Original Research

Dynamics and genesis of calcic accumulations in soils and sedimentsof the Argentinean Pampa

Alsu Kuznetsova a,n, Olga Khokhlova b

a Department of Renewable resources, University of Alberta Edmonton, Canada, T6G 2E3b Institute of Physicochemical and Biological Problems of Soil Science RAS Puschino, Moscow region, 142290, Russia

a r t i c l e i n f o

Article history:Received 6 December 2013Received in revised form18 September 2014Accepted 19 November 2014Available online 6 July 2015

Keywords:Carbonate accumulationsGypsum accumulationsPedogenic rhizolithsTosca

a b s t r a c t

Micromorphology of calcic accumulations (calcite, whewellite and gypsum) and geochemical indiceswere considered as indicators of genesis and evolution of pedogenic accumulations in soils andpaleosediments of the Argentinean Pampa. Two groups of separate and independent calcic accumula-tions were studied using scanning electron microscopy: (i) in situ Argiudolls, reflecting the current soilformation; (ii) in the layers of calcrete (locally named tosca), reflecting the past environments andconditions of these layers sedimentation. New pedogenic gypsum accumulations in Argiudolls weredescribed and possible ways of their formation were suggested. Combined analyses of morphology ofcarbonate accumulations and geochemical indices in different horizons of Argiudolls and layers of toscashowed that the tosca is paleopedocomplex with complicated formation history. Influence of currentenvironment on tosca morphology is absent, so it is possible to use these pedofeatures for paleor-econstructions in further studying.& 2015 International Research and Training Centre on Erosion and Sedimentation/the World Association

for Sedimentation and Erosion Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

Argiudolls of the Argentinean Pampa are forming on a complexpedosedimentary sequences of paleosols and sediments of diverseorigins (Amiotti et al., 2001; Zarate et al., 2009; Kemp et al., 2006).Marine sediments, usually calcareous, were deposited on fine paludalsilts 128ka ago (Kemp et al., 2006). These marine sediments were thenoverlain by fine silt deposits of fluvial/paludal genesis (before 23kaago), after that, the eolian loess deposits were formed during severalsedimentary pulses since the Late Pleistocene (Pye, 1987). In the EarlyHolocene in an arid climatic conditions, active eoline and fluvialerosion processes dominated in the region (Amiotto et al., 2001;Blanco & Stoops, 2007) and the carbonate paleosurface was exhumed.In the Middle Holocene, a loess deposit buried the carbonate paleosur-face again (Amiotti et al., 2001) and with stabilization of loess mantle,pedogenesis and sedimentation processes (pulses eolian accretion)occurred simultaneously (Kemp et al., 2006). Because of this, theparent material for the surface soils (Argiudolls) is carbonate free loess

(a mixture of eolian and fluvially or colluvially reworked material), butunderlain by the carbonate layers which may have an influence on soilformation (Paez & Prieto, 1990; Zarate & Blassi, 1991; Rabassa, 1990;Amiotti et al., 2001).

The underlying layer for the majority of soils in the ArgentineanPampa is an almost continuous calcrete, locally termed tosca (2Ckm-petrocalcic horizon), that has great variability in depth, structure,degree of indurations, hardness and CaCO3 content (Blanco & Stoops,2007). Because of its high variability it is difficult to give an averagecharacteristic of the tosca (Pazos & Mestelan, 2002). Macromorpholo-gical structure of this horizon varies with depth and from site to site:tosca can be massive, layered or laminar; silica materials may also beembeded in a calcareous matrix (Buschiazzo, 1986, 1988; Pazos, 1990;Bedogni, 1996; Blanco & Stoops, 2007). High variability in CaCO3

content of the tosca layers was shown previously (Pazos, 1990;Bedogni, 1996; Amiotti et al., 2001) with CaCO3 contents ranging from7.5 to 52.4% for different sites. Sometimes the weathered tosca can bevery friable, with a very fine blocky structure and strong evidence ofmodern processes such as clay illuviation and CaCO3 dissolution andrecrystallization. In these cases, roots can freely develop through thetosca. But strongly indurated layers occur which cannot be penetratedby roots and a dense root mat forms on top of the tosca.

