Soil mineral genesis and distribution in a saline lake landscape of the Pantanal Wetland, Brazil

Geoderma 154 (2010) 518–528

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Geoderma j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o d e r m a

Soil mineral genesis and distribution in a saline lake landscape of the Pantanal Wetland, Brazil S.A.C. Furquim a,⁎, R.C. Graham b, L. Barbiero c, J.P. Queiroz Neto a, P. Vidal-Torrado d a

Departamento de Geografia, Universidade de São Paulo, São Paulo, Brazil Soil & Water Sciences Program, Department of Environmental Sciences, University of California, Riverside, USA Laboratoire de Mécanisms et Transferts en Géologie, UMR 5563 CNRS-IRD-UPS-OMP, Toulouse, France d Departamento de Ciência do Solo, Universidade de São Paulo, Piracicaba, Brazil b c

a r t i c l e

i n f o

Article history: Received 20 August 2008 Received in revised form 6 January 2009 Accepted 20 March 2009 Available online 23 April 2009 Keywords: Nhecolândia Salinity Mineral neoformation Carbonates Mg-smectite Fe-mica

a b s t r a c t The origin of the saline lakes in the Pantanal wetland has been classically attributed to processes occurring in past periods. However, recent studies have suggested that saline water is currently forming from evaporative concentration of fresh water, which is provided annually by seasonal floods. Major elements (Ca, Mg, K) and alkalinity appear to be geochemically controlled during the concentration of waters and may be involved in the formation of carbonates and clay minerals around the saline lakes. The mineralogy of soils associated with a representative saline lake was investigated using XRD, TEM-EDS, and ICP-MS in order to identify the composition and genesis of the secondary minerals suspected to be involved in the control of major elements. The results showed that Ca, Mg, and K effectively undergo oversaturation and precipitation as the waters become more saline. These elements are incorporated in the authigenically formed carbonates, smectites, and micas surrounding the saline lake. The control of Ca occurs by precipitation of calcite and dolomite in nodules while Mg and K are mainly involved in the neoformation of Mg-smectites (stevensitic and saponitic minerals) and, probably, iron-enriched micas (ferric–illite) in surface and subsurface horizons. Therefore, our study confirms that the salinity of Pantanal, historically attributed to inheritance from former regimes, has a contribution of current processes. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The Pantanal wetland, located in the central area of South America (16°–20°S and 58°–50°W), is the largest complex of continental wetlands in the world, with an area of about 200,000 km2 (Por, 1995) (Fig. 1A). Characterized by low altitudes (b200 m) and extremely low gradients (0.02 to 0.03°), the wetland is a still active alluvial plain, partially covered by seasonal flooding mainly in the summer (November to March) (Silva, 1986; Scott, 1991). The Quaternary sediments underlying the plain are mainly sandy alluvial deposits of the Paraguai River and tributaries. Most of the flooding water and the deposited sediments are carried from surrounding highlands (200 to 900 m), which are formed by Precambrian crystalline and Mesozoic sedimentary rocks (Del'Arco et al., 1982; Alvarenga et al., 1984; Godoi Filho, 1986; Por, 1995). In spite of the recurrent presence of water, the wetland presents an annual hydrological deficit of 300 mm, resulting from a mean annual rainfall of 1100 mm and a mean annual evapotranspiration around 1400 mm (Alvarenga et al., 1984; Por, 1995).

⁎ Corresponding author. Tel./fax: +55 11 30913794. E-mail address: [email protected] (S.A.C. Furquim). 0016-7061/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2009.03.014

The Nhecolândia sub-region (27,000 km2), located in the centralsouthern portion of the Pantanal, is distinguished by the existence of about 7000 fresh water lakes (“baías”) and 1500 alkaline–saline lakes (“salinas”), generally associated with temporary channels (“vazantes”) and sand hills (“cordilheiras”) (Fig. 1) (Cunha, 1943; Brum and Sousa, 1985; Mourão et al., 1988; Fernandes, 2007). The fresh water lakes are temporary or permanent water bodies with different forms and dimensions and up to 2 m deep. Coalescence of fresh water lakes during the flooding periods forms the temporary channels, which are several kilometers long and up to 30 m wide. The alkaline–saline lakes are round depressions with permanent salty water, generally 500 to 1000 m in diameter and 2 to 3 m in depth. They are located inside the sand hills, which are narrow (200–300 m wide) and elongated stripes of dryland, 2 to 3 m higher than other landscape features and covered by savanna vegetation. Different hypotheses have been addressed to explain the origin of these Nhecolândia landforms and the subject is still a matter of debate. A set of explanations invokes eolian processes occurring in the last arid phase (Late Pleistocene) as being the responsible for the formation of sand hills and lakes. In this view, sand hills are seen as paleo-longitudinal dunes (Klammer, 1982) and fresh water and/or saline lakes are wind deflation depressions (Almeida and Lima, 1956; Tricart, 1982; Soares et al., 2003; Assine and Soares, 2004). Saline lakes

