File 47: Imprints of Pedogenesis

From a historical point of view, soil micromorphology was first used in order to decipher the expressions of pedogenic processes at the microscale (Kubiëna 1938). In the preceding chapters, the Atlas listed a series of descriptive tools to help with the identification of objects. This chapter deals with specific pedofeatures encountered in a large diversity of soils and directly related to pedogenic processes. Pedological features (Brewer 1964) or pedofeatures (Bullock et al. 1985) are “discrete fabric units present in soil materials that are recognizable from an adjacent material by a difference in concentration in one or more components or by a difference in internal fabric” (Stoops 2003, 2021). In Stoops (2003, 2021), pedofeatures are subdivided into two categories: matrix pedofeatures and intrusive pedofeatures . Matrix pedofeatures can be subdivided according to their relationship with the groundmass (depletion, impregnative, and fabric pedofeatures) and to their morphology (hypocoatings, quasicoatings, matrix infilling, intercalation, and matrix nodules). Regarding the intrusive pedofeatures, they include coatings, infillings, crystals and crystal intergrowth, intercalations, and finally nodules. The proposed nomenclature of this chapter is based on the nature and morphology of the pedofeatures, simplified from Bullock et al. (1985).

Examples of pedofeatures from left to right: a polygenetic nodule (XPL), hypo- and quasicoatings (PPL), clay infilling (PPL), infilling of needle-fibre calcite (XPL), and pellets (PPL).

File 48: Iron- and Manganese-Bearing Nodules

Nodules are defined as roughly equidimensional pedofeatures that are not related to natural surfaces or voids and that do not consist of single crystals. From a theoretical point of view, nodules can be regarded as matrix impregnative or intrusive pedofeatures (Stoops 2003). This plate presents iron-bearing nodules, and according to Bullock et al. (1985), they can be classified as amorphous or cryptocrystalline pedofeatures based on their internal fabric and external morphology. The chemical nature of nodules is often confirmed using an electron microprobe; for example, element mapping frequently shows the association of iron and manganese (see “File 7”) .

Captions from upper left corner to lower right corner.

1.:

Typic nodule : iron-bearing nodule with undifferentiated internal fabric and sharp boundaries in a loamy groundmass. Paleo-Luvisol, Piedmont region, Italy.

2.:

Concentric nodule : iron-bearing nodule with concentric layers of matter and sharp boundaries in a sandy–clayey groundmass. Chromic Luvisol, Apulia, Italy.

3.:

Aggregate nodule: such nodules are formed by an aggregation of small mostly typic nodules and are frequently found in Vertisols. Vertisol, Po Plain, Italy.

4.:

Dendritic nodule : Stoops (2003, 2021) considers them to be aggregate Fe–Mn nodules organized in a dendritic pattern. This dendritic nodule developed within a clayey micromass. Chromic Paleo-Luvisol, Sardinia, Italy.

5.:

Nucleic nodule : example of a Fe–Mn nodule precipitated around a quartz grain, i.e. an allochthonous core, in a loamy groundmass. Oxyaquic Cryosol, Italian Alps.

6.:

Geodic nodule : nodule showing an empty core, i.e. an irregular-shaped void in a silty groundmass. Loess paleosol, central Po Plain, Italy.

7.–8.:

Alteromorphic nodules : this type of nodule is usually the product of weathering. These nodules are also characterized by pseudomorphosis of mineral or organic materials. In 7., alteromorphic nodule developed from a rock fragment in a loess paleosol. Ligurian Alps, Italy. In 8., alteromorphic nodule incorporating plant residue. Paleo-Luvisol, Piedmont region, Italy.

File 49: Carbonate Nodules

Nodules are defined as roughly equidimensional pedofeatures that are not related to natural surfaces or voids and do not consist of single crystals (Stoops 2003). From a theoretical point of view, nodules can be regarded as matrix impregnative or intrusive pedofeatures (Stoops 2003). This plate presents carbonate nodules, and according to Bullock et al. (1985), they can be classified as crystalline pedofeatures based on their internal fabric and external morphology . Moreover, the size of carbonate crystals forming the nodule (i.e. micrite, microsparite, or sparite) is also a pertinent attribute of such pedofeatures.

Captions from upper left corner to lower right corner.

1.:

Typic nodule : micritic nodule with a homogeneous internal fabric and sharp boundaries in a carbonate micromass. Cambisol, Paris Basin, France.

2.:

Sparitic nodule: carbonate nodule composed of large sparitic crystals and sharp boundaries (see “File 51”) in a silty–clayey groundmass. Planosol, Po Plain, Italy.

3.:

Concentric nodule : such nodules are formed by an accretion of concentric layers of micrite. This particular nodule can result from precipitation in a swampy environment. Calcisol (Gleyic), Syria.

4.:

Septaric nodule : the void in the centre results from radiating cracks. This septaric nodule developed within a silty–clayey groundmass. They are also frequently observed in tropical Vertisols. Planosol, Po Plain, Italy.

5.:

Nucleic nodule : example of a micritic nodule precipitated around a quartz grain, i.e. an allochthonous core, in microspartic and micritic groundmass. Calcisol, Negev Desert, Israel.

6.:

Geodic nodule : nodule showing an empty core, i.e. a round void in this case, in a silty groundmass. Cambic Calcisol, Jura Mountains, Switzerland.

7.–8.:

Lithoclasts and not nodules: the shape of some carbonate lithoclasts can be confused with pedogenic nodules. In 7., example of a travertine oncoid from a Calcisol (Gleyic), Syria. In 8., lithoclast of a marine limestone with a foraminifera fragment (sparitic round feature in the centre) in a Chromic Cambisol, Apulia, Italy.

