Thermomagnetic analysis applied for identification of lithogenic and pedogenic iron oxides in topsoils from Bulgaria

Identification of magnetic minerals, which determine the magnetic properties of nat - ural rocks, sediments and soils, is of crucial importance for any further interpretation of their magnetic signature in environmental or paleogeographical context. One of the major and widely used methods for determination of kind of magnetic phases in natural materials is by obtaining the Curie/Neel temperature of the respective magnetic miner - al. In this contribution, we report a set of thermomagnetic measurements of high-tem - perature behavior of magnetic susceptibility for topsoils from the territory of Bulgaria, aiming to reveal the pedogenic and lithogenic signature. The data are considered and reported in respect to the parent rock type. Our results suggest that pedogenic magnetic minerals are represented mainly by fine-grained maghemite and/or hematite with possi - ble Al-substitutions in the crystal lattice. This phase is expressed on the heating run of the thermomagnetic curve as a “hump” with a maximum at 250–300 °C. Coarse-grained magnetite is identified as a dominant lithogenic magnetic mineral. Hematite’s presence in a sub-set of red-colored soils is confirmed by additional analyses of isothermal rema - nence acquisition curves. Two hematite coercivity components were identified, related to pedogenic and lithogenic origin, respectively.


Introduction
Iron (Fe) is a ubiquitous element in Earth's crust and especially in its surface soils cover.Fe exists in soils in various forms -it is incorporated into the structure of clay minerals, resides in the detrital rock-forming minerals, or exist in forms of secondary pedogenic (oxy)hydroxides (Cornell and Schwertmann 2003).During the modern Anthropocene era (Certini and Scalenghe 2011), Fe in soils can also have anthropogenic origin sourced from iron-containing emissions of different industrial factories, traffic and agriculture (Hoffmann et al. 1999;Magiera et al. 2011;Declercq et al. 2020).Environmental-and rock-magnetism are widely applied methods for investigation of the origin and environmental significance of iron (oxy)hydroxides in soils and sediments (Thompson and Oldfield 1986;Evans and Heller 2003), based on their magnetic properties.Soil's magnetism is due to the presence of strongly magnetic iron oxides and is a phenomenon, which is recognized long ago (Mullins 1977).The most important environmental GeoStudies 1: 27-42 (2024), DOI: 10.3897/geostudies.1.e115530 Neli Jordanova & Diana Jordanova: Iron oxides in topsoils identified by thermomagnetic analyses factors, controlling the form of iron oxyhydroxides in soils, are temperature and precipitation (Jenny 1941), being the major climate-related parameters.Aside from anthropogenic input of strongly magnetic Fe-oxides, different theories on their origin in soils are developed through time (Dearing et al. 1996), including: 1) iron oxide grains, inherited from parent rock during weathering; 2) pedogenically formed through abiotic processes; 3) pedogenically produced by Fe-mediating bacteria; 4) intra-cellularly produced by magnetotactic bacteria.Still, no unique theory is able to explain the varieties of iron forms in soils and a combination of all proposed mechanisms seem to account for the observed magnetic properties of soils from different environmental settings.
Magnetic enrichment of topsoils from the temperate climatic belt in the Northern hemisphere is a well established phenomenon (Maher 1998) and the hypotheses existing about the origin of soil's magnetic enhancement, mentioned above, were developed on the basis of their properties.Magnetic susceptibility (K) is the simplest magnetic parameter, reflecting the ability of a material to be magnetized when it is placed in an external magnetic field.In case of isotropic medium, M = K.H, where M is the induced magnetization in the material and H is the strength of the applied field.In order to entangle the most probable mechanism, responsible for soil's magnetic properties, it is of crucial importance to discriminate pedogenic magnetic minerals from those of lithogenic origin.Thermomagnetic analysis, and in particular high-temperature behavior of magnetic susceptibility, is a widely applied method for identification of the magnetic minerals through their Curie (Neel) temperatures (O'Reilly 1984).In the present contribution, we report results from thermomagnetic analysis of a collection of topsoils from Bulgaria, developed on different parent materials and in different climate conditions, in order to evaluate the relative contribution of lithogenic vs pedogenic magnetic minerals.In addition to thermomagnetic analyses, results from coercivity un-mixing analysis of curves of step-wise acquisition of isothermal remanence (Maxbauer et al. 2016), are reported, aiming at better evaluation of the contribution of hematite (α-Fe 2 O 3 ) to the soil's magnetism.

