3.1. Study Area and Sampling
The study areas are located on the south and southwestern parts of the Taquari alluvial fan, at Barranco Alto
farm, near to Aquidauana/MS, and Nhumirim
) farm, near to Corumbá/MS, respectively (Figure 1
). Three field campaigns were conducted in different seasons. The first was at Barranco Alto
farm, from 26 to 30 June 2017 at the beginning of the dry season. The second was at Nhumirim
farm, from 20 to 24 August 2018, and the third again at Barranco Alto
farm, from 17 to 21 September 2018. The last two campaigns were scheduled to happen at the end of the dry season, however, despite the low rate of rainfall in the region during the wet season and similar rainfall rate during the wet season in 2017–2018, the lakes had not dried and the water level was significantly higher than in 2017.
In Barranco Alto
farm, we studied twelve lakes and the Negro
River. In Nhumirim
farm, we studied five lakes and the Paraguay
River. In each lake and river, we collected samples of water and sediments following a sequence extending from the border of the lakes to their central part (P1, P2, P3 and P4), resulting in two to four collection points. When it was necessary, the sampling points were divided into bottom, middle and top samples (Figure 2
). We collected 160 samples of sediment, but only 95 were analyzed: 31 from the first campaign, 38 from the second and 26 from the third campaign. The samples were chosen according to the amount of sand and clay.
The water temperature (WT °C), the pH, the EC (mS cm−1
) and the TDS (mg L−1
) were measured in situ with a multiparameter equipment. The lakes were classified adapting recent criteria, which take functional biogeochemistry, pH and EC values to classify them as saline, oligosaline and freshwater lakes [25
]. In this work, we differentiate the lakes based on the names proposed by the authors; however, we used the TDS values as the main parameter to differentiate them. According to the TDS data, fresh water has less than 500 mg L−1
and we considered in this work, based on the data obtained on the field campaigns, the oligosaline lakes with TDS values between 501 and 1500 mg L−1
and the saline lakes with values higher than 1500 mg L−1
3.3. Mineralogical Analyses
The mineralogical analyses were carried out at Instituto LAMIR (Laboratório de Análises de minerais e rochas
) of the Geology Department at Federal University of Parana. The composition of the sediments was determined by means of X-ray diffraction (XRD) through a PANalytical diffractometer model Empyrean with an X-celerator detector. Scans from bulk sediments and clay minerals were run from 2θ angles of 3° to 70° and 3° to 30°, respectively, using a step-size of 0.016° and count time of 10.16 s per step [27
]. A total of 74 bulk sediment samples and 69 clay mineral samples were analyzed, since some of the bulk sediments were composed solely by sand.
To analyze the mineralogy of the clay minerals, approximately 30 g of the bulk sediment was sifted in a 350-mesh sieve with distilled water. The solution was firstly centrifuged for 7 min at 800 rpm to decant the coarser fraction. The supernatant fluid was transferred to another plastic tube and centrifuged again for 30 min at 3000 rpm. The resulting mixture was poured on a glass slide covering at least 1 cm2
of the glass slide. The samples were air-dried under room temperature and pressure (STP) to obtain oriented analyses with three different treatments: air-dried sample, ethylene-glycol vapor saturation for 8–12 h (EG) and heating at 550 °C for 2 h [27
The clay minerals were identified mostly by their 001 peak after the three treatments cited above. The dioctahedral and trioctahedral character of smectites were identified by their 060 diffraction /peaks. Table 2
shows some clay minerals and their respective 001 and 060 peaks. Sepiolite and palygorskite present 110 reflections at 12.0–12.3 and 10.4–10.5, respectively.
There is a wide variation in 060 diffraction peaks among dioctahedral and trioctahedral smectites. In dioctahedral smectites, for instance, montmorillonite and beidellite show d060 at 1.49–1.50 Å, while nontronite shows d060 at 1.52 Å [28
]. In trioctahedral smectites, stevensite and saponite present 060 at 1.52 Å and hectorite presents d060 at 1.53 Å.