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were specifically defined by Tricart (1982) as old sebkhas, where the eolian deflation was facilitated due to aggregation of fine particles by salts accumulated on the soil surface. Another hypothesis attributes the origin of sand hills and lakes to the shifting of river courses, which occurred during and after the more humid climate prevailing during deglaciation (Early Holocene). In this context, lakes would be inundated depressions located inside cutoff meander banks or isolated by embankments while sand hills would develop through transformation of shortcut meanders banks into elongated sand ribbons. The distinction between fresh water and saline lakes would depend on the recurrent input of fresh water from seasonal floodings (Wilhelmy, 1958; Ab'Saber, 1988). Eiten (1983) provided a different interpretation, attributing lake genesis to relict karstic or pseudo-karstic systems. Although these studies referred to different processes, they all agree with a past origin of the saline lakes and other typical Nhecolândia landforms. However, recent geochemical studies involving surface and subsurface waters of Nhecolândia suggested that saline water has currently arisen from evaporative concentration of subsurface fresh water (Barbiero et al., 2002, 2007). The water concentration seems to be triggered by the presence of a subsurface impervious green horizon surrounding the saline lakes, which prevents the groundwater outflow through the sand hill during the dry season, thereby retaining the water so it is concentrated by evaporation. The authors show that fresh and saline waters belong to the same chemical family and suggest that most of the chemical compositional changes in the system are related to precipitation of some elements (mainly Ca, Mg, K) as solutions become more saline. The chemical elements controlled in this way are suspected to be involved in present-day authigenic formation of carbonates (calcite or Mg-calcite) and silicates (stevensite, sepiolite, Mg-montmorillonite, or amorphous) in the soils surrounding the saline lakes. The identification of the minerals supposed to be responsible for the control of chemical elements

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during water concentration should provide insight into the role of current processes in the origin of salt waters in Nhecolândia. A detailed mineralogical investigation of the soils surrounding a representative alkaline–saline lake (Salina do Meio, Fig. 1B) was carried out by our research team, the results of which are beginning to be disseminated (e.g. Furquim et al., 2008). In spite of the extensive number of saline lakes in the Nhecolândia, our studies represent a pioneering effort in understanding the silt and clay mineralogy in these landscapes. In this paper, we further elucidate the linkages between soil mineralogy, geomorphology, and geochemistry of these lakes by providing a comprehensive view of the composition and genesis of the fine-grained minerals in the Salina do Meio landscape. 2. Study area Soils were sampled along a toposequence (T1) extending from the border of the local sand hill to the surface level of Salina do Meio (Figs. 1B and 2B). The surface water level of the saline lake varies throughout the year according to the watertable dynamics (Fig. 2B): from November to March, the level is high and the water is close to the border of the sand hill; from April to October, the level goes down, exposing most of the lake shore. Thus, the field work was done at the peak of the dry season, when the soils were at maximum exposure. The morphological and analytical properties of the soil cover along T1 (Fig. 2B) are summarized below and more fully discussed by Furquim et al. (2008). The soil consists of a pale brown (10YR 6/3), single grain, sand surface material (horizon 1), which is laterally replaced toward the lake by a dark gray (10YR 4/1), mostly prismatic, loamy sand, surface material, rich in decomposed algae (horizon 2). Beneath this, there is an organic-enriched, grayish brown (10YR 4/2), massive horizon with a sand texture (horizon 3). This, in turn, overlies a light brownish gray (2.5Y 6/2), single grain horizon, with a sand texture and abundant vertical organic-rich volumes (horizon 4). The deepest

Fig. 1. A — Location of Pantanal wetland and Nhecolândia sub-region; B — Aerial picture showing the main landscape features of Nhecolândia, the studied toposequence (T1), and the water survey transect.

