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Quaternary International 156–157 (2006) 212–223

Rock magnetism and pedogenetic processes in Luvisol profiles:Examples from Central Russia and Central Mexico

Jorge Rivasa,�, Beatriz Ortegab, Sergey Sedovc, Elizabeth Solleiroc, Svetlana Sycherad

aUniversidad Nacional Autonoma de Mexico, Posgrado en Ciencias de la Tierra, Ciudad Universitaria, 04510 Mexico D.F., MexicobUniversidad Nacional Autonoma de Mexico, Instituto de Geofisica, Ciudad Universitaria, 04510 Mexico D.F., MexicocUniversidad Nacional Autonoma de Mexico, Instituto de Geologia, Ciudad Universitaria, 04510 Mexico D.F., Mexico

dRussian Academy of Sciences, Institute of Geography. Staromonetny per. 29, 119017 Moscow, Russia

Available online 30 June 2006

Abstract

Despite a vast literature concerning magnetic properties of loess sequences, we still do not fully understand how magnetic components

and properties are related to particular soil-forming processes that can vary with each type of genetic horizon. In order to establish the

role of lithogenic factors in the link between magnetic properties and soil-forming processes, we carried out a study of two complete

profiles of buried interglacial Luvisols, one formed in loess in Russia (Alexandrovsky quarry) and the other in volcaniclastics in Mexico

(Barranca Tlalpan). In both profiles, soil genetic horizons have contrasting differences of their magnetic properties. In the Alexandrovsky

profile, the magnetic susceptibility (w) is enhanced in the paleosol compared to parent material. In the Barranca Tlalpan sequence, wenhancement is absent in the soil profile. Increase of fine-grained magnetic components in the soil is attributed to neoformed minerals.

However, this process cannot compensate for the loss of lithogenic magnetic minerals in any of the genetic horizons, and the resulting

trend is w depletion in the whole soil profile. The pedogenic environment of eluvial horizons in both Luvisols is destructive to all magnetic

components, both primary and secondary. Higher concentrations of antiferromagnetic components (hematite and goethite) found in E

horizons are related to redoximorphic processes.

r 2006 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction

Since the discovery of differences in magnetic suscept-ibility values between paleosols and loess layers in Chineseloess sequences (Heller and Liu, 1982, 1986; Kukla and An,1989; Maher et al., 2003; Tang et al., 2003), a surge ofinterest has resulted in the emergence of a vast literature onthis topic. Similar patterns of susceptibility depth variation,characterized by strong maxima in paleosol horizons, werefound in geographically distant loess–paleosol sequences inCentral Asia, Europe, and North Africa (e.g. Dearinget al., 1996; Forster and Heller, 1997; Rousseau andPuissegur, 1998; Dodonov et al., 1999). These data weresuccessfully used as a correlation tool, in particular tocompare loess and marine records (including marineoxygen isotope curve) (Kukla and An, 1989; Heller andEvans, 1995; Evans et al., 2003).

e front matter r 2006 Elsevier Ltd and INQUA. All rights re

aint.2006.05.007

ing author. Tel.: +5255 56224226; fax: +52 55 55509395.

ess: jorger@geofisica.unam.mx (J. Rivas).

At the same time, the phenomenon of susceptibilityenhancement in loess-derived paleosols drew attention as apossible paleoenvironment indicator. It was widely ac-cepted that the major part of enhancement is due to post-depositional pedogenic accumulation of fine-grained mag-netic minerals (Zhou et al., 1990; Zheng et al., 1991; Maherand Thompson, 1992), and thus could depend uponpaleoclimatic conditions of interglacials.Various approaches were developed to evaluate the

proportion of pedogenic versus lithogenic magnetic com-ponents (Banerjee et al., 1993; Fine et al., 1995; Grimley etal., 1998). Another branch of research was focused onsearching for dependence functions between susceptibilityvalues and modern climatic conditions (in particularprecipitation) in modern surface soil climosequences as atool for comparison with susceptibility patterns in paleo-sols (Maher et al., 1994; Liu et al., 1995; Maher andThompson, 1995).Although the pedogenic origin of enhancement was

widely accepted, little effort was made to understand the

served.

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interrelation between magnetic characteristics and paleosolpedogenesis in the early studies of magnetism in loesssequences. In most cases, researchers did not discusssufficiently the kind of pedogenic horizons/features thatwere present in the buried soil profile, and in many studies,the sampling was accomplished mostly on equal depthbasis without regard to soil horizonation. Furthermore,many studies failed to use a soil classification system, tofully characterize of the degree of development, or to noteif the profile was truncated by erosion before burial ortransformed by diagenesis after burial, although thesephenomena are known to be frequent in loess sequences(Bronger et al., 1998).

