Sebkhas as ecological archives and the vegetation and landscape history of southeastern Tunisia during the last two millennia

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Abstract

Sebkhas are temporary lacustrine systems depending on the number and extension of floodings. They may create laminated sediments which can be exploited as ecological archives. Contrary to those of meromictic lakes they are seasonal but not annual. After each flooding a detritus layer and a plasmo-condensed layer of bacteria and algae are formed in the water body or film and during the subsequent dessication a third subaeric layer of evaporites is build up. These laminae show a quasi-textile fabric and they can trap and conserve any dust and fine grained material. A high resolution pollendiagram covering the last two millennia as well as geochemical analysis from a southeastern Tunisian sebkha demonstrate the potentials of these ecological archives.

Introduction

History needs archives. Laminated sediments regularly provide valuable ecological archives also allowing high resolution in sampling and analysing. They normally are cored in meromictic lakes (Eugster and Kelts, 1983) but question arises whether comparable rich ecological archives may be found also in other lacustrine systems especially in semiarid and arid regions where large meromictic lakes are rare. Are saline lakes or lacustrine systems suitable to comparable investigations on the one hand and on the other hand in which way these lacustrine systems function to understand the formation of ecological archives?

Section snippets

Differences and similarities of saline lakes and sebkhas

There is a great confusion about these endorheic lacustrine systems. In semiarid and arid regions they tend to accumulate solutes in various concentrations just to the formation of evaporitic layers. The term “saline lakes” which is often used for both saline lakes and sebkhas (Williams, 1981) does not cover sufficiently these systems since they incorporate as well-permanent water bodies—real lakes—as only temporarily inundated depressions normally called sebkha—playa or pans.

The permanent

The laminae of sebkhas

However the question remains, whether and in which extension these episodically or periodically flooded systems may provide ecological archives comparable to those from meromictic lakes. Krumbein et al. (1979) reported laminated deposits of coastal sebkhas form the Sinai peninsula. They described a lamination system which was regularly formed after each flooding and which combined oxygenic and anoxygenic photosynthetic active systems of cyanobacteria and algae and which should be a modern

Landscape and vegetation history of the last two millennia

The ability of the laminated sediments of a sebkha as ecological archives is tested by a sediment core taken in a small inland sebkha in southeastern Tunisia which functions as an impluvium (Schulz et al., 1995a, Schulz et al., 1995b). A record of the last two thousand years could be analysed with an resolution of 5 mm representing about 30 years per sample (Fig. 4). The chronology is based on tephra-layers detected in thin sections. The volcanic minerals could be retraced to different

Conclusion

Salt lakes and sebkhas should be regarded as completely different lacustrine systems. Whereas salt lakes are permanent water bodies and may be found in various regions and climates, sebkhas are bound to semiarid and arid regions. Sebkhas and playas have in common that they are temporary lacustrine systems depending on number and extension of the floodings. Both salt lakes and sebkhas may create laminated sediments which can be exploited as ecological archives. Those of the salt lakes are

Acknowledgements

We are kindly indebted to L. Dupont and to an anonymous reviewer for valuable suggestions as well as to the Deutsche Forschungsgemeinschaft, Ecole Superieure d'Horticulture Chott Mariem and PICS franco-allemand OPS no. 521 of CNRS and to Laboratoire Environnement Region Aquitaine/Univ. Bordeaux and CNRS for financial and technical support.

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