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Fig. 2. A — Location of piezometers (Pz) and watertable samplers (G) in the water survey transect (Barbiero et al., 2007); B — Soil horizons and pH values along toposequence 1 (T1).

horizons consist of a gray (2.5Y 5/1), massive, loamy sand material, with common presence of millimeter- to centimeter-diameter nodules (horizon 5), and a greenish (5Y 5/2), massive, sandy loam material, which has rare to common presence of nodules and is indurated by amorphous silica (horizon 6). The amounts of clay in the soil range from 3 to 20% and are mainly concentrated in horizons 2 (8.5%) and 6 (10 to 18%). Soil pH varies from 5.4 to 11 along the toposequence, but 70% of the measurements are higher than 10. Gray and greenish horizons (5 and 6) are generally the most alkaline, with pH values up to 11 (Fig. 2B). Electrical conductivity (EC) values display a wide range (0.3 to 43 dS m− 1), but are higher toward the saline lake. Na is the main cation on exchange sites in all horizons, ranging from 3 to 190 cmolc/kg. Preliminary soil morphology surveying in different places of Nhecolândia showed that soil organization of T1 is representative of the saline lakes. The sand surface horizon (1) and the subsurface gray (5) and green (6) horizons were identified around all the surveyed saline lakes (Sakamoto, 1997; Barbiero et al., 2000; Fernandes, 2000; Silva and Sakamoto, 2003; Silva et al., 2004) while the dark surface horizon (2) and the light sand subsurface horizon with vertical organic volumes (4) were found in most of the studied saline lakes (Sakamoto, 1997; Silva and Sakamoto, 2003; Silva et al., 2004). It is

important to highlight that the green and indurated horizon typically associated with saline lakes is the feature pointed out by Barbiero et al. (2007) as being responsible for the isolation of groundwater around saline lakes during the dry season, allowing the evaporative concentration of waters. The geochemistry study of surface and subsurface waters of a transect involving T1, the adjacent sand hill, and a neighboring fresh water lake (Figs. 1B and 2A) was carried out by Barbiero et al. (2007). The results showed that fresh waters (piezometers Pz5 to Pz15 and samplers G7 to G15, Fig. 2A) were of Na–K–HCO3 + CO3 type, whereas saline waters (Pz2, Pz3, G0 to G6, and lake, Fig. 2A) were of Na–HCO3 + CO3 type. Similar to the regional water conditions measured by Barbiero et al. (2002), the geochemical control of Ca, Mg, and K in the more saline waters of the transect suggested the involvement of these elements in the formation of minerals around Salina do Meio (Fig. 3; A, B, and C). Equilibrium diagrams show that saline waters of the transect are at equilibrium or slightly oversaturated with respect to Mg-silicates (saponite, stevensite, and sepiolite) (Fig. 3; D and E), attesting that those minerals could originate from these waters. The chemical characteristics of water samples collected specifically along T1 (Pz2, G0, and G1) are listed in Table 1. These results include alkaline pH, high EC (mainly G0 and lake), high K

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Fig. 3. A, B, and C— Concentration diagrams showing the evolution of Ca, Mg, and K during the concentration of waters in the transect involving Salina do Meio. Na amounts represent the concentration factor. The solid line denotes the simulation of evaporation with precipitation of Mg-calcite and a Mg-silicate (sepiolite). D and E — Diagrams showing equilibrium conditions in solution for different minerals (Barbiero et al., 2007).

method (Nelson, 1982) and total C was determined by dry combustion (Nelson and Sommers, 2001). Organic C was calculated by the difference between total C and inorganic C, the last derived from calcium carbonate equivalent values. Fine silt (2–20 μm), coarse clay (0.2–2 μm), and fine clay (b0.2 μm) fractions of the 20 collected soil samples were separated by centrifugation and sedimentation following destruction of organic matter with NaOCl (pH 9.5) (Anderson, 1963). These fractionated samples were used in the following analyses. XRD was carried out for the 3 fractions of all the 20 samples. Silt and clays underwent 5 treatments: Mg saturation (52% relative humidity), ethylene glycol (EG) solvation of Mg-saturated clays, air dry K saturation, and heating of the K-saturated samples at 350 °C and 550 °C (Jackson, 1979). Glass slides of oriented specimens were

content (mainly G0 and lake), and low contents of Al. Silica concentrations are lower than 100 mg/L, but increase toward the saline lake. 3. Mineralogical methods Twenty whole soil samples and 17 nodules were collected along T1 from horizons of 5 pedons (P1 to P5) exposed by trenches (Fig. 2B). Soil samples and nodules were crushed with a mortar and pestle and passed through a 100-mesh sieve. These powdered materials were randomly packed in an aluminum sample holder and submitted to X-ray diffraction (XRD) for identification of carbonate minerals and measurement of d060 values of phyllosilicates. Calcium carbonate equivalent of the ground samples was determined by a manometric