Mixed-layered clay minerals are very common in natural environments [2
]. The identification of mixed-layered minerals is based on the entire diffraction pattern, like breadth, symmetry, intensity and peak position [28
]. Illite/smectite (I/S) are the most common mixed-layered clay minerals in sedimentary rocks and soils. They can be recognized by their altered diffraction pattern under EG solvation treatment and after heating the sample to 375° for 1 h, resulting in a pattern similar to that of illite [28
Non-clay minerals are very common in sedimentary rocks, such as quartz, feldspar, zeolites, carbonates, apatite, pyrite, gypsum and others [28
]. Beyond the carbonates, calcite and dolomite are normally associated with clay minerals. The d-spacings of the most intense peak of calcite and dolomite are 3.04° and 2.89° 2θ, respectively [28
]. The empirical curve of a calcite-disordered dolomite solid-solution series was used to identify the amount of mol% MgCO3
]. The semi-quantitative XRD percentages of each sample were obtained through the Rietveld multi-phase standard analysis, performed in the HighScore Plus PANanytical Software (Version 3.0, Malvern, Amsterdam, The Netherlands).
A thin section of a crust sample was examined under transmitted light using the Zeiss Imager A2m Microscope (Carl Zeiss Microscopy, New York, NY, USA) aiming to observe the contact relations between grains and cements.
Images and chemical composition of specific points in 10 samples were collected using a JEOL scanning electron microscope (SEM, JEOL Ltd., Peabody, MA, USA) model 6010LA, equipped with an energy dispersive X-ray spectrometer (EDX, JEOL Ltd., Peabody, MA, USA) EX-94410T1L11 for elemental analyses. The morphology of illite crystals consists of ribbon-like flakes projections, as well as filamentous pore-lining and pore-bridging for authigenic illites [30
]. The kaolinite occurs as stacks of pseudohexagonal plates or blocks and vermiform crystals. Smectite consists of a webby or highly-crenulated pore lining and thin ribbon of pore-bridging morphologies [30
Images of one selected fine-clay fraction sample were obtained by transmission electron microscopy (TEM) using a JEOL JEM 1200EX-II instrument (JEOL Ltd., Peabody, MA, USA) operated at 120 kV (medium resolution) at Centro de Microscopia Eletrônica (CME) in the Federal University of Parana. Images, qualitative analyses and quantitative chemical analyses of 6 samples collected in 2017 were performed by scanning tunneling electron microscopy (STEM) with a FEI TITAN G2 instrument operated at 300 kV (high resolution—HRTEM) at Centro de Instrumentación Científica (CIC) in Granada University (Spain). The chemical composition was determined by analytical electron microscopy (AEM) in the HRTEM and the data were used to calculate the chemical formulae.
The HRTM analyses usually show illite particles as straight and relatively defect-free lattice fringes, with continuous and constant 10 Å interlayer spacings, with a mottled contrast, whereas smectite particles show anastomosing and imperfect 14 Å lattice fringe images [31
]. AEM is a technique for quantitative chemical analysis of crystals of clay minerals, where data can be interpreted by phase diagrams [2
]. Muscovite, albite, biotite, spessartite, olivine and titanite were used to obtain k-factors to the correct energy dispersion X-ray data by the thin-film method [34
]. Errors for analyzed elements (two standard deviations) expressed in percentage of the atomic proportions are 6 (Na), 3 (Mg), 2 (Al), 4 (K), 4 (Ca), 5 (Ti), 3 (Mn) and 3 (Fe). Instrumental conditions for spectra acquisition were 200 s of live time for all elements except for K and Na, for which a time of 30 s was used due to volatilization problems.
Contents of the major oxides of 92 bulk-sample sediments were measured by means of X-ray fluorescence (XRF) using a PANalytical spectrometer model AXIOS MAXDY 5297 (Malvern, Amsterdam, The Netherlands), through quantitative analyses at Instituto LAMIR (Laboratório de Análises de minerais e rochas) of the Geology Department at the Federal University of Parana.