Iron oxide mineralogy in Mollisols, Aridisols and Entisols from southwestern Pampean region (Argentina) by environmental magnetism approach

Catena 190 (2020) 104534

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Iron oxide mineralogy in Mollisols, Aridisols and Entisols from southwestern Pampean region (Argentina) by environmental magnetism approach Marcos A.E. Chaparroa,

T

⁎,1

, María del Pilar Moralejob,c, Harald N. Böhneld, Silvia G. Acebalc

a

Centro de Investigaciones en Física e Ingeniería del Centro de la Provincia de Buenos Aires (CIFICEN, CONICET-UNCPBA), Pinto 399, 7000 Tandil, Argentina Instituto de Química del Sur (INQUISUR, CONICET), Av. Alem 1253, 8000 Bahía Blanca, Argentina c Departamento de Química, Universidad Nacional del Sur, Av. Alem 1253, 8000 Bahía Blanca, Argentina d Centro de Geociencias - UNAM, Boulevard Juriquilla No. 3001, 76230 Querétaro, Mexico b

A R T I C LE I N FO

A B S T R A C T

Keywords: Magnetic properties Magnetite/hematite mixtures Pedogenesis Selective dissolution techniques Soils

Ferromagnetic compounds of seven southwestern soils from Pampean region comprise iron oxides and oxyhydroxides that have an important role, because of their high surface reactivity, controlling soil adsorption and formation/conservation of soil structure among other properties. We detected minutes of iron oxides quantities by selective dissolution techniques: acid oxalate extraction (< 12% of the total Fe) and dithionite-citrate-bicarbonate (< 44% of the total Fe), moreover, we identified/quantified mixtures of magnetite, maghemite, goethite and hematite minerals by environmental magnetism approach. The magnetic particle size dependent parameters evidence a relationship with extractable Fe oxides, indicating that magnetic particle size distribution on these soils is a relevant property associated to poorly crystalline and free amorphous Fe oxides. In addition, the particle size and SP concentration of ferromagnetic iron oxides are associated with increments of cation exchange capacity and the specific surface area of soil samples. The estimated amounts of hematite are below 2.20 wt%, and lower contents of about 0.25 wt% correspond to magnetite components. Both magnetic iron oxides concentration, as well as the remanent coercivity, increase towards the western region, on the contrary, anhysteretic ratios decrease and hence relatively coarser magnetic particles are dominant in this western region. The present study provides valuable data to characterize Aridisols, Entisols and Mollisols in a region of high fertility that comprises ones of the world’s most productive agricultural soil region.

1. Introduction Soils are complex heterogeneous systems formed, during decades to millennia, under the influence of organisms, water and air from parent materials. During soil formation processes, Fe is released from Fe-containing minerals and hence iron oxides and oxyhydroxides may be formed and transformed. They are prominent soil components because of their high surface reactivity. Iron oxides in soils are of extremely small crystal size and/or low crystal order, as well as of low concentration (only tens g/kg in most soils) that depends on the type and Fe content of the parent rock and on the maturity of the soil. As soil develops, more and more of the original Fe-bearing minerals decompose and most of their Fe is precipitated as pedogenic Fe oxides (Cornell and Schwertmann, 2003). The characterization and quantification of these species are relevant for understanding adsorption–desorption processes and the formation

of soil structure dominating aggregates. Fe oxides can act as binding agents promoting organo-mineral complexes, result in electrostatic binding between positively charged oxides and negatively charged clay minerals, or produce surface coating of clay mineral (Six et al., 2004). Fe, Al and Mn oxides are involved in adsorption–desorption processes. Fe oxides and oxyhydroxides (magnetite, maghemite, hematite and goethite) are present at low concentrations occurring with varying degrees of crystallinity and Al substitution, Al3+ occurs in Al-substituted sites in soil iron oxides and Mn oxides are scarce and very sensitive to pH changes (Acebal et al., 2000). Selective chemical-dissolution techniques, physico-chemical analyses, Mössbauer spectroscopy, and X-ray diffraction are commonly applied to identify soil iron oxides and oxyhydroxides. Moreover, environmental magnetism techniques have become a very useful tool in order to investigate and understand processes occurring in different environments (Maher et al., 1999; Evans and Heller, 2003; Chaparro

⁎

Corresponding author at: CIFICEN, CONICET-UNCPBA, Pinto 399, B7000GHG Tandil, Argentina. E-mail address: [email protected] (M.A.E. Chaparro). 1 Scopus ID: 26425631300. https://doi.org/10.1016/j.catena.2020.104534 Received 19 April 2019; Received in revised form 17 February 2020; Accepted 24 February 2020 0341-8162/ © 2020 Elsevier B.V. All rights reserved.

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et al., 2006a; Bidegain et al., 2013; Quijano et al., 2014; Jordanova et al., 2018; Pulley et al., 2018). Although these iron oxides and oxyhydroxide compounds have been extensively studied in different soil types all over the world by numerous authors (Campbell and Schwertmann, 1984; Simón et al., 2000; Torrent et al., 2006; Grison et al., 2016; Poggere et al., 2018), there is little magnetic, chemical and mineralogical data available on iron oxide characterization in soils from Buenos Aires province in Argentina (Aguirre, 1987; Mijovilovich et al., 1998, Acebal et al., 1997, 2000, 2003; Bartel et al., 2005, 2011; Orgeira et al., 2008; Bidegain et al., 2009; Fernández et al., 2015). Environmental magnetism approach was combined with physicochemical analyses and selective chemical-dissolution techniques in order to give a soil characterization of Mollisols, Aridisols and Entisols from southwestern Buenos Aires province. This work focuses on: a) determining the physico-chemical and soil magnetic properties; b) establishing the spatial pattern of ratios of Fe in iron oxides and magnetic parameters in these soils from west to east; c) identifying and quantifying the ferromagnetic iron oxides that provoke the magnetic soils signal; d) carrying out a comparison between different parameters and correlate them; and e) analyzing the role of ferromagnetic iron oxides in these soils. This research contributes to gain knowledge on soil properties of Mollisols, Aridisols and Entisol and tests the potential of the applied methodologies.

Fig. 1. Soil sampling stations in the SW Buenos Aires province (Argentina). Soils from western (S1, S7, and S4), central (S2, and S6) and eastern (S3, and S5) zones have variable annual mean temperatures (14.8–15.3 °C) and mean rainfall (508–683 mm).

suspension was clear or only slightly cloudy following centrifugation (Aguirre, 1987). After this separation procedure, whole soil (< 2mm) and clay fraction (< 2µm) samples were prepared for chemical treatments and environmental magnetism studies.