Table 1 Characteristics of waters from Salina do Meio and surrounding watertable (cations with 2 repetitions).

pH EC Eh Temp. HCO3 + CO3 Cl− Si(OH)4 Al3+ Mg2+ Ca2+ Fe Na+ K+

– dS/m – °C mEq/L mmol/L

mg/L

3.2098 0.0007 1.0813 6.2313 0.0826 – 34.6911

Pz2

G1

G0

Salina do Meio

7.66 6.19 − 25.2 31.1 29.73 21.58

8.66 8.76 − 217.0 31.2 26.49 15.78

9.25 19.18 − 364.5 34.2 194.08 76.17

9.64 12.74 − 125.0 40.2 67.62 66.64

3.5155 0.0007 1.1843 6.8249 0.4466 – 37.9959

36.9463 0.0196 0.0356 1.4463 0.0319 982.2820 230.1342

EC: electrical conductivity; Temp.: temperature; Pz.: piezometer; G: watertable samplers.

38.6839 0.0594 0.0372 1.4720 0.0439 1028.479 240.9576

51.4686 0.0039 0.2202 4.2313 0.0120 5827.8413 1094.8175

52.3721 0.0040 0.2241 4.3056 0.0342 5930.1401 1114.0353

97.6426 0.0514 2.2365 8.5382 0.0553 – 945.9906

89.5579 0.0272 2.0513 7.8313 0.0880 – 867.66

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prepared by smearing a paste of clay or dropping a water–silt slurry on the slide (Theisen and Harward, 1962). All the XRD analyses were performed in a Siemens D-500 diffractometer (CuK radiation with graphite crystal monochromator), employing a step size of a 0.02° 2θ and a count time of 1.0 s per step. The XRD peak intensities from the EG- and Mg-treated clays were used to calculate the semi-quantitative amount (%) of each mineral in the assemblages (Biscaye, 1965), and to estimate the relative content of iron in the mica structure, using the formula I(001) / I(002), where I(001) and I(002) represent, respectively, the intensity of the d001 and d002 mica peaks (Brown and Brindley, 1980; Huggett et al., 2001). Results higher than 2 indicate a relatively large Fe-content in the octahedral sheet (Brown and Brindley, 1980; Deconinck et al., 1988). Quantitative analyses of major elements (Si, Al, Fe, Mg, Ca, Na, K) and rare earth elements-REE (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y) were carried out on the fine-clay fraction of 18 samples by inductively coupled plasma-mass spectrometry (ICP-MS), using a Perkin-Elmer equipment, model Elan 6000. The fine-clay fraction of 5 selected samples was analyzed using a FEICM300 transmission electron microscope (TEM) linked with a Phoenix energy-dispersive X-ray microanalyser (EDS). Six crystals of kaolinite (P1—horizon 6, P5—horizon 6), 6 crystals of smectite (P1—horizon 6; P4—horizon 2; P5—horizon 2), and 1 crystal of mica (P3—horizon 6) were examined. For analysis, a dilute suspension containing the fine clay was dropped on a standard Cu grid with carbon film. The chemical composition of individual crystals was determined by EDS for each sample. The collected data were used to calculate the chemical formulas of the individual crystals.

