The effect of weathering on clay mineralogy of Plio-Quaternary sediments and the overlying material on SE slopes of Medvednica Mt., Croatia

doi:10.21203/rs.3.rs-2499795/v1

Clay mineralogy of Plio-Quaternary sediments of SE slopes of Medvednica Mt. was compared to that of the overlying weathering horizons. Three locations of Plio-Quaternary sediments, representing past weathering products, were sampled along with the overlying material, representing more recent weathering cycles. Particle size distribution, pH and cation exchange capacity (CEC) of the samples were analyzed. Bulk mineralogy was determined using X-ray diffraction (XRD). Clay mineralogy was determined, after carbonate cement, organic matter and free Fe oxides removal, on fractions < 2 and < 0.2 µm using XRD and Fourier-transform infrared spectroscopy (FTIR). Average layer charge of the expandable layers was determined using the O-D method. Major elements content was determined using inductively coupled plasma optical emission spectroscopy (ICP-OES). Bulk mineralogy comprises quartz, feldspars, mica, clay minerals and occasional Al and Fe (oxy)hydroxides. Clay mineralogy differs subtly between samples; dioctahedral expandable clay minerals dominate with illite and kaolinite present. Hydroxyl interlayering of expandables is noted in the surface samples, corresponding to lower CEC values. Average absolute layer charge of the expandable layers is higher in the surface samples and decreases with depth on two of the investigated locations. Overall, clay mineralogy of the Plio-Quaternary sediments reflects a more intensive weathering regime, confirmed by the presence of Al (oxy)hydroxides and lower absolute charge of the expandable layers. The weathered material and soil overlying these sediments shows heterogeneity in genesis, but corresponds to less intense weathering regime, confirmed by the formation of hydroxy-interlayered minerals and higher absolute charge of the expandable layers.

clay minerals

layer charge

weathering

Plio-Quaternary sediments

Medvednica Mt

Clay minerals are commonly formed as secondary minerals in the weathering processes taking place in surface conditions. Over time, a clay mineral assemblage of an area will reflect the prevailing environmental conditions as a dynamic equilibrium is being achieved between them. This is the basis of a myriad of research using clay minerals as environmental proxies, inferring past and present climate, topography, lithology and other environmental factors’ influence on weathering and pedogenesis (e.g., Schroeder and West 2005; Hseu et al. 2007; Pal et al. 2009; Chen et al. 2014). Furthermore, the processes of transformation of clay minerals during weathering and soil formation are important for understanding their response to the future changes in environmental conditions.

The other important aspect of clay minerals research in surface sediments and soils is in understanding their influence on various chemical and physical properties/processes in the biosphere. Layer charge (LC) of expandable clay minerals is one of the most important properties affecting the chemical behavior of soils (Gillot et al. 2001). It indicates the mineral capacity to retain cations and absorb water (Pai et al. 2006), it is related to the chemical composition of the clay minerals and is one of the discriminating factors in determining the mineral species.

Plio-Quaternary (PlQ) sediments of Medvednica Mt. have not been as extensively studied as the older Miocene sediments of the same area (e.g., Grizelj et al. 2011; Pavelić and Kovačić 2018). Considering that these relatively young sediments, together with the older lithology of the area, serve as a parent material for the incipient soil forming processes on the lower slopes of Medvednica Mt., it is important to characterize their mineralogical and geochemical properties. The nature of the deposition and material provenance for PlQ sediments on the slopes of Medvednica Mt. result in a clay mineral assemblage which reflects past weathering cycles of the area. In contrast, weathered material and soil developed on top of these sediments are presumed by the authors to be formed either from the PlQ sediments as parent material or from locally deposited loess, in both cases corresponding to more recent weathering processes.

In this study, we aim to identify the differences in clay mineral assemblage between the PlQ sediments of Medvednica Mt., which represent past weathering cycles, and the materials and soils developed on them by recent weathering processes. Different mineral species in the clay mineral assemblage will presumably result in different properties of the samples (e.g. CEC, LC) and reflect different environmental conditions present during different weathering cycles.

The study area is part of the North Croatian basin in the southwestern part of the Pannonian Basin System (PBS), one of the back-arc basins formed during Miocene extension (Pavelić and Kovačić 2018 and references therein). Medvednica Mt. is one of the inselbergs of the area, consisting of exposed metamorphosed and non-metamorphosed sediments of Paleozoic-Mezozoic age surrounded by Miocene sediments (Tomljenović et al. 2008; Van Gelder et al. 2015). Medvednica Mt. took its current form during strong tectonic movements starting in Pliocene and was part of the landmass with only local fluvio-lacustrine smaller sedimentation basins. During that period, proluvial and colluvial deposition was also taking place through erosion and transport of the material from the upper parts of the mountain (Šikić 1995).