2. Sampling and laboratory methods 2.1. Study area and soil sampling According to Bartel et al. (2011), soils from this region are constituted by eolian unconsolidated Quaternary sediments, which mainly comprise sandy to silty loess that covers the landscape overlying PlioPleistocene silts and calcretes. Calcrete layers are shallow, and the parent material and soil horizons show short sections of about 1 m generally. The parent material of soils from this region shows similar mineralogical characteristics, with a predominance of minerals of volcanic origin as lithic fragments and volcanic glass; illite is the main clay mineral. Seven agricultural surface soil (0 – 12 cm depth), representing the Ap horizon, were collected from the southwest of Buenos Aires province in the Pampean region of Argentina, denoted as: S1, S2, S3, S4, S5, S6, and S7 (Fig. 1). These soils were described (the entire soil profile) in previous studies by Moralejo (2010) and Moralejo and Acebal (2014) and classified following the Soils Taxonomy Systems (USDA, 1999). Five soils were classified as Mollisols: Entic Haplustoll (S2 and S4), Typic Argiudoll (S3), Pachic Argiudoll (S5), Petrocalcic Paleustoll (S6), one soil (S1) as Aridisol: Typic Haplocambids, and other soil (S7) as Entisol: Typic Ustipsamment. At each site, about 20 topsoil samples were randomly taken from an area of approximately 0.1 ha and mixed to form a composite topsoil sample and stored in polyethylene bags. It is worth mentioning that samples from deeper horizons were not studied because they were unavailable from previous soil samplings by Moralejo (2010) and Moralejo and Acebal (2014). Although the present study focuses on (among other objectives) establishing the spatial pattern of iron oxides in these topsoils from west to east, magnetic properties of horizons B and C from some regional soils can be consulted in Bartel et al. (2011) and Bidegain et al. (2009). Soil samples were air-dried at room temperature, ground and passed through a 2-mm stainless steel sieve to obtain the < 2 mm size fraction (whole soil). The clay fraction (< 2µm) was obtained by sedimentation techniques using Stoke’s law: a 1:3 < 2 mm-soil sample/deionized water suspension was mixed thoroughly for 16 h, and then centrifuged at 500 rpm for 6 min. The < 2 µm clay fraction was siphoned from the suspension. The procedure was repeated 4 – 5 times until the

2.2. Physico-chemical measurements and selective dissolution techniques Particle size distribution was determined using the hydrometer method (Gee and Bauder, 1986). Soil pH was determined in a 1:2.5 soil/deionized water suspension and in a soil/KCl suspension using a glass pH electrode in an Orion digital ion analyzer (Model 701A). Total organic carbon (TOC) was measured by dry combustion using a LECO CR-12 Carbon System Analyzer (Model 781-600). Cation exchange capacity (CEC) was obtained by exchanging the soils with NH4OAc at pH 7 (Summer and Miller, 1996). The specific surface area (SSA) was determined using the EGME (ethylene glycol monoethyl ether) method (SEGME) (Carter et al., 1965). Samples were dried in a vacuum at room temperature prior to SEGME measurements. In addition, total Fe was determined by fusion with Na2CO3 (Hossner, 1996), amorphous Fe oxides by acid oxalate extraction (OX) (McKeague and Day, 1966), and crystalline Fe oxides by the dithionitecitrate-bicarbonate (DCB) method (Mehra and Jackson, 1960). Fe contents were measured by flame atomic absorption spectrometry (FAAS) (G.B.C. 932B Australia). 2.3. Environmental magnetism methods Detailed environmental magnetism measurements, such as magnetic susceptibility, anhysteretic and isothermal remanent magnetization (ARM and IRM, respectively) and thermomagnetic measurements, were carried out in the laboratory of Environmental Magnetism at the CIFICEN (UNCPBA, Argentina) and in the laboratory of Paleomagnetism and Rock Magnetism at the Centro de Geociencias (UNAM, México). Prior to magnetic measurements, whole soil (< 2mm) and clay fraction (< 2µm) were air-dried and then sampled using glass containers of 3.4 cm3 holding about 3.2 – 3.4 g of material. Only for thermomagnetic measurements, a small quantity of material of about 100 mg was used and placed in a special container. Magnetic susceptibility measurements were made using the 2

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soils. Soils from this region contain mainly quartz, Na-rich feldspars and plagioclase in the sand fraction, and illite, interstratified illite–smectite, quartz, Na-rich feldspars and mica in the silt and clay fraction, with Ca2+ and Mg2+ as the main exchangeable cations (Acebal et al., 2000). Higher amount of TOC (2.86 – 6.50%) was found in clay fraction (< 2μm) than in whole soil (< 2mm) samples. The CECs values in whole soil (< 2mm) samples are influenced by different TOC and clay contents. CEC values follow the same trend of TOC content (< 2mm to < 2 µm) where mainly the humic substances of the organic soil matter could contribute up to 50% of the CEC value. The lowest CEC value (9.60 cmolkg−1) was observed for sample S7, in agreement with the low TOC (0.71%) and low clay-size fraction percentage (12.5%) present in this sample (Table 1). Specific surface area SEGME in whole soil (< 2mm) samples, determined by the EGME method, varied from 49.0 to 88.0 m2g−1. The iron contents in the different soil fractions, expressed as percent oxides (%), are summarized in Table 1. Total Fe content (Fet), as well as oxalate extractable fraction (FeO) and dithionite-citrate-bicarbonate extractable fraction (FeDCB) increased as the soil (whole soil and clay fraction samples) particle size decreased. The ratio FeDCB/Fet ranged between 16 and 44% for whole soil samples, and between 25 and 57% for clay fraction, showing higher contents of crystalline and amorphous oxides and/or oxyhydroxides in clay fraction samples. FeO accounted 3 – 12% (whole soil samples) and 9 – 24% (clay fraction, Table 1) of total Fe, indicating that all samples contained small amounts of poorly crystalline oxalate soluble Fe oxides.