patterns of these carbonate-enriched nodules show 3 carbonate minerals: calcite (CaCO3), dolomite (CaMg(CO3)2) and/or nahcolite (NaHCO3) (Fig. 4). Carbonate precipitation has been observed in soils that are frequently subjected to evaporative concentration of solutions (Fehrenbacher et al., 1963; Mahjoory, 1979; Kohut and Dudas, 1995). Alkaline earth carbonates, such as calcite and dolomite, are generally the first minerals to precipitate in the concentration process. Calcite generally contains less than 5%mol of Mg(CO3) because it usually precipitates from more diluted solutions, which typically have low Mg/Ca ratios (b1). However, this ratio increases as calcite formation continues, allowing the subsequent precipitation of Mg-carbonates such as magnesite and dolomite (Eugster and Hardie, 1978; Boettinger and Richardson, 2001). As already stated, the concentration diagrams displayed in Fig. 3 (A and B) show a trend of decreasing concentration of Ca and Mg in more saline waters of the transect involving Salina do Meio (Barbiero et al., 2007), which is in agreement with the regional results obtained by Barbiero et al. (2002). The geochemical control of these elements in more saline waters suggests their involvement in the precipitation of Ca- and Mg-carbonates in the soils associated with Salina do Meio, inasmuch as the solubility limits of these minerals are the first to be exceeded in the concentration process. The presence of calcite in the surface horizons within the area of seasonal lake-level variation (P3 to P5) and the occurrence of calcite- and dolomite-enriched nodules confirm the prediction made by water analyses and support the neoformation of these carbonate minerals around the saline lake. 5. Clay minerals

4. Carbonates The studied soil samples generally have very low contents of calcium carbonate equivalent, with values ranging between 0.2 and 3.2%. The greatest values (1.2 and 3.2%) occur in the surface horizon 2 of P4 and P5. Consistent with this, XRD analysis revealed the presence of calcite (CaCO3) in the fine silt of the surface horizons of P3, P4 and P5 (Fig. 4). Most of the analyzed nodules have much higher calcium carbonate equivalents than the whole soil samples. Values vary from 0.2 to 44%, but 65% of the nodules have CaCO3eq amounts greater than 6%. XRD

X-ray diffraction patterns indicate an overwhelming dominance of quartz and kaolinite in the coarse clay, and the presence of smectite, mica, kaolinite, and quartz in the fine clay. The composition and genesis of the 3 detected clay minerals are presented below. 5.1. Kaolinite Kaolinite occurs in all the analyzed horizons of the toposequence in both coarse and fine-clay fractions. Semi-quantitative calculations

Fig. 4. XDR patterns showing the presence of carbonates in fine silt samples (air dry K-saturated peaks) and nodules (b 100-mesh powder). Identified minerals: Q — quartz; M — microcline; C — calcite; D — dolomite; N — nahcolite.

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Fig. 5. Percentages of clay minerals in coarse and/or fine-clay assemblages along T1.

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estimate 3 to 77% of kaolinite in coarse clay assemblages, with no trend of vertical or lateral distribution along the toposequence (Fig. 5). Fine-clay assemblages contain 4 to 53% of kaolinite, with a general decrease toward the saline lake (P5) (Fig. 5). TEM/EDS results were used to calculate structural formulas of 6 individual kaolinite crystals. One representative formula is listed in Table 2 (number 1). In all the analyzed specimens, tetrahedral sheet is formed by Si, with no Al substitution, and octahedral sheet is formed by Al and Fe3+. Aluminum ranges from 1.43 to 1.81 atoms per unit formula on the basis of 14 negative charges (apuf/14) and Fe3+ varies from 0.08 to 0.25 apuf/14, but the values are ≥0.13 in 5 of the 6 crystals. Iron substitution of octahedral Al in kaolinites is commonly less than 0.04 apuf/14 (Newman and Brown, 1987), which qualifies the studied specimens as iron-rich kaolinites. The octahedral sum is slightly lower than 2.00 apuf/14 in all formulas probably because of the absence of Na in the results, which is due to an interference of Cu from the Cu grid of the TEM samples holder (Hover et al., 1999). TEM images show that the analyzed kaolinite crystals are hexagonal or pseudo-hexagonal shaped, with angular or subangular vertices and smooth to rough surfaces (Fig. 6; A and B). Some evidence suggests that kaolinite is allochthonous in the study area. First, stability diagrams available in the literature showed that kaolinites are stable under low values of pH–pK and pSi(OH)4 at room pressure and temperature, typical conditions of acid, welldrained, and leached soils (Tardy et al., 1973; Bohn et al., 1985; Velde, 1995). Soils associated with Salina do Meio are strongly alkaline, poorly drained, and have high base saturations, the opposite conditions for the ideal formation of this mineral. Second, neoformed clays tend to concentrate in the fine-clay size and kaolinites of the study area are abundant in both coarse and fine-clay fractions. Therefore, the studied kaolinites were probably transported by the tributaries of the Paraguai River and deposited in the alluvial plain along with the dominant sand particles. The source area of Nhecolândia sediments is a tropical highland known as “Planalto de Table 2 Oxides and structural formulas of clay minerals and amorphous specimen. 1