The bedrock sediments of the area classified as PlQ sediments are largely assumed to be of Late Pliocene age according to their stratigraphic position. They were discontinuously deposited on eroded Upper Miocene sediments and are, at places, continuously transitional towards younger Quaternary sediments. Locally, PlQ sediments are covered by Pleistocene eolian deposits (loess), which are largely degraded into pseudogleyed loess derivates (Rubinić et al. 2018). Generally, PlQ sediments are composed primarily of unsorted gravels of quartz and older rocks (limestones and various schists), and subordinately by sands, silts and clays. The gravels are poorly rounded, irregularly mixed with clayey sands and silts that are just partly impregnated with limonite. They seem to have been predominantly deposited as fresh water proluvial to fluvio-lacustrine sediments of mollase type (Basch 1983).

According to Kurečić et al. (2021) who studied the mineral assemblage of the adjacent Pliocene Viviparus beds SE from Medvednica Mt., the change of tectonic regime from extension to compression in this part of the PBS during Pliocene (Tomljenović and Csontos 2001; Pavelić 2001; Van Gelder et al. 2015) resulted in new local sources of clastic material. Consequently, the material is poorly sorted and locally varies in composition.

The climate of the area is temperate humid (Cfwbx in Köppen classification), with mean annual air temperature of 10°C and mean annual precipitation 1000–1100 mm with no distinct dry period (Zaninović et al. 2008). Vegetation cover varies from deciduous forest (natural cover) to orchards and grassland in more populated areas.

Three locations of PlQ sediments occurrence were chosen for the study, all situated on the lower SE slopes of Medvednica Mt. (Fig. 1).

Profiles from Dotrščina and Oporovec locations were sampled as follows: PlQ sediment was sampled at the depth of its outcrop occurrence (300 and 70 cm, respectively). Another sample was taken where a change in color, texture or both was noticed (120 and 40 cm depth, respectively), while the overlying soil was sampled at the depth of 25 cm to avoid vegetation litter and anthropogenic artefacts in the topsoil. The borehole sampling followed the same principle, except that, instead of one, two samples of PlQ sediment were taken as differences in color indicated heterogeneity within this proluvial deposit.

The color of the samples was determined using Munsell Soil Color Charts (1994) in both dry and moist state. The samples were air-dried and passed through a 2 mm sieve. Particle size distribution was determined using the pipette method with wet sieving and sedimentation after dispersion with 0.4 M sodium-pyrophosphate (Na₄P₂O₇). Measurements of pH were done using Hach pH-meter EC-30 in suspensions containing 10 g of bulk sample mixed with 25 mL H₂O or 25 mL 0.01 M CaCl₂.

Mineralogical analyses were performed on bulk samples as well as clay fractions < 2 and < 0.2 µm. Bulk mineralogy was determined using X-ray diffraction (XRD) on samples micronized using McCrone mill (Glen Creston Ltd., Retsch GmbH) in ethanol with 10% ZnO (Fisher Scientific, catalog number Z/1350) added as an internal standard. Dried mixture was then passed through 0.4 mm sieve and side-loaded to obtain random powder mounts according to procedure described by (Środoń et al. 2001). Fractions < 2 and < 0.2 µm were separated by centrifugation using a Sorvall™ ST 40 Centrifuge (Thermo Scientific™, Waltam, MA, USA) after applying chemical treatments prescribed by Jackson (1958). Carbonate cement and organic matter were removed using Na acetate–acetic acid buffer and hydrogen peroxide buffered with Na-acetate buffer, respectively. Free Fe-oxides were removed using sodium dithionite-citrate-bicarbonate (Mehra and Jackson 1958). The separated clay fractions were coagulated by the addition of NaCl and split into two portions. Each portion was saturated with magnesium or potassium by washing with 1 M respective chloride solution. After saturation, all the portions were dialyzed using Nadir® dialysis tubbing (pre–washed in de–ionized water to remove glycerol) and dried. The oriented mounts were prepared from Mg²⁺ and K⁺-saturated clays. The powders were dispersed in deionized water using Vibra Cell™ Ultrasonic Liquid Processor (Sonics, VCX 130PB, Newtown, CT, USA) and deposited onto petrographic glass slides (with surface density around 10 mg/cm²) or silicon low-background wafers. XRD patterns for Mg-saturated and K-saturated clays were recorded in air–dried conditions at ambient relative humidity (24.4–62.3%). XRD patterns for K-saturated clay fractions were also recorded after heating the mounts for 1 h at 330°C and again at 550°C whereas for Mg-saturated clays were also analyzed after solvation with glycerol.