More interest towards the link between specific soil-forming processes and magnetic properties of paleosolsarose when a number of studies documented an inversepattern of the magnetic susceptibility curve in someloess–paleosol sequences of Alaska (Beget et al., 1990;Vlag et al., 1999), Siberia (Chlachula et al., 1998; Evans etal., 2003), and South America (Orgeira et al., 1998; Nabelet al., 1999). Similarly, a curve with susceptibility minimain paleosols was established in the Pleistocene volcanicsequences of Mexico (Ortega-Guerrero et al., 2004). Thereasons for this deviation were synthesized in two mainviewpoints: (i) the pattern is controlled by variations ofdepositional processes such as change of wind intensity,source area, and presence of volcanic material (e.g. Beget etal., 1990; Evans et al., 2003; Schellenberger et al., 2003), or(ii) it is induced by gleization, a post-depositionalpedogenic process occurring in a chemically reducingenvironment typical of water-logged soils. Chemicalprocesses in a reducing environment are destructive tomagnetic minerals and are responsible for susceptibilityminima in paleosols (Liu et al., 1999, 2001), as well as inpoorly drained modern soils (de Jong et al., 2000; Grimleyand Vepraskas, 2000; Grimley et al., 2004).

More extensive information about the interrelationbetween soil magnetism and type of pedogenesis wasaccumulated through detailed study of modern surfacesoils, which started earlier and independently frommagnetic research of loess sequences (Le Borgne, 1955,1960; Vadyunina and Babanin, 1972; Tite and Linington,1975; Maher, 1998).

Various works present datasets collected to establish theconnection between the behavior of magnetic parametersand the variability of soil environments (e.g. Singer andFine, 1989; Geiss et al., 2004) and soil age (Singer et al.,1992). Singer et al. (1996) proposed a conceptual model forsusceptibility enhancement in soils that considers themechanisms of pedogenic magnetic mineral neoformationand links it to the soil-forming factors climate and time.Maher (1998) developed a broader concept of interactionbetween pedogenesis and the magnetic mineral system,which includes possibilities of both susceptibility enhance-ment and ‘‘depletion’’.

Despite these studies, there has been relatively littlediscussion about how magnetic components and properties

are related to particular soil-forming processes such ascarbonate leaching/precipitation, type of weathering, clayeluviation/illuviation, humus accumulation, and redoxi-morphic processes in surface versus groundwater satura-tion conditions. These processes vary not only from soil tosoil, but within soil profiles, generating different conditionsfor magnetic minerals. Another limitation for understand-ing the link between pedogenesis and soil magnetism is thelack of knowledge about rock magnetic properties ofpaleosols and modern soils formed on parent materialsother than loess.With the aim to characterize Pleistocene paleosols

developed in different environmental conditions and withvariations in their parent materials, we initiated a multi-disciplinary study of buried interglacial Luvisols (soils,known to be unequivocally indicative of subhumid forestecosystems) in central Mexico and western Russia. Thestudied profiles were chosen on different parent materials(loess and volcanic ash) with the goal of establishing therole of lithogenic factors as well as soil-forming processeson magnetic properties. In this paper, we present a study ofsoil magnetic minerals as a function of particularpedogenetic environments within genetic horizons ofLuvisol profiles.

2. Site descriptions

2.1. Barranca Tlalpan

In central Mexico, a paleosol sequence called theBarranca Tlalpan (BT) profile is located in the Transmex-ican Volcanic Belt (19127041.300N; 98118052.500W,2580ma.s.l.) (Fig. 1a), and is comprised by a set ofpaleosols developed on Pliocene to Quaternary volcani-clastic deposits, locally known as ‘‘tepetates’’. Previouswork on the Barranca Tlalpan described in detail thepedogenic and mineral magnetic characteristics of threemajor units of Pleistocene–Holocene volcaniclastics–paleo-sol sequences, classified as Luvisols (Ortega-Guerreroet al., 2004). In one of the youngest paleosols, the humusof A horizon was dated by 14C as 38,160 yr BP. In theoldest paleosol of this sequence, the ‘‘Red Unit’’ pedo-complex (formerly labeled P6 and P7), the only horizonsfound were Bt, BC, and C. The Bt horizon was subdividedinto four subhorizons (Bt1, Bt2, Bt3, and Bt4) due todifferences in soil properties. Bt1 and Bt2 exhibit in situand thick clay cutans while Bt3 and Bt4 have reworkedpedofeatures. In later field reconnaissance, the upperhorizons Ah and EBtg were found in another exposurefew hundred meters away. The Red Unit sequence wasresampled from the two exposed profiles, labeled in thiswork as URU (at the upper part of pedocomplex profile,Ah, EBtg, and Btg horizons) and RU (at middle andbottom of the pedocomplex profile, Bt, BCt, and Chorizons), at vertical intervals of nearly 30 cm (Fig. 1band c). In URU, the unit shows a complete profile. TheAh horizon is dark gray, enriched in humus, and has

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Fig. 1. (a) Geographic location of the Barranca Tlalpan profile, near Tlaxcala city, in central Mexico. Popocatepetl (P), Iztaccihuatl (I), and Malinche (M)

are the major stratovolcanoes in the area. (b) Pedocomplex of Upper Red Unit (URU) and Red Unit (RU) profiles. (c) Photos of the studied paleosol.

