Magnetic Characteristics of Different Soil Types from Bulgaria

MAGNETIC CHARACTERISTICS OF DIFFERENT SOIL TYPES FROMBULGARIA

DIANA JORDANOVA AND NELI JORDANOVA"

Geophysical Institute, Bulg. Acad. Sci.Sofia, Bulgaria*

Summary: Magnetic studies of different soil types can provide valuable information aboutpalaeoenvironmental conditions at the time they were formed. Results of investigations of rock-magnetism of genetically different soil types, which developed over varying time intervals (MeadowChernozem - formed during the last 6000 years BP; Leached Cinnamonic soil - formed since the3rd-2nd century BC and Pellic Vertisoil - the oldest, formed since the Late Pliocene) are presented.The soil profiles of the Leached Cinnamonic and Pellic Vertisoil are characterized by lower valuesof magnetic susceptibility, as compared to that of the parent materials. It is shown that usingpercentage frequency-dependent susceptibility (Xfd%) and viscous remanent magnetization (VRM),pedogenic alteration could be detected even in such circumstances. The variations of theparameters measured along the depth of the studied Meadow Chernozem soil profile arecharacterized by gradual smooth magnetic enhancement, pointing to the absence of secondary re-distribution of pedogenic magnetites. In contrast to this case, the two other profiles (LeachedCinnamonic and Pellic Vertisoil) show magnetically enhanced lower (illuvial) horizons due toprocesses of acid destruction and re-precipitation of the original pedogenic ferromagnetic mineralsdown the profiles. These specific magnetic properties are of particular importance in usingsusceptibility variations as a palaeoclimatic proxy record.

Key words : soil, magnetic susceptibility, magnetic viscosity, pedogenesis

1. INTRODUCTION

Magnetic measurement techniques are widely used in environmental studies (Thompson andOldfield, 1986). Special attention is paid to determining the connections between different soilforming regimes (e.g., pedoenvironments) and types, properties and occurrence of Fe-oxides alongthe soil depths (Maher, 1986). This question is particularly significant in applying the magneticproperties of soils to the detection of anthropogenic pollution (Flanders, 1994; Georgeaud et al,1997; Petrovsky et al., 1998), as well as to palaeoclimatic reconstructions based on susceptibilityvariations along loess/palaeosol sequences (Heller and Evans, 1995; Maher and Thompson, 1992;1995). In this respect, knowledge of the dependence of the soil's magnetic properties, as well astheir variations, on soil type and climatic factors is necessary.

In the present study, the magnetic characteristics of three different soil types, namely, MeadowChernozem (59 samples taken along the depth profile), Leached Cinnamonic soil (23 samples) andPellic Chernozem-like Vertisoil (112 samples), are investigated.

The changes in the magnetic properties along each soil profile are represented by severalparameters, widely used in environmental studies: volume magnetic susceptibility (K), mass-specific magnetic susceptibility (X) and its frequency dependence (Xfd%); anhysteretic remanent

*

#

Studia geoph. et geod. 43 (1999), 303-318© 1999 StudiaGeo s.r.o., Prague

303

Address: Acad.BonchevStr., block 3, 1113 Sofia,Bulgaria, Fax: +359-2-700 226;E-mail: [email protected] address: Geophysical Institute ASCR, Bofinf 11/1401, 14] 31 Prague 4, Czech

Republic

D. Jordanova and N. Jordanova

magnetization (ARM) (ARM is induced by a steady field of 0.05 mT and peak AF field of 68 mT);viscous remanent magnetization (VRM), obtained after 20-days of zero-field storage in triple u,-metal boxes, and the ratios Xarm/X and SIRM/X. Saturation remanence (SIRM) is imparted by pulsesaturation of samples in a field of 2 T.

2. ROCK-MAGNETIC RESULTS

2 . 1 , M e a d o w C h e r n o z e m s o i l ( K o p r i v e t z p r o f i l e )

Description:The soil profile studied is situated on the first non-flooded terrace (To) of the Baninski

Lom river, near an Eneolithic archaeological site (Koprivetz multilevelled hill) in north-eastern Bulgaria. The soil developed on deluvial loess material, which has beenaccumulated from a near slope, mostly during the Early Holocene. Pedogenic processesact simultaneously with significant sedimentation. As a consequence, the soil profileformed grew upward during its development. Beneath the deluvium, grey clays representthe last phase of alluvial sedimentation. Horizon differentiation of the solum is notobserved clearly. The soil thickness is 120 cm, and the sampled interval of the underlyinggray clays is 80 cm.