File 50: Polygenetic Nodules

Polygenetic nodules are either nodules composed of multiple generations of cortical layers or products of different pedogenetic phases. A special type of polygenetic nodule is associated with perlitic crusts SeeSee Perlitic crust . These crusts are made of multiple polygenetic nodules, also called ooids (Durand et al. 2018), of various sizes in a monic to close porphyric c/f related distribution and almost always associated with carbonate laminar crusts .

Captions from upper left corner to lower right corner.

1.:

Polygenetic multi-layered iron-bearing nodule. The internal part is composed of a large nucleus made of iron oxyhydroxides impregnating the groundmass and clays. This nucleus is detached from a cortex formed by multiple thin layers of the same mineralogical composition. Paleosol, Liguria, Italy.

2.:

Polygenetic carbonate nodule composed of smaller nodules surrounded by a polygenetic micritic cortex in a micromass from a perlitic crust. Calcisol, Negev Desert, Israel.

3.:

Polygenetic carbonate nodule formed, first, by a sparitic typic nodule, rimmed by an iron oxyhydroxide brown layer and aggregated into a larger partially geodic and micritic nodule. Vertisol, Extreme North region, Cameroon.

4.:

Polygenetic sparitic nodule . The void in the centre is surrounded by clear sparitic cement, whereas the central part of the nodule consists of sparite with traces of iron oxyhydroxides. The cortex is also calcitic with traces of iron oxyhydroxides, but crystals are organized in a fan-like arrangement. Such nodules are frequently due to water-table fluctuations. Vertisol, Extreme North region, Cameroon.

5.–6.:

In 5., fragment of a reworked laminar crust included in a Calcisol (Gleyic), Syria. This feature is not strictly considered as a “pedogenic” nodule, but the fact that it originates from the same environment as the groundmass makes it polygenetic nevertheless, because it has been eroded, displaced, and deposited a bit further from its place of formation. In 6., in situ laminar crust showing the alternation of clear greyish microsparitic and yellowish–brownish micritic layers. The microsparitic layers are made of calcitic spherulites (see “File 16” and Verrecchia et al. 1995). PPL view, Negev Desert, Israel.

7.–8.:

Two polygenetic siliceous nodules cross-cut by the picture’s diagonal. They result from the secondary silicification of primary calcitic nodules trapping quartz grains. They are also rimmed by a very thin layer of oxyhydroxides. The groundmass is composed of quartz grains in a micritic micromass. In 8., close-up. The XPL view with a gypsum plate emphasizes the abundance of silica in the micromass (high-order colours). A quartz grain appears in bright blue. Calcisol, Chobe Enclave, Botswana.

File 51: Nodules: Morphology and Border Shape

Nodules can be characterized by their specific external morphologies. As pedofeatures, nodules also have spatial relationships with the groundmass surrounding them; this relationship is illustrated by the shape of nodule’s border.

Captions from upper left corner to lower right corner.

1.:

Sketch of the different external morphologies of nodules (Stoops 2003). The mammillate type, in the lower left corner, refers to an undulating external shape. The digitate type (centre) makes fingerlike penetrations inside the adjacent groundmass. The disjointed type, in the upper right corner, is composed of angular accommodating fragments.

2.:

Example of disjointed iron-bearing nodule. Paleo-Luvisol, Piedmont region, Italy.

3.:

Example of mammillate iron-bearing nodule. Paleo-Luvisol, Piedmont region, Italy.

4.:

Example of a digitate iron-bearing nodule. Paleo-Luvisol, Piedmont region, Italy.

5.–6.:

Example of two different iron-bearing nodules with sharp boundaries. This kind of border is often indicative of an allochthonous provenance of the nodules. Paleo-Luvisol, Liguria, Italy.

7.–8.:

Example of two different iron-bearing nodules with diffuse boundaries. In both microphotographs, the outer layers of the nodule diffuse into the groundmass, in an isotropic manner in 7. and as a gradually fading halo in 8. These kinds of borders are often related to an in situ formation process. These nodules have been observed in 7. in a Paleo-Luvisol, Piedmont region, Italy and in 8., in a Fluvisol, Jura Mountains, Switzerland.

File 52: Nodules: Orthic, Anorthic, and Disorthic

Nodules can be formed in situ, reworked to varying degrees, or inherited from the parent material. In order to identify their origin, Stoops (2003, 2021) suggests the classification proposed by Wieder and Yaalon (1974). When nodules are inherited from the parent material or are clearly allochthonous, they are called anorthic . If nodules are formed in situ and do not show any sign of reworking, they are considered as orthic . If they were locally displaced inside the soil, they are qualified as disorthic .

Captions start with the sketch in the central left column. Then, the microphotographs are described clockwise around the sketch, starting from the upper left corner to the lower left corner.

  1. 1.

    Sketch of the three types of relationships between nodules and the surrounding soil groundmass (Stoops 2003). Top: an orthic nodule is formed in situ, and not displaced, as evidenced by the linear pedofeature cross-cutting the nodule, as well as the groundmass. Middle: a disorthic nodule formed inside the soil but that has been locally reworked; the linear pedofeature is only observed inside the nodule, but not in the groundmass. Bottom: the nature of an anorthic nodule is different from the groundmass and is clearly allochthonous.

  2. 2.

    Example of an orthic iron-bearing nodule: the coarse fraction of the groundmass (silty quartz grains) shows the same distribution pattern inside and outside the nodule, emphasizing the in situ concentration of soil iron compounds. Paleo-Luvisol, Piedmont region, Italy.

  3. 3.

    Example of an orthic nodule in a carbonate-rich environment: this orthic nodule is formed by precipitation of pedogenic micrite and microsparite . Calcisol, Jura Mountains, Switzerland.