magnetic susceptibility meter MS2 (Bartington Instruments Ltd.) linked to the MS2B dual frequency sensor (0.47 and 4.7 kHz). These measurements were done on the higher sensitivity range (0.1 × 10-5 SI); and they were corrected for drift through five measurement cycles (two air readings and three sample readings). The accuracy of the magnetic susceptibility measurement is 1%. The mass specific susceptibility (χ) was computed, as well as the absolute mass specific frequency-dependent susceptibility (χFD = χ0.47kHz – χ4.7kHz), and the percentage frequency-dependent susceptibility (χFD% = 100 (χ0.47kHz – χ4.7kHz) / χ0.47kHz). The remanent magnetizations ARM and IRM were measured using a JR6A Dual Speed Spinner Magnetometer (AGICO, Ltd.), which has a high sensitivity of 2.4 × 10-6 A/m and an accuracy (absolute calibration) of ± 3%. The ARM was imparted using a device attached to a shielded demagnetizer (Molspin Ltd.), superimposing a DC bias field of 90 µT to a peak alternating field (AF) of 100 mT, and an AF decay rate of 17 µT per cycle. Mass specific anhysteretic susceptibility (χARM) was calculated using linear regression for ARM acquired at two DC bias fields of 50, and 90 µT, as well as the anhysteretic ratio χARM/χ. The IRM studies were performed using a pulse magnetizer model IM10–30 (ASC Scientific). Each sample was magnetized by exposing it to a forward DC field of 2470 mT, and backfields from −1.7 mT to −1150 mT. From these measurements, the saturation of IRM (SIRM = IRM2470mT), remanent coercivity (Hcr), S-ratios (S-300 = -IRM300mT / SIRM; S-100 = -IRM-100mT / SIRM), HIRM (=0.5 * [SIRM + IRM-300mT]) and L-ratio (=[SIRM + IRM-300mT] / [SIRM + IRM-100mT]) were calculated. In order to discriminate magnetic mineralogy contribution, the experimental IRM-AF method (Chaparro and Sinito, 2004) was applied. The method is based on the responses of different assemblages of magnetic minerals and was only carried out for backfield IRM measurements. Methodological procedures are detailed in Chaparro et al. (2005). In particular, separation was achieved using a peak AF value of 100 mT as the filter. Two magnetic phases (Phases 1 and 2) were obtained from measurements with and without the AF filter. Thus, the SIRM and the Hcr for each phase and its corresponding magnetic contribution to the total SIRM were determined. The thermomagnetic measurements, temperature dependence of high-field magnetization, were carried out using a laboratory-made horizontal magnetic translation balance (Escalante and Böhnel 2011). The magnetic field was 0.5 T, the temperature was controlled and the force compensated and recorded with a sensor that generates an output voltage. Such voltage is recorded using a PicoLog® recorder. Measurements were performed in air, and each sample was heated to a temperature of about 700 °C and subsequently cooled to room temperature (RT) with a controlled heating/cooling rate of 30 °C min−1.

3.2. Magnetic properties Magnetic properties of soil samples were studied using environmental magnetism methods, and a number of parameters concerning magnetic mineralogy, particle size and concentration is summarized in Table 1. Concentration dependent magnetic parameters χ, and SIRM ranged for whole soil (< 2 mm) and clay fraction (< 2μm) samples from 150.0 to 688.5 × 10-8 m3kg−1, and 10.1 – 90.8 × 10-3 Am2kg−1, respectively (Fig. 2a). Curie temperatures (Tc) on heating curves (Fig. 3) were calculated from the second derivative of M(T) by using the RockMagAnalyzer software (Leonhardt, 2006). Most of these heating curves show a similar behavior, evidencing the presence of maghemite, magnetite, and hematite from changes in its magnetization at about 250 – 400 °C and at higher temperatures. The first phases are observed by a loss of magnetization at about the inversion temperature of maghemite, which may occur at 250 °C up to 750 °C (Dunlop and Özdemir, 1997). This loss of magnetization may be also due to the dehydration of goethite (at 250 – 400 °C) to hematite (Martin-Hernández and García-Hernández, 2010). Goethite is often intergrown with hematite and occurs as a common constituent of soils and sediments. When heated above this temperature range, these metastable minerals will change to hematite. High temperatures of 571 – 592 °C are interpreted as magnetite (Tc = 580 °C) and hematite (Neél temperature, TN = 675 °C) minerals. The presence of hematite is possible; regarding the TN is affected by nonstoichiometry (isomorphous cation substitution and vacancies). According to Cornell and Schwertmann (2003), hematite is often not pure in natural environments and is affected by Al substitution (Al-hematite). Contribution of maghemite may be small because its magnetization is comparable to the magnetite one. Moreover, small changes of M(T) curves by contribution goethite and hematite may arise from significant differences in magnetization between these minerals, i.e. magnetization of magnetite is 220 times, and more, higher than of hematite and goethite (Peters and Dekkers, 2003). On the other hand, differences between heating and cooling curves in the range RT – 600 °C evidence distinctive differences in magnetic behavior of soils depending on their location. As visible in Fig. 3, there is an increase (of 6 – 15%) in magnetization for cooling runs at RT as consequence of a neo-formation of magnetite mineral for soil S5 and S6

3. Results 3.1. Soil properties and Fe content The soil classification and relevant properties are presented in Table 1, some of these data for soils S4, S5, S6 and S7 were reported by Acebal et al. (2003) and Fernández et al. (2015). According to the textural designation of soils or textural class, sandy loam (S1, S7, and S2), sandy clay loam (S6, and S3), and loam (S4, and S5) are identified. Historically, soil pH has been considered one of the most important criterions for soil classification because of the number of co-varying properties that are related to it. Two measures of acidity were determined by the pH (H2O) and pH (KCl), both measures the acidity of the soil solution, and the second one, the reserve acidity of the soil colloids as well. Values of pH (KCl) are lower than values of pH (H2O) (Table 1), which is in agreement with results informed by Benton (1971). That is, pH (KCl) is lower, usually by 0.5 to 1.0 units, than pH (H2O). Soils with high amounts of clay and/or organic matter (Table 1) typically have higher CECs and buffering capacities than silty or sandy 3

4

10-8m3kg−1 10-8m3kg−1 10-3Am2kg−1 10-3Am2kg−1 10-8m3kg−1 % a.u. a.u. kA/m a.u. a.u. a.u. mT mT 10-3Am2kg−1 mT 10-3Am2kg−1

χ χARM SIRM HIRM χFD χFD% χARM/χ ARM/SIRM SIRM/χ S-100 S-300 L-ratio Hcr Hcr1 SIRM1 Hcr2 SIRM2 265.9 2679.5 21.1 0.6 24.1 9.1 10.1 0.094 7.9 0.76 0.95 0.21 34.0 27.9 19.6 242.7 1.5

516.5 1775.3 78.1 0.2 7.4 1.4 3.4 0.016 15.1 0.68 0.99 0.02 36.2 32.2 70.9 194.9 7.2

688.5 1977.6 90.8 2.2 16.8 2.4 2.9 0.016 13.2 0.65 0.95 0.14 37.7 33.7 83.1 207.0 7.7

cmolkg−1 m2g−1

% % % % % % %

150.0 906.7 10.1 0.4 15.9 10.6 6.1 0.068 6.7 0.78 0.92 0.34 28.3 26.6 9.4 300.0 0.7

S7c < 2μm 4.97** 0.55** 11.1 1.59** 32.0 34.6 2.86** 8.30** 7.20** 49.70** –

S7 < 2mm 4.04** 0.19** 4.7 0.82** 20.3 23.2 0.71** 7.70** 7.40** 9.60** 49.60**

S1 < 2mm 4.77 0.18 3.8 0.82 17.2 21.9 2.56 6.45 5.68 18.05 49.00

Sample Size Fet FeO FeO/Fet FeDCB FeDCB/Fet FeO/FeDCB TOC pH(H2O) pH(KCl) CEC SEGME