Magnetic mineralogy:The type of magnetic minerals present is deduced by continuous thermal

demagnetization of the saturation remanence (Jrs), as well as the temperature dependenceof volume magnetic susceptibility (K) (Fig. 1). The "magnetite" Curie point is the onlyone expressed on the heating curve, K(T) (Fig. la). The initial slight increase in K up to~250°C probably reflects the decrease of the coercive force of magnetite particles withtemperature (O'Reilly, 1984), The subsequent K-decrease indicates a Curie point of580°C. We used the modified program for the CS-2 furnace, attached to a KLY-2susceptibility bridge in the paleomagnetic lab at St. Maur /Paris, France/, to perform smallcycling back to lower temperatures during heating, in order to verify whether the K-changes are due to some chemical alterations of the material (creation of a new phase) orwhether they are associated with the K(T) dependence of the initial ferromagnetic content.In our case, sample No37 (from the elluvial soil horizon) shows only slight increase of Kafter a 50°C-back run. This behaviour could be the result of grain growth because the K(T)curve is practically reversible during small interval of decreasing temperature, and onlyafter the subsequent temperature increase does K display slightly higher values (Fig. la).After heating to 700°C, a new strong ferrimagnetic phase appears, giving rise to anapproximately 6-fold increase in the room-temperature K-values (Fig. Ib). The probablecause of such behaviour could be the thermal decomposition of clay minerals in thepresence of a reducing atmosphere (Osipov, 1978), created by the organic matter in thesoil samples.

The results of thermal demagnetization of saturation remanence (Jrs) (imparted tosamples in a field of 2T) agree well with the K(T) behaviour, except that Jrs isdemagnetized completely at 700°C (Fig. Ic), suggesting the presence of some hematite.The kink at 120°C on the first heating curve could either be due to a dehydration effect orindicates the goethite unblocking temperature. The second Jrs(T) demagnetization curve is

Studia geoph. et geod. 43 (1999)304

Magnetic Characteristics of Different Soil Types from Bulgaria

concave with a final Tb of 600°C (Fig. 1c). It is most probably governed by the newlycreated phase during heating, since the room-temperature Jrs value after heating is 1.84times higher than Jrs1. The thermal demagnetization curves of Jrs for samples from thegrey alluvial clays underlying the soil horizon, show shapes similar to the soil samples(Fig. 1d). The one significant difference is the decrease in saturation remanence (Jrs) of~ 20% after the first heating. Several possible reasons may be given for the observedbehaviour: higher porosity, interstital water, etc., which are factors promoting thepresence of oxidizing conditions.

Fig. 1. High-temperature behaviour of magnetic susceptibility during heating (a) and both cyclesheating/cooling (b) for sample No37 from the eluvial horizon of the Meadow Chernozem. The twoshort temperature cyclings during heating are indicated by asterisks. Thermal demagnetization ofsaturation remanence (Jrs) for two samples (c) and (d) is used to obtain the unblocking temperaturespectra of the remanence carriers. The first heating is shown as the solid line, the second as thedashed line.

Studia geoph. et geod. 43 (1999) 305


Palaeoenvironmental conditions as reflected by the magnetic measurements:The variations of the magnetic parameters: magnetic susceptibility, viscous and