Materials and methods
Soil collection studies included 42 sites (Fig. 1) from which soil samples were gathered from the uppermost 0-20 cm depth.Soil material was thoroughly mixed for obtaining homogeneous sample.The samples were a selected subset of national-scale collection, consisting of 511 sampling locations for soil's magnetic characterization (Jordanova 2016;Jordanova et al. 2016).Samples for the present study were chosen as representative for the major soil groups.At each site, field magnetic susceptibility K topsoil was measured using handheld KT-5 kappameter (SatisGeo s.r.o.).Final value of K was taken as the average of 5 to 10 single measurements done in an area of 2 m 2 .Sampling procedure was described in detail in Jordanova et al. (2016).Site's geographical location, lithology, soil type and magnetic susceptibility (field-measured topsoil and rock outcrops (where available) and soil's mass-specific) was summarized in Suppl.materials 1, 2.
In the laboratory, soil material was air-dried, gently crushed and passed through 2-mm sieve.Magnetic susceptibility K was measured using KLY-2 kappabridge (AGICO Ltd., Czech Republic) and mass-specific susceptibility (χ) was obtained by dividing K to the sample's mass.Sieved soil material was used for running thermomagnetic analyses using KLY-2 kappabridge (AGICO Ltd., Czech Republic), equipped with CS-23 high-temperature non-magnetic furnace with a special platinum thermometer.Small amount of soil material (sample mass varying between 0.02-0.06g) was placed in a quartz glass test tube of the instrument.Heating run was performed by continuous heating from room temperature up to 700 °C, employing fast heating rate (11 °C/min).Fast heating rate was chosen because it provides more sensitive information about the mineralogical transformations during heating, while using medium and slow heating rate the transformations occur more slowly and are smeared on the heating curve (Jordanova and Jordanova 2016).Magnetic susceptibility was registered automatically at each ~ 3 °C.After reaching the maximum temperature, cooling run from 700 °C to 40 °C was also recorded.Data acquired was processed using specialized software CUREVAL (AGICO Ltd., Czech Republic).Isothermal Remanent Magnetization (IRM) for selected samples (showing mainly red coloring and thus potentially containing hematite) was step-wise induced up to a maximum field of 5 Tesla.After each magnetization step IRM was measured using JR6A automatic spinner magnetometer (AGICO Ltd., Czech Republic) and IRM-acquisition curves were obtained.The data were further processed with MAX UnMix software (Maxbauer et al. 2016) to separate the main remanence coercivity components using a combination of cumulative Gaussian functions (Robertson and France 1994;Kruiver et al. 2001).Parameters "relative contribution to the total IRM" (% contr.), the field at which half of the Saturation IRM is acquired ("Bh" in log-units and in "mT"), and the width of the distribution (one standard deviation of the logarithmic distribution) expressed through the dispersion parameter "DP" (Kruiver et al. 2001) were estimated for each sample.