S1c < 2μm 5.24 0.57 10.9 1.78 34.0 32.0 4.40 6.73 6.20 66.50 –

Typic Ustipsamment** 65.6** 21.9** 12.5**

% % %

14.8 507.9 Typic Haplocambid 79.0 6.4 14.7

°C mm

T Rainfall Soil classification Sand Silt Clay

S7

S1

Soil

511.0 2686.4 72.6 2.1 18.4 3.6 5.3 0.027 14.2 0.64 0.94 0.16 36.3 32.5 66.0 205.8 6.6

S4 < 2mm 3.70 0.44 11.9 1.62 43.8 27.2 1.14* 8.30 8.00 23.36* 76.00 279.7 1878.8 17.9 0.6 32.0 11.5 6.7 0.076 6.4 0.84 0.93 0.40 26.5 25.1 16.9 325.4 1.0

S4c < 2μm 3.94 0.96 24.4 2.18 55.3 44.0 3.22 7.20 6.90 48.20 –

Entic Haplustoll* 43.3* 41.7* 14.8*

S4

480.4 2438.5 72.9 0.9 15.7 3.3 5.1 0.026 15.2 0.65 0.97 0.07 35.5 32.2 66.7 200.0 6.2

S2 < 2mm 4.80 0.15 3.1 0.77 16.0 19.5 2.65 6.80 5.92 19.55 69.00 450.2 4015.1 27.9 0.8 37.2 8.3 8.9 0.108 6.2 0.81 0.94 0.29 26.3 25.0 26.3 300.0 1.5

S2c < 2μm 5.45 0.64 11.7 1.92 35.2 33.3 3.70 6.68 6.22 67.90 –

15.4 666.8 Entic Haplustoll 64.8 16.0 19.3

S2

454.8 2343.5 65.8 1.9 16.2 3.6 5.2 0.027 14.5 0.68 0.94 0.18 35.1 31.8 60.7 205.0 5.1

S6 < 2mm 4.20** 0.14** 3.3 0.72** 17.1 19.4 2.90** 7.50** 6.30** 17.10** 69.90** 222.5 1597.9 14.5 0.4 25.5 11.5 7.2 0.082 6.5 0.83 0.94 0.32 27.4 25.9 13.7 294.0 0.8

S6c < 2μm 5.83** 0.53** 9.1 1.43** 24.5 37.1 3.44** 6.54** 5.65** 51.50** –

Petrocalcic Paleustoll** 50.4** 27.7** 21.8**

S6

516.8 3768.7 75.0 2.2 25.4 4.9 7.3 0.038 14.5 0.71 0.94 0.20 34.7 32.5 70.7 210.0 4.3

S3 < 2mm 4.30 0.22 5.1 1.56 36.3 14.1 3.80 6.58 5.28 21.63 88.00

585.3 6850.8 45.5 1.4 55.3 9.5 11.7 0.111 7.8 0.80 0.94 0.31 28.0 26.8 43.4 247.4 2.2

S3c < 2μm 4.42 0.68 15.4 1.45 32.8 46.9 4.20 6.03 5.63 68.50 –

15.3 683.0 Typic Argiudoll 52.6 21.1 26.3

S3

476.5 2636.9 71.9 4.0 6.0 1.3 5.5 0.027 15.1 0.64 0.89 0.31 35.6 33.8 69.0 325.0 2.9

S5 < 2mm 3.42** 0.21** 6.1 1.25** 36.5 16.8 4.78** 6.40** 5.20** 15.00** 78.70**

257.4 1603.4 20.5 0.5 28.3 11.0 6.2 0.060 8.0 0.82 0.95 0.24 30.7 29.2 19.4 261.5 1.2

S5c < 2μm 3.86** 0.68** 17.6 2.20** 57.0 30.9 6.50** 6.80** 6.13** 51.70** –

Pachic Argiudoll** 42.0** 44.0** 14.0**

S5

Table 1 Meteorological variables, soil classification, contents of iron, physico-chemical and magnetic properties of soils from southwestern Buenos Aires province (Argentina). Part of these data/measurements were reported by *Acebal et al. (2003) and **Fernández et al. (2015), they are indicated with asterisks.

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Fig. 2. Concentration and mineralogy dependent magnetic parameters for whole soil (< 2mm) and clay fraction (< 2μm) samples. (a) Concentration dependent magnetic parameters (SIRM and χ); and (b) Mineralogy dependent magnetic parameters (L-ratio and HIRM).

from the eastern area. On the other hand, it is evident a transformation of magnetic minerals leading to a lower magnetization for the cooling run at RT for sandy loams S1, S7, S4, and S2 from western area. The latter is interpreted in terms of the maghemite transformation, its ferrimagnetic contribution to the magnetization may be estimated from the difference in magnetization of 10 – 21% by comparing the cooling and heating curves at room temperatures. Ferrimagnetic minerals (magnetite and maghemite) dominate over the antiferromagnetic (hematite and goethite) ones as observed from the S-ratio (S-100 and S-300) values. They ranged between 0.64 and 0.84 for S-100, and between 0.89 and 0.99 for S-300, higher contribution of ferrimagnetic minerals is observed in whole soil (< 2mm) samples in the following decreasing order: S7, S2, S1, S3, S4, S6, and S5 (Table 1). The influence of high-coercivity minerals, such as hematite and goethite, was analyzed from parameters HIRM and L-ratio. Results of both parameters are shown in Fig. 2b. Among these high-coercivity minerals, magnetically harder samples correspond to soils located in the eastern part (S5, S3), and relatively softer ones correspond to soils S7 and S2 (SW part). The remanent coercivity values for whole soil (< 2mm) (Hcr = 34.7 – 37.7 mT), and for clay fraction (< 2μm) samples (Hcr = 26.3 – 34.0 mT, Table 1) are indicative of: a) slight differences between soils from western and eastern Buenos Aires province; and b) appreciable variation in magnetic mineralogy is observed between whole soil and clay fraction samples, lower remanent coercivity values correspond to the clay fraction. In addition, the remanent coercivity values of low- and high-coercivity phases in mixed magnetic assemblages were determined (Fig. 4) using the experimental IRM-AF method by Chaparro et al. (2005). Lowcoercivity and high-coercivity phases with Hcr1 = 25.0 – 33.8 mT (Phase 1) and Hcr2 = 194.9 – 325.0 mT (Phase 2), respectively, were determined (Table 1). The parameter χFD% allows to give a semi-quantitative interpretation of ultrafine ferrimagnetic particles (< 0.03 μm; superparamagnetic particles, SP) contribution on soil samples (Dearing et al., 1996). On one hand, the χFD% values for whole soil (< 2mm) samples (χFD% = 1.3 – 4.9%; Table 1) indicate absence of SP particles for χFD% < 2%,

Fig. 3. Thermomagnetic measurements, temperature dependence of high-field (0.5 T) magnetization, performed in air and using a laboratory-made horizontal magnetic translation balance. Heating and cooling curves with a controlled heating/cooling rate of 30 °C min−1 were made for all whole soil (< 2mm) samples.