  4. 4.

    Although embedding the same compounds and components as the groundmass, this disorthic nodule has been locally displaced in the profile as shown by the light orange clay coatings, which are not continuous with the groundmass. Paleo-Luvisol, Piedmont region, Italy.

  5. 5.

    The identification of this pedofeature as a disorthic nodule is based not only on the similar nature of the groundmass but also mostly on the presence of a brownish rim around it. It must be noted that distinguishing orthic from disorthic (but also anorthic) matrix nodules is rather difficult and sometimes impossible. Calcic paleosol, Aquitaine Basin, France.

  6. 6.

    Typical anorthic nodule composed of a greyish micrite and clearly different from the surrounding groundmass made of silty quartz, in a carbonate–silicate micromass. Calcisol, Syria.

  7. 7.

    Typical anorthic iron-bearing nodule inherited from the soil parent material and included in a fine-grained horizon. Paleosol, Liguria, Italy.

File 53: Crystals and Crystal Intergrowths

Crystals and crystal intergrowths are individual or clusters of crystals, which are precipitated inside the soil groundmass. Such crystals are not inherited from the parent material but are the product of pedogenic processes. According to Stoops (2003, 2021), crystal intergrowths are subdivided based on their distribution and/or orientation pattern, and according to Bullock et al. (1985), they can be classified as crystalline pedofeatures based on their internal fabric and external morphology.

Captions from upper left corner to lower right corner.

1.:

Random crystals of gypsum in a Gypsic Calcisol. Such crystal intergrowth is distributed in a random pattern and is characterized by a large variety of grain sizes. Negev Desert, Israel.

2.:

Parallel gypsum crystal intergrowth in a Gypsic Calcisol. XPL view, Negev Desert, Israel.

3.:

Goethite fan-like crystal intergrowth (orange cluster in the centre of the microphotograph). PPL view, Paleosol, central Po Plain, Italy.

4.:

Crystal intergrowth of vivianite organized in a radial pattern around a central point. Note the diagnostic blue colour of the vivianite crystals in PPL. Medieval archaeological soil, Po Plain, Italy.

5.:

Crystals of calcite developed inside a micritic groundmass, where they form sparitic intergrowths. Note the euhedral tip of some crystals. Petric Calcisol, Madrid Basin, Spain.

6.–8.:

Random gypsum crystal intergrowths. The variable pleochroism of the crystals shown in 6. in PPL is due to a change in mineralogy. In 7. (in XPL), gypsum crystals in various shades of grey include some other minerals with high-order interference colours, which are calcite crystals. The use of a gypsum plate in 8. clearly reveals an ongoing process of pseudomorphosis of gypsum by calcite. Gypsic Calcisol, Libya.

File 54: Impregnations

One of the descriptive parameters of matrix pedofeatures includes the degree of impregnation . Nodules are sometimes regarded as matrix pedofeatures (see “File 48” and “File 52”). Four degrees of impregnation are proposed by Stoops (2003, 2021) according to the amount of recognizable components of the groundmass, i.e. regarding the purity of the feature: weakly impregnated, moderately impregnated, strongly impregnated, and pure. The first four microphotographs refer to a carbonate environment, whereas the last four to aluminosilicate- and iron-rich settings.

Captions from upper left corner to lower right corner.

1.:

Weakly impregnated groundmass by greyish pedogenic calcite, as a first stage of carbonate nodule formation. Calcisol (Gleyic), Syria.

2.:

Moderately impregnated groundmass by pedogenic calcite (greyish in PPL and brownish yellow in XPL) of the size of micrite and microsparite . Components of the groundmass are still visible. Chromic Cambisol, Apulia, Italy.

3.:

Strongly impregnated groundmass forming a geodic nodule . The concentration of micritic matter increases significantly in the pedofeature, but the nature of the groundmass can still be identified. Fluvic Stagnosol, Jura Mountains, Switzerland.

4.:

Pure sparitic nodule in which the micromass of the groundmass is no longer identifiable, whereas some coarse quartz grains remain visible. Chromic Cambisol, Madrid Basin, Spain.

5.–8.:

Examples of progressive impregnation of the groundmass by iron oxyhydroxides forming weakly impregnated to pure typic iron-bearing nodules . Note that the components of the groundmass are progressively less recognizable, until they totally disappear. Paleo-Luvisol, Piedmont region, Italy.

File 55: Depletions

Depletion pedofeatures are defined as lower concentrations of a given component of the micromass, e.g. calcite or iron oxyhydroxides (Bullock et al. 1985; Stoops 2003, 2021). The loss of matter can be related to either dissolution, e.g. in a carbonate environment, or redox processes, e.g. in iron-rich environments. The mobilized ions are then translocated or leached inside or outside the profile, respectively.

Captions from upper left corner to lower right corner.

1.–2.:

1. PPL view of an area depleted in carbonate emphasized by contrast and intensity of the grey colour of the micritic micromass. 2. Same view in XPL. The pore network is due to rootlets. Petric Calcisol, Galilee, Israel.

3.–4.:

3. PPL view of an area depleted in carbonate induced by a rootlet. 4. Same view in XPL. The thinner calcitic crystals display interference colours of higher orders. Petric Calcisol, Galilee, Israel.

5.–6.:

5. PPL view of the depleted area emphasized by contrast and intensity of the dark/light colours of the micromass. 6. Same view in XPL. The contrasted interference colours remain in XPL. Moreover, this colour variation is due to a loss of iron oxyhydroxides. Paleo-Luvisol, Piedmont region, Italy.

