centimetres to decimetres since the Neolithic (Hagedorn, 1979). Studies by Busche and Stengel (1993) have shown that present aeolian sand movement and dune-shaping processes of the region are less strong compared to the phases of aeolian deposition and dune formation of the Pleistocene past. REFERENCES Adams, L.J. and Tetzlaff, G., 1984, Did lake Chad exist around 18.000 yrs BP. Archives for Meteorology, Geophysics and Bioclimatology, 34, pp. 299–308. Aumassip, G., 1993, Chronologies de l’art rupestre saharien et nord africain. Gandini, Calvisson, p. 32. Ballouche, A. and Neumann, K., 1995, A new contribution to the Holocene vegetation history of the West African Sahel: pollen from Oursi, Burkina Faso and charcoal from three sites in Northeast Nigeria. Vegetation History and Archaeobotany, 4, pp. 31–39. Barth, H.K. and Blume, H., 1975, Die Schichtstufen in der Umrahmung des MurzukBeckens (libysche Zentralsahara). Zeitschrift für Geomorphologie N.F., Suppl. 23, pp. 118–129. Battarbee, R.W., Gasse, F. and Stickley, C.E., (Eds.), 2004, Past climate variability through Europe and Africa. Developments in Palaeoenvironmental Research, Suppl. 23, Springer Netherlands. Baumhauer, R., 1986, Zur jungquartären Seenentwicklung im Bereich der Stufe von Bilma (NE-Niger). Würzburger Geographische Arbeiten, 65, pp. 1–235. Baumhauer, R., 1987, Holozäne limnische Akkumulationen im Bereich der Stufen von Zoo Baba und Dibella (NE-Niger). Palaeoecology of Africa, 18, pp. 167–177. Baumhauer, R., 1987a, Holocene limic accumulations in the Great Erg of Bilma. In: Petit-Maire, N. and Vanbesien, C., (Eds.), Past and future evolution of deserts, IGCP 252, pp. 1–11. Baumhauer, R., 1987b, Das Kawar—holozäne Seen in einem Schichtstufenvorland. Verhandlungsbd. 46. Deutscher Geographentag, pp. 332–341.


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Thiemeyer, H., 2002, Maiduguri’s natural environment. In: Kawka, R. (Ed.). From Bulamari to Yerwa to Metropolitan Maiduguri. Westafrikanische Studien, 24, pp. 67–78. Tillet, T., 1983, Le paléolithique du bassin tchadien septentrional (Niger/Tchad). C.N.R.S. Paris, pp. 1–379. Tillet, T., 1989, La fin de temps paléolithiques dans le Bassin Tchadien nord-occidental. Studies in African Archeology, 2, pp. 507–511. Von Storch, H., Cubasch, U. and Gonzales-Rouco, J.F., 2000, Combining palaeoclimatic evidence and GCMs by means of data assimilation through upscaling and nudging. Proc. 11th Symp. on global change studies, pp. 28–31. Völkel, J., 1988, Zum jungquartären Klimawechsel im saharischen und sahelischen Ostniger aus bodenkundlicher Sicht. Würzburger Geographische Arbeiten 69, pp. 255–276. Völkel, J., 1989, Geomorphologische und pedologische Untersuchungen zum jungquartären Klimawechsel in den Dünengebieten Ost-Nigers (Südsahara und Sahel). Bonner Geographische Abhandlungen, 79, pp. 1–258. Völkel, J., 1991, Staubsedimentation im nordafrikanischen Sahel—Herkunft und Auswirkung auf die Landschaftsökologie eines semiariden Großraumes. Zeitschrift für Geomorphologie N.F., 89, pp. 73–85. Völkel, J. and Grunert, J., 1990, The problem of dune formation and dune weathering during the Late Pleistocene and Holocene in the Southern Sahara and Sahel. Zeitschrift für Geomorphologie N.F., 34, pp. 1–17. Warren, A., 1970, Dune trends and their implications in the Central Sudan. Zeitschrift für Geomorphologie N.F., 10, pp. 154–180. Wasylikowa, K., 1992, Plant macrofossils from the archaeological sites Uan Muhuggiag and Ti-n-Torha/Two Caves, Tatdrad Acacus, SW-Libya. Proceed. Intern. Symp. “Environmental changes and human culture in the Nile Basin and Northern Africa”, pp. 24–27.

CHAPTER 2

Comparison of proxy-based palaeoenvironmental reconstructions and hindcast modelled annual precipitation—a review of Holocene palaeoenvironmental research in the Central Sahara Brigitta Schütt and Jan Krause Department of Earth Sciences, Physical Geography, Freie Universität, Berlin, Germany ABSTRACT: The development of Late Quaternary palaeoenvironmental conditions in Northern Africa west of Sudan is described and discussed on the basis of a literature review of proxybased palaeoenvironmental research. Reconstruction of these palaeoenvironments includes references to analyses of all types of proxies from primary and secondary literature, especially of palynological, lacustrine, pedological and archaeological on-site findings. Palaeoenvironmental information given by climate proxy data is set in relation to palaeoclimates derived from hindcast modelling (Flohn and Nicholson, 1980; Kutzbach, 1979), and the difficulties of comparing the two approaches are demonstrated. Differences in lag time of system reactions can also be reflected in the palaeoenvironmental proxies, whereas hindcast modelling points to the climatic impulses triggering these reactions. It is emphasised that a valid palaeoenvironmental reconstruction needs to differentiate clearly whether proxy data provide information on pre-, syn- or post-sedimentary conditions and whether they refer to the source area or to the depositional environment.

2.1 INTRODUCTION 2.1.1 Objectives The fragmentation of Holocene palaeoenvironmental conditions of Northern Hemisphere Africa is a major topic in scientific literature. On the basis of an evaluation of primary literature, the Late Quaternary palaeoenvironmental conditions of the Sahara are analysed and collated. The resulting diachronous overview of palaeoenvironmental reconstruction discloses regional differences. The aim of this paper is to explore whether palaeoenvironmental conditions as given by climate proxy can be directly related to palaeoclimate data as given by climate hindcast modelling. The analysis is based on data from selected primary and secondary literature documenting research on different types of archives such as lacustrine sediments, palaeosoils and archaeological findings, and based on different multi-proxy approaches (pollen, chemistry, minerals). In total, 52 original publications concerning Northern Africa west of Sudan were taken into account for this analysis. In fact, many more publications exist. But an intensive review shows that many results were reproduced and published more than once, frequently in combination with

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Figure 1. Location of research areas of the analysed literature. Numbering according to figure 2.

other data. Extractions of palaeoenvironmental information from the literature analysis are spatially referenced (Figure 1), temporally itemised (Figure 2) and diachronously displayed (Figure 3a–c). On this basis it is also intended to re-assess the spatially differentiated trend of Holocene aridisation of the Central Sahara. For comparison, the CCM0-modelled hindcast of annual precipitation is used. These data are available with a resolution of 7,5 * 4,4° (Kutzbach and Guetter, 1986 and Wright et al., 1993) (Figure 3d–f). 2.1.2 Study site Environmental conditions in the Central Sahara are strongly driven by climate. On a continental scale, it is evident that the present-day distribution of ecoregions is similar to the distribution of climates (White, 1983; Schultz, 2005). In the Central Sahara the spatial distribution of rainfall and its seasonality are determined by the general circulation pattern of Africa. In the Northern Hemisphere summer the ITCZ (Intertropical Convergence Zone) and an associated low pressure cell over the Sahara lie between the moist SW monsoon and the dry NE harmattan (trade wind). During the Northern Hemisphere winter months the ITCZ is located further South and a high pressure cell is established over the Sahara. At this time of the year the northern part of Northern Africa is under the influence of the rainbearing mid-latitude westerlies, whereas its southern part is influenced by the

A review of Holocene palaeoenvironmental research in the Central Sahara

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Figure 2. Palaeoenvironmental character over the last 20 ka cal BP for selected study sites of Northern Africa, for localization compare figure 1. The chart shows the palaeoenvironment differentiated by studies and is divided into more arid (light grey), more humid (black) and transition periods (dark grey). Additionally light triangles mark arid intervals, while dark ones point to humid intervals. White triangles denote the end of more humid conditions. The boundaries of humidity and aridity phases are set based on the median probability of the lower and higher age range (given by CALIB 5.0.1).

dry NE harmattan. Hence, Northern Africa separates into a winter and a summer rainfall regime with the Sahara in between (Nicholson, 2001). Thus, the central part of the Sahara is presently under the influence of the subtropical high-pressure belt with little impact by either the westerlies or the ITCZ (Weischet and Endlicher, 2000). Rainfall is only erratic, and mean annual rainfall totals 50–100 mm with parts of the area receiving no rainfall for years. Perennial vegetation only occurs at locations where groundwater is near-surface such as oases and mountain areas with higher rainfall (Beuchelt, 1968). Ephemeral xerophytes are found after erratic rainfall events (Busche and Stengel, 1993).

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Figure 3. Left column: Spatial distributed palaeoenvironmental reconstruction for the time slices 9, 6, 3 ka cal BP referring to the literary analysis. Right column: Spatial distributed results for the CCM0 hindcasts for the time slices 9, 6, 3 ka BP (modified after Kutzbach and Guetter, 1986; Wright et al., 1993; (www.ncdc.noaa. gov/paleo/modelvis.html).

The area of interest can be clustered into five sub-regions (Figure 1): − the Mediterranean Maghreb in the climatic transition zone from a Mediterranean environment with winter rainfall to a desert environment, − the Western Sahara (including Western Sahara, Mauritania and Mali) with a dry climate due to the influence of the cold Canaries current (coastal desert) and the dry NE trade winds,


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− the Central Sahara with locally hyperarid conditions caused by high continentality and NE trade winds nearly all year round, − the Sahelian Zone with a subarid to dry–subhumid climate, receiving its moisture primarily from the SW monsoon, − the Sudanian-Guinean-Zone with a subhumid climate, also receiving its moisture from the SW monsoon. 2.2 METHODS AND SOURCES This literature analysis is mainly based on primary literature focusing on clear age determination of the palaeoenvironmental proxies and thus on palaeoenvironmental reconstruction. A further requirement is that the study site can be localised, even if it is sometimes not possible to geocode the sites. Owing to the necessity of clear age determinations the literature analysed is confined to the period since the 1970s, when 14C dating was established for palaeoenvironmental research. To make dating results comparable, data are checked to determine whether they are calibrated; if necessary calibration was performed, applying the method developed by M. Stuiver and P. J. Reimer with their calibration program CALIB 5.0.1 (copyright 1986–2005) to convert into calendar years (calBP). The time-driven processes of radioactive decay of 14C are the basis of this absolute physical dating method (Geyh, 1971, 1983). For the non-marine radiocarbon samples in the Northern Hemisphere the calibration curve IntCal04 (Stuiver et al., 2005) is used. The calibration curve error σk is set with an accuracy of 2-sigma (95% certainty) of the conventional age (Wagner, 1995). If lab errors are given in the literature these values are adopted; if not, the lab error is set at 0. The lab error is set as multiplier. The standard deviation of the conventional aging (SD years/age uncertainty) was also adopted if known, otherwise set to 1. The median of the calibrated calendar years is used to display the climate data. 2.3 LITERATURE ANALYSIS Palaeoenvironmental research was largely carried out in the Central Sahara with most sites in Mali, Niger and Tchad, indicating systematic studies over decades. By contrast, palaeoenvironmental research in the adjoining area of the Southern Sahel seems to be more erratic. To the North, in the Mediterranean Maghreb, palaeoenvironmental research also has a long tradition; thus, many publications are available. But as the regional focus of this paper is on the Central Sahara, only selected papers are considered. Regarding Libya, publications on the Sahara are frequently published in the Italian language and in journals or reports that are not internationally accessible and thus could not be considered. The same applies to miscellaneous French-language publications on the Algerian Sahara. The fragmentation of the Holocene climate in Northern Hemisphere Africa in temporal and spatial distribution is a major topic in scientific literature. Most palaeoenvironmental reconstructions focus on moisture conditions, differentiating between ‘arid’ or ‘dry’, ‘humid’ or ‘wet’ and ‘dryer’ and ‘wetter’. The definition of the dry and humid periods and the evaluation of moisture change vary depending on the proxies and timescale used. Clear definitions—e.g. by P/E-ratios or annual rainfall amounts—are lacking. Therefore the often comprehensive results given in the literature were, if necessary, attributed to three main categories (more humid, transition period and more arid) to describe the palaeoenvironment of the past 20 ka cal BP. The attribution was carried out as shown in the following two examples: − palaeolake levels: high lake level—humid, low lake level—arid − pollen records: Saharan type—arid, Sahelian type—transition period, SudanianGuinean-type—humid.

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2.3.1 From the Late Glacial period to the Early Holocene wet phase During the Last Glacial Maximum (LGM), around 18 to 20 ka cal BP, arid conditions are known to have occurred at many locations in Northern Africa, including the Sahara and its northern and southern margins (e.g. Rognon and Williams, 1977; Lézine et al., 1990; Reichelt et al., 1992). Only Flohn and Nicholson (1980) state that conditions were humid or at least transitional at the northern flank of the desert. The ‘first’ dune generation was deposited during this dry period in Saharan and sub-Saharan Africa, (Völkel and Grunert, 1990); the best known are the longitudinal Ogolian dunes in today’s Sahelian and Sudanian zone (Michel, 1973; Elouard, 1962). The length of this arid period varies in time and place from author to author, but it is repeatedly attested that humidity gradually increased after 16 ka cal BP (e.g. Völkel and Grunert, 1990; Reichelt et al., 1992). However, all palaeoenvironmental research carried out in the Central Sahara confirms that the Last Glacial Maximum arid phase was succeeded by the onset of a humid phase during the Late Glacial already: Flohn and Nicholson (1980a, b) assume a fluctuating climate between 14 and 10 ka cal BP, with a semiarid climate in the Northern Sahara (Flohn and Nicholson, 1980a), and a humid Southern Sahara (Flohn and Nicholson, 1980b). On the basis of investigations in the Zoumri-Bardagué-Arayé river system of the Tibesti Mountains, Jäkel (1979) constructs a climate curve from 19 ka cal BP onwards. He hypothesises humid conditions in the Tibesti Mountains from 19–8,9 ka cal BP with most humid conditions from 10,7–9,5 ka cal BP, caused by an overlapping of rainfall-bearing air masses occurring throughout the year and originating from both monsoonal influence during summer and westerlies influence during winter. For the neighbouring lowlands of the Massif du Termit, Gasse and van Campo (1994) set the onset of humid environmental conditions at 13,8 ka cal BP, documented in palaeolake deposits. These data almost agree with the palaeoenvironmental reconstruction for the Massif du Termit given by Servant (1973, 1983). The latter places the change from extremely arid (Kanemian/Ogolian) to moderately humid conditions ((Nigero-)Tchadien I) at around 15,3 ka cal BP. Gasse (1990) defines the first arid-to-humid transition (AHT) for Northern Algeria at around 17,5 to 13,8 ka cal BP. Lézine and Casanova (1989) assign the change in the Sahelian zone to around 14,7 ka cal BP. Figure 2 shows a shift in the onset of the first AHT from the Sudanian zone (Gasse and van Campo, 1994) to the Western Saharan region (Petit-Maire, 1988; Hillaire-Marcel et al., 1982). Finally, after 13,8 ka cal BP, during the Nigéro-Tchadien I, environmental conditions were humid nearly all over Northern Africa, and by 11,5–8,9 ka cal BP (Tchadien) the regional water balance reached its optimum (corresponding to the Early Holocene wet period) (Völkel and Grunert, 1990; Flohn and Nicholson, 1980; Servant and ServantVildary, 1980). The Sahel stretched up to 23°N, whereas today it is located around 18°N (Claussen et al., 2002). In the most recent literature humid environmental conditions during the Early Holocene are subsumed to the African Humid Period (AHP) which began between 11,5 ka cal BP (e.g. Renssen, 2006) or 14,8 ka cal BP (deMenorcal et al., 2000). During this phase Western Saharan and Sahelian climate was affected by (relatively) high summer temperatures and enhanced precipitation. For the period 9 to 7,5 ka cal BP Renssen (2006) hindcasts a precipitation of 290 mm/a due to a strong land-sea thermal gradient which increases wet air mass transport. This results in an average vegetation cover of 70%. By contrast, Early Holocene annual precipitation at Dibella (Eastern Niger) is estimated to have been at least 300–400 mm/a (Grunert et al., 1991) or 250–400 mm/a (Flohn and Nicholson, 1980). It is assumed that perennial rivers were flowing during this period even in the hyperarid desert centre between Kufra Oasis and Tibesti Mts. (Flohn and Nicholson, 1980). The improved humidity was caused by the intensification of the African monsoon due to changes of the orbital parameters (de Menocal et al., 2000). Around 10,6 ka cal BP


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the intensity of the monsoon reached its maximum and lake water levels in the Sahara peaked (Baumhauer, 1986, 1991; Gasse et al., 1990; Flohn and Nicholson, 1980). The beginning of the Holocene humid phases corresponded with the end of glacial conditions in Europe and the North Atlantic Ocean and with a high concentration of methane in the atmosphere. The July insolation at 10–11 ka cal BP was 8% higher than today. The consequence was higher precipitation, 40% higher over North Africa than it is today (de Menocal et al., 2000; Kutzbach and Guetter, 1986; Prell and Kutzbach, 1987). 2.3.2

From the Early Holocene wet phase to the present

The climatic evolution to the hyperarid conditions that determine the climate today started in the Holocene (Baumhauer et al., 2004). The Early Holocene wet phase varied in duration depending on the location of the research area and on the beginning and the progression of the successional aridisation. Gasse (1990) states another two arid-to-humid transitions (AHT). The second AHT is dated 10,5–9,4 ka cal BP, and the third AHT from 8,3 to 8 ka cal BP. This instable phase lasted locally until 6 ka cal BP (Damnati, 2000). During this phase the annual precipitation and the average vegetation cover decreased to 210 mm/a and 50%, attended by an increased climate variability (Renssen et al., 2006). During this time, ongoing or amplified aridisation finally triggered the movement of Neolithic peoples, who had settled all over Northern Africa during the Early Holocene. They headed to the margins of the developing Sahara following the shifting Sahelian zone or to oasis regions such as the Nile valley or the mountain areas of Tibesti, Air and Hoggar (Reichelt et al., 1992; Flohn and Nicholson, 1980). The second dune generation of the Sahara was deposited in this period, indicating arid conditions (Nouakchottien after Michel, 1973; Völkel and Grunert, 1990). From 5,7 ka cal BP onwards the Sahara expanded, and its fringe moved approximately 450 km to the South and less extensively to the North (Reichelt et al., 1992). However, in the literature it is stated that the Holocene aridisation included one possible and one definitive interruption, i.e. phases of humidisation or temporarily stopped aridisation. Around 7,4 ka cal BP the second (Middle) Holocene humid phase started. It extended over the whole Sahara-Sahel and ended around 5,7 ka cal BP (Flohn and Nicholson, 1980; Geyh and Jäkel, 1973; Servant and Servant-Vildary, 1980; Gasse and van Campo, 1994). Petit-Maire (1987, 1988) sets the end of the freshwater lakes at around 5,2 ka cal BP. It is assumed that around 2,5 ka cal BP the climate finally changed to presentday conditions. Völkel and Grunert (1990) date the most recent dune generation to this period. 2.4 COMPARISON OF PALAEOENVIRONMENTAL RECONSTRUCTIONS AND ANNUAL PRECIPITATION 2.4.1 Source criticism Evaluating the available literature sources on proxy-based palaeoenvironmental reconstruction is complicated by the authors’ sometimes imprecise use of the terms palaeoclimate and palaeoenvironment. Both depend on each other, but mostly the coring or sampling results indicate palaeoenvironmental conditions that are sometimes described or interpreted as palaeoclimate indicators without any further comment. In addition, owing to fluctuating climates some uncertainty attaches to the results displayed in figure 2. For example, several sources state humid conditions for a certain period, although the climate was not constantly humid within the time slice selected. These fluctuations are

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frequently mentioned but neither clearly documented nor dated. Therefore in some cases we generalised the palaeoclimatic descriptions. In the literature analysed, palaeoclimatic or palaeoenvironmental conditions for the Central and Western Sahara are reconstructed. Their temporal assignment depends on the dating provided in the respective publication and thus reflects the uncertainty of several authors cited in this text. The ages of the publications and the exact notations of sample ages, especially in terms of calibration of 14C-data, may yield inaccuracies, especially as the dates used and shown in figure 2 are rarely confined to one single site. It is not always transparent whether the dates posted in the respective publication are calibrated or uncalibrated 14C measurements or whether different dating methods may have been used. 2.4.2 Community Climate Model (CCM0) The palaeoprecipitation data displayed in figure 3 (d–f) were hindcasted by Kutzbach and Guetter (1996) and Wright et al. (1993), using the Community Climate Model (CCM0) of the National Center for Atmospheric Research. Owing to the spatial resolution of the data provided and the model setup, these data contain strong generalisations and uncertainties. CCM0 includes atmospheric dynamics, which is based on the equations of fluid motions. It utilises radiative and convective processes, condensation and evaporation. The horizontal resolution totals 4° latitude and 7,5° longitude. The input data required are insolation, atmospheric gas concentrations, land albedo and soil moisture. Additionally the model considers mountain and ice-sheet orography, sea-surface temperature, sea-ice limits and snow cover. CCM computes an array of surface and upper-air parameters. The atmospheric energy budgets are subdivided into kinetic and potential energy and into zonal, stationaryeddy and transient-eddy components. Sea-level pressure and geopotential height fields are calculated. Annual precipitation is one of the output data the model provides. Modelling results are available in 3.000-year steps up to Last Glacial Maximum (20 ka cal BP). 2.4.3 Comparison The three time slices considered in this comparison correspond to the time slices of the available CCM0 modelling results and the palaeoenvironmental age information based on proxy data. Consequently, the diachronous overview focuses on the 9 ka cal BP, 6 ka cal BP and 3 ka cal BP time slices (see figure 3). 9 ka cal BP. This time slice falls within the African Humid Period (e.g. Renssen, 2006; de Menocal et al., 2000). Proxy data explicitly point out humid conditions all over Northern Africa west of Sudan (Figure 3a). This situation is imaged for all regions examined: ranging from the Maghreb, whose present-day precipitation predominantly originates from the westerlies, over the Western and Central Sahara, which are both presently strongly affected by the trade winds, to the Sahelian zone and the Sudan-Guinean zone, where present-day precipitation is generated by the monsoon. A regional differentiation of the palaeoenvironmental conditions due to humidity-aridity cannot be derived from the proxy data. All authors agree that maximum humidity during the Holocene occurred between ∼11–8,8 ka cal BP (Early Holocene wet phase). By contrast, the precipitation data provided by CCM0 (Figure 3d) distinctly show a zonal distribution with minimum values in the Maghreb and Western Sahara. Increased precipitation is shown for the Sahelian and the Sudan-Guinean zones with a peak in the area of the Niger river course. 6 ka cal BP. A first short aridisation phase in the Early Holocene, around 7,4 ka cal BP, was followed by a humid period, the Mid-Holocene wet period. Especially the Chad