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Fig. 4. Separation of IRM backfield measurements into two phases using the experimental IRM-AF method by Chaparro et al. (2005). Sample from, curves of Total back IRM (without AF filter) and achieved phases (Phase 1 and Phase 2) for soil samples S5 and S4 are shown. All whole soil (< 2mm) samples were analyzed and their results are summarized in Table 1.

Fig. 5. Magnetic particle size distribution for whole soil (< 2mm) and clay fraction (< 2μm) samples. (a) Parameters anhysteretic magnetic susceptibility plot (χARM) and frequency-dependent susceptibility (χFD); (b) the King's plot (χARM vs. χ).

and an admixture between SP and coarser particles for χFD% = 2 –10% (Walden et al., 1999); and on the other hand, the dominance of SP particles for clay fraction (< 2μm) samples are evidenced by higher values of this parameter, χFD% = 8.3 – 11.5%. Taking into account the dominance of magnetite/maghemite over the other magnetic minerals in these soil samples, the χFD can be used semi-quantitatively to estimate the absolute SP concentration, which have higher values for clay fraction (< 2μm) samples (χFD = 15.9 – 55.3 × 10-8m3kg−1) than for whole soil (< 2mm) samples (χFD = 6.0 – 25.4 × 10-8 m3 kg−1, Fig. 5a). In addition, a quantitative size estimation of all magnetite minerals (SP, single-, pseudo-single- and multi-domain particles) is possible

through the anhysteretic ratios (χARM/χ, and ARM/SIRM) and the King’s plot, which indicate the presence of ultrafine and fine particles < 0.1 – < 1 μm (Fig. 5b). 4. Discussion 4.1. Relevant properties for Mollisols, Aridisols and Entisols Diamagnetic, paramagnetic and ferromagnetic (i.e. ferrimagnetic and antiferromagnetic) minerals have a contribution to parameter χ; however, only ferromagnetic minerals contribute to the remanent magnetization SIRM. Since both parameters are significantly correlated 6

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ferromagnetic minerals, determined by parameters χARM/χ, χFD%, and ARM/SIRM (Fig. 6b), follows values of CEC (R = 0.84 – 0.97; p < 0.01) and SEGME (R = 0.96 – 0.97; p < 0.01). It is worth mentioning that ratios ARM/SIRM and χARM/χ are designed to cancel the effects of magnetic mineral concentration, and enhance the ferromagnetic signal due to variations in particle size (Liu et al., 2012). Last but not least important, χFD is proportional to the concentration of ultrafine SP ferrimagnetic particles, as well as χARM is a parameter sensitive to the concentration and particle size of ferromagnetic minerals (Dunlop and Özdemir, 1997). Such direct relationships indicate that increase of CEC and SEGME are associated with smaller ferromagnetic particles and higher SP particle concentration.

(R = 0.87, p < 0.01; Fig. 2a), ferromagnetic minerals are identified as the main carriers that handle magnetic signal of these topsoil samples. The ubiquitous magnetite (Fe3O4) is the main ferrimagnetic carrier and it was detected in all soils (S1 – S7) through thermomagnetic studies, i.e. M(T) measurements. Other magnetic minerals such as γ-Fe2O3 (maghemite), α-FeOOH (goethite) and α-Fe2O3 (hematite) were also identified through the analysis of heating and cooling runs (Fig. 3). Oxalate has been shown to be a selective reagent for the dissolution of non-crystalline and poorly crystalline aluminosilicates, Fe oxides and oxyhydroxides (McKeague and Day, 1966). While the DCB method has been associated with free Fe oxides, amorphous coatings and crystalline Fe oxides acting as cementing agents between clay-sized particles. Besides Fe oxides, other iron compounds in weakly magnetic forms, such as diamagnetic and paramagnetic minerals, are expected from total Fe content. The Fet shows an inverse relationship with concentration dependent magnetic parameters χ and SIRM. The highest Fet content is observed in soils S1 and S2 that is only in agreement with the highest value of χ for soil S1. χ and SIRM are statistically correlated with FeO (R = -0.55 – -0.82) and FeDCB (R = -0.56 – -0.73), FeO is associated with non-crystalline and poorly crystalline Fe oxides; and FeDCB is associated to free Fe oxides and the removal of amorphous coatings and crystals of free oxides acting as cementing agents. On the other hand, Lratio, which is interpreted in terms of high-coercivity mineral distribution, is directly correlated with FeO and FeDCB (R = 0.67 – 0.73; p < 0.01). Among magnetic parameters, the particle size dependent parameters evidenced a better relationship with these Fe oxides contents. Anhysteretic susceptibility χARM follows, for whole soil (< 2mm) samples, the trend of FeO (R = 0.75; p < 0.05) that is associated with non-crystalline and poorly crystalline Fe oxides. The bivariate analysis for all samples shows that χFD, χFD% and anhysteretic ratios χARM/χ and ARM/SIRM (Fig. 6a) are significantly correlated with FeO (R = 0.59 – 0.88; p < 0.05 and p < 0.01) and FeDCB (R = 0.52 – 0.75; p < 0.05 and p < 0.01). These results indicate that ferromagnetic particle size distribution, and SP particle concentration, on these soils is a relevant property associated to poorly crystalline and free Fe oxides rather than Fe oxides into the dia/paramagnetic matrix. Soils with high amounts of clay and/or organic matter, such as soils S5 and S3, typically have higher CECs and buffering capacities than sandy loam such as S7. Higher amount of organic matter is obtained in clay fraction (< 2μm) samples (TOC = 2.86 – 6.50%) than in whole soil (< 2mm) samples (TOC = 0.71 – 4.78%, Table 1), which confirms the well known high affinity of organic compounds with clay fraction. These values are consistent with data reported by Haile-Mariam et al. (2008) indicating the high affinity of organic compounds with clays and the influence of particle size. Among whole soil (< 2mm) samples, soils S5 (Pachic Argiudoll), S3 (Typic Argiudoll) and S6 (Petrocalcic Paleustoll), all of them located on the eastern part, have the highest TOC, χARM/χ and L-ratio values, on the contrary of soil S7 (Typic Uptipsamment) located on the southwestern part (Fig. 6b). These TOC values are correlated, for whole soil (< 2mm) samples, only with parameters L-ratio (R = 0.81; p < 0.05) and HIRM (R = 0.77; p < 0.05). As mentioned, the CEC and SEGME values for whole soil (< 2mm) samples may be influenced by the different TOC and clay contents, contributing to increase the cation exchange capacity and the specific surface area. A detailed clay mineralogy analysis was carried out by Xray diffraction and Mössbauer spectroscopy in previous studies (Acebal et al., 2000, 2003; Fernandez et al., 2015), which evidenced the dominance of clay minerals such as smectite, illite, interstratified illite–smectite. The readers are referred to mentioned literature. According to the analysis of magnetic properties, among concentration dependent magnetic parameters, CEC is inversely correlated with χ (R = -0.50; p = 0.07) and SIRM (R = -0.84; p < 0.01), on the contrary, CEC is directly correlated with χFD (R = 0.78; p < 0.01), and SEGME with χARM (R = 0.91; p < 0.01). Particle size distribution of