7.–8.:

7. PPL view of an area depleted in yellow compared to the original micromass, which is reddish brown. 8. Same view in XPL. The yellow clays in the depleted area have lost the iron oxyhydroxides adsorbed on their sheets, giving them a lighter colour compared to the clays inside the original groundmass. Paleosol, central Po Plain, Italy.

File 56: Coatings with Clays I

Coatings are defined as intrusive pedofeatures that coat natural surfaces of voids, grains, or aggregates inside soils (Stoops 2003, 2021). Coatings must not be confused with infillings (see “File 61”). Coatings are constituted by various material types, i.e. clays, and coarse, amorphous , or crystalline material. This section shows coatings formed by clays, one of the earliest pedogenic features recognized in thin sections. In situ clay coatings are diagnostic features of leaching processes and are also used in soil classifications. Clay coatings are described according to their colour, the presence or absence of laminations, their thickness, and their grain size (i.e. they can be called textural pedofeatures).

Captions from upper left corner to lower right corner.

  1. 1.

    Clay coating deposited inside a void. This thick yellow clay coating is formed by two to three layers of fine clay crystals, partly orientated in the same direction. This preferential orientation of crystals is emphasized by the observation of a large extinction band in XPL. The yellow colour can be related to the presence of goethite . Paleo-Luvisol, Piedmont, Italy.

  2. 2.

    Red clay coating on both sides of a channel. There is no obvious preferential orientation of the clay crystals. In addition, the red colour can be related to the presence of hematite . Paleosol, Ligurian coast, northern Italy.

  3. 3.

    Non-laminated yellow clay coating. Paleo-Luvisol, Piedmont, Italy.

  4. 4.

    Strongly laminated orange clay coating. The large succession of layers is due to the occurrence of multiple phases of clay translocation. Erosion phases can also interrupt the regularity of the layering. Paleosol, central Po Plain, Italy.

  5. 5.

    Limpid coating formed by very fine clay crystals . This laminated coating shows some erosional internal surfaces emphasized by concave contacts visible in XPL. Paleo-Luvisol, Piedmont, Italy.

  6. 6.

    Coarse-grained coating in a void. The coating is formed by the succession of layers of coarser to finer grains upwards, from silt to clay. The uppermost layer contains the largest clay fraction. Paleosol, Libya, central Sahara.

  7. 7.

    A typic clay coating around a transversal section of a channel showing a uniform thickness in all the directions. Chromic Luvisol, Apulia, Italy.

  8. 8.

    Crescent coating characterized by a larger basal thickness compared to the sides at the bottom of a void. This laminated coating displays a sharp extinction band, emphasizing the continuous parallel orientation of deposited clays. Paleosol, Ligurian coast, northern Italy.

File 57: Coatings with Clays II

Coatings are defined as intrusive pedofeatures that coat natural surfaces of voids, grains, or aggregates inside soils (Stoops 2003, 2021). This section shows peculiar characteristics of coatings formed by clays associated to either reworking, massive deposition, or effects of waterlogging. Moreover, clay coatings must not be confused with clay neoformation in saprolite cracks, a case illustrated in the last two pictures.

Captions from upper left corner to lower right corner.

1.:

Typic clay coating around a cross-section of a channel. This situation is typical of a horizon affected by clay translocation. Chromic Luvisol, Apulia, Italy.

2.:

Fragments of laminated clay coatings isolated inside the groundmass. This type of feature indicates possible reworking of a former horizon affected by clay leaching. Such fragments have also been called “papulas” in the past (Brewer 1964) . Reworked loess, Piedmont, Italy.

3.:

Very thick clay coating showing numerous alternating laminations . The red colour is probably due to the presence of nanocrystals of hematite . Clay translocation is so intense that almost the entire thin section is occupied by clay layers. Some of the contacts between laminations correspond to erosive surfaces. Chromic Luvisol developed in a cave, Sardinia, Italy.

4.:

Very thick and convoluted clay coatings formed by multiple laminations related to different translocation phases. Some of the contacts between laminations correspond to erosive surfaces. Paleosol, Ligurian coast, northern Italy.

5.:

Fragments of clay coatings formed during intense clay translocation (similar to 3.), which have been reworked and redeposited. Chromic Luvisol developed in a cave, Sardinia, Italy.

6.:

Clay intercalation observed in bleached zones of some waterlogged soils. According to Fedoroff and Courty (2012), such intercalations differ from typic clay coatings by their whitish grey colour, their absence of both sorting and lamination, and their medium to weak orientation. Soil in a rice chamber, Piedmont, Italy.

7.–8.:

In 7., example of the presence of clays inside a crack. Such a feature is not a clay coating, but a clay neoformation layer inside a saprolite. In 8., same view as 7. in XPL. Paleosol, Ligurian coast, northern Italy.

File 58: Micropans, Coarse Coatings, Cappings, and Crusts

Coatings are defined as intrusive pedofeatures that coat natural surfaces of voids, grains, or aggregates inside soils (Stoops 2003, 2021). In terms of morphologies, coatings can be subdivided into several types, such as the typic and crescent ones shown in “File 56” and “File 57”. Micropans, crusts, and cappings are illustrated in this section. These types are often composed of material coarser than clay; consequently, this section also contains examples of coarse coatings.

Captions from upper left corner to lower right corner.

1.–2.:

A micropan is defined as “a thick sub-horizontal coating varying significantly in thickness” (Stoops 2003). In these two microphotographs, the groundmass is dominated by sandy quartz grains, emphasizing the presence of a textural pedofeature, the micropan being mainly composed of clay minerals (yellowish brown in PPL and undulated yellow with brown spots in XPL). In 2., close-up of the view shown in 1. highlighting the abundance of clay particles. Paleosol, Ligurian coast, northern Italy.