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basin was still dominated by high lake levels and freshwater conditions (e.g. Servant, 1983; Servant and Servant-Vildary, 1980; Fontes and Gasse, 1990). However, the main feeders of the Chad basin discharge from the South, where the more humid conditions of the Sahelian-Saharan and Sudanian-Guinean zones occur. By contrast, the Western Sahara features more arid conditions (e.g. Lézine et al., 1990) (Figure 3b). Also the frequent lack of proxy data for this time slice indicates more arid conditions, the hiatuses probably being due to deflation or the lack of carbon-rich sediments appropriate for dating. In the Southern Sahelian and the Sudanian-Guinean zones proxies indicate predominantly humid conditions (e.g. Fontes and Gasse, 1990b; Lézine and Casanova, 1989b). The same applies to the Maghreb, especially in sites close to mountainous or coastal regions (e.g. Faust et al., 2004; Zielhofer et al., 2004; Lamb et al., 1995; Gasse et al., 1990; Fontes and Gasse, 1990d, e; Rognon and Williams, 1977b). This palaeoenvironmental image given by the proxy data is only partly reflected by the modelled palaeoprecipitation (Figure 3e). For the Western and Central Sahara as well as for the Maghreb annual precipitation is low, totalling 2–3 mm/d (after Kutzbach and Guetter, 1986; Wright et al., 1993). The corresponding annual precipitation of 110–200 mm depends on the number of rainfall days: an average of ∼55 annual rainfall days is assumed for the Sahelian stations N`Djamena, Zinder, Sokoto, Maiduguri (after Müller, 1996). By contrast, the Sahelian and the Sudanian-Guinean zones show annual precipitation with values around 4–5 mm/d. Assuming an annual average of 100 rainfall days according to the stations at Makurdi, Batouri, Enugu and Tamale (after Müller, 1996), this value corresponds to approx. 450–650 mm annual rainfall. Also in this time slice, the course of the Niger River is displayed by increased precipitation amounts (4–5 mm/d) and extends into the Sahara. 3 ka cal BP. The shift from the Mid-Holocene humid period to extremely arid conditions occurred after 5,5 ka cal BP according to Renssen (2006), Damnati (2000) and de Menocal (2000). In the Western and Central Sahara most of the palaeoenvironmental studies point to arid environmental conditions for the time slice selected (e.g. Flohn and Nicholson, 1980a, b; Hoelzmann et al., 2000; Hoelzmann, 1992; Servant, 1983; Lézine et al., 1990; Maley, 1977). An exception is made by Lézine and Casanova (1989) who worked on a site in the Chad basin. But again it needs to be considered that the Chad basin is fed by rivers draining from the South with their headwater areas in the more humid Sudanian-Guinean and Sahelian-Saharan zones. According to Völkel (1988) the fixation of the second dune generation by weathering and soil forming processes occurred during this phase. In the Mediterranean Maghreb, too, the palaeoenvironmental picture shown is incoherent and, including information from figure 2, indicates short-term palaeoenvironmental changes (e.g. Rognon, 1988; Faust et al., 2004; Lamb et al., 1995). South of the Sahara it becomes obvious that arid conditions shifted South and affected wide areas of the Sahelian zone (e.g. Waller and Salzmann, 1999; Rognon and Williams, 1977a). Humid conditions are confirmed at the sites of the Sudanian-Guinean zone (e.g. Gasse and van Campo, 1994e; Talbot and Delibrias, 1977, 1980; Lézine and Casanova, 1989a). Again the spatial distribution of palaeoenvironmental conditions given by the proxy data is only partly reflected by the modelled palaeoprecipitation (Figure 3f ). The image is similar to the situation at 6 ka cal BP, with increased annual precipitation only in the Sahelian and the Sudan-Guinean zones and along the Niger River course, varying in the reduced total amount of modelled annual precipitation. 2.4.4 Discussion There are several reasons for a major change from arid to humid and back to arid conditions in Northern Africa. One triggering criterion is the increase in the Northern Hemisphere

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summer solar radiation. This leads to the already mentioned enhanced land-sea thermal contrast and resulting increase of convective precipitation. These increasing rainfalls caused the AHP with all its local variations (cf. Renssen, 2006; Damnati, 2000; de Menocal, 2000). The shift from humid to recent arid conditions can partly be explained by the tilt of the Earth’s axis—which, at 10,2 ka cal BP, was stronger than today—with a resulting time of the perihelion at the end of July (Berger, 1978; Claussen, 1999). Yet the quite smooth variations in these orbital parameters in the Holocene scale are in contrast to climate and vegetation change which occurred abruptly in this timescale (Claussen, 1999). Input parameters for CCM0 model runs correspond to radiation attributes, modified by local conditions affecting insolation such as relief and albedo (Kutzbach and Guetter, 1986). Thus, imaging palaeoenvironmental conditions generated by hindcast climate models such as CCM0 focus on climate as a triggering factor and are visible at the zonal scale (Kutzbach and Guetter, 1986). Results from hindcast palaeoclimatical modelling reflect the effects of solar driven forces on the climate, modified by regional particularities such as the effect of expanded wetlands on the regional evaporation and water vapour content. By contrast, palaeoenvironmental proxies reflect the synsedimentary environmental conditions as controlled by climate forces, endogenic forces and humankind. Launched processes can be observed in matter flows such as surface runoff, erosion and sediment deposition (Schütt, 2004a, b). Palaeoenvironmental proxies represent reactions of the natural system to endogenic or exogenic triggers. Palaeoenvironmental reconstruction requires differentiation between the different types of triggers and the influence exerted by climate-driven impulses (Schütt, 2004a, b). Imaging palaeoenvironmental conditions as a result of proxy data analysis always points to regional system reactions on external or endogenic forcing—and thus is of local to regional scale. Consequently, regional palaeoclimatic conditions are not appropriate to explain the different behaviours of Saharan palaeolakes (Baumhauer, 1991). When analysing palaeoenvironmental proxies we need to consider, first, that processes launched by both exogenic and endogenic forces show short-term reactions and, second, that processes are initiated whose implications underlie longer time-lag effects. Groundwater recharge and groundwater flow belong in this process group. This is convincingly pointed out by Grunert et al. (1991), who show that two neighbouring Holocene palaeolakes of the Central Sahara—Dibella and Zoo Baba, located only 70 km away from each other in the Erg of Bilma—reacted differently, with different time lags and with different process intensities, to changing Holocene climate. During Early and Mid-Holocene wet phases, Dibella had brackish water with great fluctuations in salinity, whereas Zoo Baba was a freshwater lake even up to the Mid-Holocene. Despite the proximity of both systems, Dibella is fed by a local aquifer, whereas Zoo Baba is connected to the large Bilma/Kaouar aquifer. It is now well known that the former and present-day wetland areas of the Central Sahara, in particular the depressions along the foreland of the escarpments (cuestas), were predominantly groundwater fed. Thus, if they are part of larger groundwater systems longterm reactions to climate change need to be considered. With reference to Thorweihe et al. (1984), Baumhauer (1997) estimates that groundwater flow in the Bilma depression has a velocity of approximately 6 m/year. Because of the large extent of the Bilma/Kaouar aquifer with its headwater area in the Tibesti Mountains, a period of 50.000 years is required to bring naturally replenished groundwater from the Tibesti Mountains to the foreland depressions of the Kaouar. Thus, groundwater recharge of the lake depression is still active today, although regional environmental conditions are already dry. In view of the hydrological situation of the Chad basin with its receiving Lake Chad, another kind of teleconnection may have adulterated proxy-based regional palaeoenvironmental information: Lake Chad is mainly fed by river systems of the Yedseram, Chari, Logone and Erguig, perennial rivers with their headwater areas in the Mandara Mts. or the Massif de l’Adamoua—and thus located in


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the Guinean zone with an average of 1.200–1.500 mm precipitation annually (cf. Müller, 1996). Thus, expected sensitive reactions of a desert margin area such as the Lake Chad basin to regional climate changes are masked by inflow from less sensitive areas. In consequence, it is difficult to compare palaeoenvironmental information derived from the application of both approaches because short-, medium- and long-term reactions are all reflected in the palaeoenvironmental proxies, whereas hindcast modelling points to the climatic impulses triggering these reactions. At any rate, an individual, site-specific analysis of proxies is required to assess the factors controlling the processes. 2.5 CONCLUSIONS Palaeoenvironmental reconstruction is traditionally based on proxy data obtained from sediment analysis. Depending on the sediment facies and the proxies used, different information on the palaeoenvironment is derived. For instance, the sediment’s pollen content and composition allows conclusions about the synsedimentary vegetation cover and, if transfer functions are used, even about palaeoclimatological information. In contrast, sediment structure gives information on the relief-forming processes, and clay mineral composition yields information about presedimentary weathering conditions. In addition, we need to differentiate between proxies showing − synsedimentary palaeoenvironmental conditions such as salinity of a playa lake (e.g. carbonate composition, diatom and ostracode assemblages), − presedimentary palaeoenvironmental conditions such as soil-forming processes (e.g. soil sediments in lake deposits), or − long-term reactions, for example by a lake water body in a groundwater-fed system. Auditing the different palaeoenvironmental proxies shows that the quality, accuracy and complexity of the palaeoenvironmental reconstruction strongly depends on the archives and proxies used and on the specific concepts and approaches applied for analysis. Lake archives, especially of endorheic lakes, are ranked as the most valuable sedimentary archives for reconstructing the palaeoenvironment, as they have relatively few hiatuses and are frequently even laminated, corresponding to seasonal or event layers. However, final deposition of sediments takes place in a lake, whereas temporary deposition occurs in the drainage basin. Thus, lake sediments mainly provide information on the synsedimentary palaeolimnic environments, most likely without temporal interruption, whereas the generally temporally discontinuous fluvial and colluvial deposits in drainage basin archives provide high resolution and spatially differentiated information on synsedimentary morphodynamics and flow dynamics. Integrating studies, combining palaeoenvironmental environment and process information, are carried out only rarely— but ultimately these are the basis for understanding a system. Furthermore, it is common practice for palaeoenvironmental information to be considered applicable to the entire region, while ignoring that it is generally of punctiform character. In summary, palaeoenvironmental information derived from proxy data corresponds to conclusions on system reactions to exogenic, endogenic or human induced forces. In contrast, palaeoenvironmental information derived from climate hindcast modelling corresponds to exogenic triggers forcing geomorphological system reactions. Consequently, an integrated analysis of palaeoenvironmental information provided by proxy data and derived from climate hindcast modelling shows regionally diverging information. To allow further application and utilisation of palaeoenvironmental information derived from proxy data, data analysis must include the disclosure and consideration of all system

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characters. Without this, it will be difficult to assess whether the palaeoenvironmental indicators relate to short- or long-term effects. Satisfactory results are likely to be obtained by differentiating between these effects and by comparing palaeoenvironmental reconstructions based on proxy data (indicating system reactions) with palaeoclimatic data based on hindcast modelling (showing system impulses triggering the system reactions). ACKNOWLEDGEMENTS We would like to thank the German Research Foundation (DFG) for its financial support of the ‘Limnosahara’ research project (Schu 949/8), funded since 2005. Thanks are also due to Nicole Marquardt and Katharina Ducke for their considerable support in preliminary work within this project. REFERENCES Baumhauer, R., 1986, Zur jungquartären Seenentwicklung im Bereich der Stufe von Bilma (NE-Niger). Würzburger Geographische Arbeiten, 65, pp. 1–235. Baumhauer, R., 1987, Holozäne limnische Akkumulationen im Bereich der Stufen von Zoo Baba und Dibella (NE-Niger). Palaeoecology of Africa, 18, pp. 167–177. Baumhauer, R., 1988, Das Kawar-Holozäne Seen in einem Schichtstufenland. In: 46. Deutscher Geographentag München, 46, pp. 332–341. Baumhauer, R., 1997, Zur Grundwassersituation im Becken von Bilma, zentrale Sahara. Würzburger Geographische Arbeiten Band, 92, pp. 131–146. Baumhauer, R., 1991, Palaeolakes of the South Central Sahara: problems of palaeoclimatological interpretation. Hydrobiologia, 214, pp. 347–357. Baumhauer, R. and Hagedorn, H., 1990, Probleme der Grundwassererschließung im Kawar (Niger), Die Erde, 120, pp. 11–20. Baumhauer, R. and Schulz, E., 1984, The Holocene lake of Seguedine, Kaouar, NE Niger. Palaeoecology of Africa, 16, pp. 283–290. Baumhauer, R., Schulz, E. and Pomel, S., 2004, Environmental changes in the Central Sahara during the Holocene—the Mid-Holocene transition from freshwater lake into sebkha in the Segedim depression, NE-Niger. Lecture Notes in Earth Sciences, 102, pp. 33–47. Berger, A., 1978, Long-term variations of daily insolation and quaternary climatic changes. Journal of Atmospheric Sciences, 35, pp. 2362–2367. Beuchelt, E., 1968, Niger. Deutsche Afrika-Gesellschaft e.V. Bonn, Die Länder Afrikas, 38, Kurt Schroeder, Bonn, pp. 1–143. Busche, D. and Stengel, I., 1993, Rezente und vorzeitliche äolische Abtragung in der Sahara von Ostniger. Petermanns Geographische Mitteilungen, 137, 4, pp. 195–218. Claussen, M., Brovkin, V. and Ganopolski., 2002, Africa: Greening of the Sahara Africa: a hot spot of non-linear atmosphere-vegetation interaction. In: Steffen, W., Jäger, J., Carson, D.J., and Bradshaw, C., Challenges of a changing earth. Proceedings of the Global Change Open Science Conference, Amsterdam, the Netherlands, 10.–13.7.2001, Springer Verlag, Berlin, Heidelberg, N.Y., pp. 125–128. Claussen, M. and Brovkin, V., et al., 1999, Simulation of an abrupt change in Saharan vegetation in the Mid-Holocene. Geophysical Research Letters, 26, 14, pp. 2037–2040. Damnati, B., 2000, Holocene lake record in the Northern Hemisphere of Africa. Journal of African Earth Sciences, 31, 2, pp. 253–262. De Menocal, P., Ortiz, J., Guilderson, T., Adkins, J., Sarntheim, M., Baker, L. and Yaruskinsky, M., 2000, Abrupt onset and termination of the African Humid Period:


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Rapid climate response to gradual insolation forcing. Quaternary Science Reviews, 19, pp. 347–361. Elouard, P., 1962, Etude géologique et hydrogéologique des formations sédimentaires du Guebla mauritanien et de la vallée du Sénégal. Mémoires du Bureau de recherches géologiques et minières, Paris, 7, pp. 1–274. Fabre, J. and Petit-Maire, N., 1987, Holocene climatic evolution at 22–23°N from two palaeolakes in the Taoudenni area (Northern Mali). Palaeogeography Palaeoclimatology Palaeoecology, 65, pp. 133–148. Faust, D., Zielhofer, C., Escudero, R.B. and Diaz del Olmo, F., 2004, High-resolution fluvial record of Late Holocene geomorphic change in Northern Tunisia: climatic or human impact? Quaternary Science Reviews, 23, pp. 1757–1775. Flohn, H. and Nicholson, S., 1980, Climatic fluctuations in the arid belt of the “Old World” since the Last Glacial Maximum; Possible causes and future implications. Palaecology of Africa, 12, pp. 3–22. Fontes, J.C. and Gasse, F., 1990, PALHYDAF (Palaeohydrology in Africa) program. Objectives, methods, major results. Palaeogeography Palaeoclimatology Palaeoecology, 84, pp. 191–215. Gasse, F. and van Campo, E., 1994, Abrupt post-glacial climate events in West Asia and North Africa Monsoon domains. Earth Planetary Science Letter, 126, pp. 435–456. Gasse, F., Téhet, R., Durand, A., Gilbert, E. and Fontes, J.C., 1990, The arid-humid transition in the Sahara and the Sahel during the last deglaciation. Nature, 346, pp. 141–146. Gasse, F., Fontes, J.C., Plaziat, J.C., Carbonnel, P., Kaczmarska, P., de Deckker, P., Soulie-Märsche, I. and Callot, Y., 1987, Biological remains, geochemistry and stable isotopes for the reconstruction of environmental and hydrological changes in the Holocene lakes from North Sahara. Palaeogeography Palaeoclimatology Palaeoecology, 60, pp. 1–46. Geyh, M.A., 1983, Physikalische und chemische Datierungsmethoden in der QuartärForschung. Clausthaler Tektonische Hefte, 19, pp. 1–163. Geyh, M.A., 1971, Die Anwendung der 14C-Methode und anderer radiometrischer Datierungsverfahren. Clausthal-Zellerfeld, Verlag Ellen Pilger, pp. 1–118. Geyh, M.A. and Jäkel, D., 1973, Late Glacial and Holocene climatic history of the Sahara desert derived from a statistical assey of 14C dates. Palaeogeography Palaeoclimatology Palaeoecology, 15, pp. 205–208. Grunert, J., Baumhauer, R. and Völkel, J., 1991, Lacustrine sediments and Holocene climates in the Southern Sahara: The example of palaeolakes in the Grand Erg of Bilma (Zoo Baba and Dibella, Eastern Niger). Journal of African Earth Science, 12, 1–2, pp. 133–146. Hillaire-Marcel, C., Riser, C., Rognon, P., Petit-Maire, N., Rosso, J.C. and Soulie-Marche, I., 1982, Radiocarbon chronology of Holocene hydrologic changes in Northeastern Mali. Quaternary Research, 20 (1983), pp. 145–164. Hoelzmann, P., 1992, Palaeoecology of Holocene lacustrine sediments within the West Nubian Basin, SE-Sahara. Würzburger Geographische Arbeiten, 84, pp. 57–71. Hoelzmann, P., Keding, B., Berke, H., Kröpelin, S. and Kruse, H.J., 2000, Environmental change and archaeology: Lake evolution and human occupation in the Eastem Sahara during the Holocene. Palaeogeography Palaeoclimatology Palaeoecology, 169, pp. 193–217. Hoogchiemstra, H., Stalling, H., Agwu, C.O.C. and Dupont, L.M., 1992, Vegetational and climatic changes at the northern fringe of the Sahara 240.000–5.000 years BP: Evidence from 4 marine pollen records located between Portugal and the Canary Islands. Review of Palaeobotany and Palynology, 74, pp. 1–53.

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Jäkel, D., 1979, Run-Off and fluvial formation processes in the Tibesti Mountains as indicators of climatic history in the Central Sahara during the Late Pleistocene and Holocene. Palaeocology of Africa, 11, pp. 13–36. Kutzbach, J.E., 1979, Estimates of past climate at palaeolake Chad, North Africa, based on a hydrological and energy-balance model. Quaternary Research, 14, pp. 210–223. Kutzbach, J.E. and Ruddiman, W.F., 1993, Model description, external forcing and surface boundary conditions. In: Wright, H.E., J.E. Kutzbach, Webb, III, T., Ruddiman, W.F. Street-Perrott, F.A. and Bartlein, P.J. (Eds.), 1993, Global climates since the Last Glacial Maximum. University of Minnesota Press, Minneapolis, MN, pp. 12–23. Kutzbach, J.E. and Guetter, P.J., 1986, The influence of changing orbital parameters and surface boundary conditions on climate simulations for the past 18.000 years. Journal of the Atmospheric Sciences, 43, 16, pp. 1726–1759. Lamb, H.F., Gasse, F., Benkaddour, A., El Hamouti, N., van der Kaars, S., Perkins, W.T., Pearce, N.J. and Roberts, C.N., 1995, Relation between century-scale Holocene arid intervals in tropical and temperate zones. Nature, 373, pp. 134–137. Lézine, A.M., 1987, Late Quaternary vegetation and climate of the Sahel. Quaternary Research, 32, 2, pp. 317–334. Lézine, A.M. and Casanova, J., 1989, Pollen and hydrological evidence for the interpretation of past climates in tropical West Africa during the Holocene. Quaternary Science Reviews, 8, pp. 45–55. Lézine, A.M., Casanova, J. and Hillaire-Marcel, C., 1990, Across an Early Holocene humid phase in Western Sahara: Pollen and isotope stratigraphy. Geology, 18, pp. 264–265. Maley, J., 1977, Palaeoclimates of Central Sahara during the Early Holocene. Nature, 269, pp. 573–577. Michel, P., 1973, Les bassins des fleuves Sénégal et Gambie. Études géomorphologiques. Mémoires de lÓ.R.S.T.O.M. 63, Paris, pp. 1–752. Müller, M.J., 1996, Handbuch ausgewählter Klimastationen der Erde. Forschungsstelle Bodenerosion, 5, Trier. Nicholson, S., 2001, Climatic and environmental change in Africa during the last two centuries. Climate Research, 17, pp. 123–144. Pachur, H.J., Röper, H.-P., Kröpelin, S. and Goschin, M., 1987, Late Quaternary hydrography of the Eastern Sahara. Berliner Geowissenschaftliche Abhandlungen, 75, pp. 331–384. Petit-Maire, N., 1987, Local responses to recent global climatic change: Hyperarid Central Sahara and Coastal Sahara. In: Current research in African earth sciences, edited by Matheis and Schandelmeier, Balkema, Rotterdam, pp. 431–433. Petit-Maire, N., 1988, Taoudenni Basin (Mali), Holocene palaeolimnology and environments. Würzburger Geographische Arbeiten, 69, pp. 45–52. Prell, W.L. and Kutzbach, J.E., 1987, Monsoon variability over the past 150.000 years. Journal of Geophysical Research, 92, pp. 8411–8425. Reichelt, R., Faure, H. and Maley, J., 1992, Die Entwicklung des Klimas im randtropischen Sahara-Sahelbereich während des Jungquartärs- ein Beitrag zur angewandten Klimakunde. Petermanns Geographische Mitteilungen, 136, 2–3, pp. 69–79. Renssen, H., Brovkin, V., Fichefet, T. and Goosse, H., 2006, Simulation of the Holocene climate evolution in Northern Africa: The termination of the African Humid Period. Quaternary International, 150, pp. 95–102. Rognon, P., 1986, Late Quaternary climatic reconstruction for the Maghreb (North Africa). Palaeogeography, Palaeoclimatology, Palaeoecology, 58, pp. 11–34. Rognon, P. and Williams, M.A.J., 1977, Late Quaternary climatic changes in Australia and North Africa: a preliminary interpretation. Palaeogeography Palaeoclimatology Palaeoecology, 21, pp. 285–327.


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Schultz, J., 2005, The ecozones of the world. The ecological divisions of the geosphere. Springer, Berlin, Heidelberg, New York. Schütt, B., 2004a, Reconstruction of Holocene weathering conditions from chemical character of playa-lake-sediments—a case study from Central Spain. Lecture Notes in Earth Science, 102, pp. 5–30, Springer Verlag. Schütt, B., 2004b, Zum holozänen Klimawandel der zentralen Iberischen Halbinsel. Relief, Boden, Paläoklima, 20, Stuttgart. Servant, M., 1973, 1983, Sequences Continentales et Variations Climatiques: Evolution du Bassin du Tchad au Cenozoique Superieur. Travaux et Documents de lÓ.R.S.T.O.M., 159, pp. 1–573. Servant, M. and Servant-Vildary, S., 1980, Lénvironment Quaternaire du Bassin du Tchad. In: Sahara and the Nile, edited by Williams, M.A. and Faure, H., pp. 133–163. Street, F.A. and Grove, A.T., 1976, Environmental and climatic implications of Late Quaternary lake-level fluctuations in Africa. Nature, 261, pp. 385–390. Stuiver, M. and Reimer, P.J., 1989–2005, CALIB 5.0.1 Calib Radiocarbon Calibration Program. Stuiver, M., Reimer, P.J. and Reimer, R., 2005, CALIB 5.0.1 Manual, http://www.calib.qub. ac.uk/crev50/manual/, 09/05/2005. Talbot, M.R., 1980, Holocene changes in tropical wind intensity and rainfall: Evidence from Southeast Ghana. Quaternary Research, 16 (1981), pp. 201–220. Talbot, M.R. and Delibrias, G., 1977, Holocene variations in the level of Lake Bosumtwi, Ghana. Nature, 268, pp. 722–724. Talbot, M.R. and Delibrias, G., 1979, A new Late Pleistocene–Holocene water-level curve for Lake Bosumtwi, Ghana. Earth and Planetary Science Letters, 47 (1980), pp. 336–344. Thorweihe, U., Schneider, M. and Sonntag, C., 1984, New aspects of hydrogeology in Southern Egypt, Berliner Geowissenschaftlichen Abhandlungen, Reihe A/50, pp. 209–216. Völkel, J., 1988, Zum jungquartären Klimawandel im saharischen und sahelischen Ost-Niger aus bodenkundlicher Sicht. Würzburger Geographische Arbeiten, 69, pp. 255–276. Völkel, J. and Grunert, J., 1990, To the problem of dune formation and dune weathering during the Late Pleistocene and Holocene in the Southern Sahara and the Sahel. Z. Geomorph.‚ N.F, 34, pp. 1–17. Wagner, G.A., 1995, Altersbestimmung von jungen Gesteinen und Artefakten. Ferdinand Enke Verlag. Stuttgart, pp. 1–277. Waller, M. and Salzmann, U., 1998, The Holocene vegetational history of the Nigerian Sahel based on multiple pollen profiles. Revue Palaeobotany Palynology, 100, pp. 39–72. Weischet, W. and Endlicher, W., 2000, Regionale Klimatologie. Teubner, Stuttgart, Leipzig. White, F., 1983, The vegetation of Africa. A descriptive memoir to accompany the UNESCO/AETFAT/UNSO vegetation map of Africa. UNESCO, Paris, France. Wright, H.E., Kutzbach, J.E., Webb III, T., Ruddiman, W.F., Street-Perrott, F.A. and Bartlein, P.J. (Eds.), 1993, Global climates since the Last Glacial Maximum. University of Minnesota Press, Minneapolis, MN, pp. 1–569. Zielhofer, C., Faust, D., Escudero, R.B., Diaz del Olmo, F., Kadereit, A., Moldenhauer, K.-M. and Porras, A., 2004, Centennial-scale Late Pleistocene to Mid-Holocene synthetic profile of the Medjerda Valley, Northern Tunisia. The Holocene, 14, 6, pp. 851–861.