anhysteretic remanences (Fig. 2), display high values in the upper part of the soil (top40 cm), consistent with the behaviour of other Chernozem soils in Bulgaria (Jordanova etal, 1997). As evidenced by the results from K(T) and Jrs(T) analyses, magnetite is themain ferromagnetic carrier (plus a small hematite fraction). Consequently, bearing inmind the uniform magnetic mineralogy, the behaviour of the room-temperature magneticcharacteristics in Fig. 2 could be explained mostly by concentration and grain-sizechanges along the profile. The uppermost levels (0 - 40 cm) are characterized bygradually increasing values of X, VRM and partly ARM, pointing to the presence of anenhanced fraction of "in situ" formed pedogenic magnetites of SP, SP/SD and SSD grainsizes (Mullins, 1977; Ozdemir and Banerjee, 1982; Thompson and Oldfield, 1986). Thissuggests that the soil is developing without significant processes of leaching or illuviation,which could lead to the relative enhancement of the illuvial horizon with Fe and Al oxides(Ganev, 1990; Cornell and Schwertmann, 1996). The relatively high values of thefrequency-dependent magnetic susceptibility (Xfd%) (Fig. 2) along the whole depthsuggest the presence of ultrafine pedogenic grains, even in the lower part of the soil(80 - 120 cm) and in the grey alluvial clays, otherwise characterized by low X values. Thenormalized ARM and SIRM values, used to indicate variations in the relative quantity ofstable (SD) and MD grains (XarmIX and SIRMIX) (Maher, 1988; Thompson and Oldfield,

Fig. 2. Meadow Chernozem. Variations of a set of rock-magnetic parameters along the depth ofthe profile.



1986) show relatively small changes, indicating minor influence of grain-size variationson the behaviour of these magnetic characteristics. The small systematic increase in theSIRM/X values in the grey clays could be the result of both higher relative amounts ofparamagnetic and antiferromagnetic (hematite/goethite) fractions.

The quite low values of magnetic susceptibility, observed in the interval 80 - 120 cm,may have two reasons:1) depletion of pedogenic ferromagnetic grains due to a high sedimentation rate. This

explanation is based on the mechanism of deluvial sedimentation. In the EarlyHolocene the most significant amounts of loose material were deposited during rainyperiods. Thus, the sedimentation of re-worked loess material from the slopespredominates over the pedogenic formation of new ferromagnetic phases, leading tothe observed low susceptibility values.

2) destruction of magnetic minerals as a result of temporal changes in the ground-watertable, creating an anaerobic pedoenvironment (Cornell and Schwertmann, 1996).

1.1. L e a c h e d C i n n a m o n i c S o i l ( P i s t i r o s p r o f i l e )

Description:The second soil type is a Leached Cinnamonic soil, developed on the alluvial deposits

of the Maritza river in central south Bulgaria (near the town of Septemvri). According toprevious geomorphological investigations in the area, the fluvial deposits of the Maritzariver contain a significant fraction of granitoid and metamorphic pieces from Mount Rilaand the Ihtimanska Sredna Gora mountain (Baltakov et al, 1996). The total thickness ofthe soil is only 55 cm and consists of an upper humic layer (top 25 cm), followed by anilluvial horizon with enhanced clay content. The parent material is represented by sands ofcoarse to medium size (1000 < d< 250 ujn) of yellowish-brown colour. The approximatetime of formation of the profile of Leached Cinnamonic soil is considered to be the shorttime period since the 3rd - 2nd century BC, which is the age of the most ancientarchaeological findings of the nearby Thracian archaeological site Pistiros, found at thesame geomorphologic level, where the soil has developed.

Magnetic mineralogy:The magnetic mineralogy is determined from the thermal demagnetization of

composite IRM (Lowrie, 1990) (Fig. 3a-c) and the high-temperature behaviour ofmagnetic susceptibility (Fig. 3d). All three IRM components are completely demagnetizedat the final temperature step of 620°C. The soft component is always the strongest, both inthe soil (No25) and parent material (Nos70 and 110) (Fig. 3a-c). The difference betweenthem is the slightly more clearly pronounced magnetite Tb, of 580°C for the samples fromthe parent material. The observed higher final Tb of 620°C for soil samples may well bedue to the effect of oxidation during laboratory heatings. The higher intensity of the hardcomponent (Fig. 3c) in the parent material probably indicates an increased hematitecontent. The thermal behaviour of magnetic susceptibility for sample No60 from theparent material (Fig. 3d) points to a unique magnetite contribution of coarse grains(O'Reilly, 1984). The almost reversible K curve, suggesting the absence of significantphase transformations during heating, supports the possibility that the well expressedbending point on the soft IRM component (Fig. 3a-c) of ~ 250°C reflects the unblocking.