Results and discussion
Detailed magnetic characterization of the national-scale topsoil collection (Jordanova et al. 2016) revealed that the most important factor, controlling the magnetic properties of soil at this scale, is lithology.Such a conclusion is also drawn in other works, reporting wide-scale study area or collection of soils, developed on various rock types (Fialova et al. 2006;Hanesch et al. 2007;Théveniaut and Clarke 2013;Sardoo et al. 2023).Considering the sub-collection used in the present study, a confirmation of this general observation is the obtained log-linear regression between the field-measured topsoil magnetic susceptibility (K topsoil ) and the mass-specific magnetic susceptibility of rock pieces gathered from rock outcrops at the respective sampling site (Fig. 2).In this comparison, mass-specific value of magnetic susceptibility of rock is considered in order to avoid K-dependence on the surface roughness during field measurements.Based on the finding that lithology is a dominant influencing factor, samples were grouped into six major sub-groups, as follows: 1) soils, developed on granites (7 sites); 2) soils developed on volcano-sedimentary parent materials (including tuffs) (4 sites); 3) soils developed on limestones and marls (10 sites); 4) soils, developed on sandstones (11 sites); 5) soils, developed on alluvial/delluvial deposits and loess (5 sites); 6) soils, developed on metamorphic rocks (5 sites).This general grouping will be further used for consideration of thermomagnetic analyses of soils.GeoStudies 1: 27-42 (2024)  As mentioned in the methods section, thermomagnetic analyses were carried out using relatively small amount of material.This fact has important implications for the shape and characteristics of the k-T heating-cooling curves, as discussed in detail in Jordanova and Jordanova (2016).In the latter study, the authors found that using a small sample mass in thermomagnetic runs results in weaker changes in K, provoked by thermal transformations in Fe-bearing minerals during heating.On the other hand, a bigger sample's mass creates local anaerobic conditions due to the presence of organic compounds in soil, which leads to strong expression of mineral transformations during heating.Based on the above considerations, we regard the K-behavior in the temperature range ~100-300 °C (wide maxima in K at 200-300 °C (Fig. 3b-f)) as reflecting the presence of Fe-oxide phase with such Tc (or unblocking temperature, Tub), rather than as expression of a thermal transformation.This phase can be related to the presence of pedogenic maghemite fraction of grains with sizes spanning the single-domain -superparamagnetic (SD-SP) magnetic grain size range, as proposed recently by Zhang and Appel (2023).Another possibility is to link this phase to fine-grained hematite with abundant ion substitutions in the crystal lattice (Jiang et al. 2014).Taking into account that the soil types in this group are mainly Luvisols and Cambisols (Fig. 3), characterized by pedogenic formation of maghemite (Jordanova 2016), it supports our hypothesis for the presence of pedogenic maghemite fraction.Another magnetic phase, revealed on the heating k-T curves has a Tc of ~580 °C, attributable to magnetite (Fe 3 O 4 ).This phase has bigger share in Luvisols BOGD and BLA (Fig. 3d, e) and smaller -in Cambisols DOL and VLI (Fig. 3a, f).Considering the sharp drop in K at the Tc of magnetite (Fig. 3a, d, e, f), it can be supposed that magnetite exists as coarse multidomain (MD) grains (Dunlop and Özdemir 1997).Therefore, magnetite phase can be attributed to lithogenic minerals inherited from the parent rocks (granites).The dominant presence of MD magnetite in granites from Bulgaria is reported in several works (Georgiev et al. 2009;Henry et al. 2011).Sharp drop in K at T ~100 °C (Fig. 3a, b, c, e) is related to the presence of Fe-hydroxide goethite (α-Fe 2 O 3 ) which has a Neel temperature T N of 120 °C (Dunlop and Özdemir 1997).Sample DOL shows strong thermal transformation at ~270 °C (Fig. 3a) with creation of magnetite, which can be linked to transformation behavior of Fe-hydroxides ferrihydrite or lepidocrocite (γ-FeOOH) in the presence of organic matter (Hanesch et al. 2006).Cooling curves for all samples, except BOGD and VLI show creation of new strongly magnetic magnetite phase upon heating to 700 °C (Fig. 3).
Thermomagnetic curves for soils, developed on volcano-sedimentary materials are presented in Fig. 4. Topsoils, developed on volcano-sedimentary rocks, exhibit thermomagnetic behavior (Fig. 4), similar to that of soils, developed on granites (Fig. 3).Again, two main magnetic phases are outlined -one with Tc ~ 300 °C, attributable to fine-grained pedogenic maghemite (Zhang and Appel 2023) or fine grained hematite (Jiang et al. 2014), and a second one, characteristic for coarse-grained lithogenic magnetite with Tc of 580 °C.Low Tc~100 °C is also detected in sample IRE (Fig. 4c), indicative of goethite.For this sample heating-cooling run does not cause creation of new magnetic phase and the cooling curve coincides with the heating one, except for the part, indicative of goethite.For this reason, the Tc~300 °C in this particular case can be related to the presence of titanomagnetite with high Ti content, which is a common rock-forming mineral in volcanic rocks (O'Reilly 1986).Similarity in thermomagnetic behavior of soils, developed on granites and on volcano-sedimentary rocks is consistent GeoStudies 1: 27-42 ( 2024  Thermomagnetic curves for soils developed on limestones or marls show various shapes, indicative of different magnetic minerals and/or thermal transformations (Fig. 5).Samples GI and MLIO (Fig. 5a, b) clearly indicate sharp thermal transformation at ~300 °C with production of a big amount of secondary magnetite, as revealed by one-order of magnitude stronger signal on cooling.Such strong transformation might be related to thermal destruction of the iron hydroxide ferrihydrite (Hanesch et al. 2006).Luvisol samples from sites BYA and TET (Fig. 5c, d) show similar heating-cooling curves, characterized by one well expressed wide maximum at 250-300 °C and susceptibility decrease to zero at magnetite's Tc of ~580 °C.This magnetite is preserved on the cooling curve, but without the maximum at ~300 °C.This observation suggests that most probably the thermomagnetic curves indicate the presence of fine-grained magnetite, which upon heating is subjected to grain-size change.Therefore, we attribute the phase with the maximum at ~300 °C to pedogenic magnetite.However, taking into account that both soils are of Luvisol type and exhibit reddish coloring, another hypothesis is linking it to very fine-grained hematite (Jiang et al. 2014), but it needs further investigations.
Soil samples, shown at Fig. 5f-h display similar thermomagnetic curves with two magnetic phases present -the above discussed fine-grained magnetite with unblocking temperature (Tub) ~ 300 °C and well expressed magnetite with Tc ~ 580 °C.Similarly to samples in Fig. 5a, b, again strong thermal transformation(s) occur on heating above 600 °C and new magnetite phase appears (Fig. 5f-h).Mineral transformations on cooling might partly result from the thermal transformations in Ca-bearing bedrock (limestone, marl) (Duminuco et al. 1998).
Soils, developed on sandstones (Fig. 6) are the next group of samples.As seen from Fig. 6, for all samples, except DOSE (Fig. 6a) and EFREM (Fig. 6f), the magnetic susceptibility behavior during heating is dominated by the magnetic phase, described previously for the other soils (Figs 3-5) and characterized by a wide hump with maximum at ~ 250-300 °C (Fig. 6).It is considered that this phase is related to the presence of pedogenic fraction fine-grained maghemite (Zhang and Appel 2023) or structural effects in the behavior of ultra-fine grained pigmentary hematite (Chernyshova et al. 2007;Jiang et al. 2014).Elucidation of this non-uniqueness is further tested by coercivity decomposition of Isothermal Remanent Magnetization curves and will be discussed later.In addition to this dominant magnetic phase, magnetite (Fe 3 O 4 ) with Tc of 580 °C is detected for soils VLK (Fig. 6b), KOJ (Fig. 6d) and EFREM (Fig. 6g).Most probably, it originates from the parent rock mineral composition.Low T N ~100 °C is detected in samples ELE, KOJ (Fig. 6c, d) and VOIVODOVO (Fig. 6f), assigned to goethite's presence.
Soils, developed on various kinds of metamorphic rocks (Fig. 7) show thermomagnetic curves, largely similar to those described above.Distinctly different from all other thermomagnetic curves is the behavior of sample SVETULKA 2 (Fig. 7a).Very strong alteration peak in K is observed on heating above 400 °C, which is related to secondary magnetite's formation.Such transformation behavior is typical for goethite, mixed with organic carbon (Hanesch et al. 2006).Other samples show presence of typical magnetic phases, assigned above to pedogenic maghemite/hematite and (lithogenic) magnetite (Fig. 7b-e).Soils, developed on loosely consolidated parent materials like alluvium and loess (Fig. 8) show more variable thermomagnetic behavior, compared to soils developed on solid rock parent material (Figs 3-7).Soil TRU developed on alluvial materials (Fig. 8a) displays two consecutive strong magnetic transformations -the first one taking place at ~ 200 °C and a second one, starting after 400 °C.This behavior is very similar to the one, described in Hanesch et al. (2006) for soil sample from A-horizon of a Cambisol from southern Germany.Considering the transformation behavior observed, it can be assigned to thermal alterations in ferryhidrite and goethite, respectively (Hanesch et al. 2006).Abundance of unstable Fe-hydroxides in alluvial soils is also confirmed in studies of depth soil profiles (Jordanova 2016).Very low negative K at room temperature for TRU sample is consistent with measurements of bulk standard samples (Suppl.material 2) and is due to dominant presence of quartz sand grains from the parent material, coupled with weak pedogenesis in this soil type (Jordanova 2016).Sharp transformation peak at ~300 °C on the heating curve of soil DRGI (Fig. 8b) can be related to ferryhidrite transformation in the presence of organics (Hanesch et al. 2006).Well expressed magnetite phase is observed on the thermomagnetic curves for the other samples -SPA (devel-oped on loess) and MS (developed on alluvium) (Fig. 8c, d).In addition, hump at ~ 300 °C is also observed on their heating curves, related to maghemite's behavior.All soils, developed on alluvium/loess suffer very strong thermal transformations on heating to 700 °C (Fig. 8), leading to production of secondary magnetite phase.