Bulk samples and oriented XRD patterns were recorded with a Phillips X’Pert diffractometer (Philips Electronics N.V., Almelo, The Netherlands) using CuKα radiation. The instrument was equipped with a vertical goniometer PW3020, a 1° divergence slit, a 0.2 mm receiving slit, incident- and diffracted-beam Soller slits, a 1° anti-scatter slit, and a graphite monochromator placed into the diffracted beam path. The high-voltage generator operated at 40 kV and 30 mA. The oriented mounts were scanned from 2° to 52° 2θ at a speed of 0.01° 2θ /s, and random mounts of bulk samples were scanned from 2° to 65° 2θ at a speed of 0.004°/s.

Random mounts of < 2 and < 0.2 µm fractions were scanned using Philips X’Pert PRO diffractometer employing graphite monochromatized CuKα radiation induced by 40 mA and 40 kV, with 1/8° divergent, 1/4° antiscatter slit and 7.5° receiving slit using Pixcel 1D detector with 255 active channels. The analysis was done in a continuous scanning mode from 4° to 65° 2θ with the speed of 0.05° 2θ/s.

Clay minerals were determined based on the parameters following literature data (Brindley and Brown 1980; Barnhisel and Bertsch 1989; Moore and Reynolds 1997; Środoń 2006). Kaolinite was determined by a d₀₀₁ spacing of 7 Å, unchanged with any cation saturation and glycerol treatment, but which disappeared when heated to 550°C. Illite was defined by 10 Å d₀₀₁ spacing that does not change position with any of the employed treatments. Clay mineral with a d₀₀₁ spacing of \(\approx\)14 Å in Mg-saturated air-dried form expanding to \(\approx\)18 Å with glycerol treatment was identified as smectite and clay mineral with the same spacing in Mg-saturated air-dried form not expanding with glycerol treatment, but which collapses to 10 Å after K-saturation was identified as vermiculite. Another species with 14 Å d-spacing in air-dried state (both Mg- and K-saturated) but which collapses towards 10 Å upon heating was identified as hydroxy-interlayered mineral (HIM) based on the definition proposed by Meunier (2007).

Fourier transform infrared spectroscopy in total attenuated reflectance mode (FTIR-ATR) was used for qualitative mineralogical analysis on < 2 µm and < 0.2 µm fractions of Mg-saturated samples. A Thermo Scientific™ Nicolet™ iS50 Fourier-transform infrared spectrometer equipped with the PIKE MIRacle™ single-reflection attenuated total reflectance diamond crystal was used. Infrared spectra were collected in the range 4000–650 cm^− 1 as 100-scan averages with 4 cm^− 1 resolution. The spectra were interpreted using data from literature (Russel and Fraser 1994; Madejova and Komadel 2001). Same instrument configuration was also used to measure the average layer charge (LC) of swelling minerals by means of O-D method described by (Kuligiewicz et al. 2015a, b). The principle of the O-D method is measuring the position of high-energy O-D stretching band (νO-D) on samples saturated with D₂O. Kuligiewicz et al. (2015a) found that the position of νO-D measured for dioctahedral smectite correlates with the average layer charge measured by structural formula method and alkylammonium method. Therefore, the position of the band can be used for prediction of the average layer charge of wettable interlayers. The O-D method is not sensitive to layer charge localization and non-swelling impurities within a sample. It is calibrated for the whole range of smectite LC (0.2–0.6 per formula unit (p.f.u.)). For the measurement, 5 mg of each sample was dispersed in the 0.5 ml of D₂O (Deutero, 99.9 atom% D, Kastellaun, Germany). A drop of the suspension was deposited on top of the diamond crystal and dried with dry N₂ (Air Products, purity > 98 vol%, < 0.02 ppb H₂O) under custom designed cup connected with D₂O-saturation system. After drying, the sample was re-saturated with D₂O vapor and five spectra were collected at 80, 75, 70, 65, and 60% RH(D₂O). The exact position of O-D stretching band was determined by second derivative minima calculated using Savitzky-Golay filter (12-point smoothing) in the OMNIC™ 9.6.251 software. For the prediction of LC, an equation obtained from linear correlation of νO-D and the structural formula method for a set of reference dioctahedral smectites was used (Kuligiewicz et al. 2015a). The error of the returned value is estimated to be 0.02 p.f.u.