J. Rivas et al. / Quaternary International 156–157 (2006) 212–223214

well-developed granular and subangular blocky structure.EBtg is more silty, lighter in color, with a weak subangularblocky structure, and redoximorphic features (pale-brownferruginous mottles). It also exhibits thin clay cutans invoids and pores. The boundary with the underlying Bthorizon is tonguing, which is typical for Albic Luvisols.The Btg horizon also has abundant redoximorphic featuressuch as gray and reddish-brown mottles, Fe–Mn concre-tions, and black Mn coatings on ped surfaces. No dates areavailable for the paleosol presented in this work, butaccording to its stratigraphic position, it was formedduring the Middle Pleistocene.

2.2. Alexandrovsky

The Alexandrovsky quarry site (AQ) is located in theCentral Russian Highlands some 10 km south of Kursk(Fig. 2a). This elevated (200–260ma.s.l.) part of the EastEuropean Plain was never covered by Pleistocene icesheets. During glaciations, it formed part of an extensiveperiglacial tundra-steppe megazone, characterized byintensive cryogenesis and loess accumulation. A temperateenvironment during interglacials promoted soil formation.Currently loessic sediments with paleosols form a rathercontinuous mantle (Velichko, 1990), overlying pre-Qua-

ternary rocks. The contemporary environment is temperateforest-steppe, under which thick Chernozems are formed.The Alexandrovsky profile exposure is located in the

upper near-watershed divide part of a gentle slope towardsthe valley of the Seim River and was extensively studiedfrom a geomorphological and paleopedological standpointby Sycheva (1998, 2004). In the cut of a brick quarry, weobserved a buried fluvial geoform, a ‘‘balka’’ (a smallvalley) cut into loess of the Dnepr (Riss) glaciation (Fig. 2band c). Both balka slopes are outlined by the buriedinterglacial paleosol of the Mikulino (Riss-Wurm, Eemian)interglacial. This paleosol corresponds to the lowermember of the Mezin pedocomplex, but in this paper, wewill use the name Mikulino paleosol or soil to refer to itinformally.In one place on the western side of the balka, an older

Middle Pleistocene paleosol, consisting of AB and BCthorizons strongly deformed by cryogenic processes, wasobserved below the Dnepr loess. In contrast to theMikulino soil, this earlier paleosol does not conform tothe profile of the buried balka. It marks an older landsurface that was buried by Dnepr loess and then locally cutby the balka slope.The depression was afterwards filled by Early Valday

(Wurm) colluvium that includes three weakly expressed

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Fig. 2. (a) Geographic and stratigraphic position of the Alexandrovsky quarry (AQ), near Kursk, Russia; (b) photos of the studied paleosol; (c) schematic

profile of the buried fluvial geoform balka, developed in the Dnepr loess.

J. Rivas et al. / Quaternary International 156–157 (2006) 212–223 215

paleosols formed during interstadials. Finally, this colluvialsequence is overlain by Middle-Late Valday strata includ-ing the Bryansk Paleosol, then loess that accumulatedduring the last glacial maximum; the latter is the parentmaterial of the Holocene Chernozem. In the upper part ofboth slopes of the buried balka, both the AQ Mikulinopaleosol and Early Valday colluvium pinch out. Conse-quently, the Bryansk Paleosol rests directly on Dnepr loess.

The AQ Mikulino Interglacial Paleosol, which correlatesto MIS 5, demonstrates a complete well-preserved sequenceof genetic horizons, typical for an Albic Luvisol. For therock magnetic study, we analyzed a pedocomplex profilewith Ah-E-BEt-Bt(1-3)-BCt-Cg horizons, and additionallythe AB and BCt horizons of the middle Pleistocenepaleosol.

3. Methods

Samples of 200 g were collected at roughly 20 cmintervals, homogenized, and placed in 8 cm3 acrylic boxesfor magnetic measurements. One or two samples for eachhorizon were collected, 19 samples from URU and RUprofiles and 17 in AQ profiles. All magnetic measurementswere carried out with bulk samples.

Magnetic susceptibility versus high-temperature experi-ments were used to determine the magnetic mineralogy byits Curie temperature, which is the temperature at whichminerals loose their magnetization. Curie temperatureestimations were performed on a Bartington MS2WFfurnace system, in which changes in w were measured from

