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Duricrusts in soils of the Alentejo (southern Portugal)—types, distribution, genesis and time of their formation

  • Soil Formation and Weathering in Time and Space
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Abstract

Purpose

This paper reports on extremely thick and massive duricrusts in soils of two basins in the Alentejo (southern Portugal). Since different types of duricrusts (calcretes, silcretes and palycretes) have been reported from other regions in the Mediterranean, the purpose of this study was to identify the cementing agents in the duricrusts and to compare their composition in the two basins. Moreover, the study aimed at identifying the processes involved in duricrust formation, and especially the role of topography and lateral water and element transport in the landscape, and drawing conclusions about environmental conditions and time of duricrust formation.

Materials and methods

After studying an extensive number of road cuts in the field and mapping soil patterns in parts of the two basins by manual augering, ten pedons were selected for detailed description and sampling. Thin sections were analysed under a petrographic microscope, focusing on the micromorphology and optical properties of the cementing materials. Selected samples were studied by scanning electron microscopy and energy-dispersive X-ray spectroscopy to reconfirm the optical identification. The laboratory analyses included pH, carbonate contents, and X-ray diffraction analysis.

Results and discussion

The duricrusts in the eastern Sado basin are indurated by silica. Combination of XRD and thin section analysis allowed to identify opal-CT as a major component, while opal-A is present to a lesser extent, and chalcedony is very rare. The cementing materials of the duricrusts in the Oriola basin are palygorskite and calcite, which may occur alone or in combination within a soil profile.

Conclusions

The thick duricrusts formed in the basins through precipitation of calcite, palygorskite and silica from lateral water flows, which ran from the Serra de Portel into the basins, during short moist seasons in a generally warm, semi-arid climate with strong evapotranspiration. Lithology of the upper catchment areas (element sources) and topography control the spatial distribution of the different duricrusts. Their formation took place mainly during the Pliocene. Palygorskite transformation to smectite in the upper parts of the palycretes indicates that palygorskite is unstable in the present (more humid, typical Mediterranean) climate. This study demonstrates the potential role of lateral water and element transport in landscapes that need to be considered in pedological studies and concepts, and the use of mineral indicators of past climates such as palygorskite and the ageing stage of silica precipitations as tools for reconstructing environmental conditions and possible time of duricrust formation.

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Acknowledgments

We cordially thank Prof. Dr. Gottlieb Basch (University of Evora, Portugal) for his logistical support during our work in Portugal as well as Detlev Frobel and Beate Podtschaske for their diligent and accurate work in the lab. Our thanks also go to the German Research Foundation (DFG) for financial support (project STA 146/45-1 and -2). We also thank Arnaud Temme, Reinhold Jahn and an anonymous reviewer whose valuable comments helped improving the manuscript.

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Authors and Affiliations

  1. Institute of Geography, Dresden University of Technology, Helmholtzstr. 10, 01069, Dresden, Germany

    Daniela Sauer

  2. Institute of Soil Science and Land Evaluation, University of Hohenheim, Emil-Wolff-Str. 27, 70599, Stuttgart, Germany

    Christine Stein, Mehdi Zarei & Karl Stahr

  3. Institute of Earth and Environmental Science, University of Potsdam, Karl-Liebknecht-Str. 24-25, 14476, Potsdam-Golm, Germany

    Jürgen Kühn

  4. Geoecology Group, Department of Geography and Regional Research, University of Vienna, Vienna, Austria

    Stephan Glatzel

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  1. Daniela Sauer
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Correspondence to Daniela Sauer.

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Responsible editor: Arnaud Temme

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Sauer, D., Stein, C., Glatzel, S. et al. Duricrusts in soils of the Alentejo (southern Portugal)—types, distribution, genesis and time of their formation. J Soils Sediments 15, 1437–1453 (2015). https://doi.org/10.1007/s11368-015-1066-x

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