4.2. Environmental conditions χ values of whole soil (< 2mm) samples increase from Mollisols to Entisols and Aridisols, in the following order: S6, S5, S2, S4 (χ = 511.0 × 10-8 m3 kg−1; Entic Haplustoll), S3 (χ = 516.8 × 108 3 m kg−1; Typic Argiudoll), S7, and S1 (Fig. 2a), which are comparable in χ-values with other soils classified as Typic Argiudoll (χ = 541.4 × 10-8m3kg−1), Aridic Haplustoll (χ = 557.2 × 10-8 m3 kg−1) and Lithic Argiustoll (χ = 584.6 × 10-8 m3 kg−1) reported in this region by Bartel et al. (2011). This increasing trend (of χ and SIRM) may be interpreted as a decreasing trend of weathering, as reported by Bidegain et al. (2013) for paleosols, loess and loess-like sediments from Buenos Aires province. In addition, the main magnetic mineralogy concluded from the remanent coercivity values (Hcr = 34.7 – 37.7 mT) is in agreement with those reported data that ranges from Hcr = 34.2 – 41.5 mT. The remanent coercivity values decreases slightly from western (S1) to eastern part (S5) showing a softer-coercivity pattern towards the eastern area where soils are better developed (Mollisols) according to the environmental conditions. Such coercivity pattern was also observed in surface soils across the Russian Steppe (with a rainfall ranging between 300 and 500 mm/year, Maher et al., 2003) as well as in the climosequence of soils from the southern Pampean region by Bartel et al. (2011). The authors found conspicuous Hcr differences between horizons A and C for the most developed soils, and therefore, they proposed this parameter as a proxy of pedogenesis intensity for this Pampean region. The anhysteretic ratios (χARM/χ and ARM/SIRM) are in agreement with the soil classification, indicating the presence of relatively coarser magnetic particles (1 μm) in soils S1 and S7 than in soils S3 – S5 (0.2 – < 0.1 μm), as well as the dominance of finer magnetic particles in clay fraction (< 2μm) samples than in whole soil (< 2mm) ones. Values of χFD% are higher in clay fraction (< 2μm) than for whole soil (< 2mm) samples and indicate the presence of ultrafine SP particles associated to the pedogenic magnetic minerals in soils, which is better expressed in soils located towards eastern than western part. χFD gives a semi-quantitative estimation of SP particle concentration that correlate directly with contents of hematite (R = 0.85, p < 0.01) and magnetite (R = 0.96, p < 0.01) for clay fraction (< 2μm) samples (Fig. 7). Such relationship is in agreement with Torrent et al. (2006), who observed this behavior in soils from Argentina (Buenos Aires province), China and Russia among others. The degree of transformation, of Fe-containing minerals to iron oxides, may be quantified through the ratios of Fe in iron oxides, i.e. ratio FeDCB/Fet (with age this ratio gradually approaches 100%), and FeO/FeDCB (decreasing values may be used to indicate increasing maturity of soils). These ratios can serve as an indicator of the degree of weathering (of Fe-containing minerals to iron oxides) and the maturity of a soil as function of time (Simón et al., 2000; Cornell and Schwertmann, 2003; Torrent et al., 2006). Ratio FeDCB/Fet increases values from western soils S1 and S7 (FeDCB/ Fet = 17.2 and 20.3%) to eastern soils S3 and S5 (FeDCB/Fet = 36.3 and 36.5%) where soils are better developed (Mollisols). This degree of weathering is also observed from ratio FeO/FeDCB, which decreases from western soils S1 and S7 (FeO/FeDCB = 21.9 and 23.2%) to eastern 7

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Fig. 6. Magnetic parameters (ARM/SIRM, χARM/χ, and L-ratio) for whole soil (< 2mm) and clay fraction (< 2μm) samples; and their relation with (a) total iron contents (Fet), oxalate extractable metal fraction (FeO), and dithionite-citrate-bicarbonate extractable metal fraction (FeDCB); and with (b) total organic contents (TOC), specific surface area (SEGME), and cation exchange capacity (CEC).

higher (15.3 °C and 683 mm, respectively) than in the western part. According to Bartel et al. (2011), this fact reflects the degree of pedogenesis reached in this eastern region, and higher relative humidity conditions that yield to a better development of Argiustolls and Argiudolls.

soils S3 and S5 (FeO/FeDCB = 14.1 and 16.8%). In general, soils from this study show a relatively higher weathering from FeDCB/Fet (up to 44% – 57%) than mentioned Russian and Argentinian soils (FeDCB/Fet of 33%, and 18%, respectively). Therefore, besides detrital, pedogenic iron oxides are expected to contribute to the magnetic soil signal in this region as reported in soils from NE of Buenos Aires province (Orgeira et al., 2008). The Aridisols S1 (Typic Haplocambids) is a soil developed under very arid environmental conditions and therefore poorly weatherable, as well as in the case of the Entisols S7 (Typic Psamments) that is a very young coarse soil (mean temperature of 14.8 °C and rainfall of 507.9 mm, Table 1). On the contrary, Mollisols with an argillic horizon, i.e. Typic and Pachic Argiudolls (soils S3 and S5), evidenced finer magnetic particles (< 0.1 – 0.2 μm). They are located towards the northeastern part where the annual mean temperature and rainfall are

4.3. Mixtures of magnetic iron oxides, their quantification and spatial distribution Soils from this region mainly contain hematite and goethite with varying degrees of crystallinity and Al substitution, and magnetite and/ or maghemite in minute quantities as concluded by X-ray diffraction and Mössbauer spectroscopy studies (Acebal et al., 2000, 2003; Fernández et al., 2015). In this study, magnetite, maghemite, goethite and hematite were also identified by environmental magnetism 8

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Fig. 7. Quantitative estimation of hematite (αFe2O3) and magnetite (Fe3O4) (in relation to all soil components) is based on the reference saturation isothermal remanent magnetization values of natural magnetite and hematite reported in the literature. (a) Contents of hematite and magnetite; (b) contents of hematite and χFD (semi-quantitative estimation of SP particle concentration); and (c) contents of magnetite and χFD for whole soil (< 2mm) and clay fraction (< 2μm) samples.