20 to 650 1C, during both heating and cooling in an airatmosphere. Samples with very low magnetic susceptibilitywere indurated with Omega CC high-temperature cement.Stepwise thermal demagnetization of remanences wasconducted with a Thermal Specimen Demagnetizer setModel TD-1, between room temperature and 700 1C.Antiferromagnetic goethite has a Curie temperature at80–120 1C. Pure, Ti-free magnetite has a Curie temperatureclose to 580 1C, while the content of Ti in titanomagnetites(Fe3�xTixO4, 0pxp1, represented as TM0–TM100) de-creases the Curie temperature (Dunlop and Ozdemir, 1997;McElhinny and McFadden, 2000). A Curie temperature of150–200 1C is typical of TM60, which is the primary Ti-magnetite in rapid-cooled basaltic lavas, and 300 1C forTM45. Hematite, which has a Curie temperature of 675 1C,and goethite are difficult to observe in thermal demagne-tization and susceptibility curves due to their low magneticsignal that is overshadowed by the stronger ferromagneticTi-magnetites.Magnetic susceptibility (w), which is a measure of the

concentration of magnetic minerals, was measured in allsamples at low (0.47 kHz) and high frequencies (4.7 kHz)with a Bartington MS2B dual sensor. We calculatedfrequency dependence of susceptibility wfd% as [(wlf–whf)/wlf]100, to approximate possible ultrafine (o 0.05 mm)superparamagnetic (SP) contribution. When initial suscept-ibility was o30� 10�5 SI units, samples were measured 10times in a 0.1 scale, and average values were plotted. Thenatural remanent magnetization (NRM), anhysteric rema-nent magnetization (ARM), and isothermal remanent

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magnetization (IRM) were measured with a MolspinMinispin magnetometer. ARM was imparted in a 50 mTbias field, superimposed on a peak alternating field of100mT in a Molspin AF demagnetizer. IRM was impartedwith a pulse magnetizer at a forward field of 1T and atbackward fields of 100 and 300mT. The magnetizationacquired at 1T was considered the saturation magnetiza-tion (SIRM).

Several magnetic ratios were calculated, as indicators ofmagnetic hardness and stability. ‘‘Hard’’ IRM (HIRM) isobtained by imparting a back field at 300mT (IRM�300) ona sample previously given SIRM. It is a tool to estimate theconcentration of antiferromagnetic minerals with highercoercivity, or very fine-grained ferrimagnetic grains, andwas calculated as [SIRM+IRM�300]/2 (Opdyke andChannell, 1999).

The S-ratios are useful to estimate the presence ofminerals with high coercivity, commonly hematite orgoetithe, or high stability minerals such as fine(40.05 mm) single domain (SD) magnetite. They werecalculated as Sx ¼ IRMx/SIRM, with x as the field applied,in this case we used 100 and 300mT in a backward field.S100 display grain size variations in low coercivitycomponents among coarse-grained ferromagnetic particles,although it cannot be distinguished from mixtures of ahigh-coercivity and a fine-grained low-coercivity fraction(Robinson, 1986). S300 show the proportion of low-coercivity minerals in a sample (Opdyke and Channell,1999), where values close to zero indicate antiferromagneticconcentration.

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Fig. 3. Curie temperature measurements for selected samples from Red Unit,

temperatures suggest Ti-magnetite/Ti-maghemite, almost pure magnetite plu

shown, displays the same behavior. (b) Curie temperature of 580 1C in Bt1 horiz

670 1C suggest the presence of hematite. (c) and (d) Thermal demagnetization

remanence is observed at 120 1C in both samples, followed by a large decay aro

the possible presence of antiferrimagnetic goethite, additionally to Ti-magneti

To estimate grain size, we used ARM/SIRM to estimatethe concentration of SD grains contribution parameter(Geiss, 1999). wfd% is a sensitive parameter of the presenceof SP grains (Jordanova et al., 1997), but it is toodependent on titanium present in Ti-magnetites (Walland Worm, 2000).Saturation magnetization (Ms) and coercivity (Hc) were

obtained from hysteresis loops measured with a MolspinVibrating Sample Magnetometer (VSM), at room tem-perature in all samples. Coercivity of remanence, Hcr, wasestimated by demagnetization of SIRM applying IRMbackfields. Both Hc and Hcr are useful to estimate hardergrains. As the para-, dia-, and ferrimagnetic fractionsaccount for w, the ferrimagnetic susceptibility, wf, wascalculated by subtracting the paramagnetic contributionestimated from the high field slope in the hysteresis loopsfrom the bulk susceptibility.

4. Results

4.1. Barranca Tlalpan sequence

Magnetic susceptibility (w) versus temperature curvesfrom BCt horizons presents two clear decays, at around160–300 and 580 1C, and a persistence of susceptibilityabove 580 1C (Fig. 3a). In samples of Btg and C (notshown) and most Bt horizons, w versus temperature curvesshow a larger drop near 580 1C, and a weak decay abovethis temperature (Fig. 3b). All of these curves are almostreversible. The mineralogical phases suggested by these

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Barranca Tlalpan paleosol: (a) w versus tempertarure BCt horizon Curie

s hematite as the main magnetic mineral components. Bt4 horizon, not

on sample shows pure magnetite, and the weak decay in w between 580 and

of SIRM curves for EBtg and Ah horizons, respectively. A weak decay in

und 300 1C, and a final loss of remanence at 575 1C. This behavior suggests

te (Ti-maghemite), and pure magnetite.