Genesis of tosca is still not well understood; most probably itis a thick Plio-Pleistocene layer formed by different cycles ofsedimentation and with upper part was transformed during

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International Journal of Sediment Research

http://dx.doi.org/10.1016/j.ijsrc.2014.11.0021001-6279/& 2015 International Research and Training Centre on Erosion and Sedimentation/the World Association for Sedimentation and Erosion Research. Published byElsevier B.V. All rights reserved.

n Corresponding author at: Soil Chemistry and Environmental icrobiologyResearch Laboratory, Department of Renewable Resources, University of Alberta,4-53 South Academic Building Edmonton, Alberta, Canada, T6G 2G7.Tel.:+1 780 492 4422.

E-mail addresses: alsu@ualberta.ca (A. Kuznetsova),alexkh1@sares-net.ru (O. Khokhlova).

International Journal of Sediment Research 30 (2015) 179–189

pedogenesis (Ammioti et al., 2001). Similar massive petrocalcichorizons were described throughout the south-west part of theUSA as unique set of stages V and VI calcic horizons and theirmorphology were used for paleointerpretations (Gile et al., 1966;Brock & Buck, 2009). The strong contrast in morphology betweenthe tosca and the overlying Holocene soils reflects the effect ofpolygenesis: different climatic conditions and evolution of land-scapes (Ammioti et al., 2001; Blanco & Stoops, 2007; Kuznetsova etal., 2010).

Morphology of horizons and pedogenic accumulations arevisually informative indicators of conditions during the period oftheir formation. For substantive characteristics of modern andpaleoconditions we used the geochemical methods, based on ratiosof macro- and microelements in soil mass (Nesbitt & Young, 1982;Gallet et al., 1996; Retallack, 2001; Pieter et al., 2004; Driese et al.,2005; Starr & Lindroos, 2006; Whitfield et al., 2006). The mainweathering indicators are indices of bases loss, leaching, oxidation,salinization and homogeneity (Retallack, 2001; Schilman et al.,2001). In many studies ratios of mobile elements and Ti or Zr wereused to identify enrichment/depletion of elements with time (Egli &Fitze, 2000). We can use some indices of physical and chemicalweathering as indicators of soil formation trends. The combinationof these ratios helps to assess the conditions of formation (pedo-genic and sedimentary) for each soil horizon and layer of tosca.

In the Argiudolls studied, secondary carbonate accumulations(hypocoatings and nodules) and/or inclusions of lithogenic carbonateshave been described in the lower horizons (Borrelli et al., 2009).Slow eolian addition of calcareous dust in some area or thelithogenic underlying limestone serve as source of carbonates forthe Argiudolls (Kemp et al., 2006). A biogenic calcite form-ation through calcium oxalate (whewellite СаС2О4 and weddelliteСаС2О4х2Н2О) by fungus hyphae also contributed to accumulationsinput (Osterrieth, 2005; Blanco & Stoops, 2007). Morphology of biogenic

calcite varies from needles and tubular crystals similar to fungal hyphaeto rounded and micritic forms (Verrecchia et al., 1993).

The objectives of this research is to determine the dynamics andconditions of formation of the pedogenic calcic accumulations in thesurface Argiudoll and underlying tosca and their interrelations againstthe background of recent and past geochemical processes. The majorfocus is sub-micromorphological investigation of calcic (calcite, whe-wellite and gypsum) pedofeatures and their relationship with geo-chemical indices in the Argiudoll horizons and tosca layers.

2. Methods

The profile used in this study was prepared for the 7th Interna-tional Meeting on Phytolith Research (December, 2008) and waslocated at the seaside in the south-east part of Buenos-Airesprovince, between Mar de Plata and de Mar Chiquita, Argentina(381400 S; 571100 W) (Fig. 1). This region supports subtropicalprairies; the climate is mesothermic and sub-humid, with little orno water deficiency and mean annual precipitation of about800dmm. During the year, the dry and humid seasons are stronglypronounced: winter and summer, accordingly. The mean annualtemperature is þ14 1С; the summer is warm (þ20 1С), the winteris cool (þ8 1С). The area is used for agriculture purposes. TheArgiudolls are the typical soils for the Southeastern Pampas with ablack 40–50 cm thick A horizon and a dark brown more than 50 cmBt horizon (Borrelli et al., 2009). The parent material is darkyellowish brown loess. Some properties of this profile weredescribed in other papers (Borelli et al., 2009; Borrelli et al., 2010;Kuznetsova et al., 2010). The organic matter content is high,approximately 9.39% in the A horizon, 3% in the B horizon, and1% in the parent material (Borelli et al., 2010). The pH values areslightly acid (6–6.3) in the A horizon, and neutral (6.8–7) in the

Fig. 1. Location of the study site (black point): PP – Pampas's prairies.