Fig. 3. Results of the thermal demagnetization of composite IRM (Lowrie, 1990) (a,b,c). Thebehaviour of room-temperature susceptibility, measured after each heating step, is shown on theinset graphs, (d) Susceptibility variations at increased temperatures for a sample of the parentmaterial.

of coarse magnetite grains. Moreover, this temperature corresponds well with the positionof the wide maxima on the K(T) curve (Fig. 3d). The susceptibility, measured after eachheating step during the Lowrie experiments (Fig. 3a-c, inset graphs), displays an increaseof about 20% in the interval 400 - 500°C, probably caused by the formation of a magneticphase from Fe-bearing silicate minerals during heating. The absence of a similar processduring the K(T) experiment could be due to the significantly shorter and single heatingtreatment.

Palaeoenvironmental conditions as reflected by magnetic measurements:It is evident from Fig. 4 that the soil horizons (humic 0 - 25 cm and illuvial

25 - 55 cm) are characterized by lower values of magnetic susceptibility than the parentmaterial. Variations in the X values in the parent material probably reflect successivechanges in alluvial deposition and the changes in detrital grain sizes associated with them.Consequently, we could reasonably suppose that the last sedimentation phase, precedingsoil formation, resulted in finer deposits with lower susceptibility values, similar to theinterval 80 - 95 cm (Fig. 4). Although not reflected as a bulk magnetic susceptibilityenhancement, the soil development is identified by relatively higher Xfd% values. They



are low (only up to 4%) but measurable. The observed higher values of the Xarm(X ratioalso point to the presence of stable SD grains, probably formed authigenically duringpedogenesis (Maher, 1988; Hunt et al, 1995; Verosub and Roberts, 1995).

Taking into account the results obtained, one could assume that the soil formation ofthe Pistiros profile affects the ferromagnetic mineralogy towards: 1) pedogenic formationof ultrafine grained, SP magnetite/maghemite particles, responsible for the higher Xfd%values and 2) low-temperature oxidation of the initial detrital ferrominerals.

2 . 3 . P e l l i c C h e r n o z e m - l i k e V e r t i s o i l ( S o f i a p r o f i l e )

Description:The section of the Pellic Vertisoil was sampled in the Sofia valley, where thick

Fig. 4. Leached Cinnamonic soil. Variations of rock-magnetic parameters along the depth of theprofile.

Studia geoph. et geod, 43 (1999) 309


deposits of Pliocene yellow clays, on which this specific soil type is developed, areaccumulated. The most typical characteristics of the sampled soil are: its highmontmorillonite content, black colour and the prismatic structure of the illuvial horizon.Since the sampling site is located relatively far from the valley's borders, the largethickness of the soil (170cm) suggests an enhanced pedogenesis, while colluvialdeposition is not as probable. Significant processes of leaching and subsequentreprecipitation are reflected in the formation of abundant carbonate concretions andmycelii in the carbonate horizon (180 - 330 cm). Another peculiarity of this horizon is thepresence of visible Mn concretions of diameters up to 2-3 mm, also pointing to asignificant acid destruction (at least during the initial stage of soil development) ofaluminosilicate minerals. According to pedological concepts, the initial stages in thedevelopment of Pellic Vertisoils is a waterlogged phase, followed by oxidizing conditionsof the more recent climates when the terrain was dried (Penkov, 1973).

Magnetic mineralogy:The magnetic properties of Mn nodules are revealed by the measurements of magnetic

susceptibility vs. temperature, carried out on separate Mn concretions extracted from thematerial of the carbonate horizon (Fig. 5a). The results indicate that these nodules containferromanganese minerals of the solid solution series Fe3O4 - Mn3O4 (jacobsite) which areferrimagnetic (Henshaw and Merrill, 1980). The appearance of a new strong magneticphase observed during heating beyond 300°C (Fig. 5a) may be due to the high-temperature dissolution of a primary magnetic phase of ferromanganese into one with alower Mn content and another with a higher Mn content. The presence of a magnetite Tc