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Fig. 4. IRM acquisition curves of Red Unit paleosol samples. Btg and

BCt horizons show a quick acquisition of remanence, and reach saturation

in fields close to 300mT, which suggest dominance of ferrimagnetic grains.

Ah and EBtg samples curves, in contrast, have a more gradual increment

in remanence and are not fully saturated at 500mT fields, which suggest

the significant presence of antiferrimagnetic phases, such as hematite or

goethite.

(a) (b) (c) (d)

Fig. 5. Down profile magnetic properties for Red Unit paleosol, Barranca Tla

(wf) and paramagnetic (wp) contribution to total bulk susceptibility. (c) Saturatio

abundance of ferrimagnetic minerals. (d) Frequency-dependent susceptibility,

ARM/SIRM ratio is an indicator for the abundance of fine (0.01–0.05mm) SD

such as very-fine-grained (SD) magnetite, or antiferrimagnetic minerals (hemat

or grain sizes: S100 (diamonds) is sensitive to both variables, whereas S300 (trian

hematite or goethite. (h) Coercive force Hc is sensitive to grain sizes (MD); coe

higher for antiferrimagnetic minerals.

J. Rivas et al. / Quaternary International 156–157 (2006) 212–223 217

curves correspond to Ti-magnetite/Ti-maghemite withvariable Ti content, along with pure magnetite andhematite.Due to the low w in Ah and EBtg horizons, samples were

prepared for thermal demagnetization of SIRM. Curvesdisplay a smooth decay in remanence. In the EBtg curve,subtle drops in remanence are found around 80–100 1C, asecond inflection at 300 1C, and a final decay close to580 1C (Fig. 3c). The sample from Ah horizon showsdecays at 200, 300, and 580 1C (Fig. 3d). In addition to Ti-magnetite/Ti-maghemite and pure magnetite phases in-ferred for these horizons, goethite may also be present inthe EBtg horizon.Acquisition curves of IRM show two different paths

(Fig. 4). The Btg and Bt horizons have similar behavior, sowe only present Btg and BCt horizons, which show steepercurves in low fields, and saturation is reached in fields closeto 300mT, this suggests dominance of relatively softferrimagnetic grains. Curves of Ah and EBtg samples are

(e) (f) (g) (h)

lpan. (a) Bulk magnetic susceptibility (w). (b) Percentages of ferrimagnetic

n isothermal remanent magnetization (SIRM). w and SIRM are proxies of

wfd%, is a proxy of the abundance of ultrafine (o 0.05mm) (SP) grains. (e)

grains. (f) HIRM300 reflects the concentration of hard magnetic minerals,

ite or goethite). (g) S-ratios estimate the variations in magnetic mineralogy

gles) is sensitive to hard, high-coercivity antiferrimagnetic minerals such as

rcivity of remanence Hcr is a useful guide to magnetic mineralogy, as it is

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Fig. 6. Curie temperature curves for Alexandrovsky quarry paleosol

section for A (a) and Cg (b) horizons. All curves are similar, as they drop

near 300 1C, which indicates that Ti-magnetite/Ti maghemite are present.

Weaker drops at 120 and near 580 1C suggest that goethite and pure

magnetite may also be present. The remanence between 580 and 670 1C

suggests the content of hematite.

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E

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Fig. 7. IRM acquisition curves of Alexandrovsky quarry paleosol

samples. The increase in remanence in fields above 300mT in all curves

indicates that magnetically hard minerals are important fraction of

magnetic mineralogy. The two extreme paths in these curves are

represented by the A and E horizons, in which the E horizon apparently

has the higher content of antiferrimagnetic phases, such as hematite or

goethite.

J. Rivas et al. / Quaternary International 156–157 (2006) 212–223218

harder to magnetize, the increment in remanence is moregentle, and the curves are not fully saturated at 500mTfields, all suggesting a relative increase of antiferrimagneticphases such as hematite or goethite.

Magnetic ‘‘concentration’’ parameters w and SIRMindicate higher concentrations of magnetic grains in thelower horizons and a general up-profile decay in concen-tration. The BCt has w46 mm3/kg, while Bt horizonspresent w values between 4 and 6 mm3/kg (Fig. 5a and c).The Btg, EBtg, and Ah horizons are the lowest withwo1 mm3/kg. Ferrimagnetic contribution to susceptibility(wf) in BCt and Bt horizons is 470%, while in the upperhorizons (Btg, EBtg and Ah), wf is lower than theparamagnetic contribution (wp) (Fig. 5b).

wfd% is fairly constant at around 5% in most of theprofile, suggesting moderate abundance of SP grains,except in EBtg, where wfd% is close to 1% suggesting thatSP grains are absent. In contrast, the Ah horizon is 13%,which indicates that SP particles are abundant (Fig. 5d).The abundance of fine grains (SD), according to ARM/SIRM, is higher in the upper samples of Bt4 and Bt2horizons, and in Bt1 and Btg horizons (Fig. 5e).