Cation exchange capacity (CEC) of bulk samples and < 2 µm fractions was determined using copper ethylenediamine complex as index cations (Bergaya and Vayer 1997; Ammann et al. 2005). The analysis was performed at pH 7 using Hach DR4000/U UV-VIS spectrometer with absorbance adjusted at 548 nm to measure the decrease of the index cations in the solution.

Bulk elemental composition was determined using Spectro Arcos (ICP-OES) spectrometer (SPECTRO Analytical Instruments GmbH, Kleve, Germany) with the radially (side-on) viewed torch configuration. The samples were digested in a mixture of spectrally clean and concentrated nitric, hydrochloric, and hydrofluoric acids in the presence of boric acid using the Ethos-Up Milestone microwave oven (Milestone Srl, Sorisole (BG), Italy). Certified reference material OREAS 920 (Oreas®, Melbourne, Australia) was processed and measured in the same way as the samples in the same analytical cycle to monitor the accuracy of the analyses.

4.1. SELECTED PHYSICAL AND CHEMICAL PROPERTIES OF INVESTIGATED PROFILES

The sampled PlQ sediment differs in texture, color and pH across the selected locations (Table 1). Clay content in the PlQ sediment samples varies from 17.9% (DOT 300) to 52.6% (H2 250). Whereas in the H2 borehole sediments clay is the most abundant fraction, in the remaining two it is silt. As for the overlying weathered material and soil, they contain less clay and more silt than the PlQ sediment. The color ranges from light yellowish brown to yellowish brown or brown with DOT profile samples showing a prominent difference in color between the sediment and overlying material, the former having a redder hue. Measured pH values indicate acidic environment; pH is generally higher in H2 borehole samples, than in DOT and OPO (Table 1). There is no obvious pH trend within the profiles, except that the values are generally slightly lower in the samples taken closest to the surface than in the PlQ sediment for DOT and OPO profiles.

Table 1. Selected sample properties (sample labels indicate location and sampling depth in cm)

Sample label	% of particles (sizes in mm)					Texture	Soil color (Munsell Soil Color Charts, 1994)		pH
	Coarse sand	Fine sand	Coarse silt	Fine silt	Clay
	2.0-0.2	0.2-0.063	0.063-0.02	0.02-0.002	<0.002
	2.0-0.2	0.2-0.063	0.063-0.02	0.02-0.002	<0.002		moist	dry	H₂O	CaCl₂
DOT 25	3.5	3.2	36.1	41.1	16.1	silt loam	2.5Y 4/2	2.5Y 6/4	4.3	3.8
DOT 120	2.4	2.8	34.0	39.2	21.6	silt loam	2.5Y 3/2	2.5Y 5/4	5.1	4.4
DOT 300	12.6	8.5	25.1	35.9	17.9	silt loam	10YR 3/6	10YR 5/6	4.9	4.3
OPO 25	2.3	1.8	29.3	40.9	25.7	silt loam	2.5Y 3/3	2.5Y 5/2	4.2	3.5
OPO 40	2.0	1.8	24.3	35.9	36.0	silt clay loam	2.5Y 4/4	2.5Y 6/4	4.5	3.8
OPO 70	2.0	1.4	19.4	34.4	42.8	silty clay	2.5Y 4/4	2.5Y 6/3	4.8	3.9
H2 25	6.0	2.1	32.0	46.7	13.2	silt loam	10YR 3/3	10YR 5/2	6.3	6.0
H2 70	1.4	1.9	40.1	47.1	9.5	silt	10YR 4/4	10YR 6/4	6.0	5.6
H2 150	2.0	0.7	22.4	32.5	42.4	silty clay	10YR 5/6	10YR 6/6	5.8	5.2
H2 250	0.9	0.7	19.8	26.0	52.6	silty clay	10YR 5/4	10YR 4/3	6.7	6.2

Chemical composition shows the prevalence of SiO₂ (54.91–68.58%), Al₂O₃ (12.75–18.59%) and Fe₂O₃ (3.82–6.62%) in all of the samples (Table 2). The chemical composition of the PlQ sediments is similar across all locations. The overlying material in DOT and OPO profiles shows a relative decrease in Fe, while a relative Al decrease is visible in all three profiles (Table 2). SiO₂ to Al₂O₃ ratio in OPO and DOT profiles decreases with depth, whereas in H2 borehole samples it is the highest in H2 70 sample and lowest in the lowermost sample of PlQ sediment.