3.:

Compound layered coatings formed by alternating layers of clayey, silty, and sandy material. Clays with silt layers are sometimes described as dusty clay coatings , or generally speaking, coarse coatings. Paleosol, central Po plain, Italy.

4.:

Silt coating in a Luvic Stagnosol from Syria. Such coatings are formed by silt-sized grains, which can be fine, medium, or coarse.

5.–6.:

Cappings developed on large rock fragments (garnet mica schist; garnets are the black geometrical grains in XPL, whereas mica appear as yellowish to greenish bands; see p. 80). Cappings are composed of mainly coarse material, which form on top of free or embedded grains, and are often observed in Cryosols. Cryosol, western Alps, Lombardy, Italy.

7.:

Crusts are defined as thick coatings on the soil surface, whatever their grain-size distribution and nature (hypo- or quasicoating; see p. 30). The crust shown in this microphotograph developed on a silty groundmass material. It is formed by clay minerals as accentuated in the XPL view. Chromic Luvisol, Apulia region, Italy.

8.:

Coarse crust composed of silt, clay, and a few grains of sand on the top of a carbonate layer (grey in PPL and brownish grey in XPL), overlapping a yellowish–brownish silty–clayey groundmass (in XPL). Calcaric Cambisol, Apulia region, Italy.

File 59: Hypocoatings and Quasicoatings: Amorphous

The difference between coatings, hypocoatings, and quasicoatings is the location of the material with respect to the internal soil surface. Hypocoatings refer to an accumulation of matter impregnating the soil groundmass directly adjoining the void edge (the internal soil surface). Quasicoatings are not in direct contact with the void border: there is a rim of soil groundmass material between the void and the quasicoating pedofeature. Because of their impregnative nature, hypocoatings and quasicoatings are mainly amorphous (this section) or crystalline (“File 60”). “Amorphous” refers to isotropic properties of iron and manganese oxyhydroxides , which mainly form these kinds of pedofeatures; according to Bullock et al. (1985), they can be regarded as amorphous or cryptocrystalline pedofeatures .

Captions start with the uppermost sketch, followed by the microphotographs from upper left corner to lower right corner.

1.:

Sketch illustrating the geometrical relationship between coatings, hypocoatings, and quasicoatings (from left to right), the pore limits, and the soil groundmass.

2.–3.:

Hypocoatings at two different magnifications made of an oxyhydroxide phase impregnating a loamy–sandy groundmass and associated with channel voids. Note the presence of fragments of clay coatings. PPL view, Stagnic Luvisol, Piedmont, Italy.

4.:

Quasicoating of an oxyhydroxide phase impregnating a loamy–sandy groundmass. Note the presence of a groundmass rim between the void and the quasicoating. PPL view, Stagnic Cambisol, Apulia, Italy.

5.:

Impregnating feature showing partial hypocoatings with quasicoatings resembling Liesegang rings. These hypocoatings with quasicoatings are composed of amorphous oxyhydroxides. PPL view, Stagnic Cambisol, Apulia, Italy.

6.:

An example of both superimposed SeeSee Pedofeatures and juxtaposed SeeSee Pedofeatures compound pedofeatures: a first generation of dark quasicoating to hypocoating of amorphous material is followed by the deposition of a brownish clay coating, itself affected by a hypocoating of amorphous material. Finally, an incomplete infilling of yellow clays completes the succession of pedofeatures. See also “File 66”. PPL view, Stagnic Luvisol, Apulia, Italy.

7.:

Compound superimposed SeeSee Pedofeatures pedofeature consisting of a hypocoating of amorphous material affecting a clay coating along a longitudinal void. PPL view, Stagnic Luvisol, Piedmont, Italy.

File 60: Coatings and Hypocoatings: Crystalline

The difference between coatings and hypocoatings is the location of the material in respect to the internal soil surface. Coatings are intrusive pedofeatures that cover natural surfaces of voids, grains, or aggregates. Hypocoatings refer to an accumulation of matter impregnating the soil groundmass directly adjoining the void edge (the internal soil surface). This section includes various types of such crystalline pedofeatures (Bullock et al. 1985) that are related to void edges. SeeSee Calcite

Captions from upper left corner to lower right corner.

  1. 1.

    Calcite coating around a void formed in a carbonate groundmass (with a micritic micromass ). The calcite coating is formed by small-sized crystals (microsparite ). Calcaric Leptosol, Jura Mountains, Switzerland.

  2. 2.

    Calcite coating around a void formed in a clayey groundmass. The calcite coating is formed by coarse crystals (sparite) , probably originating from calcified root cells . A secondary clear calcite coating (microsparite) is juxtaposed to the coarse coating. Paleosol, central Sahara.

  3. 3.

    Calcite pendent developed on the lower surface of a sandstone grain. The pendent is made of banded and fibro-radial small grain-sized crystals. Cryosol, Spitsbergen, Svalbard Islands, Norway.

  4. 4.

    Calcite pendent forming multiple layers, starting with a fibrous coating in contact with the carbonate micromass (top), followed by a microsparitic band, and finally by large fans of fibrous calcite . Petric Calcisol, Madrid Basin, Spain.

  5. 5.

    Calcite hypocoating formed by small-sized crystals (micrite) impregnating a carbonate-rich micromass with some quartz grains. Bronze Age archaeological soil, Po Plain, Italy.

  6. 6.

    Juxtaposed crystalline pedofeatures: a hypocoating of microcrystalline calcite (micrite) is in contact with a microsparitic coating inside a root void. Bronze Age archaeological soil, Po Plain, Italy.

  7. 7.

    Hypocoating of phosphate around a cross-section of a channel in a carbonate-rich micromass. Medieval archaeological soil, Po Plain, Italy.