2 Upper zonea

P5-hor 6 SiO2 Al2O3 Fe2O3 MgO CaO MnO K2O Cl−

53.8 39.7 5.1 0.0 0.0 0.0 1.5 0.0

P1-hor 6 52.3 35.0 10.0 1.3 0.8 0.0 0.6 0.0

3

4

5

6

Lower zonea P4-hor.2 62.0 7.0 6.3 17.7 1.2 0.0 1.6 1.3

P5-hor. 2 60.7 10.7 4.2 17.1 1.3 0.0 0.9 2.4

P3-hor.6 54.3 27.7 8.2 2.9 0.0 0.0 4.8 0.0

P3-hor. 6 73.8 10.2 9.1 2.8 0.0 0.9 4.1 0.0

Si4+ Al3+ ∑Tetrahedral

2.04 0.00 2.04

3.18 0.82 4.00

3.96 0.04 4.00

3.87 0.13 4.00

3.43 0.57 4.00

– – –

Al3+ Fe3+ Mg2+ ∑Octahedral

1.78 0.15 0.00 1.93

1.69 0.46 0.04 2.19

0.49 0.30 1.69 2.48

0.67 0.20 1.63 2.50

1.49 0.39 0.12 2.00

– – – –

K+ Ca2+ Mg2+

0.07 0.00 0.00

0.05 0.05 0.08

0.13 0.08 0.00

0.07 0.09 0.00

0.39 0.00 0.15

– – –

O5 4.00 0.00 0.16 − 0.24 − 0.08 0.07

O10 2.00 0.00 − 0.82 0.51 − 0.31 0.31

O10 1.86 0.14 − 0.04 − 0.25 − 0.29 0.29

O10 1.74 0.26 − 0.13 − 0.12 − 0.25 0.25

O10 2.00 0.00 − 0.57 − 0.12 − 0.69 0.69

– – – – – – –

Anions OH− Cl− Tetrahedral charge Octahedral charge Total charge Interlayer charge

Hor.: horizon. a Zones of smectite concentration.

Maracaju”, where soils are acid, leached Psamments (Orioli et al., 1982), ideal for kaolinite formation. Iron-rich kaolinites are very commonly formed in tropical soils (Herbillon, 1976; Newman and Brown, 1987) and this may be the case in “Planalto de Maracaju”. Experiments of kaolinite dissolution under room temperature and pressure show very high concentrations of dissolved Si(OH)4 and Al in strongly acid (≤ 3) and strongly alkaline conditions (≥10) (Huertas et al., 1999). Because most of the soil and water pH values are strongly alkaline in the Salina do Meio toposequence, kaolinite is probably undergoing dissolution and releasing Si, Al, and Fe into solution. The pseudo-hexagonal crystals of kaolinite with subangular vertices and rough surfaces are probably evidences of this weathering process. 5.2. Smectites Smectites in the Salina do Meio toposequence have been thoroughly studied by Furquim et al. (2008) and the most relevant aspects are presented here. Based on semi-quantitative estimates for the fineclay fraction, substantial percentages of smectite are present in two zones of the toposequence (Fig. 5): an upper zone, formed by all horizons of P1 and P2; and a lower zone, formed by surface horizons 1 and 2 of P3, P4, and P5. The results indicate the occurrence of different smectite minerals and mechanisms of genesis in these zones. 5.2.1. Smectites from the upper zone In this zone, XRD 00l patterns show a random interstratification of smectite with mica and vermiculite (Fig. 7) while 060 XRD peaks (0.149– 0.150 nm) indicate a dioctahedral domain in the fine-clay samples. Si, Al, and Fe3+ are the most abundant major elements of these samples, as indicated by ICP-MS analysis, with amounts varying from 49 to 63% for Si, 15 to 23% for Al (Fig. 8), and 5 to 10% for Fe3+. Results from TEM-EDS analysis of a single flake-shaped smectite crystal from P1—horizon 6 (Table 2, number 2) agree with the ICP-MS analysis in that they yielded a structural formula with high tetrahedral Al (0.8 apuf/22) and dominance of Al (1.7 apuf/22) and Fe3+ (0.5 apuf/22) in the octahedral sheet. These characteristics qualify the analyzed specimen as a ferribeidellite (Güven, 1988; Reid-Soukup and Ulery, 2002). This crystal probably has some hydroxyl-Al, -Fe in the interlayer position, raising the octahedral sum above 2.00 apuf/22 (Weaver and Pollard, 1973). The following characteristics suggest that these dioctahedral smectites originated from transformation of micas: 1) interstratification with mica, as showed by 00l XRD patterns (Fig. 7); 2) high Al tetrahedral substitution in the structural formula (Table 2, number 2), as expected for smectites transformed from micas (Borchardt, 1989); 3) dominance of iron-rich micas in the studied toposequence, as discussed in the section on micas (Section 5.3); and 4) interstratification with vermiculite (Fig. 7), which is commonly an intermediate phase in the transformation of mica toward smectite in soils (Crawford et al., 1983; Badraoui et al., 1987; Ransom et al., 1988). 5.2.2. Smectites from the lower zone In this zone, all 060 XRD patterns display a strong peak of 0.152 nm, revealing a trioctahedral domain in the fine-clay samples. ICP-MS results show dominance of Si (44–51%) and Mg (13–15%) among major elements (Fig. 8) and indicate much lower amounts of REE than samples from the upper zone (Fig. 9). Major element contents are corroborated by the calculated structural formulas of 5 flake-shaped smectite crystals. Two of these formulas are listed in Table 2 (numbers 3 and 4) and a representative picture of one crystal is displayed in Fig. 6 (C). In all formulas, tetrahedral Al is low (0–0.14 apuf/22) and Mg is the main cation in the octahedral sheet (1.49 to 2.30 apuf/22). Substitution of Al for Si in the tetrahedral sheet is zero or near zero in two crystals (e.g. Table 2, number 3) and between 0.12 and 0.14 apuf/22 in the other 3 crystals (e.g. Table 2, number 4). These are typical characteristics of the Mg-rich endmembers stevensite, which has near-zero Al substitution, and