Table 2

Chemical composition (units in %) and CEC (cmol(+)/kg) of the samples
Analyte	SAMPLE
Analyte	DOT 25	DOT 120	DOT 300	OPO 25	OPO 40	OPO 70	H2 25	H2 70	H2 150	H2 250
SiO₂	68.58	61.75	54.91	66.53	61.75	59.24	64.71	68.13	62.43	57.65
Al₂O₃	12.75	14.00	16.12	12.94	14.89	16.80	14.43	14.19	16.49	18.59
Fe₂O₃	3.82	6.01	6.62	3.86	4.82	5.16	5.57	4.31	4.93	5.15
MgO	0.81	1.09	1.66	0.59	0.73	1.20	1.01	1.16	0.71	0.97
CaO	0.47	0.51	1.58	0.29	0.34	0.47	0.67	0.67	0.49	0.61
Na₂O	1.20	1.00	1.58	0.90	0.69	0.60	1.09	1.30	0.59	0.64
K₂O	1.54	1.79	1.22	1.61	1.70	1.95	1.78	1.88	1.42	1.60
TiO₂	1.26	1.11	1.37	1.20	1.11	1.00	1.19	1.24	1.17	1.00
P₂O₅	0.10	0.12	0.12	0.08	0.06	0.08	0.11	0.11	0.02	0.03
MnO	0.13	0.47	0.01	0.05	0.05	0.08	0.14	0.07	0.03	0.02
Cr₂O₃	0.01	0.01	0.02	0.01	0.01	0.01	0.01	0.01	0.01	0.01
LOI	6.20	6.20	9.20	6.80	7.40	9.20	8.20	5.60	8.00	9.20
Sum	96.90	94.07	94.42	94.91	93.59	95.84	98.99	98.72	96.35	95.51
SiO₂/Al₂O₃	5.38	4.41	3.41	5.14	4.15	3.53	4.48	4.80	3.79	3.10
CEC bulk	10	22	25	21	29	39	11	5	23	32
CEC < 2 µm	33	40	46	42	49	56	39	33	46	50

CEC values range from 5 to 39 cmol(+)/kg for bulk samples, and from 33 to 56 cmol(+)/kg for < 2 µm fractions, separated after the removal of organic matter and free Fe oxides (Table 2). In both cases, the PlQ sediments show the highest CEC values in all profiles.

4.2. MINERALOGY

The bulk mineralogy of PlQ sediments is dominated by quartz, followed by smaller amounts of plagioclase, K-feldspar, mica and clay minerals (Fig. 2). The differences in the bulk mineral composition across all PlQ sediments are not prominent; however, the DOT 300 sample differs from the other PlQ samples in the higher content of plagioclase and the trace amount of gibbsite (d = 4.85), hematite (d = 2.68 Å), and likely amphibole (d = 8.44 Å; actinolite?).

Mineralogical composition in the upper parts of the profiles does not differ much from those of the PlQ sediments. Possible diaspore (d = 4.71 Å) is detected in DOT 25 and H2 25 and H2 70 samples. Quartz is generally more abundant in the soil and weathered material than in the PlQ sediments.

Both of the clay fractions separated from the PlQ sediments are dominated by expandable clay minerals and contain kaolinite and illite (Fig. 3). The expandable mineral is dominantly smectite except in DOT 300 sample where vermiculite is more abundant. Trace vermiculite can be discerned alongside smectite in OPO 70 and H2 150. Goethite (d \(\approx\) 4.18 Å) was detected in DOT 300 and H2 250 samples. In the < 0.2 µm fraction of the PlQ sediments there are no major differences in clay mineralogy; however, there is less vermiculite (if present in the sample) and illite. There is no distinguishable size separation trend for kaolinite.

Clay mineralogy of the upper parts of the profiles is marked by the presence of HIMs (sensu Meunier 2007) in DOT 25 and H2 25 and H2 70 samples (Figs. 4 and 6). Smectite, if present, appears only in small quantities in those two locations. OPO samples contain smectite as the dominant clay mineral, but in the OPO 25 sample vermiculite is also present (Fig. 5). Presence of HIMs in OPO 25 and OPO 70 samples is possible but inconclusive. There are no substantial differences between < 2 and < 0.2 µm fractions; the latter contains less vermiculite (if present) and less illite in some samples.

DOT 120 sample shows an incomplete collapse after heating the K-saturated sample to 550°C which can be explained by the presence of mixed-layered mineral consisting of chlorite and expandable smectitic or vermiculitic layers (Fig. 4). Possible kaolinite-smectite mixed-layering can be seen in some samples via broadening of the 7 Å diffraction maxima towards lower angles.