As revealed in the above discussion, identification of the magnetic phase, expressed as a wide "hump" with a maximum at ~ 250-300 °C is not unique when it is based solely on the high-temperature thermomagnetic analyses.It is possible to link this phase either to SP/SD maghemite or hematite.In addition, thermomagnetic analysis cannot detect hematite, when it is present in a mixture with ferrimagnetic minerals (magnetite, maghemite), except in cases of its prevailing share in the iron oxide pool (Frank and Nowaczyk 2008).In order to solve this ambiguity, a set of 10 samples was selected for analysis of their coercivity spectra through un-mixing the IRM-acquisition curves (Robertson and France 1994;Kruiver et al. 2001).Most of the samples were selected from red-colored soils in order to identify and characterize stable remanence-carrying hematite phase.Results from the coercivity unmixing are shown in Fig. 9.For most samples the IRM-acquisition curve was fitted by two coercivity components and only in samples CRV and SVETULKA1 it was necessary to consider three coercivity components for achieving reasonable data fit (Fig. 9i, j).
Coercivity components were denoted as follows: lowest coercivity C1 (Bh between 27-36 mT); moderate-coercivity C2 (Bh between 52-82 mT); high coercivity C3 (Bh between 580-1100 mT) and very high coercivity C4 (Bh above 2000 mT).They are indicated on Fig. 9 for each specimen.The lowest coercivity component C1 (Fig. 9) is identified in samples ZLI, EFREM, GI, PON and SVETULKA1.This component has a dominant contribution to IRM for samples EFREM, PON and GI, while in the others its share is up to ~20% of the total IRM (Fig. 9).Coercivity of this order is reported for fine grained magnetite/maghemite (Egli 2004).In the other samples the low-coercivity component C2 has coercivities -between 52-82 mT (Fig. 9a-c, f) and cannot be linked to magnetite/maghemite.Thus, we attribute it to a fraction of fine-grained hematite with Al-substitutions in the crystal lattice (Jiang et al. 2014), which is also characterized by low unblocking temperatures.IRM component C3 with Bh of several hundreds of mT (500-600 mT) (Fig. 9b, d, e, g, h) is typical for hematite (Roberts et al. 2020), while very high Bh (component C4) in samples TRU (Fig. 9f), CRV (Fig. 9i) and SVETULKA1 (Fig. 9j) can be attributed to goethite's presence.Therefore, unmixing analysis of IRM acquisition curves for red-colored topsoils confirmed the presence of two fractions of hematite -first one with relatively low coercivity, characteristic for fine-grained (pedogenic) hematite with Al-substitutions, and a second one, consistent with typical (lithogenic) hematite.Maghemite of low coercivity is also present in part of the samples, together with lithogenic hematite.
Thermomagnetic analysis of magnetic susceptibility of topsoils, utilizing small sample mass, allowed revealing the major mineral magnetic phases, responsible for the soil's magnetic characteristics.In the absence of micro-reducing environment during heating, created by bigger sample amount (Jordanova and Jordanova 2016), thermomagnetic curves for topsoils from Bulgaria demonstrated wide presence of a magnetic phase with low Tc (Tub) (~ 250-300 °C), tentatively attributed to pedogenic maghemite and/or finegrained hematite with abundant Al-substitutions.This phase is observed in soils, developed on various parent rocks (Figs 3-8), which further supports its pedogenic origin.Another independent confirmation of this conclusion comes from the obtained negative semi-linear regression (Fig. 10) between the calculated relative share of the "hump" phase height to the total magnetic susceptibility from the thermomagnetic curves (Figs 3-8) and the saturation magnetization (Ms), reported in a previous study (Jordanova et al. 2016).No systematic grouping according to the parent rock lithology is observed, thus supporting pedogenic origin of this phase.
Notwithstanding, it is worth mentioning that often in thermomagnetic K-T analyses of intrusive and volcanic rocks, a specific "hump" occurring in the range 200-300 °C is registered (e.g.Georgiev et al. 2014;Guo 2023).It is considered that this low-amplitude change in magnetic susceptibility could  reflect the transformation of trace amount of pyrite which converts to magnetite in this temperature range.Since pyrite is frequently identified by mineralogical methods in such rock types, this interpretation might be feasible.In case of soils reported in the present study, however, the amplitude of the thermomagnetic changes in K in the temperature range up to ~300-400 °C is much higher (Figs 3-5) and we thus favor the interpretation that pedogenic iron oxide fraction is responsible for it.Other Sulfur-containing minerals (pyrrhotite, greigite) show very strong and sharp transformation (Muxworthy et al. 2023), which is not observed for the soils in our study.
Lithogenic magnetic mineral identified through thermomagnetic analyses is most often coarse-grained magnetite.Nevertheless, hematite is detected in a number of red-colored soils, as those, developed on red sandstones (Fig. 6).Iron hydroxhides goethite, ferrihydrite, lepidocrocite are detected more rarely, mainly through their thermal transformation temperature, above which strongly magnetic iron oxide phase is created.

