Elsevier

Geoderma

Volume 406, 15 January 2022, 115447
Geoderma

Genesis of lamellae in sandy soils: A case study in a semi-arid region in NE-Brazil

https://doi.org/10.1016/j.geoderma.2021.115447Get rights and content

Highlights

  • Metamorphic rocks may structurally control the lamellae formation in sandy soils.

  • Lamellae formation in a soil topolithosequence from metamorphic rocks in NE-Brazil.

  • Clay deposition is in accordance with a physical barrier of fine sand particles.

  • Fine sand particles are strongly related to disintegration of polycrystalline quartz.

Abstract

The occurrence of lamellae in soils and sediments has direct implications for agricultural production, and for stratigraphic and geomorphological studies. Although lamellae have been studied for more than a century, their origin remains controversial. The aim of this study was to identify the origin and formation mechanisms of lamellae in autochthonous soils developed from metamorphic rocks in a semi-arid area of Northeast Brazil. As such, the spatial organization of soil profiles was evaluated in a topolithosequence, through structural analysis of the pedological coverage for subsequent soil sampling. Morphological, chemical, physical, sedimentological, mineralogical, and micromorphological analyses were carried out on the studied slope and in greater detail in four soil profiles. The results indicated that clay particles predominantly smaller than 0.2 µm are eluviated in a sandy soil matrix, infilling voids and coating quartz grains, which are organized in layers of densely packed fine sand, impeding clay translocation and helping form lamellae. Although the micromorphological properties of the lamellae demonstrate their pedogenic origin, the contribution of strips of polycrystalline quartz constitutes a petrogenic component of the illuviation process. Therefore, the genesis of the lamellae is better described based on a petro-pedogenic origin. We suggest a restructuring of the concept of petro-pedogenic origins, including the structural influence of metamorphic rocks, as illuviated clay deposition occurs in accordance with a physical barrier provoked by the increase and organization of fine sand particles, distributed horizontally and in parallel, arising from the disintegration of polycrystalline quartz grains, which indicates geological control in the genesis of lamellae, corresponding to petrogenic evidence. Furthermore, the morphological distinction of the lamellae seems to be more associated with the fine sand content (<0.25 mm), and its packing with larger grains of quartz in the soils.

Introduction

Some sandy soils have layers rich in clay defined as lamellae, which are distributed horizontally, or with slight, frequently discontinuous, undulations of varied thicknesses (Rolim Neto and Santos, 1994, Rawling, 2000, Schaetzl, 2001, Holliday and Rawling, 2006, Almeida et al., 2015). These formations are easily identified in the field due to their strong chroma (reddish colors) in relation to the adjacent, predominantly sandy, light-colored matrix (Wurman et al., 1959, Miles and Franzmeier, 1981, Schaetzl, 1992, Holliday and Rawling, 2006).

The occurrence of lamellae is reported in various bioclimatic conditions and they seem to form rapidly in soils and sediments with good drainage, deposited by the action of wind and water (van Reeuwijk and de Villiers, 1985, Rawling, 2000, Bockheim and Hartemink, 2013). Their importance is mainly attributed to increased retention of water and nutrients, due to the higher clay concentration (Hannah and Zahner, 1970, Bouabid et al., 1992, Host and Pregitzer, 1992, McFadden et al., 1994, Almeida et al., 2015), and, as such, lamellae have a strong influence on plant growth. Geomorphological and archeological studies highlight the importance of these formations as stratigraphic markers (Leigh, 1998, Prusinkiewicz et al., 1998, May and Veit, 2009, Bockheim and Hartemink, 2013). Recently, lamellae were also used as indicators of the degree of development in soils in arid and semi-arid climates (Rossi et al., 2019), and in anthropogenic soils (Obear et al., 2017).

Despite having been studied for over a century (Wilcox, 1906), the origin of lamellae remains poorly understood and controversial (Rawling, 2000, Schaetzl, 2001, Holliday and Rawling, 2006). In general, there are three modified genetic hypotheses based on Wurman et al. (1959), which are described as: (i) lamellae of petrogenic origin, formed through sedimentary characteristics, with stratifications resulting from deposition of the parent material; (ii) lamellae of pedogenic origin, formed through the action of pedogenic processes (e.g. clay translocation); and (iii) lamellae of petro-pedogenic origin, in a combination of the previous hypotheses, in which the effects of stratifications inherited from the sedimentary parent material and the occasional modifications caused by pedogenesis are considered. Rawling, 2000, Schaetzl, 2001 highlight that although some studies cite the petrogenic origin of lamellae, they do not adequately fit the definition. Similarly, there are case studies that suggest petro-pedogenic origin, but with little agreement regarding the details that support this origin (Rawling, 2000).