Overall trend is characterized by a decrease of smectite towards the surface in DOT and OPO profiles. In DOT 120 sample smectite is still present, but in DOT 25 HIM dominates with possibly some vermiculite present. In OPO samples smectite is present throughout the profile; however, its content decreases in OPO 25 sample, while the content of vermiculite, present in OPO 40 only in trace amounts, increases. H2 profile does not follow the same trend; HIM is present in both H2 70 and H2 25 samples, whereas smectite is present only in H2 25 sample.

Randomly oriented mounts of < 2 µm (Fig. 7) and < 0.2 µm show a diffraction maximum around 1.50 Å corresponding to d₀₆₀ of dioctahedral clay minerals. Trioctahedral 060 diffraction maximum (1.54 Å) is overshadowed by the presence of quartz so identification of trioctahedral clay minerals, if there are any, is difficult. Nevertheless, this diffraction maximum and another quartz diffraction maximum at 1.81 Å should be of roughly the same intensity, which is the case here. Randomly oriented mounts also show quartz and feldspar in < 2 µm fractions and some quartz presence in < 0.2 µm fractions.

FTIR spectra confirm the presence of kaolinite (3700 cm^− 1; OH stretching of inner-surface hydroxyl groups band) in all of the samples (Figs. 8–9). Bands at 3620 cm^− 1 (stretching of structural hydroxyl groups), around 915 cm^− 1 (Al-OH-Al deformation) and around 750 cm^− 1 (Al-O-Si vibrations) are characteristic of dioctahedral 2:1 clay minerals. Some Mg for Al substitution occurs, shown in the presence of Al-Mg-OH deformation band around 830 cm^− 1. The presence of quartz in the clay fraction is evidenced by 800 and 780 cm^− 1 doublet.

The absolute layer charge values of the expandable layers are in the range of 0.50 to 0.68 p.f.u. (Table 3) and they generally correspond to the clay mineralogy of the samples. Higher absolute LC values are consistent with samples containing less smectite and more vermiculite or HIM. Lower values correspond with samples where smectite is dominant. A general trend is the increased LC in the surface samples in OPO and DOT profiles, while in H2 borehole, even though the values do not show the same pattern, the lowermost PlQ sediment sample has the lowest absolute LC, same as in the other two profiles.

Table 3

Absolute layer charge values (p.f.u.) of the expandable layers in < 0.2 µm fractions measured on Mg-saturated samples
SAMPLE	LAYER CHARGE
DOT 25	0.66
DOT 120	0.63
DOT 300	0.61
OPO 25	0.59
OPO 40	0.58
OPO 70	0.50
H2 25	0.64
H2 70	0.66
H2 150	0.68
H2 250	0.59

The investigated profiles show variations in the characteristics of PlQ sediment, attesting to the chaotic proluvial sedimentation with different material available at different times and locations. The material on top of PlQ sediments could be genetically related to them in case of OPO and H2 profiles; however, particle size distribution, color and mineralogy differences in DOT profile indicate that the overlying material did not originate from the PlQ sediment, but from the loess deposited on top of it. Namely, the sand contents and coarse/fine sand ratios of the DOT 120 and DOT 300 samples (Table 1) point to a clear lithic discontinuity between the two layers (IUSS Working Group WRB 2022). Furthermore, the moist soil color is dark yellowish brown for the DOT 300 and very dark greyish brown for the DOT 120, with their hues far apart (Table 1). The upper portion of the DOT profile (samples DOT 25 and DOT 120) represents a Stagnosol, i.e. a soil in which periodic stagnation of precipitation water on/in the poorly permeable subsurface horizon causes reducing conditions and stagnic properties (IUSS Working Group WRB 2022). In Croatian Stagnosols (i.e. Pseudogleys), the textural-structural vertical contrast is attributed to the top-down soil formation on initially vertically homogeneous loess deposits, through the following sequence of major pedogenetic processes: leaching and acidification, lessivage, pseudogleying (Rubinić et al. 2014, 2015). All of the above processes are confirmed in the upper part of the DOT profile by the vertical trends in soil pH and content of clay (Table 1), SiO₂/Al₂O₃ ratio and contents of TiO₂, Fe₂O₃ and MnO (Table 2).

Particle size distribution shows high clay content in all PlQ sediments. Less clay in the upper parts of the profiles could be influenced by the local topography (slope inclination) causing washing out of the clay fraction from the surface horizons. Anthropogenic influence, especially in H2 location, which is surrounded by residential lots, could also be significant.

Low pH is related to the mineralogy and chemical composition of the samples, i.e. the absence of carbonate phases and leaching out of basic cations. The relatively higher pH values in H2 borehole are accompanied by slightly higher concentrations of basic cations.