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parent material. The soil texture is silty loam, with the silt (55–70%)and clay (20–30%) fractions being the most representative. CaCO3

content is low even in the lowermost horizons (o1%). Our studywas emphasized on detailed investigation of calcic accumulationsand their relationship with geochemical indices.

The typical Argiudoll is formed on loess underlain by an almostcontinuous calcrete – tosca, where CaCO3 content varies from 20%to 50 %. Samples were selected from two horizons of the Argiudoll,and from six horizons of the tosca. The samples were collectedtwice, in dry warm and humid cool seasons; the locations ofsamples are shown in Fig. 2 and in Table 1. Undisturbed freshlyfractured internal surfaces of samples were examined with bino-cular and scanning electron microscope Jeol JSM-6380LA equippedwith an energy-dispersive spectrometer (EDS JED-2300). Sampleswere coated with gold in Eiko-3 ion coater before analysis. Theelement composition of bulk samples was determined by X-rayfluorescence analysis (XRF) of fused discs using X-ray spectro-meter SPECTROSCAN MAX-GV. Element ratios or indices wereused to determine the degree of heterogeneity of the parentmaterials and to assess leaching and enrichment of elements.

3. Results and discussion

3.1 Micromorphological and submicromorphological observations

The major data on micro- and submicromorphological observa-tions are presented in Table 2.

3.2 Pedogenic accumulations in ArgiudollUnder the SEM, we exposed pedofeatures that were not described

in Argiudolls previously: gypsum crystalline spheroliths and collo-form films. The gypsum crystalline biogenic accumulations formedby cyanobacteria were described at the boundary of the humic A andВt1 horizons, (Figs. 3 and 4). In modern soils endolithic cyanobac-teria communities have been described in the gypsic soil curst ofvarious deserts of the world; they form laminae inside a gypsichorizon and support the biodiversity as endolithic species (Dong etal., 2007; Garcia-Pichel et al., 2001). The location of the colonies inthe soil is determined by the structure of the gypsic horizon whichcontrols movement of water, light and oxygen. Cyanobacterialcommunity form complex bi-sedimentary structures reflected thedynamic genetic history and formative processes: mineral precipita-tion, wetting–drying cycles etc. (Kamennaya et al., 2012).The directparticipation of stromatolite by cyanobacteria is also a well knownphenomenon (Zavarzin, 2003; Dong et al., 2007).

A review of studies of gypsic cyanobacteria accumulations ondifferent substrates and under natural and laboratory conditions leadus to suggest that such accumulation are possible in soils. In modelingexperiments with pure cyanobacterium culture the precipitation ofthe calcite (СаСО3), trona (Na3H(CO3)2х2(H2O)), halite (NaCl) crystalsby individual cyanobacterial cells surrounding itself was shown

Table 1Samples location and horizon morphology.

Horizon Depth fromthe top

Samples collected in drywarm season

Samples collected in wetcool season

Horizon morphology

The border of hor. A and Bt1(Argiudoll)

40 cm D1 M8 and M11 Very friable, porous, dark-grey with whitish spots and fibers.

Bt2k (Argiudoll) 60–65 cm D2 M7 Light-pale, loess with vertical fractures and root/mesofauneholes.

2K1 (top of tosca) 110 cm – M6 Whitish transitional horizon, hard and dense, a few inclusionsof silica material.

2K2 (tosca) 130 cm – M5 and M9 Whitish more friable and softer than 2K1, with thin films-cutans on aggregates.

2K3 (tosca) 150–160 cm – M4 and M10 Whitish, with laminae carbonate-silica material.2K4 (tosca) 170–185 cm – M3 Carbonate crust, grayish-white, very dense.2K5 (tosca) 195 cm – M2 Whitish sample divided on different size carbonate

accumulations with silica material.3K (tosca) 205 cm D3 M1 Whitish sample, with carbonate-silica material.

Fig. 2. Scheme of profile and samples location.