probably reflects the contribution of Fe3O4 as the end member of the solid solution. Asthe sample from the illuvial horizon shows similar K(T) behaviour (Fig. 5b), it could beassumed that the magnetic properties of these soil samples are determined by the presenceof Mn-Fe solid-solution magnetic minerals. The same thermal dissolution beyond 320°Cresults in the formation of a strongly magnetic phase with a Tb of about 420 - 450°C. Thisis clearly indicated on the cooling curve, accompanied by a "magnetite" Tc - ferromineralwhose initial magnetic mineralogy presumably remained unchanged. Another phase withTc of 250°C, probably ferromanganese containing a higher amount of Mn can also beobserved. The results of the K(T) analysis make it is easier to explain the observed Tb onthe curves of thermal demagnetization of Jrs (Fig. 5d, e). The first heating shows a widespectrum of unblocking temperatures resulting in the concave shape of the Jrs(T) curveand the final Tc of 580°C - the phase, corresponding to the one on K(T) in Fig. 5b. Afterthe new strongly magnetic (ferromanganese) phase appeared, the second saturation givesit a dominant role and that is why, in the second heating, only the final Tc of 420°C isobserved. A different thermomagnetic behaviour is observed for the susceptibility ofyellow clay sample (Fig. 5c). The heating curve has a typical "magnetite" shape with afinal Tc of 580°C. We assume that, on heating, the Mn- and Fe-ions, inherent to clayminerals, are liberated from the silicates and form a new strongly magneticferromanganese mineral with a Tc of - 400°C. Accordingly, the first thermaldemagnetization of Jrs (Fig. 5f - solid line) shows a final Tc of 580°C, as well as a low Tb

of about 120°C, which could be due to the presence of goethile (Dekkers, 1989). The

Studia geoph. ef geod. 43 (1999)310


second Jrs(T) curve exhibits a kink at 420°C (corresponding to the createdferromanganese phase), together with the magnetite Tb of 580°C.

Thus, the main difference in the magnetic mineralogy of the soil and the parentmaterial is that ferromanganese minerals are a naturally present phase in the former,wheras, in the clays, they appear as a result of heating. This observation is reasonable, asin the soil horizons, the pedogenic alterations of primary clay minerals from the parentmaterial have resulted in the liberation of Fe2+ and Mn3+ ions, which, migratingdownward or "in situ", form a new ferromanganese phase, probably capable of alsocarryng remanent magnetization.

The coercivity spectra of the magnetic phases present in the samples have been studiedby applying the procedure of obtaining "partial" IRMs, i.e. magnetizing the sample in asteady field with amplitude H1 and subsequently AF-demagnetizing at maximumamplitude H2 < H1. Examples for eight samples from different levels are shown in Fig. 6.The points on the x-axis are medians of the interval of the IRM remaining after the AF

Fig. 5. High-temperature behaviour of magnetic susceptibility for: (a) sample of extracted Mn-concretions; (b) soil sample (No39) from the illuvial horizon; and (c) - sample of yellow Plioceneclays. Results of the thermal demagnetization of Jrs for samples from different horizons (d, e, f).First heating - solid line; second heating - dashed line.



Fig. 6. Pellic Vertisoil. Coercivity spectrum derived by imparting "partial" IRMs (see the text fordetails) for samples from different levels.

treatment. As can be seen, the samples from the upper levels (humic - No7g and upperpart of the illuvial horizons) display very wide coercivity spectra and the lowest values of"partial" IRMs, At the bottom of the illuvial horizon, the samples display one softcomponent with coercivities between 5 and 30 mT, carrying up to 70% of the IRM, and asecond hard component of the remaining signal (Fig. 6). Downwards, samples fromprogressively deeper horizons show shifts towards higher values and increasing intensities(Fig. 6 - white symbols). Consequently, the pedogenic alteration in the studied PellicVertisoil is expressed by the presence of more stable pedogenic grains with relativelyhigher coercivities in the humic horizon, which become softer towards the bottom of theilluvial horizon due to illuviation processes. The parent yellow clays are characterized byhigh coercivity and magnetization probably due to the presence of detrital Fe3O4 andprecipitated aFeOOH grains.