Small differences in mineralogy between PlQ sediments and weathered material and soil developed on top of them reveal different weathering conditions. The presence of gibbsite in DOT 300 sample indicates advanced weathering (Hsu 1989). It is postulated that Al-oxides form from feldspar in conditions of excessive drainage with a clay mineral intermediate, typically kaolinite, in case of moderate drainage (Hsu 1989 and references therein). Gibbsite occurrence in DOT 300 sample is characteristic for high intensity weathering, so here it is possibly indicative of Mid-Pliocene warm period (Prista et al. 2015; Szabó et al. 2022), during which this particular PlQ deposit could have been formed. Diaspore present in DOT 25 and H2 25 and H2 70 samples indicates somewhat less intensive chemical weathering in more recent cycles.

The intensity of weathering could be also viewed through feldspar presence. Plagioclase and K-feldspar in trace amounts are present in all of the studied samples. The persistence of feldspar would, contrary to Al-(oxy)hydroxides presence in the previously mentioned samples, indicate weathering regime of lower intensity; however, abundant feldspar presence in the original rocks and initial soil formation in this area could also explain their persistence (Huang 1989).

The comparison between past and more recent weathering products on the SE slopes of Medvednica Mt. reveal subtle differences in clay mineral assemblage. As clay minerals in soils and weathering profiles reflect both the variety of parent material as well as the transformation and neoformation processes that may have occurred in previous weathering environments (Wilson 1999), these differences can serve to show the changing environmental conditions prevailing during past (Pliocene) weathering cycles and more recent (Quaternary) ones.

Persistence of goethite in clay fractions of DOT 300 sample and throughout H2 borehole with the exception of H2 150 sample even after dithionite treatment could point to high level of Al substituted Fe (Jeanroy et al. 1991). Al-rich goethite is formed in highly weathered conditions like tropical and subtropical soils, saprolites and bauxites, as well as in weakly acid reductomorphic environments (Schwertmann and Taylor 1989)

The principal finding of mineralogical analysis is the occurrence of HIMs in the topmost samples, specifically DOT 25, H2 25 and H2 70. The presence of HIMs in OPO samples is inconclusive. The wide diffraction maximum between 14 and 10 Å in K-saturated air-dried samples can indicate the presence of hydroxyl interlayering (Meunier 2007), but may also indicate smectite that does not collapse fully in case of high relative humidity. Such a finding of transitional clay mineral phases implies the current nature of weathering processes and corresponds well with acid to moderately acid pH values (Table 1) and low organic matter content. HIMs can be formed from chlorite weathering, or from the deposition of hydroxy-Al polymeric components within the interlayer spaces of expandable clay minerals (Barnhisel and Bertsch 1989; Skiba et al. 2011). The latter is the most plausible explanation for the investigated samples, as no chlorite was detected in any of the samples, and there is a trend of less expandable clay minerals in the surface samples where HIMs are detected. HIMs are likely to weather to kaolinite (Barnhisel and Bertsch 1989) so their stability here implies that current weathering conditions are not suitable for kaolinite formation. The presence of HIMs is also indicative of acid environment, initial soil development and moderate climate with interchanging wetting and drying cycles (Rich 1968; Karathanasis 1988; Barnhisel and Bertsch 1989; Skiba et al. 2011). Ideal pH range for HIMs development is between 4.0 and 5.8 (Rich 1968); however their formation was recorded even at pH > 5 if little organic matter is present (Georgiadis et al. 2020 and references therein). Based on the color of the samples (Table 1), low levels of organic matter are implied, which would explain HIMs in the uppermost parts of the profile where wetting and drying cycles are most pronounced and there is no hindrance from the organic matter. Presence of HIMs has an impact on soil characteristics, i.e. hydroxy-Al interlayer reduces expandability of smectite and vermiculite as well as CEC which decreases with increasing amount of fixed Al (Meunier 2007). In the samples studied here, clay minerals contribute dominantly to CEC as shown by the increase of CEC values when clay fraction is separated (Table 2). CEC of bulk samples is decreased in the topmost samples of all the profiles, including OPO in which presence of HIMs is doubtful. A conclusion can be made that the decrease in CEC of the bulk samples is mostly influenced by the decrease of the overall clay content.

