Links between bulk sediment particle size and magneticgrain-size: general observations and implications for Chineseloess studies

FRANK OLDFIELD*, QINGZHEN HAO*,� , JAN BLOEMENDAL*, ZOE GIBBS-EGGAR*,� ,SHIVA PATIL*,§ and ZHENGTANG GUO�*Department of Geography, University of Liverpool, Liverpool L69 7ZT, UK (E-mail: oldfield.f@gmail.com)�Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, ChineseAcademy of Sciences, Beijing 100029, China�The Cedars, 19 St Margarets Road, Hampton Park, Hereford HR1 1TS, UK§K.S. Krishnan Geomagnetic Research Laboratory (IIG), Leelapur Road, Chamanganj, Post-Hetapur,Hanumanganj, Allahabad 221 505, Uttar Pradesh, India

Associate Editor: Charlie Bristow

ABSTRACT

Using a combination of particle size analysis, magnetic measurements,

scanning electron microscopy and transmission electron microscopy

imaging, this study shows that in a wide range of depositional

environments, there is a strong link between particle size classes and

magnetic response, especially below the upper limit of stable single domain

magnetic behaviour. Ferrimagnetic grain assemblages dominated by stable

single domain magnetosomes regularly have peak susceptibility and

remanence values in coarser grades than do those containing finer-grained,

viscous and superparamagnetic secondary magnetic minerals formed during

pedogenesis. This effect is despite the fact that there is a one to two orders of

magnitude size difference between the particle size boundaries (at 1 or 2 lm)

and key domain state transitions (mostly below 0Æ05 lm). The implications of

these results are explored using samples spanning 22 Myr of loess

accumulation on the Chinese Loess Plateau. The results from the loess

sections, complemented by data from low-temperature magnetic experiments,

show that there are subtle distinctions in mean ferrimagnetic grain-size

between the Pleistocene and Miocene parts of the record, thus allowing more

refined rock magnetic interpretations of the fine-grained ferrimagnetic mineral

assemblages arising from the effects of weathering, pedogenesis and possibly

diagenesis in the sections studied.

Keywords Environmental magnetism, granulometry, loess, magnetic grain-size, pedogenesis, weathering.

INTRODUCTION

Sediments from environments as diverse as con-temporary saltmarshes in Britain (Oldfield & Yu,1994), Holocene lake sediments (van der Postet al., 1997; Chiverrell et al., 2008) and loessdeposits from the Chinese Loess Plateau (CLP)spanning the last 22 Myr (Zheng et al., 1991; Chen

et al., 1995; Hao et al., 2008b) show rather con-sistent relationships between particle size andmagnetic properties. These relationships areapparent most clearly when bulk sediment isseparated into size fractions using a combinationof sieving and pipetting. Provided that care istaken not to use chemical pre-treatments that affectmagnetic properties, the particle-sized separates

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remain available for magnetic measurement.Down to the resolution limit of scanning electronmicroscopy (SEM), and by combining imagingwith Energy Dispersive X-Ray (EDX) characteriza-tion, magnetic grain-size can often be observeddirectly in magnetic extracts. Below this limit,direct observation is more difficult and dependson transmission electron microscopy (TEM) imag-ing after magnetic extraction using more special-ized equipment (Hounslow & Maher, 1996).Magnetic extraction efficiencies are often lowand size categorization of magnetic grains withinbulk sediment, on a statistical basis, is verydifficult except where well-dispersed bacterialmagnetosomes dominate the fine-grained ferri-magnetic fraction in the sample (Hesse & Stolz,1999). Magnetic measurements themselves pro-vide an alternative approach to approximatingmagnetic grain-size because they discriminatebetween grain-size assemblages dominated bydifferent domain states which are largely sizedependent (e.g. Maher, 1988). However, the maindomain state transitions occur in grain-sizes belowca 2Æ5 lm (Fukuma & Dunlop, 1998), with some ofthe most striking and readily detectable changesoccurring within the size range below 0Æ05 lm, i.e.between one and two orders of magnitude belowthe clay/silt boundary. Therefore, any consistentlydocumented link between bulk sediment particlesize and magnetic grain-size within the fine frac-tions appears to be somewhat enigmatic.