CHAPTER 3

Holocene palaeoenvironmental changes in Central Sahara inferred from Seggedim scarp foot depression (NE-Niger) Roland Baumhauer, Jens Brauneck, Barbara Sponholz and Erhard Schulz Department of Geography, Physical Geography, Julius-Maximilians University, Würzburg, Germany Oumarou Faran Maiga and Ibrahim Sani Department of Geography, University Abdou Moumouni, Niamey, Niger Simon Pomel DYMSET/CNRS, University of Bordeaux III, Talence Cedex, France ABSTRACT: Initial research has shown that palaeolimnic sediments and palaeosoils situated in the northeastern parts of Niger are suitable for reconstructing parts of the Quaternary palaeoenvironment of the Central Sahara. Several corings in 1989, 1990 and 2005 in the sandstone depression of Seggedim revealed a composition of high-resolution sections suitable for palaeoenvironmental investigations. Stratigraphical, structural and geochemical investigations as well as the analysis of thin sections allow the characterisation of different environmental conditions. Radiocarbon dates set the beginning of an initial swamp environment at about 10,6 ka cal BP, with an exceptionally stable regime to 6,6 ka cal BP, when a major change in the sedimentation regime of the basin is recorded in the core. By mineralogical and geochemical evidence, a rapid transformation from the sapropel setting to a sequence of freshwater lake stages occurred, the latter merely lasting a few centuries. About 6 ka BP, a transition phase, determined by the accumulation of huge bedrock fragments, separated the pure lake sediments from a following sebkha environment. The subrecent and present environment is characterised by the sedimentation of a continental sebkha, where salt and sand show a rhythmic deposition mode.

3.1 INTRODUCTION As shown in chapter one, there is a general agreement between the findings on Late Quaternary landscape and climate development of the Central Sahara with those obtained for the Southern Sahara at the 10³ years time scale. However, sufficiently humid conditions for the formation of freshwater lakes only seem to have existed from the Late Upper Pleistocene to the Mid-Holocene, becoming successively less pronounced by 6,5 to 5,5 ka BP, with the intercalation of several short arid phases, until hyper-arid conditions came to prevail in the whole region since 5 ka BP at least. The evidence of, first, a continuous increase in aridity and then the rapid and quite early transition to the present hyper-arid conditions disagrees with findings from the Western Sahara, for which numerous studies indicate yet another

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wet phase around 4,5 ka BP prior to the shift to hyper-aridity. In contrast to those findings, in the Eastern Sahara again there is evidence of a single Early to Mid-Holocene wet phase only, beginning around 9,5 ka BP, interrupted by several arid phases, and ending by 4 ka BP, thus later than in the South-Central Sahara (see chapter 1). The various groups working in the different regional parts of the Sahara agree that, beyond the general Holocene climate and landscape change, there also existed a zonal palaeoclimatic pattern, together with a latitudinal shift of landscape belts (see chapter 1). At last Kuper and Kröpelin (2006), conclude from their palaeoecological studies that conditions in the Eastern Sahara, during the Early Holocene humidity optimum, were similar to those presently found at 12°N, indicating a northward shift of the Sahelian boundary in East Africa by about 600 km. This is in good agreement with the present evidence from the pattern of Early Holocene pluvial soils in the Southern Central Sahara of East Niger (FelixHenningsen, 2004). Similarly, Baumhauer et al. (2004), from the palynological analysis of a former Seggedim core, conclude that during the Late Early Holocene, at least to 20°N there existed a Saharan savanna vegetation (cf. Schulz et al., this issue) which, by 6,5 ka BP, experienced a change towards Saharan desert conditions. The aridification shown in the pollen spectrum, however, is partly attributed to anthropogenic interference (grazing, fire) in the contact region between Sudanian and Saharan vegetation with the beginning of the Neolithic. This, together with the climatic deterioration, led to the development of the Sahelian savanna vegetation further in the South, the human component possibly accelerating it (Baumhauer et al., 2004; Schulz, 2004). New analyses from a core from the Seggedim region of NE-Niger extend through the Holocene period. Using high-resolution, multi-proxy evidence, we are able to clarify palaeoenvironmental examinations of two cores collected in 1989 and 1990 as well as analyse new aspects of the landscape history in this part of the Central Sahara over the last 10.000 years. 3.2 REGIONAL SETTING The endorheic scarp foot depression of Seggedim lies at the northern margin of the Chad Basin in North-Eastern Niger (20°10' N, 12°47' E), between the Djado-Plateau and modern Lake Chad. It is located in the western scarp foot depression of a cuesta of karstified marine sand- and siltstones of Senonian age (Faure, 1966). The absolute height of the freeface escarpment is 640 m asl, the relative one around 230 m. The depression, about 10 km2 large, is partly taken up by a sebkha environment, with groundwater inflow in the centre and at the eastern margin. On average it is 2 km wide East to West, with a maximum width of 7 km. A smooth aeolian sand layer covers most of the depression around the sebkha. The adjacent plain to the West is mostly a serir, its gravel originating from some conglomerate beds within the sandstone of the cuesta. The hydrological situation of the region is largely unknown. Traces of surface runoff are restricted to parts of the escarpment foot. There is no weather station in the Seggedim area. The nearest one at Bilma, some 150 km to the South, is the only one for all of Saharan Northeastern Niger. In Bilma, the annual precipitation does not exceed 10 mm, with a potential evaporation of about 2.700 mm/yr. The rare rainfalls are very irregular, mostly connected to the interaction of the monsoon and polar front. The present plant cover of the scarp foot depression shows a concentric mosaic of belts dependent on the structure of the sediments. An outer zone bears tussocks of Panicum turgidum on coarse sand and gravel, whereas the inner dune-sand parts have stands of Imperata cylindrica, Desmostachya bipennata and Sporobolus spicatus. The inner part of the depression is colonised by trees such as Acacia raddiana, A. ehrenbergiana, A. nilotica

Holocene palaeoenvironmental changes in Central Sahara

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Figure 1. Map showing NE-Niger. The point marks the coring location of Seggedim (LANDSAT image).

or Hyphaene thebaica. Phoenix dactylifera grows on dune sands. The sebkha surface itself is plantless, but the salines and waterpoints are surrounded by Tamarix canariensis and Juncus maritima (cf. Boudouresque and Schulz, 1981). 3.3 METHODS The cores sequences described below were taken in 2005, using a modified Cullenberg corer with a core diameter of 63 mm. The core sequences were spilt: one half of the core was used for subsampling, the other half for archiving. High-resolution images were produced with a modified flatbed scanner. The main lithostratigraphical features, Munsell soil colour data and major changes in sedimentation visible with the naked eye were logged. 1 cm thick slices were taken about every 10 cm in consideration of major sediment changes. These subsamples were dried and prepared for analysis in bulk chemistry, mineralogy, palynology and microfossils with the main focus on the non-sebkha sediment (below 5 m depth). The first millimetres of the remaining archive-half were cut out for making thin sections. Bulk chemistry was measured for selected elements in aqua regia digestions using an ICPOES (Inductively Coupled Plasma Optical Emission Spectrometry). For the investigations

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of silicon and aluminium dynamics, additional XRF (X-Ray Fluorescence) analyses on fuse tablets were performed on 30 samples. Mineralogy was measured by XRD (X-Ray Diffraction). Total inorganic carbon (TIC) and total carbon (TC) were determined by using a Woesthoff Carmhograph and a carbon-sulphur analyzer. The pH values were determined in CaCl2 solution following ISO 10390. Samples for radiocarbon dating were taken from highly organic parts of the sequence and from pieces of charcoal found in the bottom parts of the core. These dates were calibrated for 95% confidence intervals using the CalPal2007 software (Weninger et al., 2007) with the calibration data set CalPal-2007Hulu. 3.4 RESULTS FROM THE 2005 CORING 3.4.1 Stratigraphic units Seven distinctive units determine the stratigraphy of the core. These units are defined by major colour changes, texture and lamination properties. Sand The most recent stratigraphic unit of the sebkha consists of sand and evaporitic material such as thenardite and syngenite. It extends from 0 to 600 cm depth and shows several dynamic changes of sedimentation. A comprehensive description of the Seggedim sebkha sedimentation can be found in preliminary studies (Baumhauer, 1986; Baumhauer et al., 2004). Clay, unstratified This unit is located at 650 cm to 740 cm depth and shows no visible stratification of any kind. Several distinct colour changes and large rock fragments give evidence of a rather dynamic sedimentation process. The main colours in this section range from 5 YR 5/8 to 10 YR 6/2 and 7,5 Y 4/2. Another homogenous unit can be found at 923 cm to 995 cm depth. Again, some sandstone fragments can be found here and no stratification is visible. The main colour is 7,5 Y 4/1. Inferred from the rock fragments deposited and the homogenous structure, these units are supposed to have originated as turbidites or from mud streams. Clay, layered The main features of these units are the fine laminations that appear in regular and ground-parallel sets of bands but show variations in thickness. One major section is located at 750 cm to 780 cm depth. It consists of finely laminated clays and shows no disturbances of any kind. The main colour is 2,5 GY 4/1. Another set of fine laminations can be found at 1.019 cm to 1.055 cm. The colours range from 7,5 Y 4/1 to 3/1. One particular feature of this segment is the occurrence of fish bones at 1.032 cm that prove the existence of a ecosystem teeming with life. Another section between 1.100 cm and 1.140 cm is comparable, yet with occasional white laminae. A third, very small unit between 1.253 cm and 1.270 cm shows variations in thickness. Few shell fragments were found in all three sections. Clay, disturbed The units referred to as “Clay, disturbed” show laminations that are thinly bedded, heavily disturbed, undulated and partly tilted. Such a major section lies between 1.140 cm and 1.250 cm. The contact between the laminae is blurred, and there are several


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Figure 2. Main properties of the 2005 core (5–15 m depth).

distinct colour changes, from 10 Y 3/1 to 7.5 Y 5/1. The disturbed bedding may be due to bioturbation during deposition. A similar segment occurs between 1.270 and 1.300 cm depth, with several white layers more than 2 cm thick. Two more small segments of disturbed clay appear at 995 to 1.019 cm and 1.055 cm to 1.083 cm depth, the latter with some ferrous lenses. Their layers are also tilted, in contrast to the adjacent segments. Tilting therefore, most have taken place during sedimentation and not coring. Sapropel This is the main unit of the lowest part of the core, from 1.300 cm to 1.470 cm depth, with continuous lamination, interrupted by a few white bands. It consists of a mixture of organic material and clay distinct from the adjacent units. Aside from white bands, its colours range from 7,5 Y 2/2 to 4/3.

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Bedrock The small unit referred to as bedrock consists of a debris of mollusc shell fragments, heavily weathered pieces of sandstone and ferrous rocks. It comprises the section from 1.470 cm to 1.500 cm, marking the bottom of the core. Gaps and disturbances During coring, a major gap occurred at 600 cm to 650 cm depth due to the massive changing sediment properties. Several disturbances are spread over the core due to the coring technique, which holds several difficulties such as falling sediment debris inside the borehole. 3.4.2 Mineralogy Quartz (silicon dioxide) and clay minerals (aluminium phyllosilicates), most likely kaolinite, are the essential elements of the mineralogical composition and are ubiquitously present. The amounts of quartz and clay sum up to more than 60% (semi quantitative) in mean throughout the core. Other abounding minerals, in a lesser amount, are pyrite and halite. Carbonates are widely spread throughout the core. They show a very heterogeneous composition of calcite, dolomite, aragonite and low-Mg calcite (in detail: figure 12). This may indicate the simultaneous occurrence of allochthonous and autochthonous carbonates followed by biogenic alteration of the carbonates (cf. ch. 6.2.) The maximum concentration of carbonates can be found at the depths of 764 cm (38%) and 1.280 cm to 1.295 cm (>30%). Evaporites, such as the sulphate minerals glauberite, syngenite and thenardite, show maximum values in the upper part of the core, deposited after the present hyperarid climate had set in. Gypsum occurs in several segments, with a maximum peak in the upper part, but other strong peaks at 11 to 12 m depth. Sylvine (KCl), another evaporitic mineral, has only been deposited in the lowest segment, from 1.300 to about 1.450 cm depth. Traces of muscovite occur at various depths throughout the core. 3.4.3 Geochemistry Performing normative calculations on the ICP-OES results (Boyle, 2002), the ascertained elements add up to about 30% in mean. With the addition of total carbon (TC), the mean value ascends to about 34%, leaving about 60% unaccounted. The essential elements of the remaining part are silicium and aluminium, compared to mineralogical analyses (quartz and clay). The results of additional XRF analyses on fuse tablets confirm the calculations. Figure 3 shows the distribution of silicon and aluminium and derived ratios in detail. SiO2 (silicon dioxide or silica) is the major component with a mean value of 41,89% (n = 30) and a maximum of 66,34% in the sandy unit. Al2O3 (aluminium oxide) shows a maximum value of 25,6% at 1.005 cm, with a mean of 15,79% (n = 30). An increase of silica in lower parts of the core and associated depletion of Al2O3 (assigned to clay) is notable. The mole ratio of Si/Al affirms the result and shows maximum values below 1.300 cm as well as a very constant ratio of about 1,6. The mole ratio of Na + K/Al, also referred to as salinization ratio, shows a pattern similar to Si/Al ratio. Potassium (K) and sodium (Na), both parts of alkali metals, represent major evaporites in the core, halite (NaCl) and sylvine (KCl) as well as thenardite (Na2SO4). The ratio of Fe/Mn is a strong indicator of reducing environments (Davison, 1993) and shows several maxima in the lower parts of the core.


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The weight percentage (wt %) maximum of sulphur amounts 31,52% and can be found at the bottom of the core at 1.496 cm, with a mean assay of 14,84%. The maximum amount of iron is again found in the lowest segment, where some heavily weathered bedrock fragments have been found. The average amount of iron throughout the core is 5,7 wt %. Calcium is spread all over the core but with differing values. Remains of gastropod shells lead to the calcium maximum of 14,74 wt % at 1.483 cm depth, immediately followed by a decline to below 1% for the following 1,5 metres. Striking is the high correlation (0,92) of inorganic carbon (TIC) and calcium throughout the core due to the amount of carbonates, although both values show high coefficients of variation (cv %) from 80 to 100%. 3.4.4 Main lake stages Regarding the macroscopic sedimentological, mineralogical, and chemical properties of the Seggedim core, six main units (Figure 2) have been classified and statistically tested. Figure 4 shows some of the main features of five units in mean percentage with their standard deviations, compared to their total values. Unit I—Initial lake (swamp) phase (1.470–1.300 cm) This sequence is characterised by its high organic carbon content (mean 6,87, n = 17) and by its extremely low inorganic carbon content ( mean 0,04, n = 17). Here, TIC and calcium content are at their lowest (0,4 wt %) compared to the mean value of the core (4,21 wt %). The same applies to strontium, which is highly correlated (0,83) with calcium throughout the whole core. Sulphur content is at its maximum (18,4 wt %). pH-values are very low and range from 2 (1.443 cm) to 4,3 (1.302 cm). Pyrite content is constantly at a high level, as is halite. Carbonates do not occur in this unit. Unit II—Transition phase 1 (1.300–1.140 cm) This unit shows rather unstable conditions. With massively decreasing LOI-values down to 1%, the pH-values increase simultaneously. CaO and TIC show maximum values that correspond to changing carbonate peaks. Gypsum occurs in the mineralogical fraction, as well as several halite peaks. Laminated material only occurs at a small sequence from 1.270 to 1.253 cm. Contrary to the initial lake phase, LOI is low and TIC as well as CaO and carbonates show a maximum peak due to fragments of carbonate shells. In contrast to the other values, FeOOH content remains stable at an average of 5,08% (n = 18): the lowest average value of iron throughout the core. Numerous sediment-colour changes and undulated sediment layers support the assumption of dynamically changing lake conditions. Unit III—Lake phase 1 (1.140–1.020 cm) Compared to underlying units, this sequence appears to be rather stable, although sedimentation conditions must have changed, as appears from the stratigraphic record. FeOOH, K2O, MgO and Na2O show low standard deviations and therefore low coefficients of variation at about 10% (n = 10). Gypsum occurs only in the lower parts in the sequence and vanishes at about 11 m depth, having led to diminishing amounts of CaO and SO3 and a high positive correlation of them. Corresponding to the decrease of gypsum, pyrite shows minimum qualitative values, whereas halite shows maximum reflections. In comparison to other units, carbonates are present throughout. Fish bones at 1.032 cm depth are evidence of life in the lake. Magnesium shows maximum values (2,6 %) with low coefficients of variation of 6,17 %. According to this, carbonate values origin solely in dolomite. TIC and iron show a high negative correlation

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Figure 3. SiO2 + Al2O3 distribution, mole ratios.

Figure 4. Five of the main chemical values in mean percentage with associated standard deviations.

(-0,81), so that peaks of carbonate result in decreased iron and sulphur values and vice versa. This interrelation results in a negative correlation of CaO and FeOOH (-0,75). Unit IV—Lake phase II (1.000–750 cm) Sulphur and iron show stable values throughout the sequence, with very low coefficients of variation at about 6% (n = 17). The stable values result in stable signals of pyrite and therefore a constant Fe/S mole ratio at 0,39, very close to pyrites stoichiometric mole ratio


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of 0,5. This difference and the bad correlation of iron and sulphur of 0,59 are evidence of a deficit of iron respectively a surplus of sulphur, as based on the pyrite values. The surplus of sulphur increases constantly towards the top of the sequence at 750 cm. TIC and iron show a high negative correlation (-0,81), so peaks of carbonates result in decreased iron and sulphur values and vice versa. This interrelation results in a negative correlation of CaO and FeOOH (-0,75). Unit V—Transition phase 2 (750–650 cm) Sediments of this period represent the transition from a freshwater lake to a sebkha environment. Just like the other transition phase, this is a very unstable sequence. All chemical values showing high coefficients of variation, multiple distinct colour changes and the presence of large rock fragments of problematic origin (see ch. 6.1). All are evidence of major changes in the environment. No laminations appear throughout this unit, which also shows that there has not been a stable phase of calm and undisturbed sedimentation such as in Unit I. Unit VI—Sebkha (600–0 cm) Only three subsamples of this unit have been studied, and therefore there will be no statistical interpretation. Nevertheless, the stratigraphic summary (Figure 2) shows some of the major traits, such as the high content of quartz and evaporites. SO3 and CaO correspondingly show increasing values, due to the development of sulphates (gypsum, glauberite, thenardite, syngenite). Pyrite and carbonates only appear as traces as do the clay minerals. A comprehensive description of the sebkha sedimentation can be found in the preliminary studies by (Baumhauer, 1986; Baumhauer et al., 2004). 3.5 THE ARCHIVE-FUNCTION OF THE SEGGEDIM RECORD Investigations on the palaeoenvironment of a region largely depend on the nature of the exploited archives. A valuable natural archive is one that, depending on its structure, will yield information on the ecological prerequisites that led to its formation, must have been able to trap and preserve remains of microorganisms or inblown dust. This will allow the reconstruction of the former environment from the life conditions of the respective organisms. Fine layered or laminated lake and sebkha sediments proved their ability in two ways: first by their quiet and regular sedimentation, pointing to a non- or less-disturbed nature of the water body, which may be permanent or periodic (Berglund et al., 1986; Clark et al., 1989; Schulz et al., 2002). The second point is the close cover of different layers ensuring all dust and remains of microorganisms trapped with them, which can thus serve as indicators of the conditions of the respective water body and characteristics of the surrounding landscape. Due to this fine layering, they consequently allow a high resolution of investigation on environmental history of the region. Investigations of this type proved to be necessary in order to understand the deposition type and possible alteration as well as to evaluate the abilitiy of sediments to reflect different types of ecosystems (cf. Mees, 1996, 1998). Here we present new micropetrographical analyses of the Seggedim cores. First results were already presented by Pomel in Baumhauer et al. (2004). As described above the 2005 coring in the depression of Seggedim revealed a composition of several units (cf. figure 2). An initial lake (swamp) environment (unit I) was replaced by a sequence of fine layered lake sediments (lake phases I and II /units II–IV). A transition phase separated the pure lake sediments from a following sebkha environment (unit V). The sub recent and actual

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environment is characterised by sedimentation of a continental sebkha (VI), where salt and sand show a rhythmic deposition mode. The initial lake (swamp) phase (unit I) The sediment consists of the remains of swamp plants and fine-grained silica material with some greater sandstone fragments. The material is monophasic deposited under wet or even subaquatic conditions over a period of several hundreds of years (cf. Chapter 6.3) and, to as typical for such swamp sediments, it trapped and well preserved the dust blown in such as pollen grains. It is of special interest that charcoal and ashes are regularly present in large quantities exclusively originating from grasses. These evidences of fires are equally present in the following sequences (units II + III). The lake phase II (unit IV) The sediments of this period are characterised by a regular alternation of two distinct layers of about 200–300 microns thickness, forming a kind of doublet (Figure 5a, b, d). A basic layer consists of diatom frustules and their remains in different states of alteration and silt and clay particles, which are often arranged in cross like patters. Small pieces or flakes of amorphous silica are an important element. Mostly horizontally aligned, they provide a textile structure for it. Charcoal and ashes are regularly present in significant quantities. Very often they form typical ash and charcoal layers in this part of the section. Pyrite as small isolated particles or clusters is regularly present as well. An upper layer is characterised by thin flakes of sesquioxides, mostly iron oxides showing a variable dimension from about 30 to 200 microns. They tend to be very flat reminiscent of parts of jellies coming from bacterial and/or algae films. These flakes have been deposited in a matrix of silt and clay also containing diatoms and their fragment as well as charcoal particles. These two layers characterise in their alternation the sediments of this period. The explanation of this alternating sedimentation however provides some difficulties. In general a deep lake with stable hypolimnion has to be assumed, rarely disturbed by lake water circulation. This would explain both the regularity of sedimentation and thin layering. The different fragmentation of diatoms and their aligned deposition as well as of flakes of amorphous silica combined with the presence of pyrite all suggest anoxic and acid conditions in the hypolimnion. The second type of layers, however, that of iron oxides and jelly particles may not be explained by this way. It calls for an oxygen-rich environment where dissolved iron II oxides could be transformed into iron-III-oxides and then be trapped by algae or bacterial layers (see Hamilton-Taylor and Davison, 1995). A possible explanation may be derived from the model of swamp-ore formation presented by Felix-Henningsen (2004), which in view of the morphology of the Seggedim depression should also apply to the former near shore environment of this palaeolake. Looking to the morphology of the Seggedim depression the model presented for the formation of swamp-ores would give a good explanation. In this shallow and highly insolated water with its fringe of reeds, one has to assume two types of iron-III-oxide formation. First it is caused by emanation of oxygen by the roots of Typha or Phragmites after its transportation via the aerenchyme cells and secondly they are formed in the water itself and trapped by the algae and bacterial jellies. Afterwards these yellies are fragmentated and transferred by the waves of the shallow water body into the centre where they could sink and form a characteristic layer of the lakes bottom together with low quantities of diatoms and charcoals. As the next diatom-, silica- and charcoallayer covers them, they are preserved from reduction in the anoxic and acid milieu.


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Figure 5. Lake sediments from the unit IV showing biphasic sediments (E. Schulz). a. Biphasic sediment containing flakes of iron oxides, silt diatoms and their fragments as well as some charcoals in the upper part, the lower part consisting of silt, charcoal and diatoms (845 cm depth); b. Double layer containing diatoms, silt, charcoal and some pyrite in the lower part and iron oxide flakes, charcoals, diatoms and silt in the upper one (790 cm depth); c. Monophasic turbidites of silt with some charcoals and pyrites (786 cm depth); d. Biphasic lake sediments containing strata of tile-like and cross-like oriented silts and thicker layers with coarse silica grains (780 cm depth).