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(Howell et al., 2005; Mikhodyuk et al., 2008). We found similarprecipitation of gypsum crystals on cyanobacteria cells (Fig. 3b).Braithwaite and Whitton (1987) have shown the formation ofstalactite-like structures from halite and gypsum having a layeredstructure: inside, around cyanobacteria colonies is gypsum, further outis a mixture of gypsum and halite and then pure halite. They did notfind evidence for a direct relationship between cyanobacterial meta-bolism and mineral crystallization, but suggested the indirect physicalcontrol by organic mucilage. Probably, in Argiudolls there is the samemechanism of spheroid accumulations formation. The cyanobacteriacolonies have a spherical form built up around crystal crust. Thesecrystals have a short life span, forming in the dry warm season but aredissolved and are rare in the wet season. Actually, in the samplescollected after the wet cool season, the spheroid accumulations aredramatically changed: gypsum crystals and spheroids become imper-fect and semi-dissolved and they are observed in the micro-pits andcracks only (Fig. 4). Hence these gypsum spheroid accumulations varyconsiderably from season to season. The dominant microbial commu-nities of cyanobacteria are replaced by diatoms. In competitionbetween the cyanobacteria and diatomic communities, diatom species

dominated all types of sediments at 10 1C and 15 1C and thecyanobacteria were more abundant at 25 1C (Watermann et al.,1999). We assume more favorable conditions for cyanobacteria exist-ing were created in summer.

Calcite accumulations in the Argiudoll are very weak colloformfilms or separate tiny crystals in the silica material at the boundarybetween A and Bt1 horizons (Fig. 5a) and individual rare large calcitecrystals and druses in the Bt2k horizon visible also under a lightmicroscope (Fig. 5b). There are no visible carbonate accumulations inthe soil for naked eye and the CaCO3 content is also low throughoutthe profile. It is connected with the absence of carbonates in theoriginal loess. All accumulations (films and crystals) in Argiudollhorizons are formed by processes of dissolution-precipitation ofunderlying tosca carbonates because parent material for Argiudoll iscarbonate free loess (Pye, 1987; Amiotti et al., 2001). We assume thatduring the wet season, water penetrates through the Argiudoll profiletill the hard tosca and dissolves a few carbonates or that a suspensionof colloidal-size carbonates in water is formed. Then, during the dryperiod, the solutions or carbonate suspensions move upward alongcapillary pores and after evaporation of the water, the carbonates re-

Table 2Micromorphology of horizons and accumulations.

Horizon Depthfrom thetop

Samplescollected in dryseason

Samplescollected in wetseason

Micromorphology of horizons and accumulations

The border of the Aand Bt1 hor(Argiudoll)

40 cm D1 – The bright-white thin powder, which weakly reacts with 10% HCl, was observed. The whitepowder coating is separated on spheroliths from 10 to 100 μm in diameter formed byperfect prismatic crystals (Fig. 3a). EDS data have shown that spheroliths are consist ofgypsum crystals, characteristic external shape also testifies it. Sometimes these spherolithsare connected in chains (Fig. 3b) and have small hollows on spherical surfaces. The gypsumspheroliths raise slightly above general surface of the sample. Between the crystals, thethin colloform (cryptocrystaline) films of calcite on aluminosilicate (Fig. 3c) have beenobserved. On the surface of the sample threads and congestions of cyanobacteria are noted.Small individual gypsum crystals on some cyanobacteria are observed (Fig. 3d).

40 cm – M8 and M11 Morphology of accumulations in the samples is slightly different than in D1 sample,collected in dry season. The sharp round form of gypsum spheroliths changed to round-likesmoothed ones without perfect crystals (Fig. 4a). The sizes of spheroliths are near 50–100 μm; they are located only in small pores and cracks. In these samples, there areabundant of two groups of diatom communities: Luticola sp. and Pinnularia borealis(Fig. 4b). These are usual soil diatomic communities in different regions.

Bt2k (Argiudoll) 60–65 cm D2 – It reacts with 10% HCL very poorly. Thin discontinuous carbonate tubules which have beenfound out under binocular, are very fragile and easily disintegrated. Under SEM the sampleis consisting of a mixture of aluminosilicate grains “pure” and covered by thin calcite films(Fig. 5a). Cyanobacteria are not revealed, there are a few separate hyphas only.

60–65 cm – M7 The most part of samples is silica material. The thin carbonate films on Si-grains are rare. Atthe some parts the Ca-oxalate tubules (6–10 μm in diameter) lay as a mat. In addition, large(5–10 μm) calcite crystals, druses and colloform films were observed (Fig. 5b).

2K1 (top of tosca) 110 cm – M6 Abundant of perfect needle carbonate crystals around the pores (Fig. 6a) and separatedimperfect crystals on surface of silica material are present. The Ca-oxalate and calcitetubules (6–10 μm in diameter) with very sharp and clear edges “kidney-shaped” crystalsare situated only in holes.

2K2 (tosca) 130 cm – M5 and M9 The mixture of imperfect needles and rhomboids crystals that broken and etched; therhizoliths-tubules are present as well but they are smaller (4–5 μm in diameter) andarranged by thin crystals (Fig. 6b). The carbonate colloform films are cover some surface.The most part of tubules and imperfect crystals are broken and recrystallized (Fig. 6c).