HIRM300 measurements (Fig. 5f) suggest that the higherabundance of high-coercivity minerals is in the Ah, EBtg,and C horizons. S300 ratios and Hcr have constant values,except in EBtg, where S300 is close to 80% andHcr450mT. These values suggest that antiferrimagneticminerals such as goethite or hematite are present in thishorizon (Fig. 5g and h). This interpretation agrees with themagnetically harder phases detected in these horizons byIRM acquisition.

4.2. Alexandrovsky quarry sequence

Low initial magnetic susceptibility in most of thesesamples (o 0.20 mm3/kg) prevented our obtaining reliablewarming curves for Curie temperature determinations,except for the A horizon. Curie temperatures were thusestimated from thermal demagnetization in cementedsamples. Most curves from all horizons showed inflectionsat 300 and 580 1C, and a weak remanence until 675 1C. ACurie temperature close to 120 1C is suggested in samplesfrom the A and Cg horizons (Fig. 6a and b).

Acquisition curves of IRM point to magnetically hardcomponents, as remanence is not fully saturated at 500mTfields in all samples (Fig. 7). Only the A horizon samplepresents a steeper acquisition curve, which suggestsrelatively softer magnetic grains.

Paramagnetic contribution (wp) to w is apparently higherthan the ferrimagnetic contribution (wf) to w in most of thesamples with wo0.2 mm3/kg. This might be misleading, as wand wp are obtained from two different instruments, anddifferences in sensitivity may be significant in these lowinitial w samples. A more reliable proxy for ferrimagneticconcentration may be the SIRM. Higher concentration offerrimagnetic minerals are found in the BEt, E, and Ahorizons (Fig. 8c). wfd% was measured repeatedly, and gave

consistent values. This parameter does not give a clearpattern of high concentration of SP grains in most horizons(wfd%47%). However, one E horizon sample resulted incomparable values than those to Bt (Fig. 8d).ARM/SIRM indicates that the fine SD grains are more

abundant in the A and E horizons, and less in the Cg and

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(a) (b) (c) (d) (e) (f) (g) (h)

Fig. 8. Down profile magnetic properties for Alexandrovsky quarry paleosol. Captions as in Fig. 5.

J. Rivas et al. / Quaternary International 156–157 (2006) 212–223 219

BCt horizons (Fig. 8e). HIRM300 is higher in the Bt1, BEt,and E Alexandrovsky profile horizons (Fig. 8f). S-ratios donot indicate significant differences in magnetic hardness,except in one sample of E horizon (Fig. 8g). In contrast, thehighest coercivity Hcr is found in Cg and lower BCtAlexandrovsky samples (Fig. 8h).

Two samples of the Middle Pleistocene paleosol areplotted in order to compare the magnetic behavior of theupper AB horizon and minimally weathered BCt horizon,to the respective horizons of the AQ paleosol (Fig. 8). Bothpaleosols have in common higher w and SIRM in the AB/Aupper horizons than in the minimally affected horizons, arelationship that is much higher in the AQ Mikulinopaleosol (Fig. 8a and b).

5. Discussion

The Red Unit of the Barranca Tlalpan and theAlexandrovsky quarry paleosols have different magneticmineral characteristics. The Curie points indicate that themain ferrimagnetic carriers are Ti-magnetite or Ti-maghe-mite with variable Ti content, for most of the RU profile.The dominance of relatively soft ferromagnetic grains isalso suggested in IRM acquisition curves of samples fromBtg, BCt, and C horizons. For these samples, the presence

of hematite is also suggested. In addition to Ti-magnetite/Ti-maghemite, in the EBtg horizon, the Curie point around100 1C suggests the presence of antiferrimagnetic goethite,which is supported by the IRM acquisition curve.In the URU and RU sections, parent material and soil

horizons weakly affected by pedogenesis are characterizedby a higher concentration of magnetic minerals, dominatedby MD Ti-magnetites or Ti-maghemites. Pedogenic pro-cesses seem to be responsible for the formation of fine, SDmagnetite, as observed in Bt4 (Fig. 5). However, aprogressive upward destruction of magnetic mineralsresults in the decrease in magnetic concentration andcoarsening of the magnetic fraction. Although the con-centration of ferrimagnetic grains is very low in the EBtgand Ah horizons, antiferrimagnetic goethite accountssignificantly to the magnetic signal. Ultrafine SP minerals,magnetite or goethite, are present in the Ah horizon. If theSP fraction in the upper horizons corresponds to goethiteor hematite, this could be due to either the neoformationby pedogenesis, or the oxidation of ferrimagnetic minerals,such as Ti-magnetite or Ti-maghemite.In previous work in the Red Unit, direct observation of

magnetic minerals in the P6 and P7 pedocomplex found Ti-magnetites and ilmenite grains weakly affected by corro-sion and with several degrees of hematization in Bt and C

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horizons (Ortega-Guerrero et al., 2004). Ti-magnetites,hematized Ti-magnetites, pure magnetites, and scarcehematite were identified by rock magnetism methods.