methods verifying the previous findings. Parameters HIRM and L-ratio are useful, when magnetic signals are dominated by ferrimagnetic minerals such as magnetite and maghemite, to determine the coercivity values of high-coercivity minerals in natural samples (Chaparro et al., 2017). In addition, it is also possible, while not easy, to determine this high-coercivity phase by an experimental IRM-AF method as reported by Bidegain et al. (2013), Quijano et al. (2014) and Mejia-Echeverry et al. (2018). Although HIRM parameter is often interpreted as a proxy for the absolute concentration of high-coercivity minerals (hematite and/or goethite), in the case of whole soil (< 2mm) samples, such interpretation may not be valid because L-ratio correlated significantly with HIRM (R = 0.97; p < 0.01) and therefore the HIRM may not provide useful quantitative information about variations in the absolute concentration of antiferromagnetic minerals (Liu et al., 2007). This correlation between L-ratio and HIRM indicates that the observed HIRM values may be controlled by changes in the coercivity distribution rather than by the concentration of high-coercivity minerals. Magnetic properties of these Fe oxides, as well as magnetite/hematite mixing ratios were determined (Table 1). Low-coercivity and high-coercivity phases correspond to characteristic values of magnetite (Hcr = 8.0 – 69.5 mT), maghemite (Hcr = 16.9 – 31.0 mT), and hematite (Hcr = 30.0 – 821 mT) as reported by Peters and Dekkers (2003). In addition, values of Phase 2 belong to the Hcr values for hematite ranging between < 5 μm and 250 μm in size (Dankers, 1978), and although higher values were reported for goethite (Hcr = 500 – 4100 mT), Chaparro et al. (2006b) reported Hcr = 268 mT for natural goethite. According to studies on artificial magnetite/hematite mixing ratios reported by Chaparro and Sinito (2004) and by Frank and Nowaczyk (2008), a rough estimation for whole soil (< 2mm) samples gives magnetite/hematite mixing ratios of 0.16/0.84 – 0.07/0.93. Although Phase 2 contributions (SIRM2) to the SIRM are low (4 – 9%), the per cent magnetic fraction (wt%) of this phase has to be higher if the typical SIRM values for hematite (0.24 – 0.35 Am2kg−1) and magnetite (22.0 – 33.1 Am2kg−1; Maher et al., 1999; Peters and Dekkers, 2003) are considered, note that the SIRM for magnetite is

about 100 times higher than that of hematite. Thus, a quantitative estimation of both main Fe oxides (in relation to all soil components) may be given based on the reference SIRM values of natural magnetite and hematite reported in the literature. Such estimations of magnetite and hematite give higher contents for whole soil (< 2mm) samples (0.18 – 0.25 wt% and 0.82 – 2.20 wt%) than for clay fraction (< 2μm) samples (0.03 – 0.13 wt% and 0.19 – 0.61 wt%, Fig. 7), which are within the ranges of iron contents, e.g. Fet (3.42 – 4.77 wt%) and FeDCB (0.72 – 1.62 wt%), determined by independent methods. The contents of magnetite for whole soil (< 2mm) samples vary in a narrow range and there is an increasing trend towards the SW region (soils S7 and S1) as reveals parameters χ and SIRM as well. The highest contents of hematite (2.20 – 1.89 wt%) correspond to soils located in the western part (S1, S7, and S4; mean temperature of 14.8 °C and rainfall of 507.9 mm), following values (1.77 – 1.46 wt%) of soils from the center part (S2 and S6), and the lowest ones (1.23 – 0.82 wt%, Fig. 7) for soils in the eastern part (S3 and S5; mean temperature of 15.3 °C and rainfall of 683.0 mm). Such hematite contents are within the range reported (Fernandez et al., 2015) for hematite + goethite (1.4 – 1.6 wt%) in clay fractions of soils S5, S6, and S7 using the Rietveld method. It is worth mentioning that this high-coercivity phase for whole soil samples not only vary slightly in concentration, but also in its coercivity distribution as concluded from the L-ratio analysis and the Hcr2 values determined for the Phase 2. The Hcr2 values increase (higher-coercivity) towards the eastern part; such values varied between 194.9 and 210.0 mT, except for soil S5 (Hcr2 = 325.0 mT; Table 1). Such increase in remanent coercivity values may be indicative of finer hematite particles as reported by Dankers (1978), that is, Hcr values ranging from 143 mT (size of 75 μm) to 447 mT (size < 5 μm). Soils with same/similar pedological characteristics, such as S4 and S2 (Entic Haplustoll), and S3 (Typic Argiudoll) and S5 (Pachic Argiudoll), show similar contents of magnetite but different hematite contents, especially S3 and S5 (Fig. 7). Such differences follow an inverse relation with total organic carbon, being 1.14% (loam S4), 2.65% 9

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(sandy loam S2), 3.80% (sandy clay loam S3), and 4.78% (loam S5).

Declaration of Competing Interest

5. Conclusion

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The analysis of iron contents, determined by selective dissolution techniques, indicates that Fet, FeO and FeDCB increased as the soil particle size decreased, in addition, the highest Fe amounts are present in these soils as a non-extractable form, that is, they are in appreciable extent as aluminosilicates, hematite and magnetite. The FeDCB/Fet and FeO/Fet for whole soil samples accounted for < 44% and < 12%, respectively, indicating small amounts of amorphous oxides and/or oxyhydroxides, and poorly crystalline oxalate soluble Fe oxides. Some of the magnetic parameters follows the trend of extractable Fe, i.e. FeO and FeDCB, in particular, particle size (χARM/χ, χFD%, ARM/ SIRM) and SP concentration (χFD) dependent parameters evidence significant direct correlations with these Fe oxides contents. This fact indicates that ferromagnetic particle size distribution on these soils are associated to poorly crystalline and free amorphous Fe oxides rather than to Fe oxides in aluminosilicate phases. These particle size and concentration dependent magnetic parameters are also directly correlated with the cation exchange capacity; indicating that larger CEC values are associated with smaller ferromagnetic particles and higher SP particle concentration. This result proves the important role of such smaller ferromagnetic iron oxides together with physico-chemical parameter CEC on the soil adsorption–desorption processes. Iron oxides in form of magnetite, maghemite, goethite and hematite mixtures are present in these Mollisols, Aridisols and Entisols from southwestern Buenos Aires province. The amounts of magnetite (0.18 – 0.25 wt%) and hematite (0.82 – 2.20 wt%) for whole soil (< 2mm) samples are higher than for clay fraction (< 2μm) samples; however, they are low with relation to soil components. This may not be well characterized if detailed environmental magnetism methods are conducted. Ratios of Fe in iron oxides, as well as particle size, mineralogy and concentration of these magnetite/hematite mixtures vary along the studied areas. Ratios FeDCB/Fet increase and FeO/FeDCB decrease values from western soils S1 and S7 to eastern soils S3 and S5 (Mollisols), which may be interpreted in terms of the degree of weathering or maturity of soils. Although most of the magnetic particle sizes belong to the range ultrafine–fine category (< 0.1 – 1 μm), sandy loam soils S1 and S7 located on the SW part have coarser magnetic particles, than the central and eastern zones (S3 – S5) where climatic conditions favor the development of Argiudolls. Pedogenic SP iron oxides are expected to contribute to the magnetic soil signal, which are better expressed in Mollisols. The remanent coercivity values decrease from western part showing a softer-coercivity pattern towards the eastern area where Mollisols are better developed according to the environmental conditions. Concentration dependent magnetic parameters χ and SIRM, as well as contents of magnetite and hematite, show an increasing trend towards the SW region where Entisols (S7) and Aridisols (S1) are developed. The clearest difference among these soils is obtained analyzing the contents of hematite, decreasing from western zone (S1, S7, and S4) to the eastern zone (S3 and S5) where soils are better developed. Such changes on these soil components –influenced by climatic conditions, lithology, and degree of pedogenesis, among others– lead to differences of soils in terms of magnetic properties. These results show that the environmental magnetism approach applied here is a valuable tool to analyze iron oxide mineralogy in soils developed under different environmental conditions.