The pedogenic origin of lamellae is generally associated with processes of clay dispersion, translocation, and accumulation, without the influence of stratifications of the parent material. Dispersion and translocation refer to the movement of clay particles in suspension in percolation water, and their organization into the form of lamellae (Gile, 1979, Torrent et al., 1980, Berg, 1984, Kemp and McIntosh, 1989). In this sense, Bockheim and Hartemink (2013) emphasize the importance of hydric flow in lamellae formation in soils with udic or ustic moisture regimes. Lamellae may also result from clay flocculation through increased pH in specific zones due to the occurrence of carbonates and a greater concentration of Fe oxihydroxides (Smith et al., 1950, Miles and Franzmeier, 1981, Schaetzl, 1992). Moreover, their formation is also associated with decreased porosity due to the arrangement of sand grains, which causes a positive mechanism for clay deposition and continuous lamellae growth (Bouabid et al., 1992). In addition to clay translocation being widely reported in the literature (Wurman et al., 1959, Holliday and Rawling, 2006), Furquim et al. (2013) pointed out another mechanism for lamellae formation occurring in the middle segment of the hillslope. The authors suggest that there are steeper gradients and a shallower fine loamy Ultisol dominates, with degradation of the Bt horizon and clay eluviation through subsurface lateral flow, developing mainly in the contact between the Bt horizon and the more permeable overlying horizons. In addition to reduction-oxidation processes promoting clay instability, the subsurface lateral flows play an important role for the formation of lamellae through Bt degradation due to its influence on the eluviation of suspended fine particles downslope, and the leaching of Fe2+ and other dissolved ions from the redox affected soils (Furquim et al., 2013). The variety of factors and mechanisms in their formation, which makes it difficult to establish a general genetic model, also enable lamellae to be good examples of pedogenic equifinality, that is, the same feature may have been developed in different ways (Rawling, 2000).

Considering that lamellae are most frequently found in marine, coastal, eolian, fluvial, and glacial sandy sediment facies, and, less frequently, in soils developed from sedimentary rocks (Paisani, 2004), the soils with lamellae in the present study differ by originating from Precambrian rocks (Rolim Neto and Santos, 1994, Almeida et al., 2015). Furthermore, due to the hydrological and geological conditions of the semi-arid region of Northeast Brazil, soils with and without lamellae may be observed under the same pedoenvironmental conditions. As such, their formation processes should be investigated and understood specifically from a local perspective. The hypothesis is that the petro-pedogenetic model of lamellae origin is not exclusively associated with sedimentary stratifications, but is controlled also by other parent material morphologies, parallel to pedogenetic modifications. Thus, petro-pedogenic origin would constitute the genetic model of the studied lamellae. The aim of this study was to identify the origin and formation mechanisms of lamellae in autochthonous soils developed from metamorphic rocks in the semi-arid region of Northeast Brazil.

Section snippets

Description of the physical environment

The semi-arid region of Northeast Brazil occupies approximately 800,000 km2, which corresponds to around 11% of Brazilian territory (Drumond et al., 2000). The climate has high evapotranspiration potential (2,000 mm ano−1), with mean annual temperatures of 23–27 °C, and mean annual rainfall of 700 mm, concentrated between three to five months of the year (Sampaio, 1995). The predominant vegetation is caatinga type, which is a biome exclusive to Brazil (Oliveira et al., 2013). Caatinga

Classification of the soils

The soils have an Ap horizon with a structure of predominantly single grains, and less expressive medium-sized subangular blocks, with a poor degree of development (Table 1). Lamellae were identified in the underlying horizon with differing macromorphological patterns (Fig. 1), with the exception of P4.

In P1, the lamellae are wavy, parallel to the surface and discontinuous, with some sections with less than 5 mm in length, thickness varying from 1 to 4 mm, and greater definition in the central

Pedogenic processes

The occurrence of clay oriented in the form of link capping and capping clay coatings and dense complete or incomplete clay infillings between the sand particles in the lamellae, are indicative of the argilluviation process frequently associated with the pedogenic origin of lamellae (Wurman et al., 1959, Bond, 1986, Holliday and Rawling, 2006, May and Veit, 2009, Furquim et al., 2013). The elluviation of clay for the formation of lamellae is preceded by processes of dispersion and

Conclusion

The concept of petro-pedogenic origins of the lamellae is suggested based on the incorporation of the structural contribution of metamorphic rocks associated with pedogenesis. Illuvial clays contribute to lamellae formation; their dispersion and transport being favored by the torrential rains of the semi-arid climate and sandy texture of the soils. However, illuviated clay deposition in accordance with a physical barrier provoked by the increase and organization of fine sand particles,

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

This study was financed in part by the Coordination for the Improvement of Higher Education Personnel – Brazil (CAPES), and by the National Council for Scientific and Technological Development (CNPq) through grants CNPq 308946/2019-7 and 308029/2018-6. The authors would also like to thank José Fernando Wanderley Fernandes de Lima for help with fieldwork.

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