Results that document links between particlesize and magnetic grain-size become more usefulwhen the combination of particle size separationand magnetic measurements makes it possible todistinguish between components of the bulksample that reflect different processes or sources(e.g. Hao et al., 2008b). The present paper seeks toexamine these relationships empirically usingdata from a range of sedimentary environments.The main focus is on the magnetic properties ofparticle-sized separates below 4 lm in diameterbecause, as noted above, it is in the finer gradesthat some of the most interesting and puzzlingrelationships are apparent. In particular, the linksbetween particle size classes in the fine silt–clayrange are explored, together with some keymagnetic parameters, including frequency-depen-dent susceptibility (vfd), anhysteretic remanentmagnetization (ARM) and quotients that includethese measurements, as these parameters areoften indicative of magnetic domain state, hencegrain-size (Maher, 1988). Then considered are theimplications of these results for studies based onthe magnetic properties of Chinese loess.

SAMPLES AND METHODS

The following samples have been included in thepresent account.

Loess and palaeosol samples

Twelve samples were analysed from theDadongling section which is close to Xining inthe western part of the CLP (Chen et al., 1995).These samples, which range from late marineoxygen isotope stage (MIS) 6 to the Holocene inage, were dispersed using calgon and ultrasonictreatment. Samples were separated into up tonine grades from <1 lm (/10) to sand.

Two samples were analysed from the Luochuansection in the east-central part of the CLP (Zhenget al., 1991). Pre-treatment and separation were asfor the Dadongling samples. One sample repre-sents the palaeosol contemporary with MIS 5,while the other is from the underlying loess fromlate MIS 6.

Six samples were studied from the Xifengsection in the west-central part of the CLP (Haoet al., 2008a). These samples range in age fromPliocene (2Æ69 Ma) to Late Pleistocene (0Æ51 Ma).All were dispersed using buffered acetic acid toremove carbonates and calgon. Only pipettingwas carried out on these samples and four sizeclasses were separated: <2, 2 to 4, 4 to 8 and>8 lm.

Four samples were analysed from the Dongwansection in the western part of the CLP (Hao et al.,2008a). These samples range in age from LateMiocene (6Æ96 Ma) to Pliocene (4Æ26 Ma). Pre-treatment and separation were as for the Xifengsamples.

Sixteen samples were studied from the Qinan(QA-I) section in the western CLP (Hao et al.,2008a). These samples range in age from Early(21Æ83 Ma) to Late Miocene (6Æ41 Ma). Pre-treatment and separation were as for the Xifengsamples.

Lake sediments

Two samples were analysed from the Late Holo-cene sediments of Blelham Tarn in the EnglishLake District (van der Post et al., 1997), onedating from AD 1975 to 1990; one from thecentury preceding this. Pre-treatment and sepa-ration were as for the Dadongling loess samples.

Four undated Late Holocene lake sedimentsamples were analysed from Semer Water, asmall lake in the Pennines, Northern England

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(Chiverrell et al., 2008). The samples were aggre-gated from 20 cm depth intervals from an 80 cmlong Mackereth minicore. Pre-treatment andseparation were as for the Dadongling loesssamples.

Saltmarsh silts

Nine samples were studied from sites along theeastern coastline of the Irish Sea from Angleseyto Southern Scotland (Oldfield & Yu, 1994). Allwere surface samples taken from coastal marshessubject to tidal inundation. Pre-treatment andseparation were as for the Dadongling loesssamples.

The results summarized here arose from workcompleted by several authors over a period of17 years and, as noted above, the methodsused have varied. The main differences in pre-treatment methods are between those whereonly calgon was used for dispersal and thosefrom the three loess sections (Xifeng, Dongwanand Qinan) studied by Hao et al. (2008b) whereacetic acid was used for carbonate removal. Allthe samples dispersed by calgon alone wereseparated into particle size classes that distin-guished between the <1 and 1 to 2 lm gradesand between all / classes up to sand size.Samples subjected to acetic acid pre-treatmentwere divided into only four size classes as notedabove.