Uniform and unstructured silt layers containing a few charcoal fragments and pyrites interrupt this alternation of thin layers. They are up to 5–7 cm thick and represent turbidites having slid down the steep slopes to the lake bottom (Figure 5c). There, short time and monophasic sedimentation stands in contrast to the biphasic deposition of the thin layers of diatoms, charcoal and silt. Unlike these, they contain very hardly any particles of inblown dust and thus their archive function is a very limited. Figure 6d shows a more differentiated sequence in this period. Fine layers of totally aligned silt and clay particles are in a tile like deposition. Thicker layers overlie them with

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Figure 6. Landscape and sediment conditions during transition phase 1 and lake phase I (unit II–III) (E. Schulz). a. The Emi Bao escarpment and Seggedim lake; b. Shallow lake margin with burning reed; c. Phragmites-Typha reeds, the formation of iron oxide flakes around their stems and roots and their transport into the centre of the lake; d. The formation of the fine layered sediments in the hypolimnion of the lake; e. Turbidite formation in the centre of the lake; f–j. Mode of sedimentation of the fine layers and the turbidite in the hypolimnion of the lake.

some coarse quartz grains in mixed with the diatom remains and charcoals. Some beds again show the pattern of the cross-like layering indicating a quiet sedimentation milieu (cf. Pomel, 2004). The model of the deep lake at the foot of Emi Bao (Figure 6a–j) summarises the formation of this type of line-layered deposits. The slopes are covered by an open savanna of the Acacia-Maerua type. A dense stand of Phragmites-Typha reed fringes the lake (Figure 6a). Here, the iron oxide flakes were originate before being washed to the centre of the lake. Figure 6d–j again summarises the two modes of sedimentation in the deeper part of the lake and the regular formation of the fine layers as an alternation of diatom-siltcharcoal and iron oxide sedimentation occasionally interrupted by turbidites. It present the discussion remains open, whether we have to assume a regular deposition of doublets indicating seasonal differences or whether it was a continuous sedimentation of diatom, charcoals and silt regularly interrupted by the inwash of iron-III-oxide flakes at the end of summer season. Transition from lake to sebkha (unit V) This represents the transition phase from a freshwater lake to a sebkha environment. It is an unstable period. All chemical values show high coefficients of variations. The presence of large rock fragments evoke the question of their origin. Figure 7 shows two different modes of sedimentation. The first one is the continuation of the fine layered deposition


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Figure 7. Sediments from the Transition Phase 2 (unit V) (E. Schulz); a. Transition from lake sediment of diatoms, diatom shell, silt and charcoal as well as pyrites to a silica slurry of clay and silt with pyrite and some charcoal. Apparently, there is no hiatus in between these two sediments. In the upper part the slurries are horizontally arranged containing with pyrites and some iron oxide flakes as well as sharp-angled quartzes (683 cm); b. Irregular slurry deposit of sharp-angled quartzes, silt and clay and some charcoals (677 cm); c. Dense slurry of clay, silt and some sharp angled quartzes and charcoal (629 cm); d. Dense slurry of clay, silt, sharp angled quartzes some charcoal and iron oxides. The silts are cross-like layered indicative of a calm sedimentation environment. Presence of some post sedimentary gypsum crystals (606 cm); e. Irregularly deposited clay slurry with isolated sharp- angled quartzes some iron oxide flakes and charcoal. Apparently, the sediment contains material from eroded soils (585 cm). Dense clay slurry with sharp- angled quartz grains and some iron oxide flakes. Some quartzes already show an alteration around their margins (543 cm).

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in a stable hypolimnion as described above. In Figure 7b a definite change takes place. The lower part shows the lake sedimentation. This is replaced by a totally different type of deposit consisting of a silica slurry of clay and silt with some coarse quartz particles and pyrite which will prevail from this time on. Remarkably, diatoms are no longer present, but flakes of iron oxides are still common. This change stands for the change to a shallow water body, being filled with slurry or dust. Initially there was still an open water body most likely a shallow lake or some pools. Further sedimentation still was of the slurry type (Figure 7f, g) with charcoal and iron oxides particles still being present. Coarse grained and sharp-angled quartz particles became more and more characteristic of this sedimentation period. At times deposition took place in a very calm environment as shown by a cross-like arrangement of the silty particles (Figure 7d). In the uppermost part of this section rounded quartz grains as reworked material from the cuesta sand stones were found with surficial weathering indicating a post-sedimentation alteration (Figure 7c). Figure 8 depicts the landscape during this period. The former lake has been reduced to some open pools of open water swamps. The plant cover on the slopes of Emi Bao escarpment has changed to the desert type. In contrast to the preceding period linear wadi vegetation made of trees shrubs and grasses of the Acacia-Panicum type was dominant. The Phragmites-Typha reed belt was still present though less dense than in the previous periods. Thus, more and more sediments could be washed into the centre of the depression. The formation of iron oxides in the swamp and charcoal from burning of the reed appears to be as in the previous period. The sedimentation, however, has become a mixture of sub aquatic and windborn deposits as dust storm have been more and more important (d, e, f). The character of the archive function changed definitely too in this period. The lower part of the sequence still allows and analysis for pollen and diatoms. The change of sedimentation (from the biphasic anoxic to the monophasic oxic) goes along with the end of the preservation of pollen or diatoms mainly due to the permanent oxic nature of the sedimentation milieu. Sebkha (unit VI) The recent and present phases are those of continental sebkha conditions. Figure 9 shows samples taken along a west-east transect through the sebkha, sample c was taken near the coring site. The profiles were drawn from soil cuts in the field and from a short core (c). The transect starts at both and ends at the contact with the dune belt surrounding the sebkha basin (Figure 9a). Common to all sections are the alternating sand layers. There is a pellicular silt cover on top occasionally crusted. Another regular phenomenon is the coarse-grained salt, which in the core taken at the centre core also appears as nests. In the central part of the sebkha (b–d) the salt layers either lie on top of a sand layer or at the base of the next stratum. Flooding of all or parts of the sebkha floor seems to be of extremely rare occurrence. The people of Seggedim could not recall a single one, but there are traces of running water from the Emi Bao escarpment to the centre of the sebkha. Fluctuations of the groundwater table could be observed over the year 2005/06. This points to an extreme continental type of sebkha dynamics, where inundation by runoff or precipitation did not occur or where it was extremely rare. Thus the sedimentation is done with inblown material, which secondly is incrusted by salt crystallisation according to the changing water table. This is in contrast to sebkhas from semiarid areas, where regular or periodic inundations and subsequent short time lacustrine conditions lead to the formation of threefold sequence consisting of in washed material at the base, a layer of clay and algae/ bacteria and finally an upper salt crystallisation (Abichou, 2004; Schulz et al., 2002).


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Figure 8. Landscape and sediment conditions during the transition from freshwater lake to sebkha (E. Schulz). a. The Emi Bao escarpmet and the sebkha with its swamp lake and sebkha sediments; b. The lake margin with burning reed; c. Phragmites-Typha reed with the formation of iron oxide flakes and their transport into the centre of the lake; d–f. Subaquatic-subaeric sedimentation in the sebkha environment.

Figure 9. Subrecent and present sebkha sedimentation in the Seggedim depression (E. Schulz). a. Silt coated alternating sand and salt layers near the southwester margin of the sebkha; b. Silt coated alternating layers of coarse and fine sands with salt emanations; c. Sequence of sand and salt layers near the coring site. It is visible, that the salt emanations are formed in the sand layers either as nests or strata. A pellicular crust with a tendency to crack and to form again covers the sequence; d. Alternating layers of sand and salt in the southwestern part of the sebkha. There is a comparable rhythmicity to the centre (c) with a similar coat and crust formation; e. Scheme of sand-silt layers near the present salines. Inblown/inwashed silt with some fine sand dominates the layers. Some salt nests are visible. The fine layering may be due to changing near surface groundwater horizon.

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Figure 9 resumes the conditions of the subrecent and present sebkha sedimentation. Figure 9a shows the sebkha in front of the Emi Bao escarpment. In contrast to the preceding period no vegetation is present in the wadis and runnels, whereas a dune belt around the sebkha bears an oasis vegetation of various trees and the remnants of an outer grass belt (Schulz, 1994). The Seggedim depression is completely filled and shows the four different sediment periods and their respective deposits. Figure 10b, c depict a transect from the outer dune belt to the interior of the sebkha explaining the dust sedimentation and some rare situation of surface run off. Figure 10d explains the formation of the sand layers and their subsequent salt formation due to lateral or vertical groundwater supply. A superficial pellicular crust cracks and due to different humidity it wells up giving space to dust accumulation. Figure 10e–g describe the formation of the sand layers, the salt crystallisation and the final crust formation. Comparable to the sediments of the transition phase these deposits are again monophasic and due to their permanent oxygen influence, they do not preserve any pollen or diatoms (Baumhauer et al., 2004) in contrast to the sebkhas of Northern Africa or the Mediterranean, which by their fine layering represent an excellent archive. Finally, figure 11 summarises the steps of the landscape history from first lake phase (unit III) on. The change from savanna environments in unit III to the desert conditions in the following periods is visible.

Figure 10. Present sedimentation in the Seggedim depression (E. Schulz). a. The Emi Bao Escarpment and the Seggedim sebkha with the swamp lake and sebkha sediments of the cored sequence. Not the dune belt with an oasis vegetation around the sebkha; b/c. Blockdiagrams showing a transect form the dune belt to the sebkha. Main sedimentation is done by blowing sand and dust and by rare inwash from the mountainside; d. Blockdiagram showing the sedimentation in the upper part of the sebkha sequence. Lateral and vertical groundwater influence controls the emanation of salt. The pellicular crust falts up. Cracks are filled with dust and sand; e–f. Blockdiagrams showing the subaeric sedimentation mode of the sebkha including the crust formation.


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Figure 11. The landscape history of Seggedim (after Schulz and Merkt, 1996 modified).

3.6 INTERPRETATION 3.6.1 Conditions of the initial sapropel phase (unit I) The initial lake phase referred to as sapropel, unit I or swamp is different in many ways from the other/later ones (Figure 4). It shows high organic carbon and low pH-values as well as extremely low content of calcium and a total lack of carbonates. The “carbon pump”, which has been described by Dean (1999), is a suitable model for describing the sedimentary conditions during this initial lake phase. The decomposition of a high amount of organic detritus in a hypolimnion will lead to a high oxygen demand and thus

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to anoxic conditions. CO2 will be produced in high amounts due to this decomposition, in turn leading to decreasing pH-values. CO2 production and the decreasing pH-value will cause the destruction of carbonate remains. The fine, undisturbed laminae in this segment suggest a lifeless lake bottom sealed by a certain depth of water, as the preservation of the organic content can only have been possible under anoxic conditions. This is supported by the high ratio of Fe/Mn, a strong indicator of reducing environments (Davison, 1993), which shows several maximum peaks in this segment (Figure 3). The increase of SiO2 and the decrease of Al2O3 (clay) in the sapropel stage (Figure 3) originate either in a massive increase in aeolian deposited sand (quartz) or in high populations of diatoms. The texture characteristics of the sediment indicate the latter, but ongoing investigations on the grain size distribution and microfossils will show what causes this effect. The maximum values of the salinization ratio originate in the unique occurrence of KCl in this segment and the characteristically constant content of halite, which supports the assumption of a halocline. As no other evaporitic minerals appear at this depth, sylvine and halite may have originated by solution from underlying sandstone. It seems likely that this initial and very stable lake phase was enabled by groundwater discharge from a salt-containing marine aquifer within the Emi Bao formation (Baumhauer, 1986). The saltworks not far from the coring site and freshwater wells right next to it indicate how complicated—and yet not understood—the hydrogeology in this depression is. There is as yet no way to estimate the share of inflowing salty groundwater during the initial lake phase. 3.6.2 Carbonates—distribution and origin Figure 12 shows the heterogeneous distribution of carbonates throughout the core. So far it has been assumed that there are no carbonatic rocks in the Seggedim area (Faure, 1966). Consequently, and due to the different conditions under which aragonite and dolomite would have formed, their presence in the sequence may only be explained by aeolian input. The Mg/Ca mole ratio is an approved indicator of changes in carbonate distribution. It shows two large peaks in the deeper parts of the core, but only the one at 12,50 m coincides with aragonite or low-Mg-calcite. Due to the large amounts of calcite in the upper parts of the core, the ratio remains low. The mole ratio of Sr/Ca is commonly used as an indicator of palaeosalinity (Gasse et al., 1987; Goschin, 1988; Hoelzmann et al., 2000). The authors state that a Sr/Ca mole ratio above 0,003 points is indicative of increased salinity. This threshold value is not exceeded throughout the core, thus freshwater conditions can be assumed. Ongoing investigations on microfossils may confirm the statement. Comparing the amounts of total inorganic carbon and calcium, the values are quite constant. Only in the sapropel stage, where the calcium content has decreased due to the low pH, the mole ratio by far exceeds the ideal one of 1. Other fluctuations will be due to changing calcium content throughout the core (Figure 2). As to the origin of carbonates, several fragments from transition phase II create some problems. The largest one is a massive piece, about 7 cm long, of dark dolomitic carbonate rock, coated by brighter solid crust of most likely biogenic origin. This is the first evidence of any carbonate rock in this area, calling for further investigations. Faure (1966), who studied the geology of Emi Bao and Seggedim formations, did not report any carbonate strata from the catchment. A second piece from the core consists of calcite and aragonite with a calcium content of nearly 30%, and thus a CaCO3 content of more than 50%. This material seems to be of biogenic origin, like the coating of the larger piece. The unusual carbonate composition may best be explained by the transformation of allochthonous carbonate from


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Figure 12. Distribution of carbonates and mole ratios of determining chemical features.

the catchment area, probably dolomite, to aragonite by biogenic activity and the regular weathering of primary carbonates to calcite. This is supported by the simultaneous presence of low-Mg-dolomite and calcite (and aragonite as well at about 12,50 m). 3.6.3 Age-Depth-Model Figure 13 shows the distribution of calibrated AMS radiocarbon (14C) dates compared to two conventional uncalibrated dates from the previous coring. 21 radiocarbon dates were analyzed in the Poznan Radiocarbon Laboratory with accelerator mass spectrometry (AMS). The amount of dates was necessary to provide evidence of the complex sedimentation history of the record. For 17 samples radiocarbon dating was performed on bulk organic matter and revealed a rather chaotic pattern of the upper dates, interpreted as repeated contamination by translocated organic material. To receive dates from other sources than organic material, radiocarbon was additionally measured on carbonates. The mineralogical composition revealed singular signals of aragonite at the beginning and the end of the lake phase which are interpreted as remnants of gastropod shells. Detailed studies on gastropods revealed the reliability of various gastropod taxa for radiocarbon data (Fritz and Poplawski, 1974) especially those terrestrial snails incorporating aragonite in their shells (Brennan and Quade, 1997; Pigati et al., 2004; Zhou et al., 1999). When analysing biogenic carbonates by radiocarbon methods, shifts of dates have to be considered, either caused by the contamination of carbon containing groundwater (hardwater effect) or by carbonate bedrock (carbon reservoir effect) (Fontes and Gasse, 1989; Snyder et al., 1994; Zhou et al., 1999). The results show that no nameable hardwater effect contaminated the carbonates. Therefore, it can be assumed that

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Figure 13. Calibrated radiocarbon dates of the 2005 coring compared to the two dates from 1984 (squares).

adjacent radiocarbon dates from organic material can be considered to show the true age of the sediment. The lowermost dated sample was taken from the bottom of the core, close to the bedrock. It places the onset of continuous sedimentation at about 10.410 a cal BP ± 180. Radiocarbon dates from within the first stage set the beginnings of the initial lake phase at about 10.050 a cal BP ± 200. Several dates from higher up are evidence of a very low accumulation rate in unit I. Only about 0,6 mm/a were deposited for over 3.000 years, at a very constant rate. Probably this phase has to be extended to even more than 4.000 years, if the date at 12,61 m depth is included (6,6 ka cal BP). This may be justified by the fact that the accumulation rate of approximately 0,6 mm/a remained the same, even though there had been a change in sedimentary conditions (cf. figure 2). The low accumulation rate is evidence of a very low minerogenic input, which might be interpreted by very low surface wash in the catchment for several millennia, due to the savanna-type vegetation cover inferred from the pollen spectra. The high organic content of the sediment suggests a supporting argument, namely that a closed belt of reed vegetation fringing the lake, as discussed above (Chapter 5), filtered the surface wash from the adjacent slopes. This could have led to accumulation of sediment and development of sediment banks. The following sequences (boxed) show rather confusing variations. Starting at 7.080 a cal BP ± 140 at 1.184 cm depth, all the radiocarbon dates lie inside a small range from 7.400 a cal BP to 6.250 a cal BP and, unlike those from the lower sequence, show no linear distribution. It may be assumed that these variations originate from translocated sediment, originally deposited higher up on the sloping lake bottom during the high-organic sapropel stage. With increasing surface wash and increased sediment input, turbidite movements are likely to have been triggered.


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The two dates from 6 and 8 meters depth (squares) are uncalibrated conventional radiocarbon dates from a preliminary coring in 1984 by Baumhauer (1986). In a preliminary study of another core from Seggedim (Baumhauer et al., 2004), thin-section analysis revealed the presence of large amounts of charcoal. Such fragments can be preserved unaltered in the sediment, whereas other freshly deposited organic matter will be decomposed almost immediately under oxic conditions. Most likely, such surviving charcoal flakes from burnt grass are the source of improper dates from 1984 and 2005 coring. In addition, the time of frequently discussed fires can be localized in a relatively short period from 7.500 to 6.200 a cal BP (boxed dates), but can be extended to the beginning of Initial Phase due to charcoal presence at the lowermost parts of the core. In conclusion, the five dates from the lowermost part of the core appear to be reliable. The majority of those from the upper section, as well as the earlier ones published (Baumhauer, 1986; Baumhauer et al., 2004), seem to be the result of mixing by translocation of sediments of various ages. Additional radiocarbon dating of biogenic carbonates set up a framework for validating the results obtained from bulk organic matter. 3.7 CONCLUSION The coring of 2005 revealed some new evidence and helped to elaborate on some known facts concerning the landscape history of Seggedim. An initial swamp environment came into existence in the Early Holocene and existed for approximately 4.000 years. No evidence was found of the formerly assumed Pleistocene lake stages. A high organic content, with organic fragments partially preserved due to extremely reducing conditions, and very low accumulation rates within the sediment are characteristic of the sapropel/swamp environment. The botanical record reveals the presence of Saharan savanna vegetation in the area, which may have been the main cause of diminished surface wash, but, in addition, a belt of reed vegetation may additionally have filtered any surface wash. Radiocarbon dates set the beginning of the stage at about 10,6 ka cal BP, with an exceptionally stable regime to 6,6 ka cal BP (at 12,6 m depth), when a major change in the sedimentation regime of the basin is recorded in the core. By mineralogical and geochemical evidence, there was a transition from the stable sapropel stage to a sequence of freshwater lake stages showing huge variations in all attributes investigated (cf. figure 2 and figure 4). The distinct increase in Al2O3 (Figure 3) indicates an ascending detritus input due to enhanced surface wash or a reduced belt of reed vegetation. Variations in the thickness and appearance of layers confirm variations in sedimentation most likely reflecting changes of precipitation. The occasional occurrence of gypsum suggests phases of desiccation. The discrepancies in the sequence of radiocarbon dates may best be explained by underwater sliding processes within the deposits. Variations among the freshwater lake stages may reflect variations in amount and seasonality of precipitation. The duration of lake stages has not yet been determined because of irregularities in the sequence of radiocarbon dates. From an earlier coring, the transition from freshwater to sebkha conditions had been assumed to lie further back in time. The new and more reliable dating, however, now indicates a somewhat longer existence of the Early Holocene freshwater lake(s), to at least about 5.310 ± 40 B.P. (uncalibrated, 728 cm depth; corr. 6.098 cal a B.P.), as compared to 6.850 ± 345 B.P (600 cm depth; 1984 bulk date). Additional radiocarbon ages from biogenic carbonates validated some of the dating results obtained from bulk organic matter and set up a framework for the reconstruction of palaeoenvironmental conditions within the Seggedim basin (Figure 14). Due to the lack of dateable material in the upper core section the termination of the lake stage and the onset of the subsequent sebkha stage cannot be determined precisely but

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Figure 14. Palaeoenvironmental succession scheme of the Seggedim basin.

can be narrowed to a period around 6 ka BP. Ongoing investigations will hopefully permit to determine more precisely the transition to the following sebkha stage, likely to indicate the onset of the present hyperarid phase. By isolating any aquifer impact, the results obtained from the core should then be linked with those from terrestrial and lacustrine sediments from outside the depression, situated a few hundred kilometres further to the North. Schütt and Krause (see chapter 2) have reviewed the literature on palaeoenvironmental research in the central Sahara. Several authors assume humid conditions from about 11 to 6,5 ka cal BP, with the maximum intensity of the monsoon activity around 10,6 ka cal BP. This is exactly when sedimentation appears to have started in the Seggedim core. Whether this was due to an increase in local precipitation or the rise of a replenished aquifer is still open to discussion, as the role of groundwater recharge in the hydrological balance of the depression is still unknown. The results of the radiocarbon dating show that no noticeable hardwater effect contaminated the carbonates and the adjacent radiocarbon dates from the organic material can be considered to show the true age of the sediment. However, as the aragonite is considered to be of terrestrial origin (gastropods), conclusions concerning groundwater charge during the initial stage cannot be drawn. The mineralogical and geochemical study of rock fragments of the core revealed the presence of carbonatic rocks in the Seggedim area. So far, the nearest known carbonatic outcrops are those of the Carboniferous Dembaba limestone of the Djado basin, at least 100 km northwest and at Ezerza North of Achegour in the Southwest. Evidence of frequent fires raises the question as to their causes. Despite the irregularities in the radiocarbon record, the dates from the translocated and redeposited charcoal pieces can be narrowed down to a tight time slice of 1.200 cal years, during which the regular burnings should have occurred. Two models are discussed for the causes of the fires: natural fires caused by lightning (Dolidon, 2005; Sponholz, 2004), versus intentional seasonal burning of the belt of reeds, which is still a regular procedure among traditional Sahelian herdsmen for providing access to the water for their animals (cf. Schulz, 2004, Figure 2c, d; Schulz and Merkt, 1996). ACKNOWLEDGEMENTS We would like to thank the German Research Foundation (DFG) for its financial support of the ‘Limnosahara’ research project (Ba 1000/21-1,2), funded since 2005. REFERENCES Abichou, A., 2002, Les changements des paysages du basin versant de lóued Tataouine Fessi (Sudest Tunisie). Etude multiscalaire et micromorphologie des remplisages des sebkhas et études des états des surfaces. These Université Michel de Montaigne, Bordeaux, pp. 1–1207.


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Baumhauer, R., 1986, Zur jungquartären Seenentwicklung im Bereich der Stufe von Bilma (NE-Niger). Upper Pleistocene and Holocene evolution of lakes in the Bilma region, Northeastern Niger. Würzburger Geographische Arbeiten, 65, pp. 1–235. Baumhauer, R., Schulz, E. and Pomel, S., 2004, Environmental changes in the Central Sahara during the Holocene. The Mid-Holocene transition from freshwater lake into sebkha in the Seggedim depression, NE-Niger. In: Smykatz-Kloss, W., Felix-Henningsen, R. (Eds.), Palaeoecology of quaternary drylands. Lecture notes in earth sciences, 102, Springer, Berlin, pp. 31–45. Berglund, B.E. and Ralka-Jasiewiczowa, M. (1986), Handbook of Holocene Palaeoecology and Palaeohydrology. John Wiley and Sons, Chicester. Boudouresque, E. and Schulz, E., 1981, The flora and vegetation of NE-Niger (Djado, Kaouar and Ténéré). Willdenovia, 11, pp. 363–394. Boyle, J., 2002, Inorganic Geochemical Methods in Palaeolimnology. Tracking Environmental Change Using Lake Sediments, pp. 83–141. Brennan, R. and Quade, J., 1997, Reliable Late-Pleistocene stratigraphic ages and shorter groundwater travel times from 14C in fossil snails from the Southern Great Basin. Quaternary Research, 47, 3, pp. 329–336. Clark, J.S., Merkt, J. and Mueller, H., 1989, Post-glacial fire, vegetation and human history on the Alpine forelands, Southwestern Germany. Journal of Ecology, 77, pp. 997–925 Davison, W., 1993, Iron and manganese in lakes. Earth-Science Reviews, 34, pp. 119–163 Dean, W.E., 1999, The carbon cycle and biogeochemical dynamics in lake sediments. Journal of Palaeolimnology, 21, pp. 375–393. Dolidon, H., 2005, Léspace des feux en Afrique de lÓuest. Thèse de lÚniversité de Caen, pp. 1–345. Faure, H., 1966, Reconnaissance geologique des formations sedimentaires post-paléozoiques du Niger Oriental. Direction des Mines et de la Géologie, Paris, pp. 1–587. Felix-Henningsen, P., 2004, Genesis and palaeo-ecological interpretation of swamp ore deposits at Sahara palaeo-lakes of East Niger. In: Smykatz-Kloss, W., Felix-Henningsen, R. (Eds.), Palaeoecology of Quaternary drylands. Lecture notes in earth sciences, 102, Springer, Berlin, pp. 47–72. Fontes, J.C. and Gasse, F., 1989, On the ages of humid Holocene and Late Pleistocene phases in North Africa—remarks on “Late Quaternary climatic reconstruction for the Maghreb (North Africa)” by P. Rognon. Palaeogeography, Palaeoclimatology, Palaeoecology, 70, 4, pp. 393–398. Fritz, P. and Poplawski, S., 1974, 18O and 13C in the shells of freshwater molluscs and their environments. Earth and Planetary Science Letters, 24, 1, pp. 91–98. Gasse, F., Fontes, J.C., Plaziat, J.C., Carbonel, P., Kaczmarska, I., de Deckker, P., SoulieMarsche, I., Callot, Y. and Dupeuble, P.A., 1987, Biological remains, geochemistry and stable isotopes for the reconstruction of environmental and hydrological changes in the Holocene lakes from North Sahara. Palaeogeography, Palaeoclimatology, Palaeoecology, 60, pp. 1–46. Goschin, M., 1988, El Atrun (Nubien)—Ein frühholozänzeitlicher See. Freie Univ. Berlin, pp. 1–221. Hamilton-Taylor, J. and Davison, W., 1995, Redox-driven cycling of trace elements in lakes. In: Lerman, A., Imboden, D. (Eds.), Physics and chemistry of lakes. Springer, Berlin, pp. 217–263. Hoelzmann, P., Kruse, H.J. and Rottinger F., 2000, Precipitation estimates for the Eastern Saharan palaeomonsoon based on a water balance model of the West Nubian Palaeolake Basin. Global and Planetary Change, 26, pp. 105–120. Kuper, R. and Kroepelin, S., 2006, Climate-controlled Holocene occupation in the Sahara: Motor of Africa’s evolution. Science, 313, pp. 803–807.

