Elsevier

CATENA

Volume 187, April 2020, 104290
CATENA

Assessing geomorphological and pedological processes in the genesis of pre-desert soils from southern Tunisia

https://doi.org/10.1016/j.catena.2019.104290Get rights and content

Highlights

  • Climate changes and geomorphic processes drove soil formation in southern Tunisia.

  • Soil accretion and deflation were the main pedogenetic processes.

  • Byy horizons formed from accretion of windblown materials and salt-rich water-table.

  • Hardpan formed from sedimentation of repeated mudflows.

  • Bk horizons formed from cycles of sedimentation and strong wind erosion.

Abstract

Arid environments are fragile and the associated soils are subject to serious threats like water deficiency, erosion, salt accumulation, and loss of fertility. In this context, understanding the processes involved in soil genesis may contribute toward protecting land from degradation. This study highlights the interconnection between geomorphic and pedogenetic processes in soil formation of the Jeffara Plain, a pre-Saharan area of southern Tunisia. To reach this goal, one coastal oasis (Chenini Nahel) and two inland environments (Matmata Nouvelle and Menzel Habib) were studied. After geomorphological and pedological surveys, the soils were sampled by genetic horizons and characterized by physical, mineralogical, and chemical analyses, and by microscope observation. Field observations and laboratory data suggest that soil formation in the Jeffara Plain was a combination of additions and losses controlled by climate changes. At Chenini Nahel, the soils developed by accumulation of wind-blown sediments coming from a close area dominated by gypsum-bearing rocks. At Matmata Nouvelle, the soils mainly formed from sedimentation of repeated mudflows during a rainy period between 9000 and 5000 years before present, followed by drought periods. Finally, the soils of Menzel Habib developed from an early gypsum formation in the presence of a salt-rich water table and repeated cycles of sedimentation/deflation of wind-blown materials. The different genesis of these pre-desert soils produced characteristic B horizons: Byy horizons with poorly developed soil structure at Chenini Nahel, Bw horizons with a hard rupture-resistance at Matmata Nouvelle, and Bk horizons at depth due to intense sedimentation with CA + BC horizons at the surface due to the accretion of wind-blown materials at Menzel Habib.

Introduction

Soils affected by water deficiency and consequent salt accumulation cover ≈12% of the global ice-free land area (Sombroek, 1987). Such soils are particularly prevalent in the Sahara desert, sub-Saharan area, Turkestan desert, Gobi and Taklamakan deserts, central Australia, southern Argentina, south western Africa, south western USA, Mexico, Pakistan, and many countries of the Middle East. In many places, pedogenesis of drought-affected soils has been studied since the beginning of the last century with investigations based on soil survey, photo-interpretation, remote sensing, and laboratory analysis. For example, Khresat et al. (2004) investigated soils of arid environments from north eastern Jordan to assess how geomorphic features interact with soil forming processes. Yamnova and Golovanov (2010) studied how geomorphology influenced the formation of gypsum in Aridisols from Uzbekistan’s Turkestan desert. In the Saharan and pre-Saharan areas, geomorphological and pedological studies are of great concern because of the connection with themes of planetary interest such as desertification, global warming, famine, and human migrations. Many studies performed on soils from arid environments concern formation processes as an understanding of the phenomena involved in soil genesis may contribute to protecting them from degradation (Mtimet, 1983). As such, soil study in northern Africa has been fairly extensive. Morocco (e.g., Hofmann et al., 2000, Linstädter and Kehl, 2012), Algeria (e.g., Renac and Assassi, 2009), Tunisia (e.g., Kamoun et al., 1999, Zaaboub et al., 2005, Henchiri, 2007, Mannaï-Tayech, 2009, Gallala et al., 2010), Libya (e.g., Hunt et al., 2010, Zerboni et al., 2011), and Egypt (e.g., Wanas, 2002) all feature considerable studies of arid soils. Soil formation in such arid areas has mostly been led by a combination of climatic and geomorphic features, with the main processes represented by wind erosion/accumulation, evapotranspiration, and salt precipitation. Valley floors also feature alluvial sedimentation due to ancient flooding events. However, an understanding of the interrelationships among climatic changes, geomorphic processes, and pedogenesis in arid environment remains scarce.

As such, this study focuses on pre-Saharan areas of the southern part of Tunisia, where previous soil studies have provided information on both general and site-specific soil forming processes (e.g., Bureau and Roederer, 1961, Pouget, 1963, Mtimet, 1983, White et al., 1996, Fromm et al., 2005, Bismuth et al., 2009, Hannachi et al., 2015). However, details on the interconnection between climatic changes, development of geoforms, and pedogenesis remain rare. Especially lacking are considerations of the influence of the groundwater at shallow depths, ancient mudflows and aeolian transports, and palaeosols on pedogenetic processes like the development of horizons or mineral alteration. Knowledge of these aspects is essential for reconstructing soil developmental factors such as hydrology and historical climate, the results of which will help to protect the current soil from degradation. As such, this research seeks to further understand the processes involved in soil formation in the pre-Saharan area of southern Tunisia, considering both coastal oasis and inland environments.

Section snippets

Study area

The Jeffara Plain (JP) is a coastal plain of north Africa with a rough semicircular shape, extending from the low plains located south of the Gafsa Mountains (Tunisia) to Tripoli (Libya). The area is ≈37,000 km2 (Fig. 1) and geologically belongs to the northern sector of the Saharan platform, characterized by a sedimentary sequence associated with Lower Triassic to Upper Cretaceous material (Rigane and Gourmelen, 2011). The JP is separated from the Sahara by a series of cuestas running from

Chenini Nahel

At the soil surface, diffused saline efflorescences of gypsum and halite were observed (Table 1). The soil was mostly dominated by gypsum and showed a pale brown to light yellowish brown color, a poorly developed and friable structure (with the exception of the Ayy horizon), and a very low diffusion of the finer roots. Below 60–70 cm, redoximorphic features (e.g., Mn concretions, mottling) were present due to alternating periods of reduction and oxidation. No grains with paramagnetic behavior

Chenini Nahel

The development of weak soil structure was ascribed to the low diffusion of the fine roots (e.g., Cocco et al., 2013, Cocco et al., 2015, Fageria and Stone, 2006, Walley et al., 2005). However, scarce aggregation and poor root diffusion were rather expected as they are generally limited by the high salinity/sodicity of the soil (Al-Barrak and Rowell, 2006, Horneck et al., 2007, Khresat and Qudah, 2006, Seelig, 2000). Clues about possible sodicity of this soil were represented by the abundance

Conclusions

The dominant process of soil formation in the Jeffara Plain (southern Tunisia) is a combination of additions and losses controlled by climate changes over the last 10 K years. Some difference was noted among the three study sites, which all exist today in a similar aridity index. The soils at Chenini Nahel mainly formed from the accumulation of wind-blown sediments coming from a close area dominated by gypsum-bearing rocks. Further, the formation of gypsum-enriched horizons was also due to the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We are grateful to the Arid Regions Institute (IRA) of Gabès (Ministry of Agriculture, Hydraulic Resources and Fisheries, Tunisia), and in particular with Rachid Boukchina and Naceur Haj Ahmed for their help during soil surveying. We are indebted with Naceur and Nader Hannachi for their assistance during field operations and Carlos Villamil for laboratory assistance. We also thank Alessandra Negri and Luigi Gobbi for their help in microscopy observations. We acknowledge the financial support of

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