Additional data are presented for samples fromthe Irish Sea saltmarsh sediments, based on TEMstudies by Gibbs (1997). Additional informationon the magnetic grain-size assemblages present inthe particle-sized loess material was derived fromlow-temperature magnetic measurements using aMagnetic Properties Measurement System(MPMS) (Quantum Design, San Diego, CA, USA)(cf. Liu et al., 2005), and from SEM and EDXstudies.

RESULTS

Figure 1 presents typical results from each of thesample sets listed above. Wherever possible, thefollowing magnetic properties have been plotted:

• The low-field, low-frequency (0Æ47 kHz)magnetic susceptibility (vlf) which was measuredusing a Bartington Instruments MS2 susceptibil-ity meter (Bartington Instruments, Oxford, UK).

• Frequency-dependent susceptibility (vfd)which is defined as the difference between

susceptibility measurements at 0Æ47 (vlf) and4Æ7 kHz (vhf) using a Bartington Instruments MS2susceptibility meter; these data can also beexpressed as a percentage of vlf (vfd%).

• The ARM or, where it has been normalized tothe strength of the DC biasing field, ARM sus-ceptibility (vARM), which was imparted using apeak alternating field (AF) of 100 mT generatedby a range of adapted AF demagnetizers, with aDC bias field of 0Æ1 mT and measured on aMinispin Slow Speed Spinner Magnetometer(Molspin Limited, Newcastle upon Tyne, UK).

• The quotient vARM/vlf, which increases withmagnetic grain-size provided that all otherevidence points to a mean grain-size within orbelow the stable single domain (SD) size range,i.e. grain diameters less than ca 0Æ1 lm (Maher,1988; Oldfield, 1994, 2007).

• vARM/vfd, provided vfd can be measured reli-ably, will increase with increased grain-sizewithin the superparamagnetic (SP) to SD sizeranges (Oldfield, 1994, 2007).

• vARM/SIRM, where SIRM is the saturationisothermal remanent magnetization imparted in afield of 1 T using a pulse magnetizer and mea-sured using a Minispin Magnetometer. Changesin this quotient most often reflect the balancebetween remanence carrying minerals above orbelow the upper size limit for true SD grains(Maher, 1988), with higher values sometimesexceeding 2 · 10)3, indicating dominance by SDgrains (e.g. Oldfield et al., 2003).

Table 1 sets out the number of times in whicheach property peaks in a particular size class ineach of the sample sets. Table 2 summarizes theaggregate results for each size class for the maintypes of sample measured. Note that not allsamples yielded results for all magnetic proper-ties and quotients. In most cases, this reflects lackof data from earlier studies or unreliable mea-surements of vfd as a result of small sample mass.In rare cases, the peak value for the magneticproperty is in a size class greater than 4 lm.

The data presented for the loess/palaeosolsamples in Tables 1 and 2 include some notablefeatures:

• vfd peaks below 2 lm in 36 of the 40 samples;vfd% peaks below 2 lm in 35 samples;

• vARM peaks below 4 lm in 36 of the 40 sam-ples;

• none of the quotients indicative of SD domi-nance peaks in the <1 lm size class; all peak mostfrequently in the 2 to 4 lm size class;

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Fig. 1. Plots of selected magnetic properties versus particle size for representative samples from each sample setreferred to in the text and in Table 1.

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Fig. 1. Continued.

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• vfd and vARM peak in the same size class inonly three of 40 cases. In the remaining cases,vARM peaks in the size class one or two gradesabove.

In the tidal sediments from shoreline salt-marshes, reliable vfd measurements on individualsamples were not possible. Calculations based onaggregate values for each size class indicate peakvalues of around 3% in each size class below4 lm. In the case of the lake sediments, thepreferred size for peak vfd values, where reliablymeasurable, is in the <1 lm class; peak values for

the other measurements are scattered betweenclasses.