In the AQ sequence, the magnetic mineralogy inferred byCurie temperatures consist of ferrimagnetic minerals suchas Ti-magnetites and pure magnetites, and antiferrimag-netic phases such as hematite and goethite are suggested.However, the IRM acquisition curves, the S300 ratios, andHIRM300 strongly suggest that the antiferrimagneticphases are the main constituents of the AQ sequence.

The Mikulino paleosol profile at the Alexandrovsky siteportrays only minor differences in magnetic concentrationbetween the different pedogenic horizons along the profile.Here, only the upper sample of E horizon has lowermagnetic concentration. The major difference in magneticproperties is in the distribution of small, SD grains, whichare more abundant in the horizons where pedogenesis hasbeen more intense (horizons Bt3 up to A). One of theremarkable characteristics of this sequence is the magnetichardness of its components. Although there is lowconcentration of magnetic minerals, the magnetic para-meters indicate that antiferrimagnetic phases, both goethiteand hematite, are present throughout the profile. Anapparent magnetic enhancement in susceptibility in the Ahorizon is not mirrored in the proxies of ferrimagneticconcentration as SIRM, or in the ultrafine fine fractionwfd%. This enhancement is only coincident with the increasein the abundance of small SD grains. A possible explana-tion of these characteristics is that paramagnetic iron-bearing minerals are being formed in the upper horizon,and only a small fraction of them are ferrimagneticminerals, such as SD magnetite.

As shown above, within both Albic Luvisol profiles,there are layers having contrasting differences of theirmagnetic properties, which coincide with soil genetichorizons. This provides evidence that these properties arelinked to the specific set of soil-forming processes operatingin each horizon. In the Kursk profile of the buried AlbicLuvisol developed on loess, the well-known phenomenonof magnetic susceptibility enhancement in the paleosolcompared to parent material is expressed.

The high values of magnetic susceptibility in this profileare probably mostly due to accumulation of fine-grainedmagnetic components, which we assume to be neoformedin the soil environment. However, this phenomenon isobserved only in some parts of the paleosol profile. It isstrongest in the topsoil, in the Ah of the AQ paleosol, andthen to a lesser extent in the Middle Pleistocene ABhorizon at the base of the studied sequence (Fig. 8). Thiscorresponds to the zone of highest past biological activityand accumulation of organic material, a zone that is ratherthin in Albic Luvisols.

Less-pronounced w enhancement was detected in theuppermost horizons of the illuvial part of the profile,namely the EBt and partly the Bt1. The values decreasedownwards in the Bt2, Bt3, and BCt horizons, althoughthey are still somewhat higher than in the Cg horizon. In

general, this zone is characterized by carbonate leaching,weathering, and clay illuviation (associated with precipita-tion of iron oxides). However, we refrain from directlylinking of the second susceptibility maximum just withthese processes. It should be taken into account that thismaximum is located not in the middle part of illuvial zone(Bt2 and Bt3) where clay accumulation due to illuviation isgreatest, but in the upper horizon, where illuviation is stillmoderate and eluvial pedofeatures (concentrations ofbleached skeletal material) are frequent. The distributionof different kinds of clay illuvial pedofeatures could behelpful to explain the susceptibility curve.Targulian and Bronnikova (2002) report in their recent

study of an Albic Luvisol cutan complex that EBt and Bt1horizons are characterized by a high concentration ofspecific iron–clay and iron–manganese cutans, whichquickly diminish downwards. These pedofeatures indicateintensive precipitation of iron oxides together withmanganese and/or clay (which still persists in this horizonin minor quantities, despite eluviation).Finally, somewhat higher susceptibility values in the Bt2,

Bt3, and BCt horizons compared to the Cg horizon shouldbe partly controlled by carbonate leaching. The parentmaterial of Mikulino paleosol at AQ, the Dnepr loess, isknown to contain 20% and more of calcium carbonate,which ‘‘dilutes’’ magnetic minerals present in this sediment.The decarbonatization (leaching) front is located at theupper Cg horizon boundary, where an increase of mineralmagnetic components is apparent.Two major maxima of the susceptibility curve for the Ah

and EBt horizons are separated by a strong minimum inthe eluvial E horizon. We believe that this minimum islinked to a specific pedogenic environment of an E horizon,in which simultaneously operate strong acid weathering,leaching, and surface redoximorphic (stagnic) processesthat are likely aggressive towards magnetic minerals, bothinherited (lithogenic) and neoformed. The specific role ofstagnic conditions, caused by periodic water saturationabove low permeable clay-illuvial horizons, consists ofmobilization of iron in the form Fe2+. This iron, mobilizedin eluvial horizons and capable of migration with soilsolutions, is believed to be the source for iron oxides thatprecipitate in the specific illuvial pedofeatures of thesubadjacent EBt and Bt1 horizons (considered above as apossible reason for susceptibility enhancement).