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Mees, F., 1996, Petrological studies of saline deposits of a perennial saline lake and a dry lake basis, and of calcareous deposits of small freshwater basin. Ph. D. thesis, University of Ghent, pp. 1–318. Mees, F., 1998, The alteration of glauberite in lacustrine depositsd of the Toudenni-Agargott basin, Northern Mali. Sedimentary Geology, 117, pp. 193–205. Pigati, J.S., Quade, J., Shahanan, T.M. and Haynes, C.V., 2004, Radiocarbon dating of minute gastropods and new constraints on the timing of Late Quaternary springdischarge deposits in Southern Arizona, USA. Palaeogeography, Palaeoclimatology, Palaeoecology, 204, 1–2, pp. 33–45. Schulz, E., 1994, The southern limit of the Mediterranean vegetation in the Sahara during the Holocene. Hist. Biol., 9, pp. 137–156. Schulz, E., 2004, Landscape and diseases. The Middle Holocene Sahara: A development of cultural landscape as reusult of a risk oriented behaviour. Riscure si catastrofe, 1/2, pp. 1–24. Schulz, E. and Merkt, J., 1996, Transsahara. Die Überwindung der Wüste. Würzburger Geographische Manuskripte, 38, pp. 1–116. Schulz, E., Abichou, A., Hachicha, T., Pomel, S., Salzmann, U. and Zouari, K., 2002, Sebkhas as ecological archives and the vegetation and landscape history of Southeastern Tunisia during the last two millenia. Journal of African Earth Sciences, 34, pp. 223–229. Snyder, J.A., Miller, G.H., Werner, A., Jull, A.J.T. and Stafford, T.W., 1994, AMSradiocarbon dating of organic-poor lake sediment, an example from Linnevatnet, Spitsbergen, Svalbard. The Holocene, 4, 4, pp. 413–421. Sponholz, B., 2004, Fulgurites as paleoclimatic indicators- the proof of fulgurite fragments in sand samples. In: Smykatz-Kloss, W., Felix-Henningsen, R. (Eds.), Palaeoecology of quaternary drylands. Lecture notes in earth sciences, 102, Springer, Berlin, pp. 73–78. Weninger, B., Jöris, O. and Danzeglocke, U., 2007, CalPal-2007. Cologne Radiocarbon Calibration and Palaeoclimate Research Package. http://www.calpal.de/, accessed 200711-14. (calibration data set is CalPal-2007Hulu ). Weninger, B. and Jöris, O., 2007 (in press), Towards an absolute chronology at the Middle to Upper Palaeolithic transition in Western Eurasia: a new GreenlandHulu time-scale based on U/Th ages. Journal of Human Evolution. Zhou, W., Head, M., Wang, F., Donahue, D. and Jull, A., 1999, The reliability of AMS radiocarbon dating of shells from China. Radiocarbon, 41, 1, pp. 17–24.

CHAPTER 4

The desert in the Sahara. Transitions and boundaries Erhard Schulz Department of Geography, Physical Geography, Julius-Maximilians University, Würzburg, Germany Abdelhakim Abichou Département de Géographie, Université de Tunis, Tunisia Aboubacar Adamou Département de Géographie, Université Abdou Moumouni de Niamey, Niger Aziz Ballouche and Issa Ousseïni Département de Géographie, Université d’Angers, France ABSTRACT: The Sahara is a system of three main landscape types. It comprises the semidesert in its northern part, the desert in the centre and the savanna in the South. The semidesert is a sparse but diffuse shrub vegetation, which is fed by winter rainfall and contains several Mediterranean affiliations in its floristic composition. A sharp boundary separates it from the desert, which is to define by the dominance of linear or contracted permanent vegetation. Aleatoric resources such as rainfall are exploited by achabs, short time vegetation plots of therophytes, which may cover large surfaces. A savanna covers the southern part of the Sahara. It is also separated from the desert by a sharp boundary. This savanna is alimented by monsoon summer rain. The floristic composition remains Saharan. The boundaries of the desert are climatic dependent because the annual rainfall is no longer sufficient enough to allow a diffuse plant cover. They can be followed over the whole continent. Even in the climatically favoured region of the Oceanic Sahara in Mauritania these boundaries are clearly identifiable. In general the vegetation shows two different strategies to cope with the severe physical conditions of the Sahara. The first is to equip the individual organisms with protections against drought and frost but also to adjust their number to the amount of water available. The second is to exploit aleatoric resources by short time organisms in a great number without special protection mechanisms. A great number of pioneer elements in the vegetation describes its general vulnerability but also a great resilience. Especially the repeated development of extended achabs is a sign of rapid reaction to slight modifications of the rainfall conditions without changing the general ecosystem. A vegetation map is presented as a modern model for the interpretation of former environments in the Sahara and adjacent areas.

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4.1 INTRODUCTION The Arabic term “Sahara” means desert or steppe (Wehr, 1968) and describes a waste land being permanently poor in water and hostile for human existence. These estimations are easily to understand if one realises the limited number of water points along the main caravan routes of the Sahara regardless if they crossed the diffuse pastures of the North or the dune fields of the Central Sahara. The use of the term “desert” is wide. The only common feature in this variety of landscapes is that they are arid regions and poorly suitable for human existence. The permanent lack of water demands a highly elaborated organisation of life. Finally, both “Sahara” and “desert” have a more or less emotional dimension. Difficulties arise in that moment when one tries to transfer them into scientific terms. Both terms are largely employed but there is hardly any consensus neither on the nature of Sahara nor on its limits in space. Moreover the use of the terms “steppe”, “semidesert” and “savanna” is often contradictory if one compares the descriptions of Barry et al. (1976), Le Houerou (1989) or White (1983). Anyway, it is necessary to find common criteria in order to differentiate the waste areas especially for the monotony of plant cover (Barry et al., 1976). Regarding the wide discussion on an expanding Sahara or desert (cf. Stebbing, 1935, Tucker et al., 1991) one should choose phenomena which in principal could change in order to decide, whether there might be an extension of an ecosystem on the cost of another. Several attempts were made to define these regions. Describing the limits of ecosystems is a necessary way to understand their internal nature to define indicators of past changes in landscape to be read from various archives. 4.2 SAHARA AND DESERT. ATTEMPTS TO DIFFERENTIATE THE UNKNOWN AREAS The first attempt is to look for visible “real” limits in the landscape. This is the way voyagers proceeded along the main commercial routes during the last and the first half of this century (Denham, 1822–25; Barth, 1857–1858; Nachtigal, 1879; Vischer, 1910). They reported clear boundaries in the landscapes of the Southern Sahara, but in general these indications were not accepted later on to define or limit these regions of the desert itself. These landscape descriptions as well as those from the Western Sahara (Monod, 1954) and from Southwestern Africa (Walter, 1964) had one thing in common. They reported an important change within short distances either from forest to steppe or woodland to grassland. They also coincided in describing these changes in the same regions during the last century where they can be recognized still today (Schulz and Hagedorn, 1994). Monod (1954) and Walter (1964) described the general transition from outside into the desert in explaining it by the gradual diminishing of annual precipitation to a threshold, where the basic needs of plants could only be satisfied by a combination of runoff, groundwater and precipitation in wadis or depressions. These descriptions provided a suitable tool to classify the immense complex of Sahara with help of criteria, which easily could be followed and detected in the landscape. In this sense the desert is a region and an ecosystem where permanent life is only possible under special conditions e.g. the combination of run off, precipitation and of groundwater in depressions and wadis. This is valid both for vegetation and wildlife. It also has the advantage to deal with permanently present phenomena and not only with the areas of certain organisms Uromastix acanthinurus the Bell’s dab lizart, Cerastes cerastes, the Horned viper or the Dorcas gazelle Gazella dorcas (Le Berre, 1989, 1990) for the whole Sahara or the change from the Had (Cornulaca monacantha) to the Cram Cram (Cenchrus biflorus) for the southern limit of the Sahara (Capot-Rey, 1953).

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In a second way one tries to impose an arrangement from predefined limits and their values. These were as well isohyets, aridity indices or percentages in the floristic composition (Klaus and Frankenberg, 1980). From the time Dubief (1959, 1963) presented the isohyetmaps of Northern Africa these maps were taken as suitable tools to define landscapes. There were parallels between landscape/vegetation boundaries and isohyets and inverse and consequently to limit the Sahara and desert at once by the isohyets of 100, 50 mm in the North and 150 mm in the South (Giessner, 1988; Hagedorn, 1985). However, the restricted number of meteorological stations and the great variability of rainfall set a principal limit for the use of these definitions. Anyway, a general consensus on the climatic conditions of the Sahara comprises: (1) The general diminishing of the annual amount of precipitation from the external regions to the centre. (2) The characterisation of the Northern Sahara by the Atlantic–Mediterranean cyclones and of the Southern Sahara by the monsoon and their general interaction in the Central Sahara. (3) The increasing irregularity of rainfall and the rising part of aleatoric precipitation towards the centre. Especially the last point makes it very difficult to use isohyets for a limitation of the desert and the Sahara (Barry et al., 1972, 1973). Anyway, the variability of precipitation and consequently of the annual isohyets often lead to the ideas of moving frontiers of the desert still lasting into most recent times (Tucker et al., 1991; Reichelt et al., 1992). In the end one took the most variable and fluctuating phenomenon to limit the main landscape units. If one compares these two ways to describe and to limit the landscape units, the characterisation of the desert by the repartition of vegetation has several advantages. It frees from the emotional use of the term “desert” as a barren hostile land but also from the exclusive connection to variable criteria as the amount of precipitation or the presence of single organisms. The vegetation summarises both climatic and soil conditions and also provides information of the rainfall regime in regions where records are not available. Moreover it represents phenomena, which could also be detected in the remnants of former landscapes as sediments or palaeosoils. In that way the vegetation provides chances to reconstruct the past and compare it to the present based on the same material. 4.3 THE LANDSCAPE UNITS IN A NORTH–SOUTH TRANSECT THROUGH THE SAHARA In a North–South-transect we will follow the landscape formations and their changes in the Sahara in order to detect, whether and where there are boundaries between ecosystems and in which way they can be recognized. A vegetation map of Northern and Western Africa (Figure 1) was conceived and compiled as a modern model to be compared with the Holocene vegetation. It refers to former regional maps (Pomel et al., 1994; Schulz and Lueke, 1994) and tries to correct the too simplistic model given by White (1983)—at least for the Saharan part. It combines own investigations in the central part of this region and a compilation of the recent vegetation mapping projects. An extensive bibliography is given in Schulz et al. (2000). A special attention is given to the dynamic of the southern part of the Sahara, which is widely discussed as an indicator of “Desertification” (see Tucker et al., 1991). The different formations incorporate remnants of formerly denser vegetation types and the whole spectra of the derivations and transformations by man. In this way it does not deal with “natural”, “potential-natural” or “near to nature” formations or their reconstructions (cf. Anhuf and Frankenberg, 1991). This concept follows the way already indicated by Trochain (1940), who pointed out that most of the vegetation types of these regions could only be characterised as “pseudo climaxes” (depending on soil- or groundwater types)

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Figure 1. Schematic vegetation map of Northern and Western Africa (from Schulz 2000, modified) (for colour map and legend see pages xiv–xv in the preliminary section).


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or even more “peni climax formations” being created by man. Difficulties based on differences of regional vegetation mapping projects are evident in the border region of the former English or French dominions. The limits of the Sudanian region differ in Cameroon (Letouzey, 1985) from those in Nigeria (Barbour et al., 1982; Keay, 1959). The map we present in this article refers to the system of Letouzey (1985) because it seemed to be more coherent and also comparable to own observations in Cameroon and Togo. Moreover, Ballouche and Dolidon (2000) and Dolidon (2005) could confirm the northern limit of the Sudanian zone by its frequency of annual fires. In addition there is a series of block diagrams (Figure 2), which may explain the physiognomy of the main vegetation units including their altitudinal zonation. To understand the Saharan system it is necessary to know about the adjacent areas too. Thus, the Mediterranean not only shows a sharp borderline to the Sahara it also interfingers with it. The southern part of the Mediterranean is characterised by the transition of forest and shrub communities to those of open grasslands (the steppe units). These plant communities are steppe units in the original sense as winter cold continental tussock grass communities of the genus Stipa (cf. Walter, 1964). Floristically and also climatically they still range into the Mediterranean system. A. The Mediterranean macchia and oak forest (from the coast to the Ain Draham Mts./Northern Tunisia Quercus faginea, Qu. suber, or the macchia units of Qu. coccifera, Qu.ilex, Pistacia therebinthus or Olea europea). B. The Djebel Chaambi near Kasserine (Central Tunisia) represents the Stipa steppe and the transition to the Pinus halepensis, Rosmarinus communis, Juniperus oxycedrus forests and on the top (about 1.500 m) the southernmost stands of Quercus ilex. 4.3.1 Northern Sahara and the semidesert. Two strategies of life Parallel to the so-called “Sahara line” south of the Atlas chain from Southwestern Morocco to Southern Tunisia the shrub communities of the Gymnocarpus-Rhanterium-Atractylisunits—rarely exceeding a soil cover of 30%—replace the tussock grass communities of the Stipa-steppe. This transition represents the change to the semidesert (Bornkamm und Kehl, 1985). The transition is characterised by two different physiognomic vegetation units but also by a change in the floristic composition. Whereas the Stipa tenacissima-units are characterised by Mediterranean floristic elements, the semidesert is dominated by Saharan elements. But we have to admit that both vegetation types include units characterised by Artemisia: A. campestris in the steppe units and by A. campestris and A. herba-alba in the semidesert. Stokker (1962, 1976) explained the characteristic composition of the semidesert as a treeless shrub vegetation type caused by the double stress against drought and frost, which impedes a noteworthy tree growth. Stokker also stated that the tussock grass Stipagrostis pungens physiologically behaves as a small shrub in colonising dune sands. However, the questions of a former content of trees evoked by Le Houerou (1969) remain open and may be solved only by records of vegetation history covering the complete Holocene. Perhaps one time this will explain the presence of Acacia raddiana-tree communities in the northern part of the semidesert area. Some principal problems limit the understanding of this borderline. The first is a simple case of nomenclature since the schools around Quezel (1965), Barry (Barry et al., 1973, 1974, 1981, 1985, 1986) and Gaussen and Vernet (1958) regroup these two


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Figure 2. Block diagrams showing the main vegetation types along a North–South transect between the Mediterranean and the Northern Sudan. 1. Pinus pinea, Pinus halepensis, 2. Juniperus phoenicea, 3. Quercus ilex, 4. Quercus faginea, 5. Quercus suber, 6. Quercus cocciferea, 7. Pistacia lentiscus, P. terebinthus, 8. Olea europaea, 9. Phillyrea europaea, 10. Acacia ehrenbergiana, 11. Acacia raddiana, 12. Acacia nilotica, 13. Acacia laeta, 14. Aacia albida, 15. Maerua crassifolia, 16. Balanites aegyptiaca, 17. Salvadora persica, 18. Commiphora africana, 19. Hyphaene thebaica, 20. Guiera sengalensis, Combretum micranthum, C. nigricans, 21. Adansonia digitata, 22. Khaya senegalensis, 23. Parkia biglobosa, 24. Annona senegalensis, 25. Stipagrostis pungens, 26. Panicum turgidum, 27. Artemisia herba alba, 28. Salsola baryosma, Cornulaca monacantha, 29. Stipagrostis plumosa, 30. Aristida sp. 31. Andropogon sp., 32. Mediterranean forest and steppe, 33/34. Sahara, desert, 35. savanna. a. Macchia, cultivated land and oak forests near Ain Draham (N-Tunisia, Quercus coccifera, Qu. ilex, Qu. suber, Qu. faginea, Pistacia terebinthus, Olea europaea). b. Steppe, cultivated land and Pinus-Quercus forest at Djebel Chaambi as altitudinal change of vegetation (Central Tunisia, Stipa tenacissima, Rosmarinus officinalis, Juniperus phoenicea, Pinus halepensis, Quercus ilex). c. Semidesert at Dj. Dahar and Jeffara (S-Tunisia, Artemisia herba alba, Gymnocarpus decander, Rhanterium suaveolens, Olea europaea). d. Southern limit of semidesert and transition to desert south of El Golea (Central-Algeria, Ephedra alata Chenopodiacea, Retama raetam, Acacia raddiana, Panicum turgidum). e. Altitudinal change from desert to semidesert at Atakor/Ahaggar (Southern Algeria, Acacia raddiana, Panicum turdidum, Artemisia herba alba, Chenopodiaceae, Pistacia atlantica). f. Altitudinal change from desert to savanna at Bagzan Mts. (Northern Niger, Acacia raddiana, A. laeta, Panicum turgidum, Commiphora africana). g. Change from desert to savanna and the southern part of Sahara at Tigidit (Northern Niger, Acacia raddiana, A. ehrenbergiana, Maerua crassifolia). h. North-Sahelian savanna south of Tigidit (Northern Niger, Acacia raddiana, Maerua crassifolia, Commiphora africana, Aristida mutabilis). i. Mid-Sahelian savanna and cultivated land north of Tahoua (Central Niger, Maerua crassifolia, Balanites aegyptiaca, Acacia laeta, Aristida mutabilis). j. South Sahelian savanna near Dosso (Southern Niger, Combretaceae savanna- fallow bush, parks, alluvial vegetation, Acacia albida, A. raddiana, Combretum micranthum, Hyphaene thebaica, Adansonia digitata). k. North Sudanian savanna and parks near Gaya (Southern Niger, Khaya senegalensis, Parkia biglobosa, Adansonia digitata, Combretum micranthum).

vegetation types as “steppe”-units (grass-steppe and shrub-steppe). Differences between them are explained as secondary phenomena. The main point is presented by Le Houerou (1969). He describes both vegetation types—as steppe phenomena—in a degradation series starting from Pinus halepensis-Juniperus-(Quercus)-forests into the Stipa tenacissima-steppes and finally into the shrub formations of Gymnocarpus-RhanteriumAtractylis. Artemisia-communities are regarded as degradation stages of both principal units. It is obvious, that use and overuse of these landscapes are omnipresent and may go back to the last ten thousand years (Gabriel, 1984). Consequently pioneer elements, as Artemisia will dominate several communities. Moreover, they will make “steppe” and “semidesert” units similar in the first stages of recovering. However, in a time

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of about twenty years they may be overtaken by Stipa or Gymanocarpus respectively (Abichou, 1988). One may characterise the semidesert as a landscape dominated by dwarf and small shrub communities. They range from Rhanterium-Gymnocarpus-Atractylis-units on medium to coarse-grained substratum to Calligonum-Retama-Ephedra-units colonising fine-grained material or Stipagrostis pungens on loose dune sands. The first units often are dominated by Artemisia sp., the Mediterranean pioneer shrubs. Silts and clay are normally colonised by Chenopodiaceae (Suaeda, Salsola or Arthrocnemum), which also may be explained by the salt accumulations in this material by evaporation of the soilwater. The semidesert is dominated by Saharan floristic elements, either Saharo-Sindic or with a Mediterranean affiliation (see Barry et al., 1973ff.). The vegetation remains diffuse with a soil cover of about 30% in the North. The distances between the individual shrubs are directed by the extended root system, which exploits all kind of humidity ranging from rainfall to fog and dew. There still is another strategy of life in the Sahara. Whereas these shrubs are equipped with a complicated physiological system in coping with drought and frost events in their life span, therophyte seeds answer immediately to the variable rainfalls in creating achabs of different extension. These achabs or short time rain floras have only a short life span and they have to fulfil their life cycle in the time the amount of water allows. They take a regular part in the semidesert system. Nomads know them and exploit them systematically. The achabs consist of grasses and herbs with a floristic composition belonging to the Sahara as well as to the Mediterranean (Ammodaucus leucotrichus, Convolvulus arvensis, Diplotaxix harra, Eragrostis papposa a.o). The semidesert is also represented in figure 2 by the unit C, which describes the situation of the Dahar Mts. in Southeastern Tunisia. On the plateau the southernmost steppe units are bound to sand cover and fog. The area is characterised by the intensive Olea-cultivation either in small gardens (jesur) or in plantations similar to those around Sfax in the Tunisian Sahel. The southern limit of the semidesert is astonishingly clear. Around 30°N in the Central Sahara the diffuse shrub vegetation comes to an end within a distance of a few km. This boundary is sharp, it traverses a variety of substrata, and it can be recognised over the continent. Near the boundary the semidesert is represented by the Ephedra- Arthrophytumunits on coarse material and Retama-Calligonum-units on loose sand. In the Western Sahara it is dominated by Anabasis- or Fredolia-units. In the Libyan or western desert of Egypt it is reported to find an end already by 31°N (Stahr et al., 1985), whereas in the Atlantic region it follows the Atlas chain to the Southwest and continues as a parallel vegetation band parallel to the coast down to 19°N (Quezel, 1965; see also below). The sharpness of the semidesert boundary and the fact, that it transverses several substrata, characterises it as a clear climatic boundary, where the sum of regular precipitation, runoff and dew is no longer sufficient enough to allow a diffuse permanent soil cover. Unit D in figure 2 explains this situation south of El Golea in Central Algeria. 4.3.2 The desert South of that sharp boundary a change in vegetation and landscape takes place. The vegetation is contracted and dominated by trees, tussock grasses and shrubs. The term “contracted” means that permanent plant life does only exist in special places where the basic needs of water are fulfilled by the combination of precipitation, run off, dew or groundwater. These are wadis or depressions (Figure 3). Physiognomically the vegetation type belongs to the so called “desert-savanna-units” (Quezel, 1965) and phytosociologically to the Acacio-Panicion alliance (Barry, 1982).


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Figure 3. The contracted desert vegetation with Acacia raddiana, Fagonia aegyptiaca, Cassia italica and Colocynthis vulgaris in the Enneri Achlouma/Northeast Niger.