Figure 2 shows results of low-temperatureexperiments for the <2 and 2 to 4 lm size classesfor loess and palaeosol samples ranging from 4Æ2to 22 Ma in age from the Dongwan and Qinansections. Low-temperature vfd was defined asthe difference between susceptibility measure-ments at 1 and 10 Hz in AC fields of 240 A m)1,or 0Æ3 mT in order to recognize the existence ofsuperparamagnetic ferrimagnetic grains finer than20 to 25 nm (Liu et al., 2005). The measurementswere made over a temperature range of 10 to300 K with an increasing step of 5 K using aQuantum Design superconducting quantum inter-ference device (SQUID) MPMS.

Table 3 summarizes the mean values of vfd forthree temperature intervals (10 to 50, 200 to 250and 200 to 300 K) using the MPMS system.Figure 3 shows TEM images of extracted mag-netic grains from Irish Sea saltmarsh sedimentsobtained by Gibbs (1997), using the extractionmethod described by Hounslow & Maher (1996).These plates show that magnetosomes dominatedthe extracts. For the four samples studied, extrac-tion efficiencies, based on measurements of vlf

before and after extraction, ranged from 45%to 94%. Figure 4 shows SEM images with, inthe centre of each, a representative iron-oxideparticle (see figure caption for sample sites andages).

Figure 5A plots the results for vARM/vlf

versus vARM/vfd for all the particle-sized loessand palaeosol samples from Xifeng, Dongwanand Qinan (Hao et al., 2008a) on the bilog-arithmic graph used to discriminate betweenfine-grained ferrimagnets of pedogenic and(magnetotactic) bacterial origin (Oldfield, 1994,2007). Figure 5B compares the envelopes ofvalues for several subsets derived from Figure 5A

Table 1. Incidence of peak values by site in each size interval in micrometres.

Magneticproperties

Dadongling(n = 12)

Xifeng(n = 6)

Dongwan(n = 4)

Qinan(n = 16)

Luochuan(n = 2)

Blelham Tarnand SemerWater (n = 6)

Irish Sea(n = 9)

< 1 1–2 2–4 < 2 2–4 < 2 2–4 < 2 2–4 < 1 1–2 2–4 < 1 1–2 2–4 1–2 2–4

vfd 6 2 6 4 16 2 3vfd% 6 1 6 4 16 2vARM 7 2 1 5 4 15 1 1 3 2 1 8 1vARM/vfd 5 3 6 4 15 2vARM/vlf 3 5 5 3 8 2 2 1 3 8 1vARM/SIRM 9 1 2 2 14 1 1 3 3

Table 2. Incidence of peak values by sample type ineach size interval in microns.

Magneticproperties

Sizeclasses(lm)

Loess andpalaeosolsamples

Marinesediments(n = 9)

Lakesediments(n = 6)

vfd < 1 8 (n = 14) 31–2 2 (n = 14)< 2 26 (n = 26)

vfd% < 1 8 (n = 14)1–2 1 (n = 14)< 2 26 (n = 26)

vARM < 1 0 (n = 14) 31–2 8 (n = 14) 8 2< 2 1 (n = 26)2–4 27 (n = 40) 1 1

vARM/vfd < 1 0 (n = 14)1–2 3 (n = 12)< 2 0 (n = 26)2–4 23 (n = 40)

vARM/vlf < 1 0 (n = 14) 21–2 5 (n = 14) 8 1< 2 0 (n = 26)2–4 30 (n = 40) 1 3

vARM/SIRM < 1 0 (n = 14)1–2 10 (n = 13) 3< 2 7 (n = 26)2–4 18 (n = 40) 3

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with previously published data. In interpretingthese plots, the results should be consideredin the light of the criteria set out in Oldfield(1994). It should also be borne in mind that the

quotients will discriminate less effectivelywhere samples are influenced strongly by eitherimperfect antiferromagnetic or paramagneticcontributions.

0

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Particle fraction: <2 µm Particle fraction: 2−4 µm

+

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QW-3481 (P, 21·73 Ma)

QW-1612 (L, 15·48 Ma)

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QW-3481 (P, 21·73 Ma)

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QW-1603 (P, 15·44 Ma)

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χfd

χ1Hz

χ10Hz

Fig. 2. Results of low-temperatureexperiments determined using theMagnetic Properties MeasurementSystem (MPMS) on particle-sizedseparates from five samples from theDongwan and Qinan sections (Haoet al., 2008a).