Paleoenvironmental conditions as reflected by magnetic measurements:The significantly lower susceptibility values along the whole soil depth (Fig. 7) are

most probably the result of destruction of magnetic and clay minerals, which occurredunder anoxic conditions. The variations of the viscous remanence along the profile wereobtained after 20 days of storage in a zero field. Contrary to the susceptibility values, thehighest viscosity is displayed by the samples from the soil horizon accompanied byslightly higher Xfd% values (Fig. 7). As regards the Pellic Vertisoil studied, the strongestdistinction between soil and parent material is reflected in the contrast of the X and VRMvariations, rather than Xfd%. One may thus assume that the destruction of magneticminerals caused the disappearance of very fine (SP) grains, so that the fraction of unstableSP/SD grains is the best magnetic reflection of the presence of pedogenic enhancement.At the same time, normalized ARM values (Xarm/X) show little variations between the soil



Fig. 7. Pellic Chernozem-like Vertisoil. Variations of rock-magnetic parameters along the depthof the profile.

horizon and yellow clays which, compared to the ARM-behaviour, suggests that thefraction of stable SD particles varies only in concentration, being higher in the soil. TheSIRM/X ratio displays very consistent values through the soil depth, and progressivelyincreases in the yellow clays. Bearing in mind the results of the coercivity spectra, thisincrease may be due to a higher content of high-coercivity ferromagnetic minerals(goethite), but also to a higher amount of MD particles (Thompson and Oldfield, 1986),which could explain the enhanced susceptibility in the grey clays as well.

3. DISCUSSION AND COMPARISON OF THE STUDIED SOIL PROFILES

The three presented soil profiles have been compared, because each of them showssome departure from the widely accepted soil magnetic properties used inpalaeoenvironmental reconstructions, as well as in detecting industrial pollutions. In bothof these environmental aspects of magnetic studies, low background susceptibilities of theparent material are a necessary condition for obtaining straightforward interpretations.However, the enhanced values of frequency dependent magnetic susceptibility are in mostcases considered to be unambiguous proof of the presence of pedogenesis. We haveshown that in our three cases these assumptions are not always correct. The interpretationof Xfd% in terms of detection of the "in situ" formation of pedogenic ultrafine grainedmagnetite is based on the assumption of a single, constant mineralogy of the frequency



Fig. 8. Plot of frequency-dependent magnetic susceptibility (K lf-Khf,) vs. low-field magneticsusceptibility (Klf) for the three profiles. Black symbols relate to soils, and white symbols to theparent material of the corresponding profile.

independent fraction (Forster et al, 1994) in the soil and the parent material. If this is thecase, a linear relationship between Xlf and (X l f-Xh f) should be obtained. In our case, thismethod of estimating the "background" susceptibility Xb yields contradictory results(Fig. 8). For all three profiles studied, the soil samples plot as separate populations of thecorresponding parent materials. This suggests that different "background" susceptibilitiesshould be calculated for different horizons. However, this assumption points to thepresence of different populations of frequency independent fractions in the soils andparent materials. If the latter is represented by MD ferrimagnetic, paramagnetic andcanted antiferromagnetic minerals, their relative contribution should be considered.

Para- and antiferromagnetic minerals can dominate Xlf soils where Xlf< 10-6 m3/kg(Dearing et al., 1996). For the Koprivetz and Sofia soil profiles the minimum Xlf is quitelow (see Figs. 2,4) and the above reason could explain the observed data scatter. Asregards the Pistiros profile, Xmin is higher (- 6 x 10-6 m3/kg), so that MD fractionprobably plays the main role. The absence of a strong linear correlation between Xlf

(X l f-Xhf), especially in the parent material in the Pistiros and Sofia profile as a whole, asevidenced by the low values of the correlation coefficient (R2), is most probably due to thequite high variability in MD content along the depth. This effect can be reasonablyexpected if a large number of coarse pieces of volcanic rocks is present, as in the Pistirosprofile, or if a variable degree of iron immobilization/precipitation on a small scale and



Table 1. Summary of magnetic susceptiblity and Xfd% values in soil and parent material for thestudied profiles.

Profile/horizon

Koprivetz soil/parentSofia soil/parentPistiros soil/parent

Xmax

[10-8m3/kg]

82.6/45.514.8/31.983.8/97.7

Xmean

[10-8m3/kg]

64.5/38.011.6/17.873.9/83.1

Xfd% mean

9.8/9.32.9/1.93.2/2.1

(K l f- Khf) mean

[10-5 SI]

6.96/3.910.49/0.494.28/3.10

secondary Fe and Mn concretions in the carbonate rich horizon occur along the solum, incase of the Sofia profile. A variability of the mineralogy in the B horizons similar to thatin the upper O and A horizons was observed by Dearing et al, (1995) in their study ofacid gley soils. The effectiveness of the destruction of inherited ferrominerals by theprocesses of gleying due to the waterlogged environment, probably accompanying theinitial stages of pedogenesis in the Sofia profile and most probably occurring in theMeadow Chernozem soil at Koprivetz, can be seen from the absolute and relativeenhancement parameters listed in Table 1.