Two main types can be separated. Wadis and depressions with coarse sand are colonised by Acacia (mostly A. raddiana) and Panicum turgidum whereas areas with fine sand are settled by Tamarix aphylla or T. saharae and Stipagrostis pungens. The general vegetation map (Figure 1) shows that the contracted Aciacia Panicum- or Tamarix-Stipagrostis-vegetation is mainly restricted to the mountain areas and the wadis radiating from them into the plains. The main components of these vegetation units are trees like Acacia raddiana, A. ehrenbergiana, A. nilotica, Maerua crassifolia and Balanites aegyptiaca, Capparis decidua, Tamarix aphylla together with shrubs like Salvadora persica, Aerva javanica, Zilla spinosa or Leptadenia pyrotechnica. Tussock grasses are represented by Panicum turgidum Stipagrostis pungens or Cymbopogon schoenanthus. From the floristic composition also a lot of pioneer elements from the Sahelo-Sudanian regions are present like Balanites aegyptiaca, Aerva javanica or Solenostemma oleifolium and Cassia italica. Aerva javanica and Balanites aegyptiaca reach the Ahaggar and the Arak gorge north of Ahaggar respectively. The dominance of trees can be explained by the presence of the dualistic root system of the Acacias. A taproot may reach to ground water horizons as deep as 55 m (Abadie, 1927) whereas horizontal roots beneath the soil surface exploit precipitation and run off. In that way trees may become rapidly independent from the actual climatic conditions when they reach a ground water horizon (Ullmann, 1985). The intensive root system of the tussock grasses will act in the same way. Thus, acacias may react in two different ways to the climatic constraints. As often visible saplings from horizontal roots will form genetically identical groups whereas in favorable years a mass of seedlings will occur as one could observe in Northern Niger in 2006. Only when these seedlings will reach the first groundwater lenses or horizons they can continue to grow. Outside the area of the contracted vegetation the achab represent the second strategy of plant life. As already shown for the semidesert the presence of therophytes or even biennial grasses and herbs is directly connected to aleatoric rainfalls. Cyperus conglomeratus, Aristida acutifolia, Stipagrostis plumosa, Schouwia purpurea, Indigofera coerulaea, Aerva javanica etc. are the main components of these vegetation plots, which may extend from

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some square meters to several tens of square kilometres. Observations in the Ténéré (Niger) showed a regular sprouting of grasses and Cyperaceae from their seed bank in layers of fine sand covered by coarse sand (Figure 4). In this way the humidity of rainfalls can be stored and used for the germination. Kehl and Bornkamm (1993) pointed out, that these aleatoric rainfalls also may lead to an installation of permanent plant communities for several years if they manage to reach the first ground water level. In the Southern and Southeastern Sahara these achabs are known as “gizu” (Murat, 1936; Wilson, 1978). Even if they depend on the aleatoric rainfalls and they are not present every year they characterise large regions in Northern Sudan, Chad and Niger between 14°30'N to about 16°N (Murat, 1936; Schulz and Adamou, 1988; Schulz et al., 1999). Wilson (1978) described their appearance as extended grass areas in the Southern Sudan and as limited and isolated plots of herbs and bushes in the Northern Sudan. These “gizu” pastures are regularly exploited by camel herders and regarded as an important part of their pasture area. Out of their economic value the appearance of “gizu” pastures serve as an important step to the stabilisation of soil surfaces and moreover as an indicator of the interaction of Monsoon and Mediterranean air masses. The “gizu” rainfalls last from the end of summer just to the end of winter. Observation in Northern Chad during spring 1997 evidenced repeated rainfalls in March and proved their existence from December 1996 on. This was based on the age estimation of “gizu” pastures. In March 1997 these rainfalls were connected to the interaction of cold air masses coming from the

Figure 4. Achab vegetation in the eastern Ténéré, Eastern Niger. Stipagrostis generally roots in fine sand layers, which store the humidity filtered down through the coarse sand layers.


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North and intrusions of Monsoon air masses (Schulz, 1999). Southwest of Adrar des Iforas, Air Mts. and Tibesti these achabs regularly evolve into large areas providing the image of savannas combined to the extended wadi vegetation (see figure 1). Thus, the permanent vegetation characterises and defines the desert as a region, where permanent life is restricted to depressions or wadis where beside of rainfall, dew and run off groundwater is available. Even smaller changes in substratum or relief are followed by a change of vegetation. Sandy patches in rocky areas normally are colonised as the first places by achabs or small bushes. Units D and G in figure 2 describe these changes in the North between semidesert and desert and in the South between desert and savanna. 4.3.3 The change with altitude There are two modes of change with altitude. The first is represented by the “Mediterranean type”, visible in the Ahaggar, Tibesti and in parts of the Tassili-n-Adjjer. From about 2.000 m altitude (Ahaggar) or 2.900 m (Tibesti) the contracted vegetation of the Acacia-Panicumtype is replaced by diffuse shrub communities of the Artemisia-Ephedra type representing the change to semidesert environments (Quezel, 1958, 1964, 1965, 1968) In addition the Ahaggar shows relict tree- and shrub units as Olea laperrini and Pistacia atlantica, whereas the Tassilli-n-Ajjer is known for the presence of Cupressus duprezziana-stands (Leredde, 1957). Erica arborea is described for the summit area of the Tibesti Mountains (Quezel, 1958). Climatically these environmental changes depend on the regular presence of Mediterranean winter rainfall in the higher altitudes. The second type of altitudinal change is manifested in the Air Mts./Northern Niger. Isolated High Mountain massives expose the transition to a diffuse tree and shrub vegetation over 1.500 m asl (Schulz and Adamou, 1988; Schulz et al., 1999) consisting of Acacia raddiana, A. laeta, Commiphora africana and Rhus tripartita. These savannas depend on elevated summer rainfall and this model represents the “Sahel type” of altitudinal change. A variation of this type is the appearance of Acacia-Indigofera-savanna on the high plateaus of the Ennedi in Northern Chad (Gillet, 1968). The Adrar des Iforas in Northern Mali however does not reach the necessary altitude for a change of vegetation and remains a Saharan desert mountain area (Schulz et al., 2001; Voss and Krall, 1994a, b). The units E and F in figure 2 describe the two types of altitudinal change.

4.3.4 The southern boundary of both Sahara and desert. The Saharan savanna The southern boundary of the desert is the most disputed limit between landscape zones or over regional ecosystems. It caused the misunderstanding of degradation, desertification and extension of desert reaching back to the alarm cries from Stebbing (1935), even it was corrected very soon (Aubreville et al., 1937/1973). However, information on this boundary is still very scarce. For long times the Sahel was regarded as an poorly defined transition zone between the Sahara and the (Sudanian) savannas proper, forming more or less an ecotone (Barry et al., 1986). It took a long time until the Sahelian savannas were accepted as consistent vegetation units (cf. Monod, 1986; Le Houerou, 1989). The Maerua-savannas as the step of vegetational adapatation to rising humidity in the Southern Sahara In the lowlands of the Southern Sahara we could observe a change from the contracted desert vegetation to a diffuse tree formation in Northern and Eastern Niger. The figures

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Figure 5. Block diagram showing the depression and oasis of Agadem, Eastern Niger. Note the Maerua-savanna on the slopes and on the plateau.

Figure 6. The Tigidit escarpment, Northern Niger. The linear wadi vegetation contrasts to the sparse Maerua-savanna on the sandstone plateau.

5 and 6 show the examples of the sandstone cuesta of Agadem and Tigidit in the East and North of Niger. The depression and oasis of Agadem represent the southernmost parts of the cuesta system of Bilma in Eastern Niger. The oasis of Agadem as well as the plateaus of Homodji and Tcheni Tchadi South of it consist of deeply fissured sandstone, which, is iron crusted in its upper part. In the oasis of Agadem (Figure 5) the centre is sand covered and it bears a group of Acacia ehrenbergiana, A. raddiana, Phoenix dactylifera and Hyphaene thebaica trees with some Salvadora persica bushes. The sandy plain supports some Capparis decidua and Maerua crassifolia trees, Panicum turgidum tussocks and some Salvadora bushes. Tussock grasses like Stipagrostis vulnerans or therophytes like St. adscensionis or Cyperus conglomeratus colonise the dune covered lower slopes. Trees like Maerua crassifolia, Aacia raddiana or Capparis decidua grow in the wadis accompanied


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by Leptadenia pyrotechnica-bushes. The fissured sandstone of the cuesta as well as the block accumulations of the slopes expose a sparse but diffuse tree vegetation composed of Maerua crassifolia. In 1984 we observed, that the trees on the slopes were all living, including great and rather old exemplars. On the plateau however, trees were small and about the half of them were dead. The plateau may have been colonised in the 1960ies, but during colonial times there were many old trees on the slopes (A. Kelle, personal communication). The plateaus of Homodji and Tcheni-Tchadi Southeast of Agadem showed a similar picture. The sandstone is fissured and covered by an iron crust. The plateaus supported at that time a diffuse tree vegetation consisting of Maerua crassifolia accompanied by a few Acacia ehrenbergiana and Fagonia-bushes and grasses of Panicum turgidum, Stipagrostis vulnerans, St. adscensionis. Here, all Maerua and Acacia trees were alive. In the southern foreland of the plateaus some extended achabs of Stipagrostis adscensionis were present as well as isolated Capparis deciduas-trees in some dune depressions. Another example is the Tigidit escarpment in the Southeast of the Air Mts., which rises about 80 m above the foreland. The cuesta consists of sandstone of Cretaceous age. It is highly dissected and the sandstone shows a mosaic of fissures and has a thin sand cover. Figure 6 shows the different types of vegetation. The wadi in front of the cuesta follows the scheme of the contracted vegetation of the desert with an arrangement of large Acacia and Balanites trees and Salvadora bushes. Tussok grasses (Panicum) form an understorey. On the fine-grained outer parts grows an extended population of Schouwia purpurea (Cruciferae) providing an important pasture. The plateau bears a sparse tree vegetation of Maerua and Acacia (ehrenbergiana). In spring 2005 the Maerua-trees were all living and we estimated an age of 20 years. This sparse but diffuse tree formation with a variable grass component is to classify as a savanna. It is also present in North-Mali (see map, figure 1). These Maearua-savannas colonise fissured sandstone environments, where finegrained material and humidity is collected in the cracks and exploited by the flat root system of Maerua crassifolia. During long-lasting drought periods survival is difficult. Thus populations of the same age might be explained. These Maerua-savannas are well restricted to a substratum providing a cistern situation and consequently a diffuse colonisation in a desert environment, where else permanent life is still restricted to wadis and depressions. The Acacia-Panicum-savannas as the definite change from desert to savanna If one proceeds at about 16°30'N in Niger from the North to the South both in the Tigidit area southwest of Agadez and in the Tintoumma in the East, South of Bilma, one can observe a densification of the tree lines and within a distance of about one km a change to the diffuse repartition of the Acacia-Panicum-vegetation. This represents the transition from the desert into the savanna, a diffuse tree and grass-vegetation dependent on tropical summer rain. The floristic elements of the vegetation still remain Saharan. A similar change is performed in Northern Mali (Barry, 1982) and in Northern Mauritania (Barry et al., 1987) Observations in Northern Chad (Schulz, 1999) showed a modification by substratum. The large inundation plains are devoid of plants but the sand ridges between them bear a treevegetation and at about 16°N a diffuse tree and tussock grass savanna occurs in the sandy plains. A comparable transition belt characterised by substratum changes is announced for the Northern Sudan by Akthar-Schuster (1995). Contrary to the Maerua-savannas described above this transition takes place on the same substratum and it is no longer totally dependent on the underground conditions. Several kilometres South of this boundary another transition takes place. The plant cover remains as a savanna but the floristic components are different. Beside Acacia Commiphora-trees become characteristic. The tussock grass Panicum turgidum disappears and annual grasses like Aristida mutabilis or Chenchrus biflorus are dominant. This is the

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Figure 7. Block diagram showing the southern margin of the Sahara in Niger. Above the change from linear desert to the diffuse savanna vegetation at about 150 km south of Agadez, below in the Erg of Bilma (Schulz, 1999, modified).

transition to the Sahelian savanna system, which for its northern part is phytosociologically characterised by Barry (1982) as the Acacio-Aristidion alliance in contrast to the Saharan Acacio-Panicion. Other tree elements in these savannas are Maerua crassifolia, Balanites aegyptiaca and shrubs like Grewia tenax, Ziziphus mauritiana or Boscia senegalensis. As the general map (Figure 1) shows, the Sahelian savannas change southwards to Acacia-LeptadeniaBalanites-Piliostigma-units and finally to Combretaceae-savannas. The Units G to K in figure 2 describe the southern boundary of the desert and the succession of different savanna types of the Sahel and the northernmost Sudan. It is necessary to mention again that the Sahelian and Sudanian savannas represent cultural landscapes in different intensity and history. 4.4 THE SPECIAL CASE OF THE ATLANTIC SAHARA Travelling in Southern and Western Mauritania along the road from Aleg, Boutilimit, Nouakchott and Nouadibou in April 2005 we observed the transition in the Southern Sahara where humidity is largely provided by fog. Hitherto only little information is available on this region. Quezel (1965) characterised the oceanic Sahara by shrub formations of Nitraria, Zygophyllum, Gymnocarpus, in the littoral zone also with Tamarix and Artrocmenom trees and bushes. On dune areas he noted a mixed tree and shrub formation of Balanites, Acacia (raddiana) or Salvadoraand Cornulaca-bushes. He described the presence of an Acacia raddiana, Euphorbia balsamifera, Lycium intricatum-tree and shrub land for these hills. Barry (1989) distinguishes in his floristic analysis between the alliances of Acacio Panicion and the


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Figure 8. The degraded Acacia-Leptadenia savanna near Aleg, Southern Mauritania.

Figure 9. The semidesert vegetation around Nouakchott/Mauritania.

Aervo-Fagonion both of Saharan character. Diak (1977) presented a map showing a change from the Commiphora-Acacia-savannas of the Sahel to Acacia (senegal)-savannas and to the coastal Tamarix-Salsola-Salicornia formations giving place to the North to tussock grass (Stipagrostis pungens) and Acacia raddiana formations of the dune areas. Culmsee (2002a, b) described the savannas and semideserts around Nouakchott and Akjoujt and Tabia et al. (2005) analysed the recent vegetation changes in the area of Lake Aleg. In 2005 the dune areas between Aleg and Nouakchott were monotonously covered by a loose and degraded savanna of Acacia senegal, A. ehrenbergiana, Balanites and Leptadenia pyrotechnica (Figure 8). Some Panicum turgidum-tussocks remained in the dune depressions. The whole region was totally overgrazed, which explains the presence of this pioneer vegetation, where trees are generally replaced by Leptadenia-bushes. A diffuse scrubland of Salsola and Zygophyllum presents the image of an extended semidesert on the disturbed sand and marls north of Nouakchott (Figure 9), which is also described by Culmsee (2002). It certainly depends on the intensive overgrazing of the region. About 160 km north of Nouakchott, a diffuse tree and grass vegetation is present in the area of disturbed dunes (Figure 10). Some Zygophyllum and Salsola bushes, mainly along the roadside accompany these savannas.

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Figure 10. The Acacia-Panicum savanna about 160 km north of Nouakchott, Western Mauritania.

Figure 11. The Maerua savanna about 180 km north of Nouakchott/Western Mauritania.

At about 180 km north of Nouakchott the type of the savanna changes again (Figure 11). The open Maerua crassifolia-savanna appears with an understorey of Panicum turgidum tussocks may reach to about 20% of soil cover. Zygophyllum or Salsola bushes are also presen (Figure 11). 90 km to the North the dunes and extended wadis bear some achabs of the Stipagrostis adscensionis-type together with some Chenopodiaceae bushes. Trees are only present as Acacia (raddiana) in lines representing the change to a contracted vegetation. This desert formation dominates over 150 km with a varying density but without changing its main character (Figure 12). From about 400 km north of Nouakchott on (Figure 13) the dune area is again colonised by a diffuse vegetation of small bushes of Zygophyllum, Salsola- and Linariabushes, some annual grasses and sparse Acacia (raddiana)-trees. Hyoscyamus stands are present. The maximum soil cover of this vegetation is 20%. It represents the change to the semidesert, which characterises the westernmost Sahara from this region on to the North.


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Figure 12. The linear desert vegetation with some achabs about 270 km north of Nouakchott/Northwestern Mauritania.

Figure 13. The semidesert vegetation about 350 km north of Nouakchott/Northwestern Mauritania.

Thus the general mosaic of vegetation types in the Southern Sahara manifested by the change from diffuse to contracted vegetation can be found also in this region. But the semidesert shrub vegetation widely interfingers with the plant cover of desert and savanna. This may be due to the special climatic conditions of the Oceanic Western Sahara. However, it is also evident that an extreme overgrazing has influenced both the floristic composition and the density of the different vegetation types (cf. Tabia et al., 2005). From the inland part of Northern Mauritania Quezel (1965) described a savanna vegetation on the inselbergs of the Zemour region. A diffuse tree-shrub cover mainly consisting of Maerua crassifolia, Rhus tripartita, Ephedra rollandi and Lycium intricatum colonises isolated granitic massives. This points to the special ecological conditions of isolated mountains providing cisterns for humidity and fin grained material as well as it indicates an interfingering with the northsaharan plant units as it is visible in the Oceanic part of the Sahara. It is certainly necessary to invest a lot of fieldwork to verify these observations. However, it will probably be impossible for certain areas because the region was a theatre of war in the past and still it is mined over large distances.

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4.5 THE CHANGE OF VEGETATION IN TIME AND SPACE As explained above the double strategy of life in the Sahara allows a twofold reaction to severe or favourable conditions. Short time fluctuations are answered by the therophytes. Achabs may rapidly cover large area out of their seed bank. Trees react in a double way too: vegetative with root saplings or generative with seeds. In good years a mass production of seeds may lead to uniformous groups of the same age. Trees equipped with tap-roots may survive long and even severe periods, if they can rely on a groundwater horizon. This causes a certain vulnerability and resilience of these Saharan ecosystems. Vulnerability, because periods of decreasing amount of water and/or accelerated exploitation may easily lead to a decline of individuals or greater groups, especially when they have a similar age. Resilience, because they will recover from their own resources (vegetative way) or because pioneers will replace them for the time of one generation or for a longer period. It could provoke degradation-regeneration systems like the replacement of Acacia nilotica, A. raddiana, A. ehrenbergiana, Leptadenia pyrotechnica and inverse or the disappearance of Commiphora from the Southern Sahara and the Northern Sahel for a certain period. The latter is caused by the unique lateral root system of the Commiphora trees making it sensitive to drought as well as the intensive grazing. During the last years the permanent vegetation recovered in many regions such as in the Mangueni in NE-Niger. This region showed the general background of regeneration in the Sahara and Sahel. Periods of renovelation and regeneration are connected to climate as well as to human interference. Man exploits intensively both Sahara and Sahel (Adamou, 1979; Spittler, 1989; Schulz et al., 1999). Pastoralism, either as a full- or as a semi-nomadism, still is the most adapted exploitation system for the desert and the North Sahelian savannas (Ousseïni, 1996). Agrior horticulture is necessarily restricted to favoured places following the oasis structure of these ecosystems. The intensive human exploitation reaches back to the Neolithic and the question remains open whether the whole savanna system of the Sahel did not evolve from the contact of Sudanian and Saharan savanna-systems during the Middle Holocene due to human impact and pressure (Pomel et al., 1994). In periods of human absence the vegetation may recover to the level the respective climatic conditions will allow. It is the case for the Mangueni-Achelouma region (cf. figure 3), which is part of the traditional pasture area of the Tubu. However, since the last decade after the last rebellion the local Tubu clan did not exploit these pastures (A. Kelle, pers. comm.). Olson et al. (2005) showed that the long war in Central Sudan caused a regeneration of vegetation. It goes parallel to the fact that the renovelation periods of acacias in the Air Mts. in Northern Niger coincided with the evacuation of the population during the Kaoucen upheaval (Spittler, 1989) or during the present civil war. Thus, the question of stability–lability and vulnerability–resilience cannot only be evaluated from the physical conditions of these regions. 4.6 DESERTIFICATION AND REGENERATION As the southern boundary of the Sahara is still connected to the discussion on “Desertification”, this subject merits the same remarks as for the savannas (Cole, 1978): most discussed and least understood. From the 1930ies there was a steady warning against the spreading of the Sahara and /or desert ranging from the first cries of Stebbing (1935) to the complete misunderstanding of the vegetation dynamic by Tucker et al. (1991). Annual changes of the plant cover in Sahara and Sahel as detected by satellite images


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were interpreted as a spreading of the Sahara to the South. However, these ideas are still active. The NASA (2007) states that expansion and shrinking of the Sahara resume to a net stability during the period of 1984 to 1994. For this area we have valuable informations on the landscape development during the last 200 years. The descriptions given by various voyagers for the Southern Sahara during the last centuries (s.a.) had one thing in common. About 100 km north of Lake Chad in the region of Ngourti they reported an important change within short distances either from forest to steppe or woodland to grassland. They all pointed to a stability of the borders between the major ecosystems. These are the same regions where the principal changes can still be observed today (Schulz and Hagedorn, 1996). The present debate on the “greening” of the Sahel resumes the whole debate on “Desertification” of the last decade (Anyambo and Tucker, 2005; Hellden, 1991; Hutchinson et al., 2005; Herrmann and Hutchinson, 2005; Mainguet, 1991; Mortimore and Turner, 2005; Nicholson, 2005; Olson et al., 2005). Herrmann and Hutchinson (2005) characterised one fact as very problematic, that of the low resolution of the satellite images. However one states a general improvement of vegetation since 1982 and the discussion is, whether it represents a return to past humid periods or to the development of a new equilibrium. There is a general change in understanding of climatic variability, of social processes or on the question whether the humid periods of the last centuries were more an exception than normality in the regional climatic system. After all this reasoning and disinvestments it became evident, that combat this degradation is only possible when the human impact is modified and combined with a general acceptance of the people concerned. The Southwestern Niger furnished good examples (Bender and Ousseïni, 2000; Ousseïni, 2002). Figure 14 may explain these interdependencies. After an intensive overuse of the Combretaceae savanna system and

Figure 14. Block diagrams showing the soil erosion risk in the Acacia albida-parks of Liptako/Southwestern Niger. The necessity of an integral antierosional intervention is evident (after Ousseïni, 2002, modified).

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of the Acacia albida-parks (1, 2) it was obvious that an interference of anti-erosion measurements, which was only restricted to slopes (3) could be dangerous and also could provoke retrogressive and deep cutting erosion. Only a complete and long time assured interference may be successful and be maintained by the local population (4). The regeneration potential of the Saharan and Sahelo-Sudanian ecosystems is still present (Schulz and Hagedorn, 1994). This is based on the experience of fence and observation experiments (such as Maas, 1991) and observations on the regeneration of the South Saharan and North Sahelian savannas in Northern Chad (Schulz, 1999). Also the behaviour of soil surfaces shows the regeneration potential of these regions (Casenave and Valentin, 1989, Malam Issa et al., 1999). 4.7 DISCUSSION These observations were done during the last 30 years on various expeditions, which mostly were for coring of sediments in order to reconstruct palaeoenvironments. Out of logistical reasons these expeditions were conducted in the dry period (February to April) with the exception of one project in the Air Mts., which allowed investigations during the rainy season (cf. Schulz and Adamou, 1988). Thus, it only allowed a restricted picture of the herb vegetation but the observations are comparable for the permanent vegetation. However, the detection of the continental wide distribution of these formations allows a discussion of the nature of these ecosystem changes. The general reasoning on boundaries between ecosystems focussed on “ecotones” as regions of interactions between the respective units (Di Castri and Hanssen, 1992), as contact zones of two different ecosystems with marginal effects or as continua between them (Duvigneau, 1984), as transit areas (Odum, 1991) or simply as border biotopes between biozones of a certain extension (Frey and Lösch, 1998). This also involves the classification of the Sahelian steppes/savannas as transition units between the Sahara and the real (Sudanian) savannas (Barry et al., 1976,) or even as a “zono-ecotone” (Walter and Breckle, 1984). However, Di Castri and Hansen (1992) admit, that there are sudden changes in these interaction zones. These classifications all come from regions of a permanent and diffuse plant cover. Question arises, whether one can transfer them to areas, where long living organisms come to their definite limits of existence and where biocoenosises are not only determined by concurrence between them. Thus, the observed clear boundaries between two basic distribution types of permanent live—as diffuse or contracted—show that the whole system of the Sahara has a basic threshold-ecology. These thresholds are hydric, based on regular annual rainfall or on the combination of rainfall or fog, run off or humidity storage in soil or on the availability of groundwater. These are primary thresholds and they determine the basic chances of life in a certain area. The discussion of ecotones or transit formation is not adapted for these regions. Even if one admits, that achabs may connect desert and savanna for short periods, as animals do it too, the distribution of permanent life does not follow a gradient but is directed by these principal thresholds. The continental-wide presence of these boundaries on various substrata characterises them as climatic boundaries. Semidesert and savanna as northern and southern part of the Sahara landscape system are principally different and should not be combined to one unit (cf. White, 1981). However, it is unknown whether the present shape of these formations represents their full nature or only a type of basic survival. Our observations are done in the period of recent droughts and only the investigations of Leredde (1957), Monod (1954) or Quezel (1965) refer to the last humid period of these areas. Comparing their descriptions there is a basic agreement on the contracted mode of the permanent vegetation in the desert and of the clear boundaries with


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diffuse formations of savanna and semidesert even other terms are often employed for them such as “steppe” for the semidesert. On the other side some historical descriptions point out, that the Maerua-savannas had a much wider distribution in the Central Southern Sahara at the time Barth (1854), Nachtigal (1879) or Tilho (1911) travelled on the Borno–Tripolis road (Schulz and Hagedorn, 1994). These authors announced them for the dune areas south of 16° N. It seems, that what we could observe in Mauritania will represent a model, which we may transfer to the central part of Southern Sahara too. Certainly, the intensive overgrazing during the last decades will have modified the shape and the floristic and faunistic inventory of these formations. It may be comparable for the semidesert too, where the presence of trees south of the Atlas chain in Morocco and in Tunisia raises the question of the principal treeless nature of these formations. It is certain, that all these questions demand much more fieldwork—also in the rainy season. However, the present situation of the Sahara will not allow it. Wide areas are a theatre of war and the distribution of landmines will exclude many regions from travelling and investigations.