  8. 8.

    Transparent coating made of a cryptocrystalline siliceous compound (possibly silica) around quartz grains and carbonate-rich aggregates. The coatings are visible due to their high relief. PPL view, Duric Kastanozem, Manga, Niger.

File 61: Mineral Infillings

Infillings are formed by soil material or some fraction of it, which fills any void other than packing voids (Stoops 2003, 2021). These infillings are formed by mineral material originating from either biological (see “File 62”) or physicochemical processes. This plate shows examples of mineral infillings, which can be coarse or fine and of different mineralogical natures. They are termed textural or crystalline pedofeatures according to their composition.

Captions from upper left corner to lower right corner.

  1. 1.

    A former planar void filled with coarse quartz grains, constituting a dense incomplete infilling. It can be considered as a textural pedofeature due to the grain-size distribution of the quartz grains. Agricultural layer of a Vertisol, Po Plain, Italy.

  2. 2.

    Well sorted and coarse infilling made by quartz grains forming a dense complete infilling, but which has undergone cracking during pedoturbation (central plane). In XPL, some slickenside features can be observed at the bottom of the microphotograph. Agricultural layer of a Vertisol, Po Plain, Italy.

  3. 3.

    Textural clay infilling in a silty groundmass. The clay nature of this dense incomplete infilling is accentuated by the colours and laminations in PPL as well as in XPL. Moreover, in the XPL view, the micromass is characterized by a circular striated b-fabric. Paleo-Luvisol, Piedmont, Italy.

  4. 4.

    Textural clay infilling in a silty to sandy groundmass. The clay nature of this infilling is accentuated by the colours and laminations in PPL as well as in XPL. The clays form a dense complete infilling, but which has undergone cracking due to pedoturbation. Paleo-Luvisol, Piedmont, Italy.

  5. 5.

    Infilling of micritic granular aggregates in a desiccation crack in a pre-existing carbonate-rich groundmass. These aggregates are associated with coarser calcite crystals (microsparite) as a secondary phase. Both features form a dense incomplete infilling. Petric Calcisol, Chobe Enclave, Botswana.

  6. 6.

    Large planar void developed in a carbonate-rich groundmass filled by a first generation of micritic aggregates, coated by microsparitic calcite cement deposited during phreatic events. These features form a dense incomplete infilling. In the upper part of the microphotograph, another planar void is filled by coarse-grained sparite forming a dense complete infilling. Petric Calcisol, Madrid Basin, Spain.

  7. 7.

    Large planar void filled with gypsum crystals, easily identified by their characteristic lenticular shape and their birefringence colours. These crystals form a loose discontinuous infilling. Gypsic Regosol, Swiss Alps.

  8. 8.

    Channel void in a carbonate-rich micromass filled with a crystal of vivianite , recognizable by its blue colours in PPL. This infilling is dense and incomplete. Reductive conditions are necessary for vivianite formation. Note the presence of phosphate hypocoatings . Medieval archaeological soil, Po Plain, Italy.

File 62: Mineral Infillings of Biological Origin

Infillings are formed by soil material or some fraction of it, which fills any void other than packing voids (Stoops 2003, 2021). These infillings are formed by mineral material originating from either biological (see “File 61”) or physicochemical processes. This plate shows examples of biomineral infillings. They are termed crystalline pedofeatures according to their composition.

Captions from upper left corner to lower right corner.

1.–2.:

Dense incomplete infillings formed by calcified root cells (Becze-Deak et al. 1997; Durand et al. 2018) around cross-sections of (1.), and along (2.) channels in a siltic groundmass. In 1., most of the crystals originate from the biomineralization of epidermis cells, whereas in 2., the crystals were precipitated in cortex cells. Loess paleosol, Shaanxi, China.

3.:

Same type of feature as in 1. and 2. This infilling, due to calcified root cells, also preserved some biomineralized root hairs appearing as long styloidic calcite crystals overgrowing specific cells. Loess Plateau, northern central China.

4.:

Dense complete infilling in a carbonate-rich micromass, formed by the complete biomineralization of a root fragment, showing calcified cells from the epidermis, endodermis, and cortex of a rootlet. Petric Calcisol, Galilee, Israel.

5.:

A loose discontinuous infilling by secondary acicular crystals of calcite between micritic lithoclast of chalk. This type of infilling shows a convoluted fabric according to Rabenhorst and Wilding (1986). Petric Calcisol, Champagne, France.

6.:

Close-up of 5. showing the structure of the convoluted fabric made of acicular crystals of calcite. These crystals are not randomly organized but tend to form bundles. The origin of such features is related to fungal activity and biomineralization (Bindschedler et al. 2016, 2012). Petric Calcisol, Champagne, France.

7.:

Magnification of microphotograph 6. emphasizing the needle-fibre calcite nature of the acicular crystals (Durand et al. 2018). Petric Calcisol, Champagne, France.

8.:

A cross-section of a channel filled with a dense incomplete infilling made of calcified root cells (biomineralized epidermis cells), as well as a loose discontinuous infilling of needle-fibre calcite. In addition, the channel section is rimmed by a micrite hypocoating. Petric Calcisol, Galilee, Israel.

File 63: Pedoturbations

Pedoturbation is defined as any kind of physical mixing of soil material accomplished by the following mechanisms: shrinking and swelling of clays, freeze–thaw activity , and bioturbation by animals or plants. It is not because the soil material is mixed that it becomes homogenized (Schaetzl and Thompson 2015). However, even if some homogenization seems obvious at the macroscale, at the microscale, the effects of pedoturbation are always visible because of the imprint of the various processes. In addition, in Stoops (2003), most of the pedoturbations are included in fabric pedofeatures (belonging to matrix pedofeatures ), which are “recognizable from the groundmass because of difference in fabric only”.