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Fig. 6. TEM pictures of: A— Kaolinite crystal with angular vertices and smooth surfaces; B— Kaolinite crystals with subangular vertices and smooth to rough surfaces; C: Smectite crystal with flake shape; D: Mica crystal with lath shape; E: Amorphous silica-rich phase with mineral crystallization.

saponite, which presents slightly higher Al substitution (Weaver and Pollard, 1973). However, because of the presence of certain properties not found in stevensite and saponite, such as very low octahedral sum and Cl substitution for OH, the studied specimens are designated as stevensitic and saponitic minerals. The considerably lower octahedral sum is mainly attributed to the presence of a dioctahedral component in the crystals and, to a lesser degree, to the absence of Na in the analysis due to an interference of Cu grids of TEM samples holder (Hover et al., 1999). Some lines of evidences show that stevensitic and saponitic minerals originate by direct precipitation from the saline lake: 1) minerals formed by chemical precipitation from waters have much lower REE values than inherited or transformed minerals (Torrez Ruiz et al., 1994; Jamoussi et al., 2003), and this is true of the Mg-smectites of the studied area (Fig. 9); 2) the concentration diagram displayed in Fig. 3(B) shows that Mg precipitates during the concentration process,

potentially as Mg-smectites; 3) equilibrium diagrams reveal that saline waters of the transect at Salina do Meio are at equilibrium or oversaturated with respect to Mg-silicates (saponite, stevensite, and sepiolite) (Fig. 3; D and E), attesting that these minerals can precipitate from the saline waters; and 4) horizons having Mgsmectites occur only on the surface and exactly within the area of seasonal lake-level variation (Fig. 5), indicating that these minerals are directly associated with the lake dynamics.

5.3. Micas XRD patterns from oriented clays show that mica is present in all horizons analyzed for the toposequence. The higher percentages (67 to 84%) clearly occur in the gray (5) and green (6) horizons of the pedons nearest to the saline lake (P3, P4, and P5) (Fig. 5).

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Fig. 9. Sum of REEs in upper and lower zone samples.

Fig. 7. Example of smectite peaks behavior under different treatments. Mixed-layer mica smectite is identified in the upper zone (P1, horizons 1, 3, 5, and 6; P2, horizons 1, 5, and 6) by the partial coalescence of the d001 mica and smectite peaks in K (22 °C) and/or Mg patterns and a clear separation of these peaks in the EG patterns. Mixed-layer vermiculite–smectite is recognized in this zone by total or partial collapse of smectite peaks to 10.0 Å in K-saturated samples (22 °C) (P1 and P2, horizons 1 and 6).