2K3 (tosca) 150–160 cm

– M4 and M10 Surface of sample is the strong recrystallized calcite. The carbonate colloform films andperfect needle-crystals are absent. The rare calcite tubules are formed by imperfect crystals.Some tubules are dissolved and broken (Fig. 6d).

2K4 (tosca) 170–185 cm

– M3 Most part of the sample is crystalline: “kidney-shaped” calcite tubules (o7 μm indiameter); needle and imperfect crystals (Fig. 7a); the colloform films are absent

2K5 (tosca) 195 cm – M2 The wide diversity of calcite forms is present: abundant tubules similar with those in theprevious samples but different sizes (5–30 μm in diameter), more needles and lessrhomboid crystals (Fig. 7b). Most part of the sample is covered by colloform film of calcitewith the crystals which have grown onto it.

3K (tosca) 205 cm D3 – The matrix is a mechanical mixture of silica grains (with thin colloform films) andimperfect scattered calcite crystals inside (Fig. 7c). In small holes calcite crystals transformsto a colloform film which smooth out a relief.

3K (tosca) 205 cm – M1 There are abundant tubules (10 μm in diameter) made with “kidney-shaped” crystals andthe separate crystals (the most of them are rhomboid, needles are less). The most part ofsample is covered by colloform film of calcite. All forms of carbonate crystals are imperfect;there are abundant signs of recrystallization and dissolving (Fig. 7d).

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precipitate as thin colloform films or separate crystals (sometimes,druses). According to our previous studies, the powder carbonates andcarbonate filaments (mini-tubes) are the usual forms of carbonateaccumulations in soils on loess (Khokhlova et al., 2001; Khokhlova &Kouznetsova, 2004). Carbonate colloform films precipitate from satu-rated colloidal solutions during seasonal change of water regime(Kuznetsova & Khokhlova, 2010).

The thin tubules composed of Ca-oxalate observed in the Bt2khorizon in the wet season may be explained on the basis of fungalactivity in accordance with the concept proposed by Verrecchiaet al. (1990). In the sample collected during the dry season, weobserve rare remains of tubules only, they are practicallydestroyed. It is clear evidence of their low resistance to theunfavorable environmental conditions.

In the Argiudolls all pedogenic accumulations are formed bycurrent soil processes. Gypsum spheroliths and Ca-oxalate tubulesare formed by the microbiological community and change duringthe year; calcite colloform films and crystalline accumulations areformed by chemical dissolution-precipitation processes.

3.3 Accumulations in toscaThe main accumulations that we observed in tosca were

neddle-crystals and tubules – rhizoliths (Figs. 6 and 7). Klappa(1980) defined these as “organo-sedimentary structures resultingin the preservation of roots of higher plants, or remains thereof, inmineral matter”. Rhizoliths form relatively rapidly in exposedcalcareous sediments under suitable conditions, but the

Fig. 3. SEM-photos and microprobe analyses (marked by squares) of sample D1: (a) the gypsum spheroliths (some are marked by arrows) on the aluminosilicate matrix;(b) the gypsum spheroliths are connected in chains; (c) the thin collomorphic (cryptocrystalic) film of calcite (is marked by arrow) on aluminosilicate; (d) the chain ofgypsum spheroliths formed by perfect crystals (some are marked by arrows).

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mechanisms of calcification of rhizoliths remain unclear (Wright,1994). It is possible that uptake of water by the roots is the maincause of precipitation, but the area near a root is the site ofcomplex biological and biochemical processes which may be theactual causes of carbonate precipitation. Brock and Buck (2009)described this type of accumulations in petrocalcic horizons atMormon Mesa (south west USA) as calcified fungal hyphae. Theabundance of such tubules suggests that the tosca was once a soilhorizon. As suggested by Klappa (1980), tubules are evidence ofpaleo-root systems which underwent mineralization, i.e. differentlayers of tosca were the horizons of soils at various periods:pedogenic and sedimentological processes changed each other'sand tosca was formed as complex pedosedimentary sequences(Blanco & Stoops, 2007). Similar model of morphological develop-ment of petrocalcic soils was shown by Robins et al. (2012) as fivemajor abiotic processes: atmospheric deposition; dissolution andinfiltration; precipitation; dissolution of primary grains and re-precipitation. Relationships between calcite and others mineralsare controlled by these processes. Ca-oxalate tubules that occur inthe top layer of tosca are replaced by calcite tubules in low layers.This process of rapidly destroying metastable whewellite byoxidation for semi-arid regions was described by Verrecchiaet al. (1990, 1993) as diagenesis.