4.8 CONCLUSIONS The tripartition of the Sahara in semidesert, desert and savanna illustrates the strategies of living organisms in coping with severe conditions mainly the lack of water. In the interaction space of Mediterranean/Atlantic cyclones and the monsoon the semidesert as well as the Saharan savanna receive enough precipitation to maintain a diffuse cover of permanent shrubs or trees and tussock grasses respectively. The desert is characterised as a region where permanent life is restricted to favoured places where additional water is provided by runoff and groundwater. Thus the plant cover of mainly trees and tussock grasses appears as linear or contracted. In all these ecosystems there is a second type of exploiting aleatoric resources such as irregular rainfalls. Achabs or gizu appear as short time but diffuse plots of therophytes, which have to fulfil their life cycles according to these irregular rainfalls. The boundaries of the desert are sharp as for the North between semidesert and desert and for the South between desert and savanna. These can be followed over the continent on several substrata and thus they are climatic boundaries. Substratum, relief or exploitation cause changes and variations within these limits, as it is visible in the appearance of an intermediate savanna formation of Maerua, Capparis and Acacia on the sandstone plateaus of the Southern Sahara. There is a distance between the Maerua- and the Acacia-Panicum-savannas in the continental Sahara, whereas in the Atlantic province they have a direct contact. These ecosystems have an elevated vulnerability but also resilience is high both in Sahara as in the desert resulting into a general tendency of regeneration of vegetation and soil. Regeneration of vegetation is often restricted to the vegetative mode and only in favoured period of precipitation—or protection—a generative renovelation is visible. Thus the desert remains stable—in a secular scale—within these boundaries and the Sahara seems to be stable too. The sharp boundaries of desert and Sahara and the shape of the Saharan and Sahelian savannas explain that one has to revoke ideas of only transition between the desert and the savannas—often understood as Sudanian savannas. The Sahel too stands as a consistent savanna system and not as an ecotone interfingering Sahara and Sudan. The general limited resources force all living organisms to adjust to this situation. Neglect, overuse or mining of non-renewable resources may easily lead to long-lasting devastations.

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As the vegetation map is conceived as a modern model to interpret palaeo-environments, question arises, to which extension the actualism may work. We have to accept, that we dispose on only very limited descriptions and investigations for these waste areas. However, several historical reports confirm the basic features of clear and sharp boundaries between a diffuse and contracted mode of permanent life in the Sahara. The present dynamic of vegetation points to the function of threshold. Short time fluctuations of rainfall are immediately answered by the achabs or gizu, which may cover large areas and also stabilise them for a certain time. A repeated formation of them will reduce deflation. A favourable combination of rainfall may cause a formation of tree formations in a vegetative or generative way. If their roots reach to a groundwater level, they will survive even if the rainfall diminishes again. It means that a small change in the precipitation regime will cause important changes in the landscape without changing its principal nature. The comparison of the continental distribution of the South Saharan savannas in Niger with the oceanic one in Mauritania already points to this fact. A next step would follow a longer period of slightly increased precipitation. This would be an extension of the Saharan savannas, which could conquer large areas. This still would remain in the Saharan dynamic and also include the sharp boundaries of savanna areas to a desert environment. During these periods plants—trees, shrubs or herbs—may invade and settle on favourable places like valleys, niches in cuestas or around springs and lakes. Remnants of these elements may be found in suitable sediments. They will allow a reconstruction of the former landscape type but the knowledge of the present dynamic of the vegetation will point to the narrow limits to take them as indicators for a different climate in a greater area. Landscapes may react very fast on slight changes in the precipitation regime, but several features of them will remain, even if the climatic modifications are already terminated. The present observations and the description from the last two hundred years all point to the stability of the Saharan ecosystems. They are vulnerable but they have a great recovering potential even if we still know very little on their dynamic in time. This also includes the various modes of human interference in the desert as in the savanna. ACKNOWLEDGEMENTS We are grateful to I. Ullmann for valuable suggestions and to K. Wepler for help in cartography. REFERENCES Abadie, M., 1927, La colonie du Niger, (Paris: Societé d’ Edition Geographique Maritime et Coloniale). Abichou, A., 1988, El Bahira (Sud tunisien) deux géofacies dans un milieu aride. (analyse et cartographie). (Caen; Mémoire de maitrise de Géographie). Adamou, A., 1979, Agadez et sa region. Etudes Nigriennes, 44, pp. 1–358. Akhtar-Schuster, M., 1995, Degradationsprozesse und Desertifikation im semiariden randtropischen Gebiet der Butana/Rep. Sudan, Göttinger Beiträge Land- und Forstwirtschaft in den Tropen und Subtropen, 105, pp. 1–166. Anhuf, D. and Frankenberg, P., 1991, Die naturnahen Vegetationszonen Westafrikas. Die Erde, 122, pp. 243–265. Anyambo, A. and Tucker, C.J., 2005, Analysis of Sahelian vegetation dynamics using NOAA-AVHRR NDVI data from 1981–2003. Journal of Arid Environments, 63, pp. 596–614.


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Reichelt, R., Faure, H. and Maley, J., 1992, Die Entwicklung des Klimas im randtropischen Sahara-Sahelbereich während des Jungquartärs—ein Beitrag zur angewandten Klimakunde. Petermanns Geographische Mittleilungen, 136, 2–3, pp. 69–79. Republique du Senegal, 1985, Carte du couvert végétal 1:500.000, (Dakar). Schliephake, K., 1995, Libyens Großer Künstlicher Fluß—Lösung der agrarischen Versorgungsprobleme? Mitteilungen des Deutschen Orientinstituts, 52, pp. 165–180. Schulz, E., 1994, The southern limit of the Mediterranean vegetation in the Sahara during the Holocene, Historical Biology, 9, pp. 1347–156. Schulz, E., 1999, The southern margin of the Sahara in the Republic of Chad. Vegetation, soil and present pollen rain. Zentralblatt Geologie. Paläontologie, I, pp. 483–496. Schulz, E., 2000, Present vegetation. In: The Holocene landscape and vegetation history of Northern and Western Africa. A palaeoecological atlas, edited by Schulz, E., Agwu, Ch., Dupont. L., Jahns, S. and Salzmann, U., www.uni-wuerzburg.de/geographie/fachi/pal_ atlas_afrika/index_atlas.htm Schulz, E. and Adamou, A., 1988, Die Vegetation des Aïr-Gebirges in Nord-Niger und ihre traditionelle Nutzung. Giessener Beiträge zur Entwicklungsforschung, I, 17, pp. 75–86. Schulz, E., Adamou, A. and Ousseïni, I., 2001, Air et Adrar des Iforas: une comparaison de deux montagnes du Sud du Sahara, et leur evolution actuelle, Espaces Tropicaux, 16, pp. 219–232. Schulz, E. and Hagedorn, H., 1994, Die Wüste, wächst sie denn wirklich? Die Geowissenschaften, 12, pp. 204–210. Schulz, E. and Himsel, H., 1993, Sahara-Sahel-Sudan. Honige als Spiegel der Vegetation des Niger, Würzburger Geographische Arbeiten, 87, pp. 373–388. Schulz, E. and Lueke, M., 1994, A two years pollen calendar for traditionally produced honey types from Gaya, Southern Niger. Grana, 33, pp. 254–259. Spittler, G., 1989, Dürren, Krieg und Hungerkrisen bei den Kel Ewey (1900–1985) (Wiesbaden: Steiner), pp. 1–199. Stahr, K., Bornkamm, R., Gauer, J. and Kehl, H., 1985, Veränderungen von Böden und Vegetation am Übergang von Halbwüste zur Vollwüste zwischen Mittelmeer und Quattara Depression in Ägypten, Geoökodynamik, 6, pp. 99–120. Stebbing, E.P., 1935, The advance of the desert. Geographical Journal, 91, pp. 356–359. Stokker, O., 1962, Steppe-Wüste-Savanne. Veröffentlichungen des Geobotanischen Institutes ETH Zürich (Rübel), 37, pp. 234–243. Stokker, O., 1976, The water-photosynthesis syndrome and the geographical plant distribution in the Saharan deserts. In: Lange, O., Kappen, L., Schultze, E.D. (Eds.), Ecological studies, analysis and synthesis, 11, pp. 506–521. Tabia, A.N., Gassanib, J., Elghadi, A.V., Ballouche, A. and Moguedet, G., 2005, Diagnostic de la dynamique des resources hydriques de surface et des processus de “desertification” du Lac d’Aleg et son basin basin versant (Brakna, Mauretanie) par teledetection multidate. Teledetection, 5, 1–3, pp. 123–137. Trochain, J., 1940, Contribution à l’étude de la végétation du Sénégal, Mémoires de l’IFAN, 2, pp. 1–433. Tukker, C.J., Dregne, H. and Newcomb, W.W., 1991, Expansion and contraction of the Sahara desert from 1980 to 1990, Science, 253, pp. 299–301. Ullmann, I., 1985, Tagesgänge von Transpiration und stomatärer Leitfähigkeit Sahelischer und Saharischer Akazien in der Trockenzeit, Flora, 176, pp. 383–409. UNESCO/FAO, 1968. Carte de la végétation de la région méditerranéenne 1:5000.000, (Paris). United Nations, 1992, Convention on desertification. (New York), pp. 1–44. Vischer, H., 1910, Across the Sahara from Tripoli to Bornu, (London), pp. 1–308.


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Voss, F. and Krall, P., 1994a, Principaux biotopes du criquet pelerin dans le Nord dur Tilemsi (Mali), (Berlin, Eschborn). Voss, F. and Krall, P., 1994b, Principaux biotopes du criquet pelerin dans l’Adrar des Iforas (Mali), (Berlin, Eschborn). Walter, H., 1964, Die Vegetation der Erde. In ökophysiologischer Betrachtung, (Jena: VEB, G. Fischer, 2 vols). Walter, H. and Breckle, S., 1984, Ökologie der Erde, 2, (Stuttgart, G. Fischer), pp. 1–461. Wehr, H., 1968, Arabisches Wörterbuch, (Wiesbaden). White, F., 1983, The vegetation of Africa. Natural Resources Research (Paris: UNESCO), pp. 1–356. Wilson, R.T., 1978, The “gizu” grazing in the South Libyan desert. Journal of Arid Environments, 1, pp. 327–344.

CHAPTER 5

Palaeo-climatic evidence of soil development on Sahelian ancient dunes of different age in Niger, Chad and Mauritania Peter Felix-Henningsen and Peter Kornatz Institute of Soil Science and Soil Conservation, Justus Liebig Universität Giessen, Germany Einar Eberhardt Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, Germany ABSTRACT: Ancient dunes of the Sahel reflect lengthy arid climatic phases in which the desert margin of the Sahara extended southwards over a distance of several hundred kilometers from its present position. During the following semi-humid to humid climatic phases the dunes and sediments were fixed by vegetation and soil formation. The extent and duration of the humid phases determined depth and intensity of weathering and soil formation. In order to show the relationship between degree of soil development and age of the ancient dunes, soil profiles in the Sahel of Mauretania, Niger and Chad were investigated pedologically and by OSL dating. Due to the limited number of investigated sites and dated samples, general conclusions concerning the stratigraphy of ancient Sahel dunes are not possible. However, the examples described in this paper show that a relationship exists between dune age and depth and intensity of soil formation. The sedimentation of the Upper Pleistocene dunes of the Sahel in Niger started at least around 30 ka and ceased at ∼10 ka with the transition from the Upper Pleistocene to the Holocene, while the Middle Holocene dunes in Chad and Mauritania were sedimented between ∼5 and ∼3,5 ka. Due to the accumulated processes in the humid periods of the Early Holocene and the Mid-Holocene during soil formation on Upper Pleistocene dunes, the rubefication, accumulation of fines and leaching depth of soluble salts is much more pronounced than on Middle Holocene dunes, which were only affected by weathering from the Mid-Holocene until present. The alkaline pH values of the soils reveal that under recent climatic conditions in the Northern and Central Sahel, with annual precipitation up to 400–500 mm a−1 and deposition of dust containing carbonates and soluble salts, the progress of silicate weathering is inhibited. Only under higher precipitation in the southern Sahel weathering of silicates proceeds due to stronger soil acidity. Therefore the soils on ancient dunes of the Northern and Central Sahel are today relict soils, which mainly weathered in periods with conditions of higher precipitation than ∼500 mm a−1 during the Early and Middle Holocene humid periods.

5.1 INTRODUCTION In Sahelian countries ancient dunes form a several hundred kilometer broad belt south of the present southern desert margin of the Sahara, at approximately 16° northern latitude and stretching from East to West Africa. These morphologically more or less denudated dune fields, lee dunes at escarpments or inselbergs, eolian sand sheets and valley fillings

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indicate lengthy arid climatic phases, in which the desert margin of the Sahara extended southwards over a distance of several hundred kilometers from their present position. During subsequent periods with semi-humid to humid climatic conditions, depending on the geographical position, the eolian sediments were covered by vegetation and fixed by soil formation. The latter is characterized by root channels, animal burrows, a brown to reddish brown colour and a cementation by silica and soluble salts, which indurates or stabilizes the normally loose eolian sand. In valleys the dune relief was dissected by water erosion due to the incision of wadis. Apart from archaeological findings or organic material, which allows radiocarbon dating, soils and palaeosols are also valid proxies for the stratigraphical and palaeo-climatical reconstruction of humid periods. With regard to soil characteristics, e.g. thickness, weathering intensity, depth of leaching or accumulation of soluble constituents, the influence of climate and biosphere accumulate over the time of soil development. Therefore soils in arid regions always represent combined information about the duration and intensity of humidity as the main factor. First investigations of soils on ancient dunes in the Goz-region of Sudan by Warren (1970) and Felix-Henningsen (1983, 1984) showed that two ancient dune generations existed, which can be subdivided by the intensity of soil development. On the older dunes of the “Low Goz”, which according to Warren (1970) are of Upper Pleistocene age, a thick rubefied palaeosol is developed, overlain by longitudinal dunes of the “High Goz”, on which shallow, weakly weathered soils have developed. The Holocene age of these dunes (Warren, 1970) is discussed by Gläser (1987). In Northern and Central Sahel of Niger and Burkina Faso, two different ancient dune generations also exist, which can be subdivided by the relief forms and the degree of soil development (Grunert, 1988; Völkel, 1988, 1989; Pfeiffer and Grunert, 1989; Pfeiffer, 1991; Völkel and Grunert, 1990; Felix-Henningsen, 2000). In addition to radiocarbon datings, which established an Early Holocene and a Mid Holocene humid period, the stratigraphy (supporting the previous results of French geologists) from three humid periods during the Upper Quaternary must be considered in the Southern Sahara of East Niger: (a) an Upper Pleistocene (Ghazalien) humid period between 40.000 and 20.000 BP (Servant, 1983), (b) a Late Pleistocene to Early Holocene (Tchadien) humid period between 14.000–7.500 BP with a maximum humid period between 10.000 and 7.500 BP (Servant, 1983), and (c) a Middle Holocene (Nouakchottien) humid period (Young Neolithic) between 4.500–3.000 BP (Michel, 1973). In arid periods prior to the humid climatic phases, ancient dunes and sand sheets were sedimented. Ancient dunes of the Sahel extend to the Southern (Völkel, 1988, 1989) and Central Sahara (FelixHenningsen, 2000) of East Niger. Relict and fossil palaeosols indicate the northward transgression of the Southern Sahara margin during the Early Holocene humid period over a distance of at least 600 km. In order to complete the stratigraphy of palaeo-climatic changes at the southern margin of the Sahara, OSL datings of ancient dunes of the Sahara and Sahel in Chad were carried out. The eastern Manga dunes, east of Lake Chad in the Northern Sahel, as well as the ancient dunes with palaeosols in the Sahara of Chad up to the oasis of Faya in the North, exhibited Mid Holocene ages between 4,7 and 3,1 ka and a development of rather weak and shallow soils (Mauz and Felix-Henningsen, 2005; Felix-Henningsen and Mauz, 2005). In the Sahel of Niger, west of Lake Chad, ancient dunes with a thick reddish brown soil, rubefied due to the formation of hematite, are distributed. A first OSL age of ∼9,5 ka (Mauz and Felix-Henningsen, 2005) indicates an end to dune sedimentation and consequently the beginning of soil formation in the following humid period of the Early Holocene. In order to show the relationship between degree of soil development and age of the ancient dunes, further investigations were carried out during the Sahara expeditions in 2005 and 2006 at one site in the Central Sahel of Mauritania, and at two sites in the

Palaeo-climatic evidence of soil development on Sahelian ancient dunes

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Southern and Northern Sahel of Niger. Although general conclusions regarding the ancient dune stratigraphy are not possible due to the limited number of investigated sites and dated samples, the examples described in this paper reveal that the soils of the Sahel are partly relictic and, due to the duration and intensity of the weathering in previous humid periods, a relationship between the age of the ancient dunes and the degree of soil development does exists. Apart from palaeoenvironmental aspects, the results may have practical implications for land-use planning and combating desertification. Soil fertility decreases while soil stability against deflation, a common process of desertification as a consequence of overgrazing, increases with the duration, depth and intensity of soil development. 5.2 METHODS 5.2.1 Pedology At selected sites (Figure 1) soils were mapped by digging. Soil characteristics were described (Munsell colour, texture, structure and bioturbation, hardening and cementation, carbonatization and gleyification) and sampled for further pedochemical, physical, mineralogical and micromorphological investigation. The following physical, chemical and mineralogical methods were used: The Redness Ratio was calculated from the values of the Munsell moist colours RR = (10-Hue) × Chroma/Value according to Schwertmann et al. (1983). Texture: Amounts of sand >63 μm and fines 1.500 grains), in the following referred to as “aliquots”. Per sample about 12 up to 50 aliquots were measured to obtain equivalent dose (De) values, being an estimate of the amount of radiation dose that the sample accumulated within its crystal lattice since it was shielded from sunlight by subsequently deposited sediments. The equivalent dose values were estimated following the single-aliquot regenerativedose protocol (SAR) as proposed by Murray and Wintle (2000) that enables De determination for each individual sub-sample. Measurement of a range of aliquots allows a more detailed investigation of the dose distribution within the sediment, which is further improved by a reduction of the aliquot size or number of grains per sub-sample, respectively. All luminescence measurements were carried out on automated risø TL/OSL readers TL-DA12 or -15, equipped with 90Sr/90Y beta sources for irradiation and EMI 9235 photomultiplier tubes for luminescence detection. The BLSL signals (blue light stimulated luminescence) were measured for 50 s at constant temperature (125 °C) and the UV emission of quartz was detected in the in wavelength window 330 ± 40 nm generated by a Hoya U340 filter. The preheat temperature for all measurement cycles was set to 220 or 240 °C, respectively, according to preheat plateau tests and held for 10 s. The signal intensities were calculated from the luminescence signals integrated over the first second of stimulation with the signal measured during the last 5 s subtracted as background. To calculate the annual dose derived from the decay of lithogenic radionuclides in the sediment the concentration of uranium, thorium, and potassium was determined by laboratory gamma-spectrometry and/ or inductively coupled plasma–mass spectrometry (ICP-MS, Preusser and Kasper, 2001). A water content variation of 0,5 to 3,5 weight-percent was assumed. The cosmic dose contribution was calculated according to the sampling depth. It was assumed, that the thickness of the overburden sediments which serve as a shielding did not change throughout the burial time.


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The dating of the samples from profiles 3, 5 and 6 is described in detail in Mauz and Felix-Henningsen (2005). 5.3 STUDY SITES, DUNE AGES AND MORPHOLOGICAL SOIL CHARACTERISTICS 5.3.1 Northern Sahel, Mauretania (Site 1) In the Northern Sahel of Mauritania flat, shield-shaped ancient dunes are widely distributed in valleys and form ramps, stretching towards the valleys mainly on the lee-side of cuestas and inselbergs, which expose Carboniferous sandstones. In this area, intensive grazing under recent climatic conditions has generated an extensive mobilization of the ancient dunes. A site without such characteristics of desertification, situated about 70 km east of the village Kiffa (16°30'21,0" N, 10°46'20,8"W, 206 m asl), was investigated in a middle slope position of a weakly inclined ancient dune ramp. The investigated soil profile represents a shallow (Chromi-)Cambic Arenosol, which developed from the Upper Middle Holocene until today. The OSL age of 3,74 ± 0,31 ka of a sample from 60–70 cm depth indicates an end of dune deposition at about 3,5 ka BP. Since that time a 15 cm thick Ah horizon developed, followed by a reddish brown (7,5 YR 4/4) Bw horizon down to 80 cm depth. 5.3.2 Northern Sahel, Niger (Site 2) In the Northern Sahel of Niger flat shield-shaped ancient dune ridges are wide-spread. They also form valley fillings and dune ramps at cuestas. In contrast to Mauritania and Chad, the soils on those ancient dunes are thick and intensively red-brown coloured ChromiCambic Arenosols. Due to intensive grazing, exposed sites in the crest of dune ridges are especially subject to re-mobilization of the dune sand. A site was investigated 5 km SW of the village Abalak, situated between Tahoua and Agadez (15°25'54,4"N, 6°14'32,4"E, 408 m asl). Here, ancient dune plateaus and ridges cover the bottom of broad valleys between shallow cuestas. The profile of the ChromiCambic Arenosol was exposed at the cut bank of a wadi. Shallow erosion channels at the surface in the surrounding of the profile site indicate that, originally, the soil was thicker and stronger, weathered soil horizons were removed. Soil erosion under dryer climatic conditions led to a flattening and dissection of the dune relief. A sample from 240 cm depth shows an OSL age of 10,48 ± 0,73 ka and indicates an Upper Pleistocene age of the dune and an end of sedimentation in the transition period between Pleistocene and Holocene. 5.3.3 Central Sahel, Niger (Site 3) In the Central Sahel of Niger, north and northeast of the capital Niamey, extensive ancient eolian sand sheets and shallow dune ridges cover red sandstones and gravel beds with ferricretes of Tertiary age (Continental Terminal). The dunes are the parent material of Chromi-Luvic Arenosols. The higher precipitation in this region causes land use of agricultural fields mainly for the cultivation of sorghum. A soil profile of a Chromi-Luvic Arenosol was investigated 10 km north of Niamey (13°36'N, 02°05'E, 235 m asl). The soil is characterized by a > 1 m thick red-brown (5 YR 6/8) and weakly indurated Bwt horizon. Within this horizon thin dark redbrown clay bands occur as a consequence

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of lessivation. An OSL sample, taken at 150 cm depth displays an age of the top layer of 9,9 ± 1,2 ka and indicates that the sedimentation ended in the transition period between Pleistocene and Holocene. 5.3.4 Southern Sahel, Niger (Site 4) Southwest of Niamey the relief is dominated by dissected plateaus and ridges of sedimentary rocks with lateritic crusts of the Continental Terminal (Tertiary). The bottoms of wide trough valleys are partly covered with ancient dunes, especially in lee-positions. In the deeper positions the valleys are dissected by wadis. The occurrence of the dunes at a lesser density as compared to the region north of Niamey, indicates the southern margin of the ancient dune distribution. While grazing mainly occurs on the poor and shallow soils of the rocky plateaus, the ancient dunes of the valley bottoms are frequently used by agriculture. A profile was investigated of a thick, red brown Chromi-Luvic Arenosol, exposed at the cut bank of a wadi, about 50 km southwest of Niamey near the village Kobadi (13°12'39,4"N; 01°51'34,9"E, 223 m asl). Three OSL dates were obtained from this profile. A sample from 280–290 cm depth shows an age of 29,31 ± 4,93 ka, which indicates the beginning of the eolian sand deposition. A sample from 170–190 cm depth shows an age of 17,34 ± 1,28 ka, while a sample from 110–115 cm depth has an age of 11,87 ± 1,42 ka, indicating that the eolian sedimentation terminated at the end of the Upper Pleistocene. With such a low number of samples it cannot be proven that eolian sedimentation occurred continuously under an arid climate between 30 and 10 ka BP. In this profile, however, as well as in other investigated profiles of that region, no interruption in sedimentation caused by a phase of fluvial erosion or formation of a palaeosol was detected. 5.3.5 Northern Sahel, Chad (Sites 5 and 6) The landscape is characterized by ancient longitudinal dunes forming plateau-like flattened ridges of more than 40 meters relative altitude and of a width from 1–2 km. The plateaus and rather steep slopes of the dunes are sparsely vegetated by grass and trees, mainly Acacia species. The soil surface is covered by a layer of young, weakly-weathered, mobile sand. A strew of stone artefacts and fragments of pottery indicates that the dunes were settled in former times. The interdunal valleys are up to some hundred meters wide and filled with dark grey colluvial flood sediments, rich in fines (about 60 mass-%) and organic matter (about 0,5–1 mass-%). At two sites soil catenas, each with several profiles, were studied and dated by OSL (sites 5 and 6 in Mauz and Felix-Henningsen, 2005). In order to compare the Sahelian soil formation on ancient dunes of different ages, the two sites from Chad are included in this study. Profile 5, situated 22 km north of Moussoro (13°50'31"N; 16°28' 43"E, 298 m asl) and profile 6, 30 km west of Moussoro (13°37'56"N; 16°11'52"E, 289 m asl), are both situated in a flat plateau position on top of ancient dune ridges. At two sites soil catenas, each with several profiles were studied and dated by OSL (sites 5 and 6 in Mauz and Felix-Henningsen, 2005). OSL dating of dune sand from a weakly developed Cambic Arenosols, situated at both sites in a flat plateau position on top of the dune ridges, reveals Middle Holocene ages. A sample from 90 cm depth in profile 5 displayed an age of 3,4 ± 0,2 ka while a sample from 80 cm depth in profile 6 had an age of 3,65 ± 0,74 ka. The dark grey (10 YR 3/3, moist) topsoil of a weakly developed Cambic Arenosol is covered by a thin layer of loose sand from redistribution of aolian sand in the course of desertification. A dark brown (7,5 YR-10 YR 4/3, moist) Bw horizon changes gradually at