As we have shown earlier, the viscous magnetization, carried by unstable grains ofsizes just at the critical SP/SD threshold, can be used to discriminate the presence ofpedogenic alterations, even if frequency-dependent susceptibility is not a straightforwardindicator. Bearing in mind that both magnetic characteristics reflect the existence of finegrains with characteristic relaxation times, we calculated the VRM/(X l f-Xhf) ratio for theKoprivetz and Sofia profiles, and plotted it as a function of Xlf(Fig. 9). Our considerationsare based on the idea that the ratio will be constant for different samples if the proportionof viscous and SP grains does not change. If an increase of the ratio is observed withincreasing X, then a prevailing contribution of the viscous fraction is most probable. Thisis observed for the Koprivetz profile (Fig. 9a). As regards soil samples, the trend ofVRM/(Xlf-Xhf )values increasing with X (e.g. from the parent material to the top of theprofile) indicates a significant viscous enhancement. The samples of alluvial clays displaya high scatter, corresponding to a narrow X interval (30 - 40 x 10-8 m3/kg) probably as aresult of random occurrence of SP and viscous grains due to the influence of a fluctuatingwater table.

The case of the other profile - Sofia, again shows the useful role of viscousexperiments in proper identification of grain-size variations. Most of the values of theVRM/(Xlf-Xhj) ratio for soil samples are low although X increases. This suggests that theincrease is caused either by a proportional increase of the SP and viscous populations, orby a true MD fraction, which does not contribute significantly to VRM. Again, thecarbonate rich horizon and the underlying clays display a high scatter in susceptibilityaveraging 18 - 20 x 10-8 m3/kg. This indicates that concentration changes, mostly of theMD fraction, are responsible for the changes in magnetic properties.


D. Jordanova and N, Jordanova

Fig. 9. Biplot of VRM/(Xlf-Xhf) vs. Xlf for the a) Koprivetz and b) Sofia profiles. Black symbols- soil, white symbols - parent material.

4. CONCLUSIONS

On the basis of the results and discussion presented, the following conclusions havebeen drawn:1. The action of soil-forming processes can be unambiguously identified either from the

higher values of the percentage frequency-dependent susceptibility, or from theviscous remanence of the soil samples as compared to the parent material.

2. Two equally important factors could be responsible for the formation of soil profilescharacterized by magnetic susceptibilities lower than that of the parent material: a) acoarse-grained parent material with high content of minerals, resistant to weathering,and b) a water-logged stage in the soil development.

3. Magnetic susceptibility, used as a palaeoenvironmental magnetic proxy parameter, isnot always a straightforward indicator and depends on the particular soil forming



conditions. Additional magnetic parameters (as Xfd% and VRM, used in our study)should be investigated for a more unambiguous interpretation in specific cases inwhich the soil shows lower magnetic enhancement, as compared to the parentmaterial.

4. The thermomagnetic results for the Vertisoil Sofia profile reveal the presence of apedogenic secondary Fe-Mn ferrimagnetic phase, which is responsible for thesusceptibility and remanence properties. Therefore, the pedogenic processes in soils,characterized by a water-logged stage during their development, could lead to thecreation of ferromanganese ferrimagnetic minerals.

Acknowledgments: We are grateful to Dr. B. Henry and Dr. M. Le Goff of the PalaeomagneticLaboratory in Saint Maur (Paris, France) for providing access to the CS-2 equipment and helpduring the experiments. Thanks are due to our colleagues at the Paleomagnetic Laboratory inThessaloniki (Greece) for measurements of frequency-dependent magnetic susceptibility. Veryhelpful and constructive referees' comments contributed to the improvement of the paper and arehighly appreciated,

Manuscript received: 16 August 1998; Revisions accepted: 23 June 1999

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Magnetic Characteristics of Different Soil Types from Bulgaria

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