Although the shape and size of tubules are similar in differentlayers, we noted some differences on the sub-microlevel. Three typesof these carbonate tubules were distinguished: (i) tubules formedwithneedle crystals (Fig. 7b); (ii) tubules formed with “kidney-shaped”

roundish crystals (Fig. 7d); (iii) tubules covered by carbonate colloformfilm (Fig. 7c). We assume that the conditions of their formation wereconnected with their morphology. Stepanov (1970) have describedinfluence of the kinetics of crystallization on morphology. Withincreasing saturation of solution (or, what is the same, rate of crystal-lization), calcium carbonates are crystallized in the followingsequence: (i) large perfect crystals (very low degree of saturation);(ii) small imperfect crystals (medium degree of saturation); (iii)acicular and skeletal crystals (high degree of saturation); and (iv)

Fig. 4. SEM-photos of the samples M8 and M11 (Argiudoll): (a) gypsum spheroliths(some are marked by arrows) without sharp round form and perfect crystals;(b) abundant of diatom community Luticola sp.

Fig. 5. SEM-photos and microprobe analyses (marked by squares): (a) thincarbonate collomorphic films (marked by arrow) on aluminosilicate grains insample D2; (b) large crystals (cr), druses (dr) and collomorphic films (cf) insample M7.

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suspension of small skeletal crystals in the case of volume crystal-lization (very high degree of saturation). In the M5 and M2 layers,tubules formed with needle-like crystals: perfect, tiny needle crystalswere formed from saturated solutions and a stable temperatureregime (Sunagawa, 1999). According to Wright (1984) preservationof needle-fiber calcite in soils indicates that the weakness of pedogen-esis and/or arid to semiarid climate. In the M6, M3 and M1 layerstubules made of “kidney-shaped” crystals formed, most likely fromdiluted solutions, but also in a stable temperature without sharpchanges in concentration (Stepanov, 1970).

The tubules coated by colloform film (M1, M9, M10 and D3layers) were formed in the conditions with significant seasonalfluctuations of temperature and water regime (Kuznetsova &Khokhlova, 2010). The differences between samples are con-nected with the variations in the conditions during the forma-tion of different layers and in the initial content of CaCO3 andsilicate materials in the sediments. In these samples there was afast re-crystallization of carbonates from strongly saturated

solutions, as such, most of the material did not move butprecipitated in situ.

The abundance of tubular rhizoliths is related with past periods oftosca when it was exposed on surface and was well-permeable forroots. Due to the gradual deposition of carbonate material the lowerlayers were buried; then they hardened and became impervious forroots. So in the buried layers of tosca ideal conditions for preservationof the carbonate accumulations were created.

3.4 Geochemical indices

Data of bulk chemical composition is necessary for geochemicalindices calculations. We used several geochemical indices forinterpretation of genesis and sedimentary conditions of differenthorizons in the section studied. A summary Fig. 8 shows theintensity of each index; the relationships of the indices with somepedofeatures and horizons are shown in Table 3.

Fig. 6. SEM-photos and microprobe analyses (marked by squares) of tosca samples: (a) perfect needle carbonate crystals around the pores in sample M6; (b) imperfectcrystals and broken tubules in sample M5; (c) wide variety of carbonates forms in sample M9; (d) recrystallized tubules and different forms of crystals in sample M4.

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The morphology of described calcic accumulations is well corre-lated with geochemical indices. In the A/B horizons of the Argiudoll,where carbonate accumulations are rare and weakly expressed,indices of weathering, calcification and salinization reflect humidclimatic conditions (Fig. 8 and Table 1). In contrast, the same indices(weathering (Ti), calcification and salinization) have the lowest valuesin the tosca 2K1 and 2K5 layers. The morphology of carbonateaccumulations in those two layers is indicative of the weakness ofpedogenesis and/or drier climate conditions as well. The perfect thincrystals of carbonate accumulations could only form in conditions ofundisturbed precipitation without excess of water. Herewith the 2K1,2K2 and 2K5 layers are the least weathered (indices of oxidation,leaching and chemical weathering (Sr)), i.e. pedogenesis processeswere weaker than in other tosca horizons. It might be related withmore arid conditions or with less period of pedogenesis theyfunctioned as soil horizons.