Conclusions
Thermomagnetic analysis of high-temperature behavior of magnetic susceptibility for a collection of topsoils from Bulgaria, developed on various lithologies, showed that it is a powerful tool for identification of iron (oxy)hydroxides' s origin.Using small sample mass, pedogenic fraction of maghemite and/or fine-grained hematite is revealed.This pedogenic fraction is identified in soils, developed on various parent rocks, showing that its origin is related to climatic-and biologically-induced processes in soils.Lithogenic magnetic minerals identified are mostly coarse grained magnetite and hematite.

Figure 1 .
Figure 1.Sampling site's location depicted on the map.

Figure 2 .
Figure 2. Mass-specific magnetic susceptibility of rocks (Xrock outcrop) vs. field-measured magnetic susceptibility of topsoils (K topsoil ) for the collection studied.

Figure 3 .
Figure 3. High temperature behavior of magnetic susceptibility for representative samples of soils, developed on granites.Soil type is indicated for each sample next to the sample's name.Heating performed in air, using fast heating rate (11 °C/min) and small sample's mass (20-60 µg).Heating run is shown in red, cooling run -in black.Righthand Y-axis relates to k-values on the cooling curve.

Figure 4 .
Figure 4. High temperature behavior of magnetic susceptibility for representative samples of soils, developed on volcano-sedimentary rocks.Heating run is shown in red, cooling run -in black.Right-hand Y-axis (when present) relates to k-values on the cooling curve.

Figure 5 .
Figure 5. High temperature behavior of magnetic susceptibility for representative samples of soils, developed on limestones/ marls.Heating run is shown in red, cooling run -in black.Right-hand Y-axis relates to k-values on the cooling curve.

Figure 6 .
Figure 6.High temperature behavior of magnetic susceptibility for representative samples of soils, developed on sandstones.Heating run is shown in red, cooling run -in black.Right-hand Y-axis (when present) relates to k-values on the cooling curve.

Figure 7 .
Figure 7. High temperature behavior of magnetic susceptibility for representative samples of soils, developed on metamorphic rocks.Heating run is shown in red, cooling run -in black.Right-hand Y-axis relates to k-values on the cooling curve.

Figure 8 .
Figure 8. High temperature behavior of magnetic susceptibility for representative samples of soils, developed on alluvial deposits and loess.Heating run is shown in red, cooling run -in black.Right-hand Y-axis relates to k-values on the cooling curve.

Figure 9 .
Figure 9. Unmixed coercivity components of IRM-acquisition curves, using MAX UnMix software by Maxbauer et al. (2016).Data points are denoted by dots; cumulative fit together with its confidence band is shaded in yellow.Fitted components with their confidence band are shown in blue, pink and green colors.Bh value (in mT) and component assignment (C1 -C4) for the corresponding component is denoted on each graph.

Figure 10 .
Figure10.Relation between saturation magnetization (Ms) and the calculated relative share of the "hump"-shaped phase with maximum at 250-300 °C from the thermomagnetic curves.Soils, developed on different parent material, are denoted in different color, according to the legend.