Captions from upper left corner to lower right corner.

1.–2.:

Pedoturbation related to clay swelling and shrinking in a Vertisol. The brown and dark groundmass is separated by iso-oriented clay domains, i.e. slickenside , usually identifiable both in PPL (light greyish coloured) and XPL (first order birefringence colours), inducing a porostriated b-fabric . In 2., detail showing a clay domain at higher magnification. Vertisol, Po Plain, Italy.

3.:

Pedoturbation due to freeze–thaw activity and ice segregation forming lenses. Lenticular aggregates are covered by silt cappings . Both aggregates and cappings include fine sand grains. Cryosol, Alps, Italy.

4.:

Pedoturbation related to freeze–thaw activity in an organic-rich micro-granular horizon. A sub-lenticular aggregation is visible at the top of the image. Another level of aggregation, consisting of a network of zigzag planes, affects the groundmass in the lower part of the picture. The presence of micro-granular silty aggregates enhances the identification of frost pedoturbation. Paleosol, northern Apennines, Italy.

5.:

Pedoturbation made by animals. It can be considered as a passage feature, senso Stoops (2003). The groundmass is organized in a concentric manner but has been mechanically reworked and dispersed. Calcisol, Jura Mountains, Switzerland.

6.:

Pedoturbation due to animals. Note the yellow-coloured fabric hypocoating due to mechanical forces that have compacted the micromass. In this case, the groundmass has not been dispersed. Loess Plateau, northern central China.

7.:

Pedoturbation due to the presence of a rootlet (central part of the microphotograph in brown), which has created a large void and compacted its bottom part. Reworked loess deposit, northern Po Plain, Italy.

8.:

Pedoturbation due to a dense network of rootlets (brown rounded organic features in PPL) resulting in a disturbance of the groundmass. In XPL, a long channel is partly filled by the longitudinal cross-cut of a root . Fluvisol, Jura Mountains, Switzerland.

File 64: Faecal Pellets

SeeSee Faecal pellets Sometimes referred to as “excrements of the mesofauna” (Stoops 2003, 2021), faecal pellets (in the sedimentological sense of the word) do not only belong to a special type of pedofeature but can also create granular and/or vermicular microstructures (see “File 20” and “File 21”). They can form infillings as well. The faecal pellet’s shape can refer to specific mesofaunas, as listed in Stoops (2003, 2021) and Bullock et al. (1985).

Captions from upper left corner to lower right corner.

1.–2.:

Layer extremely rich in faecal pellets forming a vermicular microstructure. Almost the entire horizon is composed of such excrements, resulting in a mixture of organic matter and clay-size mineral material. In 2., detail of microphotograph in 1. Paleosol, northern Apennines, Italy.

3.:

The channel tip in the central part of the microphotograph is partially infilled with faecal pellets and shows a sharp limit separating it from the groundmass. Fluvisol, Jura Mountains, Switzerland.

4.:

Ellipsoidal faecal pellets of Oribatid mites. The faecal pellets are weakly coalescent and loosely infill the hollow centre of a decaying plant fragment. Fluvisol, Swiss Plateau, Switzerland.

5.:

Faecal pellets found in a root , partially decayed by mesofaunas, which fed on plant organic matter and produced faecal pellets in situ. Faecal pellets are almost exclusively composed of organic matter. Calcisol, L’Isle-Adam, Paris Basin, France.

6.:

Faecal pellets found in a root, partially decayed by mesofauna, forming a mixture organic and mineral matter. The mesofaunas responsible for these faecal pellets are different from those which produced the faecal pellets in microphotograph 5. Cambisol, northern Apennines, Italy.

7.:

Large faecal pellets formed inside earthworm casts. These large faecal pellets are weakly coalescent and porous. It is sometimes difficult to distinguish them from the soil groundmass. However, these casts can be associated to calcitic biospheroids (see “File 17”). Cambisol, Swiss Plateau, Switzerland.

8.:

Coalescent faecal pellets of earthworms forming a partial infilling of a void. There is a clear difference in composition between the silty groundmass (yellowish brown in PPL) and the faecal pellets, which are impregnated by calcite (greyish colour in PPL). Loess Plateau, northern central China.

File 65: Dung and Vertebrate Excrements

Dung and excrements of large animals are mainly of interest to archaeological micromorphology (Nicosia and Stoops 2017), whereas faecal pellets can be observed in both natural soils and archaeological settings. Regarding excrements, they are characterized by different external shapes, colours, basic constituents, and internal fabrics, depending on the genera of the animals (Stoops 2003).

Captions from upper left corner to lower right corner.

1.:

Dung from a rock shelter in the Libyan Sahara showing the presence of undisturbed excrements of herbivores (capriovids), one in PPL and another in XPL. They are characterized by a sub-rounded mass with convoluted fabric, almost extinct in XPL (very low birefringence) due to the high content in organic matter. They might include opal phytoliths, faecal spherulites, oxalate druses, and crystals, depending on the diet of the animals (see “File 36”).

2.:

Dung from a rock shelter in the Libyan Sahara showing a laminated fabric due to trampling by penned herbivores. Therefore, it is impossible to recognize any shape of excrement, although constituents of the faecal matter and biominerals (see “File 36”) are still present but totally dispersed in the groundmass.

3.–4.:

Herbivore dung is composed of organic fragments at different stages of preservation (due to digestion) in both undisturbed and laminated deposits. In 3., the large oblong longitudinal shape is a seed in cross-section with long and thin plant fragments, whereas in 4., plant debris at various stages of preservation are observed along their radial section (see also “File 41”).