The index of hydromorphism for the samples is stronglycorrelated to the morphology of the calcic accumulations: in layerswith the highest value of this index (3K, 2K4, 2K3, 2K1) most ofthe carbonate accumulations are covered by collomorphic films,rare crystals that are imperfect and with signs of dissolution.Stoops and Delvigne (1990) have shown large, coarse grainaggregates of calcite forms in the relatively moist zones of thesoils of the tropics. The value of the index of hydromorphism inthe 3K horizon is so high that we regard it as hydromorphic

horizon; but abundance of carbonate accumulations indicate thathydromorphic conditions existed only periodically.

In the uppermost tosca 2K1 layer, the geochemical indicesconnected with humidity/aridity differ from each other: indices ofweathering, calcification and salinization are high and testify thatthe horizon formed in dry conditions; the index of hydromorphismis high as well and testifies that the horizon formed in humidconditions. We consider that horizon has undergone the diageneticchanges after burial because of recent soil formation. It wasreflected in the presence of Mn in the 2K1 layer as an indicator ofbiological productivity. At the same time the morphology ofcarbonate accumulation is characterized by perfect needle carbo-nate crystals and sharp tubules proper for arid conditions.

Geochemical indices show that the А/В horizon of the Argiu-dolls is highly weathered, so all the processes of element redis-tribution in the top horizons are certainly faster than in the Btkhorizon and obviously the Btk horizon is less weathered. Thegeochemical indices in tosca layers are proof that carbonatesaccumulation of different layers were formed gradually in differ-ent periods under different conditions: at first calcareous sedi-ments were deposit and, probably, dissolution, infiltration,precipitation processes were occur, then next layer deposited onsurface and processes started again in new layer. Simultaneousanalyses of the morphology of carbonate accumulations and thegeochemical indices for bulk soil samples allowed for division and

Fig. 7. SEM-photos of tosca samples: (a) tubules – rhizolits with signs of recrystallization in sample M3; (b) the diversity of calcite forms: tubules – rhizolits, needles andrhomboid crystals in sample M2; (c) the aluminosilicate grains mix with chaotically scattered crystals and druses and uncontinued collomorphic film in sample D3; (d) roottubules – rhizolits (tb) made from “kidney-shaped” crystals and collomorphic film of calcite (cf) on the most part of sample M1.

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characterization these layers. We agree with authors who assumedthat the relic tosca in the Argentinean Pampa have formed mainlyin an arid climate and was then exhumed after the erosion ofpreexisting soils of the Late Pleistocene (Blanco & Stoops, 2007),but we suggest that the history of tosca formation was morecomplicated. To summarize, we consider that the Argiudoll andthe 2K3, 2K4, 2K5, 3K layers of tosca were formed mainly inrelatively humid conditions; the 2K1, 2K2 and 2K5 layers wereformed mainly in relatively arid conditions. But 2K1 layer hasfeatures of humid conditions because of recent influences, diage-netic for the buried layer.

4. Conclusions

Two groups of calcic accumulations in the section studied areoccurring: (i) in the Argiudolls, reflecting the current soil formation;(ii) in the layers of tosca, reflecting the paleoenvironments and howthese layers sedimentation formed. These accumulations were formedin different conditions and time and do not have a significant influenceon each other. Modern climate and water regime do not causechanging of accumulations in tosca. In the Argiudolls, calcic accumula-tions of different genesis are revealed: (i) spheroid gypsum accumula-tions of biogenic origin on the lower boundary of the A horizon;

0 0.5 1 1.5

A/B

BtCa

2K1

2K2

2K3

2K4

2K5

3K

Al2O3/(CaO+Na 2O+K2O+MgO)0.0 20.0 40.0 60.0 80.0

A/B

BtCa

2K1

2K2

2K3

2K4

2K5

3K

CIA0.00 0.12 0.24

A/B

BtCa

2K1

2K2

2K3

2K4

2K5

3K

Rb/Sr

0 0.5 1 1.5

A/B

BtCa

2K1

2K2

2K3

2K4

2K5

3K

Ba/Sr0 0.005 0.01 0.015 0.02

A/B

BtCa

2K1

2K2

2K3

2K4

2K5

3K

MnO/Al2O3

0 5 10 15 20

A/B

BtCa

2K1

2K2

2K3

2K4

2K5

3K

(CaO+MgO)/Al2O3

0.0 2.0 4.0 6.0

A/B

BtCa

2K1

2K2

2K3

2K4

2K5

3K

Na2О/K2O 0 0.5 1 1.5

A/B

BtCa

2K1

2K2

2K3

2K4

2K5

3K

(Na2О+K2O)/Al2O3

0 0.02 0.04 0.06

A/B

BtCa

2K1

2K2

2K3

2K4

2K5

3K

TiO2/Al2O

Fig. 8. The geochemical indices in the different horizons of Argiudolls (dark bars) and in the layers of tosca (light bars).