6. Conclusions

In general, the samples of a Luvisol profile formed in‘‘tepetates’’ from Barranca Tlalpan show much highervalues of magnetic susceptibity than the AQ Albic Luvisolformed in loess. A conspicuous feature of the BT profile isthe complete absence of enhancement in the soil profile.The values in the parent material are higher than in any soilhorizon. At the same time, we observe considerableincrease of fine-grained magnetic components in the soil,compared to the C horizon where these components are

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lower. We conclude that the high susceptibility values inthe parent material are controlled by abundant coarsevolcanogenic magnetic minerals (mainly titanomagnetite)and the susceptibility decrease in the soil results from thepartial loss of these minerals due to intra-soil weathering.At the same time, the neoformation of magnetic compo-nents in soil takes place, which accounts for accumulationof fine-grained magnetic minerals.

However, this process cannot compensate for the loss oflithogenic magnetic minerals in any genetic horizon, andthe resulting trend is magnetic susceptibility depletionthroughout the soil profile. This model was alreadyproposed earlier for volcanic paleosols of Mexico (Orte-ga-Guerrero et al., 2004) and supported by microscopicobservations of weathering features. A significant point ofthis finding is that it is critical for researchers to fullycharacterize a paleosol profile including a detailed mor-phological description and also analysis of both genetic soilhorizons and the parent material (C horizon). This shouldalso include quantitatively establishing that the described Chorizon is the parent material for the genetic soil horizonsand is not a different lithologic unit.

Within this hypothesis, the divergence between AlbicLuvisols on tephra (depletion case) and loess (enhancementcase) can be explained by the differences in originalquantity of the lithogenic coarse magnetic components.

Since coarse lithogenic magnetic components are few inloess, their loss due to weathering is of minor importancecompared to neoformation in A, BEt, and Bt1 horizons,which results in susceptibility enhancement, as was shownwith the help of CBD test by TenPas et al. (1999).

Behavior of magnetic properties within the soil profileshows their remarkable correlation with the type of genetichorizon. Susceptibility is higher in Bt horizons, decreases inthe BEt, reaches a minimum in the E, and increases a littlebit in the A horizon. It is interesting also to trace thedifferences in fine-grained magnetic components accumu-lation, which have two maxima—a major one in the Bthorizons (especially in the lowest Bt3 horizon) and minorpeak in the topsoil A horizon, with a sharp decrease in theE horizon. We speculate that the neoformation of fine-grained magnetic minerals is associated with two pedoge-netic environments: (1) topsoil area of humus accumulationand highest past biological activity, and (2) area of clayilluviation and weathering in the middle part of the profile.

A question arises concerning the accumulation of finemagnetic components in the Bt horizons: Since the finecomponents are well expressed in the paleosol on tephra,why are they weak or absent in the loess-derived paleosol(except the uppermost illuvial horizons)? We attributethis divergence to the differences of parent materialcomposition.

Pyroclastic sediments provide abundant (up to 25% ofcoarse fractions) primary minerals containing iron (crystal-line Fe–Ti oxides, Fe–Mg silicates) which generate fine-grained iron oxides in the course of weathering. On thecontrary, Dnepr loess has rather ‘‘poor’’ mineralogical

composition of primary minerals, dominated by quartz andK–Na feldspars, and low concentrations of iron-bearingminerals.Magnetic susceptibility curves of both studied profiles

coincide in demonstrating strong minima in eluvialhorizons, which shows also low concentrations of fine-grained components. We conclude that the pedogenicenvironment of eluvial horizons, which experience strongweathering, leaching, surface water redoximorphic pro-cesses, and low biological activity, is destructive for allmagnetic components (both primary and secondary) anddoes not support neoformation. Our data and interpreta-tion contradict to some extent the results of Singer andFine (1989), who found greater susceptibility in the eluvialpart of the profile than in the illuvial part or subsoil in anumber of soils of California.Probably the detected phenomenon of E-horizon mini-

mum is specific for Albic Luvisols and is related to thecombination of eluvial processes and surface gleying(stagnic conditions) responsible for bleaching in this layer.Terhorst et al. (2001), who also observed this phenomenonin the E horizon of a buried Eemian soil in southernGermany, report minima of magnetic susceptibility in thebleached zones of the Bt horizon of the same soil, alsoaffected by eluviation and gleying.Finally, the higher concentrations of antiferromagnetic

components (hematite and goethite), which were found inE horizons of both studied soils, also seem to be related toredoximorphic processes. Such processes involve bothdissolution of iron oxides and also their local fastprecipitation as ferruginous mottles and nodules that areobservable in thin sections.

Acknowledgments

This work was funded by ICSU (International Councilof Science) project ‘‘Polygenetic models for Pleistocenepaleosols’’. Partial support also was given by UNAMDGAPA IN107902, IX102104, and CONACyT G-28528T.D. Hernandez collaborated in technical support. Theauthors thank Dr. D.A. Grimley and an anonymousreviewer for their helpful comments on the paper.

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