5.–6.:

Bird excrements are yellow amorphous phosphatic masses, which can also contain uric acid spherulites (see “File 37”), as well as oxalate crystals (see “File 37”). In these thin sections, secondary precipitations of calcium carbonate form early diagenetic features, e.g. in 5., as high-order birefringence coloured clusters. Cave wall deposit, Libya, central Sahara.

7.–8.:

Carnivore excrement (Hyaena sp.). In 7., a large sub-rounded yellowish phosphatic mass containing bone fragments, as well as feather and hair imprints, as shown in detail in 8. Paglicci Cave, Apulia, Italy.

File 66: Composite Pedogenic Features

Pedofeatures made up of several parts or elements (i.e. composite) encompass compound and complex pedofeatures, as defined by Stoops (2003). They consist of a mixture of two or more pedofeatures resulting from different pedogenic processes, either contemporaneous or successive. If each single pedofeature lies side by side, they are defined as juxtaposed, whereas if they overlap, pervade, or affect one another, they are considered to be superimposed.

Captions from upper left corner to lower right corner.

  1. 1.

    Compound pedofeature constituted by three generations of clay coatings, juxtaposed to one another. The three coatings are distinguishable by their respective colours, from orange, at the bottom, to bright, and finally pale yellow. These juxtaposed clay coatings form a clay infilling inside a channel. Paleo-Luvisol, Piedmont region, Italy.

  2. 2.

    Succession of three different pedofeatures around a channel in a loamy groundmass. First, an amorphous hypocoating (orange with coarse material) impregnated the micromass. A red and laminated clay coating formed the second step. Finally, a light yellow and non-laminated clay coating ended the sequence. The three pedofeatures are juxtaposed to one another. Paleosol, Liguria, Italy.

  3. 3.

    Reworked fragment of two juxtaposed clay coatings clearly distinguished in PPL due to their different colours (light and dark orange). Paleosol, Liguria, Italy.

  4. 4.

    Reworked fragment of a clay coating surrounded by microsparitic nodules. These two juxtaposed pedofeatures refer to two distinct and successive pedogenic processes. Paleosol in a doline infilling, Syria.

  5. 5.

    Dark amorphous iron-bearing nodule SeeSee Oxyhydroxide partially overlapped by a yellow clay coating. In this case, the relationship between the two pedofeatures refers to juxtaposition. Paleo-Luvisol, Piedmont, Italy.

  6. 6.

    Amorphous iron oxyhydroxide hypocoatings superimposed on large sparitic crystals of a calcium carbonate nodule. Paleosol in a doline infilling, Syria.

  7. 7.

    The groundmass has been first depleted (colour gradient from the side to the centre). A new clay infilling in its central part (bright orange) superimposes this depletion pedofeature. Paleosol, central Po Plain, Italy.

  8. 8.

    Oxyhydroxide hypocoating superimposes a phosphate coating (yellow in PPL and extinct in XPL). Medieval archaeological site from northern Italy.

File 67: Uncommon Features

The list of uncommon features encountered in soil thin sections could be very lengthy. In this section, only eight have been selected and concern specific minerals, forms of organic matter, and finally some micromorphological effects produced by termites and tropical trees. The examples chosen are sometimes confused with other features.

Captions from upper left corner to lower right corner.

  1. 1.

    Secondary calcium carbonate forming random crystal intergrowths of needles. The needle shape does not refer to needle-fibre calcite but to acicular crystals of aragonite . The presence of aragonite is due to the high concentrations of ions, such as Na+ and Mg2+ in the soil solution. Vertic Solonetz, Dead Sea, Israel.

  2. 2.

    Crystalline coatings of sodium silicate forming a palisade fabric , i.e. juxtaposed fans of magadiite (Sebag et al. 2001). These minerals form under high-alkaline conditions in brines. Chernozem, Lake Chad, Niger.

  3. 3.

    Soil developed on a marine limestone in which dark crystal intergrowths of iron oxyhydroxides formed . These crystal infillings have the rhombohedral shape of former dolomite crystals, which have been dissolved during an early stage of pedogenesis. Calcic Cambisol, Provence, France.

  4. 4.

    Sapric horizon from a peat bog soil in which less than one-sixth of the groundmass is recognizable as original plant material (Chesworth 2008). Only a few fragments can be ascribed to cells and fibres of plants; most of the thin section is composed of an undefined and brownish material, forming a micromass affected by zigzag planes , delimiting aggregates. Histosol, Jura Mountains, Switzerland.

  5. 5.

    River deposit with a coarse monic c/f related distribution. The long and black shards in PPL and extinct in XPL are fragments of graphite and not present-day humified organic matter. Fluvisol, Rhone Valley, Switzerland.

  6. 6.

    A specific pedofeature encountered in paleosols, mostly from the Tertiary, called Microcodium . These crystal intergrowths are fossil features and must not be confused with present-day or Quaternary calcified root cells . Their shape resembles a corn cob with a single layer of elongated cells around a hollow or infilled (recrystallized) central axis. Paleosol, Corbires Massif, France.

  7. 7.

    Perturbed soil reworked by termites . The groundmass includes quartz grains and the micromass is a mixture of calcite and diatomite , the former being brownish grey coloured and the latter extinct in XPL. Aggregates are also present. Kastanozem, Chobe Enclave, Botswana.

  8. 8.

    Biomineralization of iroko wood cells forming calcified cells. The micromass is micritic SeeSee Calcite with some large sparitic crystal intergrowths SeeSee Calcite . Such features are associated to the oxalate–carbonate pathway (Cailleau et al. 2005). Calcite layer in a Ferralsol, Biga district, Ivory Coast.