A. Kuznetsova, O. Khokhlova / International Journal of Sediment Research 30 (2015) 179–189 187

(ii) thin Ca-oxalate tubules in Bt horizon; (iii) carbonate colloformfilms on a surface of aluminosilicate matrix in all soil horizons.Formation of the gypsum spheroliths is an accumulation of crystal“crusts” by spherical cyanobacterial colonies; these gypsum cyano-bacterial spheroliths have seasonal dynamics of formation and occur-rence in the soil studied. They are formed and exist in the profile onlyduring the dry seasons, whereas in the humid seasons, they are partlydissolved and/or even disappear. The thin Ca-oxalate tubules formwith fungal activity. The colloform films precipitate from dilute andcolloidal solutions when water moves upward from the underlyingtosca and evaporates in the surface soil horizons.

Different layers of tosca have different kinds of calcic accumu-lations. In the uppermost tosca layers Ca-oxalate tubules areabundant. Their occurrence and preservation are connected withthe recent root activity. In the lower tosca layers, the tubules arerecrystallized and decomposed and calcite tubules are predomi-nating.The different morphological forms of calcite and whewellite(tubules-rhizoliths, colloform films, association of separate crystalsand druses) and distinction of geochemical indices in the toscalayers allows interpreting the tosca as paleopedocomplex withcomplicated formation history: some layers formed in drier con-dition than others.

Acknowledgments

We sincerely thank Dr. M. Osterrieth for organization of thefield trip and providing some samples.

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Table 3Geochemical indices and their variability.

Name ofindex

Formula of index Soil/sediment features that reflected by index Value in the studied section

Chemicalindex ofalteration

[Al2O3/(Al2O3þCaOsilikatþNa2OþK2O)]n100

Index represents a ratio of primary and secondary minerals in the bulksamples (Nesbitt & Young, 1982) and is based on the relationship of thenon-mobile aluminum to the mobile alkali metals and alkaline earths.

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Index of basesloss(hydrolysis)

Al2О3/(СаОþNa2OþK2OþMgO)

Index represents the ratio of Al2O3 (clay component) to the main cationsput off in the soil solutions (Retallack, 2001).

The maximum is in the А/В horizon, thenmuch smaller for the 2К3 layer, all otherare substantially less.

Index ofchemicalweathering(Rb)

Rb/Sr Index proposes on the basis of difference in the stability of various mineralsto weathering, namely, micas and feldspars, with which the association isRb, and carbonates, which is associated with Sr (Gallet et al., 1996).

The maximum is in the А/В horizon, andthen a little less for the 3K layer, theminimal value is in the 2К2 layer.

Index ofleaching

Ba/Sr Index represents leaching of mobile elements Ba is associated withK-feldspars and is removed from the soil weaker than Sr, which isassociated with carbonates (Gallet et al., 1996).

The highest and very close values are inthe A/B, Btk horizons and 2К4 layer.

Index ofoxidation

Fe2O3þMnO/Al2O3 Index characterizes intensity of the soil material oxidation (Retallack, 2001) The maximum value is in the 3K layer,then is in 2К3 and 2К4, and theminimum ones are in the 2К2 and the2K5 layers.

Indices ofoxidation

MnO/Al2O3; Fe2O3þMnO/Fe2O3

and MnO/Fe2O3

Indices represent biogenic productivity in automorphic conditions andoxidation – in hydromorphic ones (Pieter et al., 2004)

Value in the 3K layer is the highest andsharply different from other horizons.

Index ofcalcification

CaOþMgO/Al2O3 Index shows the accumulation of pedogenic calcite and dolomite (Retallack,2001).

The highest values are in the 2K1 and2K5 layers and the lowest ones in the A/Band 2K3 layers

Indices ofsalinization

Na2O/K2O, (K2OþNa2O)/Al2O3,Na2O/Al2O3

Indices show the migration of soluble salts in soil (Retallack, 2001; Retallacket al.,2003).

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Indices ofweathering(Ti)

TiO2/Al2O3 and Zr/TiO2 Indices characterize a degree of homogeneity of the parent material(Schilman et al., 2001)

The material of the different horizons andlayers in the section is heterogeneous.

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