The measured layer charge represents an average value of expandable clay minerals in the samples, and since the calibration of the measurement method was done only for smectites (Kuligiewicz et al. 2015a), absolute values above 0.6 p.f.u. should be interpreted with caution. However, the clay minerals composition and CEC of the clay fraction are in line with the obtained LC values. The relative increase of absolute LC values in the surface samples corresponds to clay mineralogy showing less smectite and more vermiculite and/or HIMs. Considering the LC values measured in the samples, it is possible to assume smectites to be high-charge, according to the classification criteria proposed by Emmerich et al. (2009). This corresponds somewhat with Wilson (1987) postulating that soil smectites tend to be more Fe rich and show intermediate charge characteristics. Gillot et al. (2001) suggested that high-charge smectites are present in younger soils because further weathering and/or pedogenesis, which desaturates smectites and leads to lower absolute LC values, has still not taken place. On the other hand, absolute LC values of the samples being around 0.6 p.f.u., which is the arbitrary border separating smectite and vermiculite group (Środoń 2006), would suggest the vermiculite layers present in the samples are on the lower end of the absolute LC spectrum for this group of clay minerals. Pai et al. (2006) have linked low-charge vermiculites with areas of more intense weathering, so the possible explanation here could be that vermiculite layers are inherited and recycled from the past weathering events.

The relationship between measured LC and CEC of the < 2 µm fraction in the studied samples is inversely related (Fig. 10). Although the correlation is not too strong, it suggests that LC values above 0.6 obtained with O-D method are likely to be correct. The only exception is H2 150 sample, where the LC is the highest among all the samples, while CEC is moderate. This inconsistency is additionally surprising because of the lack of vermiculite in XRD patterns of clay fraction separated from H2 150 sample. Nevertheless, in general, low CEC is connected with high LC, and the clay mineral composition of the samples. For DOT 25 and DOT 120 samples low CEC is most likely caused by the abundance of HIMs, where cation exchange is hindered by hydroxyl interlayers. In OPO profile, where the LC corresponds to high-charge smectites, the CEC is higher than in DOT and H2 profiles. In samples OPO 25 and OPO 40, even though the LC is almost the same, the difference in CEC might be explained by possible hydroxyl interlayering in the OPO 25 sample.

The shape of O-D stretching envelope and the fact that νO-D is not well separated (Figs. 10–11), could point to some illite-expandables mixed layering, but could also be due to small particle size (< 0.2 µm) used in the analysis (Kuligiewicz et al. 2018).

Overall, mineralogical composition of the studied samples can be used to differentiate the weathering products formed in different environmental conditions. Plio-Quaternary sediments, formed largely in Late Pliocene, also containing older weathering products, speak to a more intense weathering regime visible in the occurrence of Al (oxy)hydroxides, Al-rich goethite and the dominance of low-charge expandables in the clay fractions. More recent weathering material shows less intensive weathering regime, mainly visible in the formation of HIMs. Additionally, layer charge values can also be interpreted in the same sense; even though this trend is not observed in all of the studied profiles, there is an abundance of high-charge expandable layers in the surface samples, in line with their hindered transformation in the conditions of less intense weathering.

PlQ sediments can be viewed as representative of past weathering regimes and can be compared to the overlying material representing more recent weathering. Intermediary clay transformation phases such as HIMs suggest moderate recent and current weathering conditions, while the mineralogy, and especially appearance of Al (oxy)hydroxides in PlQ sediments locally implies more intense weathering in the past cycles. Dominant expandable clay mineral in the samples is reflected in the XRD patterns. The measured average layer charge of the expandable layers is in agreement with the clay mineralogy and CEC of the samples. The highest absolute layer charge values in the surface horizon can be related to a lower intensity weathering regime in more recently developed weathered material and soil.

ACKNOWLEDGEMENTS

The authors would like to thank Dr Dorota Salata for the chemical composition analysis.

STATEMENTS AND DECLARATIONS

Funding

This work was partly financed by the University of Zagreb [grants no. 20286304 and no. 20286480] and by National Science Centre Poland [grant number: UMO-2018/31/B/ST10/02192].

CEEPUS (network RO-0038 Earth-Science Studies in Central and South-Eastern Europe) financed two 1-month grants (in 2018 and 2019) for the study visit of ZG to Jagiellonian University in Kraków, Poland.

Competing Interests

The authors have no relevant financial or non-financial interests to disclose.

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No competing interests reported.

The effect of weathering on clay mineralogy of Plio-Quaternary sediments and the overlying material on SE slopes of Medvednica Mt., Croatia

Status:

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Abstract

Figures

1. Introduction

2. Geological Setting

3. Materials And Methods

4. Results

4.1. SELECTED PHYSICAL AND CHEMICAL PROPERTIES OF INVESTIGATED PROFILES

4.2. MINERALOGY

5. Discussion

6. Conclusions

Declarations

References

Additional Declarations

Status:

Journal Publication

Version 1