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Palaeoecology of Quaternary Drylands (Lecture Notes in Earth Sciences)

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In various periods throughout the younger earth history comparable changes in climate occurred globally and simultaneously. Such global events can be reconstructed with the help of reliefs, sediments and palaeosoils and their specific morphological, chemical and mineralogical properties. Desert margins represent inter-sections between arid and humid ecosystems. Their geographical position will react very sensitively on climatic changes. The broad regions of transformation between recent humid ecosystems and the fully arid deserts are the proper areas where palaeoclimatically different phases will be remarkably recognized and interpreted.
Aeolian sediments, e.g. dunes, can be used as palaeoclimatic indicators: palaeodunes in todays more humid climate may indicate arid conditions at the time of their deposition. As an example, fossil dunes are widely distributed in the Sahel south of the Sahara. In resting periods of sedimentation a cover of vegetation appears, and chemical weathering processes and hence soil formation takes place on the sediments in humid climates (see Felix-Henningsen, Heine, Rögner et al., Smykatz-Kloss et al.). In deeper positions of the relief fluvial sediments in wadis and limnic sediments in palaeolakes and playas were deposited. They can be recognized by their sedimentary structures and by characteristic mineral associations, such as for example transformed evaporites (see Rögner et al., Schutt, Heine), by diatomites and lacustrine sediments (see Baumhauer et al.) or by bog ores (see Felix-Henningsen). At some rare occasions the coastline of a former lake is traced by fulgurites (see Sponholz). The organogenic components of soils and sediments mirror the palaeoecological conditions and changes (see Smykatz-Kloss et al.). The pollen communities in upper soils and sediments show the spectrum of the vegetation and thus deliver important criteria for palaeoclimates and relative ages (see Baumhauer et al.). Anthropogenic relicts in soils and sediments are a proof for humid phases. The existence of humid phases and their relative occurrences in the stratigraphical context and the kinds of sediments and palaeosoils allow the re-construction of the frequency, relative age and character of palaeoclimatic changes (see Rögner et al., Mischke et al.). Absolute dating of aeolian sediments by using luminescence methods such al TL or OSL (see Jäkel, Smykatz-Kloss et al.) and organic substances (14C) - where present - indicate the age position. If the sets of data are sufficiently dense, a picture can be obtained about the time periods of the humid and arid climate phases (Eitel et al., Rögner et al., Smykatz-Kloss et al.).
The signals of arid periods can partly be discovered widely distributed, e.g. over the desert margins to off-shore regions in the oceans. Thus, Leuschner, Sirocko et al. describe layers of (aeolian) dust from Saudi-Arabia in drilling profiles of the Arabian Sea: the geochemical and sedimentological evaluation of these palaeo-loesses in the marine sediment cores contributes to the reconstruction of palaeo-monsoon movements (Leuschner et al.).
Questions on the palaeo-ecological interpretation of drylands and desert mar-gins are explored in the German working group "desert margins" and in many interdisciplinary projects. The group conferences are held annually in January at the Rauischholzhausen castle near Gießen. This working group, which has also acted as the German representation for several international geological correlation programmes (all concerned with desert research: IGCP 250, 349, 410), was established seven years ago by the editors of this volume. It is made up of approximately 50 geoscientists of (nearly) all disciplines: geomorphologists, geologists, mineralogists, geochemists, soil scientists, geochronologists, sedimentologists - as well as several palynologists, geobotanists and archaeologists.
At the beginning a pilot project built the core of the research (group) comprising nine projects from the edges of the Sahara (Reichelt, Baumhauer et al., Felix-Henningsen, Rögner et al., Schulz et al., Smykatz-Kloss et al., Sponholz) and of the Namib (Eitel et al., Heine). After a while the study areas were extended to-wards the north-west (Spain: Schutt, Günter) and - primarily - (north-) eastwards across the Arabic world (Leuschner, Sirocko et al.) towards Central Asia (Grunert & Lehmkuhl; Mischke, Hofmann et al.; Walther). Methodical questions on age analysis (dating of young sediments and aridic soils) and the correlation between chemical weathering (geochemistry, soil science) and palaeoecology are the themes that raise the regional and subject specific results onto a global scale (Jäkel; Eitel, Blümel & Huser; Felix-Henningsen; Heine; Leuschner, Sirocko et al.; Rögner et al.; Schutt; Smykatz-Kloss et al.).
The investigation of the desert margins as suitable indicators for global climatic fluctuations belongs to the basic research in palaeoecology. The obtained results contribute to the efforts of several earth scientific disciplines in order to under-stand and reconstruct the causes, frequencies and time periods of palaeoclimatological events and changes. This is especially important on the background of the recent global temperature increase, which is mainly anthropogenetically initiated, and of regional climatic catastrophes. The prognosis of long-term consequences on the base of modeling exhibits many uncertainties concerning the frequency, duration and amplitude of natural climatic fluctuations.
Additionally, the results of studies on desert margins enrich our knowledge on the complexities of landscape formation and on the distribution pattern of their re-sources (e.g. soils and groundwater) in dependence on extremely different climatic conditions and changes. Ecosystems of savannahs and semi-deserts in the regions of desert margins and the people living there are endangered in their existence by short- and long-term climatic fluctuations. The research data of the working group contribute to a more pronounced understanding of these ecosystems: not only the studied structures and processes, but their development in time, their formation and disappearance under the influence of global climatic changes have to be regarded. Geomorphological research in these climatic regions (e.g. the desert mar-gins) will only be effective if the various geo- and bioscientific disciplines will work together. The contributions to this volume may prove this.