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0,8 to 0,9 m depth into a yellow C horizon. Krotowinas, filled with humic topsoil material, and root channels are frequently developed within the Bw horizon. The Ah, which also contains fragments of pottery, and the upper part of the Bw horizons are strongly indurated, due to accumulation of salts and amorphous silica, while a weaker cementation of the lower part of the Bw horizon gradually decreases towards the loose C horizon. The upper soil horizons in profile 6 were disturbed by a prehistoric pit, refilled with sand containing fragments of pottery and bones. As the dating of two samples from the refilled material displayed ages of 900 and 400 years, respectively, the younger age may reflect the minimum age of the soil disturbance. 5.4 PHYSICAL AND CHEMICAL SOIL CHARACTERISTICS The morphological differences between the soils on ancient dunes of different ages are also reflected by their physical and chemical properties. The analytical data of the physical and chemical soil properties are presented in table 1. Correlation coefficients in the text refer to sample numbers of n = 20 for the soils on Pleistocene dunes and n = 24 for soils on Middle Holocene dunes. 5.4.1 Redness Ratio According to Schwertmann et al. (1983) the Redness Ratio is significantly correlated with the proportion of hematite in the total amount of pedogenic oxides. The formation of hematite is an indicator of high weathering intensity due to a relatively high rate of dissolution of silicates, which leads to a fast release and oxidation of iron followed by the formation of ferrihydrite as the precursor of hematite. A highly significant but rather loose correlation (r = 0,58**) exists between redness ratio and the content of pedogenic iron oxides (FeDCB). Therefore the depth and intensity of rubefication (RR values) of the Chromi-Cambic-Arenosols of the Northern Sahel is weaker developed than in the Central or Southern Sahel (Table 1). 5.4.2 Amounts of fines and pedogenic oxides The source of the fines is mainly dust from the Sahara, which sedimented together with the dune sand in arid periods. Furthermore, dust deposition in the Sahel is very effective during the dry season and supplies the ecosystems with bases and nutrients (Stahr et al., 1994; Herrmann, 1996). Apart from silicates the dust contains carbonates and soluble salts (Herrmann et al., 1994). After deposition of dusts on the soil surface or on the vegetation, which acts as a filter, the fines are washed into the soil horizons with the infiltrating rain water. While the amount of clay 140 3,13 6,21 14 29 9 4

FeDCB (g kg−1)

2 3 3 2

MgCl2 (mg kg−1)

19 13 12 12

Site 1: Cambic Arenosol—Northern Sahel, Kiffa (Mauritania), 270 mm * a−1 Ah 15 0 7,14 16 207 108 BwAh 35 2,50 7,31 11 115 21 Bw 80 1,67 7,49 11 47 14 BwC 100 0 7,44 10 54 10

Horizon

9 13 9 9

7 6 6 6 7 4

6 7 7 6

SiNaOH (g kg−1)

5 7 6 6

4 3 3 3 3 3

2 3 4 4

AlNaoH (g kg−1)

19 25 21 20

14 12 12 12 12 8

10 13 14 12

Sum. ped. Ox. (g kg−1)

Table 1. Contents of fines and soil chemical characteristics of the representative soil profiles of the Sahel in Mauritania, Niger and Chad; RR: Redness ratio; TDS: total dissolvable salts; FeDCB: pedogenic iron oxides; SiNaOH, AlNaOH: amorphous Silica and Al-oxides.

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4 4 4 3 4

5 4 4 4 3

1 1 1 1 1

Profile 5: Cambic Arenosol—Northern Sahel, 22 km north of Moussoro (Chad), 300 mm * a−1 Ah1 7 0 8,45 14 680 149 8 Ah2 15 0 8,16 4 172 25 5 Bw1 50 3,75 8,46 6 148 4 1 Bw2 70 3,13 8,25 6 35 3 1 C 100 0 8,02 3 33 5 0

Profile 6: Cambic Arenosol—Northern Sahel, 30 km west of Moussoro (Chad), 320 mm * a−1 Ah 20 0 8,16 6 303 70 7 1 Bw1 60 0 7,97 4 100 8 3 1 Bw2 80 4,17 8,02 5 72 4 1 1 Bw3 100 3,13 8,30 3 57 6 2 500 g L−1), profile 1: Cambic Arenosol, Northern Sahel, Mauritania; profile 2: Chromi-Cambic Arenosol, Northern Sahel, Niger; profile 4: Chromi-Cambic Arenosol, Southern Sahel, Niger. Location of the profiles see figure 1.

relative high amounts of amorphous silica and soluble salts in the refilled soil sediment (bABw horizon, Figure 3) indicate that the accumulation of these elements in the topsoil horizons may have previously existed. This would mean that the underlying brown 3bBw horizon must have developed, as in profile 15, during a long period with more humid conditions than today, when salts and carbonates were completely leached and the pH was below 7.

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5.5 PALAEO-CLIMATIC INTERPRETATION The thicknesses, morphological, physical and chemical characteristics of the soils on ancient dunes are related to the humidity of the climate, and the duration of the humid period suitable for soil formation. The OSL ages of the ancient dunes give references of the maximum age of the soils, because sustainable soil development started with the end of eolian sedimentation. The real date of the end of sedimentation can only be approximated because the sampling depth was below the Bw horizons, which had been influenced by mixing due to bioturbation and rooting. Therefore it is not possible to use the youngest sediment layers for dating. Although the number of OSL data of ancient dunes of the Sahel in Mauritania, Niger and Chad is very limited and cannot be interpreted with respect to a general ancient dune stratigraphy for the entire Sahel, nevertheless they reflect evidence that only two ancient dune generations existed, which can be clearly subdivided by the thickness and weathering intensity of the soils. The sedimentation of the Upper Pleistocene dunes of the Sahel in Niger started at least around 30 ka and ceased at ∼10 ka with the transition from the Upper Pleistocene to the Holocene, while the Middle Holocene dunes in Chad and Mauritania were sedimented between ∼5 and ∼3,5 ka. The existence of two dune generations in the Sahel is in concordance with previous results of investigations in the Sahel in Sudan (Warren, 1970), the Sahara and Northern Sahel in Chad (Felix-Henningsen and Mauz, 2005), and the Northern Sahel and Southern Sahara in Niger (Völkel, 1988, 1989; Völkel and Grunert, 1990; Felix-Henningsen, 2000). With respect to older stratigraphical conclusions of French researchers, the data show that an Upper Pleistocene humid period (Ghazalien), which was assumed between 40.000 and 20.000 BP by Servant (1983), must have ceased before ∼30 ka if indeed such a humid period occurred. From the Sahara and the Sahel, Gasse et al. (1990) report an abrupt transition from arid to humid conditions at > 12 ka and at ∼9,3 ka, with a reversal (from humid to arid) at ∼10,5–10 ka. The climate changes took place within a few centuries or even decades. The latest eolian activity is confirmed by the OSL data of the ancient dunes in Niger. Servant (1983) dates the Late Pleistocene to Early Holocene humid period (Tchadien) between 14.000–7.500 BP, with a maximum of humidity between 10.000 and 7.500 BP. In view of the OSL data of the ancient dunes, this would mean that between 14.000 and ∼10.000 BP the climate of the Sahel in East Niger was only semi-arid and the low intensity of soil development still allowed sedimentation or local redistribution of eolian sand. The ages of the optically dated palaeo-dunes of Chad and Mauritania indicate that the uppermost sand layers were deposited in the Mid Holocene between ∼5 ka and ∼3 ka. The southernmost ancient dunes at Site 5 show the youngest age of 3,1 ka BP. If the ages of the “Nouakchottien” humid period (between 4.500 and 3.000 BP) provided by Michel (1973) are reliable, then the OSL data of the Middle Holocene ancient dunes in Chad, Mauritania and the Central Sahara (Felix-Henningsen, 2000) show that sufficient humidity existed in the Sahel after ∼3.500–3.000 BP, which led to fixation of dunes by vegetation and soil formation. The fact that palaeosols of this humid period occur in the Southern to the Central Sahara (Völkel, 1988, 1989; Felix-Henningsen, 2000) proves that the climatic conditions since the Middle Holocene humid period must have also changed in the Sahel to more arid conditions. The depth and intensity of weathering of the soils seem to be associated with the stratigraphical unit of the dune generation. Due to the accumulation of the processes of soil formation in the Early and the Mid Holocene humid periods until present, on Upper Pleistocene dunes the rubefication, accumulation of fines and leaching depth of soluble salts is much more pronounced than on Middle Holocene dunes, which were only affected


103

by weathering from the Mid Holocene until present. In soils of both dune generations the latest climatic changes are expressed in the physical and chemical soil characteristics. The weathering of silicates indicates that acid conditions must have prevailed for a sufficient time. Especially in the rubefied soils on Upper Pleistocene dunes, the intensity of weathering was high due to intensive leaching and acid conditions, because the rate of iron release by weathering of silicates is one important factor in the formation of hematite (Schwertmann, 1985). Under recent climatic conditions, leaching of bases, which are deposited with dust, is limited. Therefore the pH values of soils in the Northern Sahel are neutral to alkaline and in the Central Sahel neutral in the uppermost part. Consequently, under recent climatic conditions with annual precipitation up to 2 mm) and fine soil. Fine soil was subject to further analysis according to Herrmann (2005).

Are there valuable pedological palaeoenvironmental indicators in Northern Chad?

111

6.4 GRADIENTS OF SURFACE SAMPLE PROPERTIES 6.4.1 Texture With respect to texture no latitudinal gradients could be detected. Sandy and clayey surfaces were present in all landscapes. Figure 2 shows the overall dominance of sandy surface textures (ca. 70%). Sands are omnipresent, even in the lacustrine domain of the Bodele Depression either as dunes, sand sheets or only thin veils on serir-type surfaces. This is a clear sign for the active redistribution of materials under the given environmental conditions. Main cause is eolian activity, which in its intensity leads to the fact that the areas south of the Tibesti are net exporters of fines (mineral dust), which are transported with the Harmattan in south-westerly directions (Herrmann et al., 1999). About 10% of the samples are very fine and have a maximum either in the clay or fine silt fraction. These are in their origin lacustrine samples. They were detected on the whole transect, transformed to soils in the Sahel and as deflated surfaces in the Sahara with vast occurrence in the Bodele Depression. Intermediate texture is either typical for moving sand, which has incorporated lacustrine material or mineral dust (coarse to medium silt dominance) accumulated under serir cover. However, the texture distribution as presented in figure 2 cannot be interpreted as representing coverage of textural surface types, especially since skeleton—significant for the serir—is not presented, and sampling was not based on spatial representativity. Typical for the great majority of sampled sites is strong layering as indicated by the comparison of grain size fraction ratios (i.e. medium/fine sand). Exceptions occur only at Sahelian sites on stable sand dunes (Cambic Arenosols) and levelled pedogenically transformed lacustrine sediments (Vertic Gleysols), indicating relative geomorphologic stability. With exception for the lacustrine sites, the second stratum sampled is always enriched in clay and fine silt. This is again a hint towards deflation as dominant recent process impoverishing the uppermost stratum. In conclusion, the granulometric data together with field observation indicate that complete pedological records as in situ developed profiles from rather uniform or

Figure 2. Frequency of sand concentration in surface samples on the visited Sahel-Sahara transect.

112

Ludger Herrmann et al.

reconstructable parent materials are to be expected mainly in the Sahelian environments, especially with dunes. 6.4.2 Kaolinite as a weathering indicator Though kaolinite can also be formed under initial weathering i.e. from feldspars, it is a good indicator for weathering processes if present in higher concentrations. It could be shown, that under Sahelian conditions (560 mm annual rainfall) in SW-Niger freshly deposited smectite (from mineral dust) was quantitatively transformed to kaolinite under the given leaching conditions in Arenosols (Herrmann et al., 2002). On the first look, kaolinite concentrations in the clay fraction are quite scattered over the whole N–S-transect (Figure 3). Looking closer, it can be remarked, that the minimum concentration stays the same in all areas (ca. 15%). On the other hand, maximum concentration increases with an inclination of about 7,5% per degree northern latitude. It remains the question, whether these trends are due to weathering or due to inheritance of the parent rock. Comparing the sites with minimal and maximal values and crosschecking with other data reveals in both groups lacustrine as well as terrestrial sediments and all texture classes occur. Low values occur more often where also carbonates (rhizo-concretions) were detected at the surface. In conclusion, kaolinite cannot be used ex ante as indicator for Holocene palaeoweathering, but must be interpreted in its local context here, i.e. very high kaolinite concentrations (60%) where Fe-sandstone outcrops appear. This does not undermine the interpretative value of kaolinite if the parent material for soil formation is known. 6.4.3 Fe-fractions Since frequent features of Fe-redistribution via the solute pathway on the landscape scale were observed (swamp ores, rhizo-concretions etc.), also Fe-fractions were investigated. The hypothesis behind is, that where high absolute iron accumulation (total iron) or

Figure 3. Kaolinite concentrations (estimated semi-quantitatively from XRD-diffractograms of sedimented samples) in the clay fraction of the top-stratum in dependence of geographical location (latitude N) in N-Chad.


113

Figure 4. Fe-compounds (Fet: rhomb, Fed/t: bullet) of top strata on a latitudinal (N) gradient in N-Chad.

high Fe-transformation (Fed/t ratio) is detectable, pedogenic (palaeo-) processes could be responsible. The results obtained yield a similar picture as already for kaolinite. Low Fe-concentrations (10 (bicarbonate buffer) or lower 6,5 (silicate buffer) and concern (mixed) lacustrine materials. The same is true for the low values (pH < 6,5) around 16° northern latitude along the Bahr el Ghazal. Samples with pH around 5,5 represent mixtures of eolian sands with lacustrine sediment flakes, extreme low values with pH < 4 represent pure lacustrine surfaces. South of 15,5°N, entering the Sahelian domain the pH is depending on landscape position, which at the same time means petrography. Here, several catenas have been sampled from the dune top positions (pure sands) towards the interdunal inundation planes (lacustrine sediments, partly fluvially redistributed). Dune sites with Arenosols show neutral pH around 7,5. Landscape lows (Vertic Gleysols to Gleyic Vertisols) are characterized by pHvalues around 9. In conclusion, average pH-values of free draining sites are increasing towards the North with increasing aridity from 7,5 with ca. 350 mm to 9 with 10, bicarbonate buffer), elevated EC and dominance of water soluble Na, high total Na and the highest kaolinite shares in the clay fraction (>40%). In addition, Mn- and Fe-carbonate (rhodochrosite and siderite) could be detected in the lowermost Bwk-horizon of site 39a. Since the sites lie north of the main borderline of the Bodele Depression, the characteristics show strong local influences. The iron-impregnated sandstone occurring to the West should influence the high kaolinite share. The lateral influences on the landscape scale are also documented through the Mn- and Fe-carbonates, which indicate the former existence of Bg-horizons in the former terrestrial surroundings (Fe-/Mn-mobilization and solute transport to the depression). That high pH occurs also in the eolian sand cover of 39a is taken as a hint, that alkalinisation is a post-sedimentary process, since it is out of the zonal pH-range (see figure 5). Here palaeoenvironmental information concerning an aridisation phase might be available, but should be investigated in more detail at the landscape scale. Again, easily distinguishable from the samples before by the extreme low pH (< 4, Al-buffer) are those collected along the Bahr el Ghazal (45a, 47b). Very fine textured, high total Si (39–44%) and LOI (8%) argue for a higher share of diatoms in the sample. The presence of gypsum and texture are comparable to those of samples 12 and 13. So the question is, why are these samples that acid? A hint comes from the chemical characteristics: higher than average water soluble Ca and Mg concentration was detected. An explanation could be, that the lacustrine sediments contained reduced Fe-compounds, which oxidized consecutively and produced sulphuric acid (as known from acid sulphate soils) that was then buffered by dissolution of Ca- and Mg-carbonates. At least, this process could explain low pH as well as high water soluble Ca- and Mg-values at the same time. Also over-average free iron (Fed) values in all these samples hint towards this process. Unfortunately sulphur analytics, which could have helped to support the given hypothesis, was not possible in this framework. Why this process is especially intense in this area is not understood, but might be in relation to geological strata (Pliocene/Pleistocene) with high sulphur concentration (gypsum, Kusnir, 1995). If the given hypothesis is accepted, low pH-values are an effect of post-sedimentary processes during a desiccation phase.

116

Ludger Herrmann et al. Table 1. Characteristics of (mixed) lacustrine materials in N-Chad.

Surface Munsell

pH H 2O

EC μS/cm

Ca (H2O) mval kg−1

Mg

Na

17,45 17,38 17,95 17,95 17,95 17,93 16,02 15,89 13,84 13,84 13,84 13,62

2.5 Y8.2 2.5 Y8.2 7.5 YR7/6

10 YR6/1

5,7 5,2 10,5 10,7 10,3 10,8 3,7 3,9 9,3 8,2 8,5 9,3

512 1015 99 116 8780 2275 502 467 144 2500 800 183

17 119 17 19 2037 530 20 29 28 28 589 35

49 40 30 −0,5 −7 >7 >4 >30 −10 −3 −12 −40 −1

Sample No.

18 24 42 – – 62 28 35 – – – 17

10 YR8/1 7.5 YR7/2 7.5 YR7.2 10 YR6/1

61 42 32 – – 9 45 36 – – – 47

5 3 0 – – 0 2 1 0 – – 0

0 0 0 – – 17 0 0 4 – – 0

1 3 94 94 87 40 0 0 91 51 10 26


117

The Sahelian samples take an intermediate position, with pH in the carbonate buffer (8–9), low EC in the topsoil increasing with depth, elevated total Al and Fe and an expected kaolinite to smectite ratio. Exceptional are only the low Fed/t ratios. All in all the determined values are in the expected range taking the given climate and landscape position into account. Since the sites are under present soil formation (development of vertic and gleyic properties), pedological palaeoenvironmental information can hardly be expected. 6.6 ARE THERE VALUABLE DUNE RECORDS? Since this question is treated by long and large in other sections of this volume, only some general remarks are made here. Soils on dunes, which do not seem to match with the recent climate, can successfully be used as indicators of palaeo-climates as shown in the bordering Sudan and E-Niger by various authors (i.e. Felix-Henningsen, 1984; Völkel, 1989). However, there are some terrain characteristics and processes present, which should be carefully considered before drawing conclusions from soils on dunes in the visited region. First of all, complete terrestrial soil profiles could only be detected in the Sahelian domain. Due to anthropogenic disturbance even there eolian redistribution of materials is a common phenomenon. Apart from these recent phenomena, layering is very common in the described soils as indicated by texture data (Felix-Henningsen, 1999; Mounkaila, 2006). In order to derive conclusions about the weathering intensity in the sense of quantifiable characteristics we need to reconstruct the parent material. In order to do so, we have to consider three processes. The first is incorporation of fines into the dunes during dune formation, i.e. by transgressing lacustrine sediments, as it could be often observed in the Saharan domain. The second is recent dust deposition as a continuous ubiquitous process with a gradient depending on aridity and distance to sources (Herrmann, 1996). The third one is local disturbances of unknown origin (i.e. fires) or short term arid phases (several years) which enable re-translocation of fines from the finer textured inter-dune depressions. All these processes can influence the presence of fines (clay) and free iron (Fed) which are normally used to identify in situ weathering intensity. In order to separate these influences detailed analytical work, especially concerning mineralogy and geochemistry is indispensable but rarely executed. In the Saharan domain, apart from the recently moving sands, suspected older dune sand formations where only discovered in protected situations, i.e. under lacustrine sediments. Occurrence is erratic and state and setting allow only very limited conclusions. Normally, C-material could not be sampled or is not present, which means that no reliable reference to establish the weathering intensity exists. The locally different colour of barchans in the Saharan domain impressively shows, that potential C-material can be very different, especially with respect to iron minerals. Consequently, in situ weathering intensity and extent cannot be determined. So pedological information is scarce, it rests dating as a possibility to determine arid events. However, pre-requisite for interpretable TL and OSL dating is complete bleaching before sedimentation, which needs a certain transport distance. This information is normally derived from landscape context and sedimentary information but can hardly be collected under the given circumstances for the very localized “older” dune materials. In conclusion, Northern Saharan Chad with its extreme deflation activity (negative regional mass balance) has only low potential for valuable pedological information from dune palaeosoils. In the Sahelian domain exploitable sites are present. In future the sites in major eolian transport direction, i.e. SW of the Bodele depression where materials begin to accumulate should be of interest in order to find the connection between findings of this expedition (east of Lake Chad and Bodele Depression) and those established in E-Niger.

118

Ludger Herrmann et al.

6.7 A PALAEOSOIL IN THE SAHARAN REGION UNDER SERIR A rather unusual finding was a nearly complete palaeosoil (Figure 6a) several kilometres north of the Angamma escarpment. The site was visited due to a very sharp linear boundary visible on a Landsat image (Figure 6b). The profile is situated under a serir surface on the banks of a palaeo-riverbed. The river drained the Tibesti towards the Bodele Depression. The visit in the terrain showed a fluvial morphology. A natural N–S oriented dam is separating the fluvial domain with relatively unconsolidated greenish fresh and mica rich sandy (3% clay) sediments—establishing the magmatic Tibesti influence—and the terrestrial domain with significant signs of terrestrial weathering (BC-horizons present). The latter materials are brownish to reddish, finer (ca. 20% clay) and consolidated. From the morphological point of view the profile can be separated into three parts. The uppermost is characterised by a serir surface and a yermic phase. The central part contains coarse components and has a reddish colour. Finally, the finer textured subsoil is characterised by brown to greenish colours, impregnated along coarse vertically oriented pores with material from the reddish overlying horizons.

Figure 6a. Palaeosoil profile (Aridi-gleyic Luvisol) in a river dam north of the Angamma escarpment.


119

Figure 6b. Landsat 5 TM image showing an erosion event in the Bodele Depression mainly on lacustrine sediments (bright area), and the location of Angamma escarpment and palaeosoil sampling site.

The granulometry (Table 2a) indicates—with exception for the two lowermost horizons—strong layering with an increase of the skeleton content towards the topsoil, which is rather unusual for riverbank profiles developing under constant environmental conditions. A distinct downward increase of the clay content between 0,4–0,7 m depths indicates beside clay cutans clay illuviation, though the whole profile contains small

120

Ludger Herrmann et al. Table 2a–d. Analytical data of a palaeosoil profile (Aridi-Gleyic Luvisol) north of the Angamma escarpment in Northern Chad.

Horizon

Depth cm

Skeleton wt.%

cS %

mS %

fS %

Sand %

Silt %

Clay %

Corg %

pH H2O

Serir C E Btg B(t)kg B(t)gr

0–3 −7 −40 −70 −120 −140

100 63 61 3 2 2

10 35 12 5 6

18 20 23 17 25

24 7 13 16 18

53 62 47 38 48

24 12 21 33 27

23 26 32 30 25

0,10 0,02 0,03 0,02 0,02

9,1 8,1 7,9 8,1 8,1

Horizon

KAKpot

Exchangeable cations Ca

Mg

K

Na

−1

Horizon

256 207 343 470 407 EC 1/20 μS cm−1

222 148 192 329 265

BS

Ca

18 25 58 75 65

12 6 4 4 3

3 3 3 5 2

Mg

K

P

−1

%

mmol c+ kg C E Btg B(t)kg B(t)gr

Weatherable elements (HCl)

mmol kg 100 88 75 88 82

161 74 90 208 133

115 91 129 166 146

96 71 57 55 49

11 4 2 2 2

Water soluble ions Na

K

Ca

Mg

NO3

SO4

Cl

PO4

0 19 12 3 4

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