XRD 001 diffraction peaks suggest an interstratification of mica with smectite layers in all samples. This interpretation is based on very broad and asymmetrical mica peaks from Mg- and/or K-saturated clays and a decrease of width and asymmetry of mica peaks accompanied by a better definition of smectite peaks upon EGsolvation of the Mg-clays (Fig. 10). Results of the I(001) / I(002) index shows values higher than 2, suggesting that all fine-clay samples contain Fe-rich micas. TEM-EDS analysis of one lath-shaped mica crystal from P3— horizon 6 (Fig. 6D; Table 2, number 5) confirms the trend pointed out by the I(001) / I(002) index. The calculated structural formula of this crystal defines Al (1.5 apuf/22) and Fe3+ (0.4 apuf/22) as the main

cations of the octahedral sheet. This amount of octahedral Fe3+, intermediate between illite and glauconite minerals, qualifies the studied specimen as a ferric–illite (Fe-illite) (Porrenga, 1968; Baker, 1997; Hugget et al., 2001). Abundant crystallites associated with Sirich amorphous materials were also detected in the analyzed sample (Fig. 6E; Table 2, number 6). Although research about this mica is still underway, the following water chemistry (Barbiero et al., 2007) and mineralogy data suggest an authigenic formation: 1) geochemical control of K in the more saline waters of the transect involving Salina do Meio (Fig. 3C) shows that the involvement of this element in the precipitation of micas is possible; 2) synthesis experiments under room conditions carried out by Harder (1974) verified neoformation of mixed-layer illite–smectite from waters that had similar chemical characteristics to those of the study area, such as alkaline pH, Si(OH)4 concentration lower than 100 mg/L, and high K concentration; 3) the mechanism of neoformation observed by Harder (1974) involved initial precipitation of amorphous hydroxides from solutions, followed by mineral crystallization after aging. The presence of amorphous materials associated with mica crystals in the sample investigated by TEM-EDS might be evidence of the same mechanism of mica neoformation in the study area. Further investigation will probably give more consistent information about genesis of mica around Salina do Meio. 6. Conclusions The current evaporative concentration of waters is promoting precipitation of Ca, Mg, and K from the more saline solutions and their consequent participation in authigenic mineral phases (carbonates, smectites, and probably micas) around Salina do Meio. The

Fig. 8. Relationship between Si%–Al% and Si%–Mg% in the upper and lower zone samples.

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historically attributed to inheritance from former regimes, has a contribution of current processes. Acknowledgments The authors wish to thank Capes for the doctoral fellowship to S.A.C.F. and for the financial support to the research through a CapesCofecub cooperation (412/03); Dr. Arnaldo Sakamoto, Dr. Rosely Pacheco Dias Ferreira, Dr. Sônia Furian, Dr. Nádia Nascimento, MS Ary Tavares, undergraduate and graduate students of the University of Mato Grosso do Sul (UFMS), and employees of the Nhumirim Farm for their assistance in the field; Paul Sternberg, Marcos Pinheiro, and Luís Silva for general help with the laboratory work; and Dr. Krassimir Bozhilov for TEM assistance. References

Fig. 10. Example of mica peaks behavior under different treatments: broad and asymmetrical mica peaks on Mg- and/or K-saturated patterns, decrease of width and asymmetry of mica peaks and better definition of smectite peaks on EG patterns.

involvement of Ca and part of the Mg in the precipitation of carbonates is confirmed by nodules containing up to 44% of CaCO3eq, consisting of calcite (CaCO3) and dolomite (CaMg(CO3)2). The major control of Mg, however, occurs by precipitation of Mg-smectites (stevensitic and saponitic minerals) within surface horizons (1 and 2) of the area of seasonal lake-level variation (P3 to P5). Potassium probably participates in the neoformation of Fe-micas in the deepest horizons (5 and 6). The dissolution of kaolinites, which are allochthonous in the study area, probably increases the supply of Si, Al, and Fe of the solutions, contributing to the neoformation of the other clays. The identification of these mineralogical processes within the soils surrounding the studied alkaline–saline lake corroborates the hypothesis formulated by Barbiero et al. (2002, 2007) that chemical variation between fresh and saline waters of Nhecolândia is related to mechanisms triggered by present-day evaporative concentration of fresh waters. Therefore, our study confirms that the salinity of Pantanal,

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