Acknowledgements The authors thanks to UNS, UNCPBA, CONICET and UNAM for their financial support. This contribution was partially supported by the Bilateral CONICET/CONACYT Project No. 207149 (Harald Böhnel) and Res. 1001/14 - 5131/15 (Marcos Chaparro). The authors thank to both anonymous reviewers whose comments greatly improved this manuscript. The authors also thank to Ing. J. Escalante (Centro de Geociencias, UNAM, México) and Mr. P. Zubeldia (Tech. CICPBA, Argentina) for their help performing measurements. References Acebal, S.G., Rueda, E.H., Aguirre, M.E., 1997. Extracción de las distintas formas de hierro presentes en un Haplustol Petrocálcico. Estudio comparativo de los métodos Ditionito-citrato-bicarbonato (DCB) y Ditionito-EDTA. Agrochimica XLI 3–4, 155. Acebal, S.G., Mijovilovich, A., Rueda, E.H., Aguirre, M.E., Saragovi, C., 2000. Iron-oxide mineralogy of a Mollisol from Argentina: a study by selective-dissolution techniques, X Ray diffraction and Mössbauer Spectroscopy. Clays Clay Miner. 48, 322–330. https://doi.org/10.1346/CCMN.2000.0480303. Acebal, S.G., Aguirre, M.E., Santamaría, R.M., Mijovilovich, A., Petrick, S., Saragovi, C., 2003. Selective Dissolution Techniques, X-Ray Diffraction and Mössbauer Spectroscopy Studies of Forms of Fe in Particle-Size Fractions of an Entic Haplustoll. Hyperfine Interact. 148 (149), 3–12. https://doi.org/10.1023/B:HYPE.0000003758. 14157.df. Aguirre, M.E., 1987. Rol de los minerales amorfos en el proceso de cementación a la agregación. MSc. Thesis. Universidad Nacional del Sur, Bahía Blanca, Argentina. Bartel, A.A., Bidegain, J.C., Sinito, A.M., 2005. Propiedades magnéticas de diferentes suelos del partido de La Plata, provincia de Buenos Aires. Revista de la Asociación Geológica Argentina 60 (3), 591–598. Bartel, A.A., Bidegain, J.C., Sinito, A.M., 2011. Magnetic parameter analysis of a climosequence of soils in the Southern Pampean Region, Argentina. Geofísica Internacional 50(1), 9-22. http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S001671692011000100003&lng=es&nrm=iso. Benton Jones, Jr. J., 1971. Laboratory guide for conducting soil test and plant analysis. The relationship between soil pH and base-saturation percentage for surface and subsoil horizons of selected mollisols, alfisols and ultisols in Ohio. Ohio J. Sci. 71, 43-55. Bidegain, J.C., Rico, Y., Bartel, A., Chaparro, M.A.E., Jurado, S.S., 2009. Magnetic parameters reflecting pedogenesis in Pleistocene Loess deposits of Argentina. Quat. Int. 209, 175–186. https://doi.org/10.1016/j.quaint.2009.06.024. Bidegain, J.C., Jurado, S., Chaparro, M.A.E., Gómez Samus, M., Zicarelli, S., Parodi, A.V., 2013. Magnetostratigraphy and environmental magnetism in a Pleistocene sedimentary sequence, Marcos Paz. Argentina. Environ. Earth Sci. 69, 749–763. https:// doi.org/10.1007/s12665-012-1958-7. Campbell, A.S., Schwertmann, U., 1984. Iron oxide mineralogy of placic horizons. J. Soil Sci. 53, 569–582. https://doi.org/10.1111/j.1365-2389.1984.tb00614.x. Carter, D.L., Heilman, M.D., González, C.L., 1965. Ethylene glycol monoethyl ether for determining surface area of silicate minerals. Soil Sci. 100, 356-360. https://journals. lww.com/soilsci/Fulltext/1965/11000/ETHYLENE_GLYCOL_MONOETHYL_ETHER_ FOR_DETERMINING.11.aspx. Chaparro, M.A.E., Sinito, A.M., 2004. An alternative experimental method to discriminate magnetic phases using IRM acquisition curves and magnetic demagnetisation by alternating field. Revista Brasileira de Geofísica 22 (1), 17–32. https://doi.org/10. 1590/S0102-261X2004000100002. Chaparro, M.A.E., Lirio, J.M., Nuñez, H., Gogorza, C.S.G., Sinito, A.M., 2005. Preliminary magnetic studies of lagoon and stream sediments from Chascomús Area (Argentina)magnetic parameters as indicators of heavy metal pollution and some results of using an experimental method to separate magnetic phases. Environ. Geol. 49, 30–43. https://doi.org/10.1007/s00254-005-0049-4. Chaparro, M.A.E., Gogorza, C.S.G., Chaparro, M.A.E., Irurzun, M.A., Sinito, A.M., 2006a. Review of Magnetism and Heavy Metal Pollution Studies of Various Environments in Argentina. Earth, Planets Space 58 (10), 1411–1422. https://doi.org/10.1186/ BF03352637. Chaparro, M.A.E., Sinito, A.M., Bidegain, J.C., de Barrios, R.E., 2006b. Magnetic studies of natural goethite samples from Tharsis, Huelva. Spain. Geofísica Internacional 45 (4), 219–230. http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S001671692006000400001&lng=es&nrm=iso. Chaparro, Marcos A.E., Suresh, G., Chaparro, Mauro A.E., Ramasamy, V., Sundarrajan, M., 2017. Magnetic assessment and pollution status of beach sediments from Kerala coast (South-western India). Mar. Pollut. Bull. 117 (1–2), 171–177. https://doi.org/ 10.1016/j.marpolbul.2017.01.044. Cornell, R.M., Schwertmann, U., 2003. The Iron Oxides: Structure, Properties Reactions, Occurrences and Uses. Wiley, New York.

Conflict of interest There is no conflict of interest.

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