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DISCUSSION

Lack of evidence for multi-domain grains inthe fine fractions

Peak values are taken for vfd and vfd% as indic-ative of the particle size class within whichferrimagnetic grains at or below the SP/SD tran-sition make their maximum contribution. Peakvalues for vARM and vARM/SIRM indicate the sizeclass with the maximum contribution from SDgrains. Where, as in the present set of samples,the criteria set out in Oldfield (1994, 2007) aremet, higher values of vARM/v and vARM/vfd are alsolikely to indicate the maximum contribution fromSD grains. The results as a whole are remarkablefor the consistency with which all the indicatorsof dominance by magnetic grain-sizes below theupper limit of the SD range (Tables 1 and 2) peakin particle size classes below 4 lm. This observa-tion points to a strong link between ferrimagneticgrain-size and bulk sediment particle size asdetermined using the pipette method. This linkappears to be common to all the sediment typesused in the present study despite the fact that theupper size limit for SD magnetite is probablyaround 0Æ07 lm and is certainly not greater than0Æ2 lm, which is at least one order of magnitudebelow the upper threshold of the bulk sedimentparticle size classes. What is remarkable thereforeis not so much the presence of fine magneticgrains in the fine size classes, but the lack ofevidence for coarser magnetic grains in these size

classes, despite the fact that ferrimagnetic grainsup to ca 1Æ6 lm in diameter would still be withinthe size range of magnetite grains to be found inthe 2 to 4 lm fraction if density differences alonewere influencing the particle separation usingthe pipette method. The results summarized byOldfield & Yu (1994), Chen et al. (1995), van derPost et al. (1997), Hao et al. (2008b) and Fig. 4indicate that the apparent paucity of pseudo-single domain (PSD) and multi-domain (MD)grains in the fine fractions of the bulk sedimentdoes not arise from their absence from thesamples as a whole as, in all cases, the magneticproperties of the coarser grades confirm theirpresence, often in quantities that dominate themagnetic properties of the bulk samples. It istherefore inferred that the difference in specificgravity between quartz and magnetite/maghemitealone cannot account for the results common tomost of the data considered here.

The only SEM/EDX studies on the presentsample set were carried out on the >8 lm fractionfrom the Xifeng and QA-I sections. These studiesreveal iron oxide grains with maximum dimen-sions ranging from ca 4 lm to over 12 lm, whichis fully compatible with the particle size classwithin which they occur once specific gravity istaken into account. This compatibility is alsolikely to be so in the case of the saltmarsh samplesbecause particle size separation clearly distin-guished between a biogenic, magnetosome com-ponent predominantly in the clay fraction andthe coarse heavy mineral assemblages found in

Table 3. Mean frequency-dependent magnetic susceptibility of particle-size separates at low temperatures.

Sample numberand age Lithology

Sizeclasses(lm)

v1Hz,10K )v1Hz,100K*(·10)8 m3 kg)1)

v1Hz,280K )v1Hz,100K�(·10)8 m3 kg)1)

Mean vfd� (·10)8 m3 kg)1)

Differencein mean vfd

10–50K

200–250K

200–300K

DW-738 (4Æ26 Ma) Loess 0–2 105Æ55 85Æ37 0Æ32 15Æ24 16Æ81 16Æ49QW-1603 (15Æ44 Ma) Palaeosol 0–2 137Æ15 22Æ69 1Æ85 8Æ35 8Æ32 6Æ50

2–4 119Æ31 0Æ70 1Æ28 1Æ62 1Æ63 0Æ35QW-1612 (15Æ48 Ma) Loess 0–2 152Æ19 )5Æ49 0Æ26 2Æ51 3Æ06 2Æ80

2–4 135Æ82 )6Æ80 0Æ52 1Æ38 1Æ09 0Æ86QW-3481 (21Æ73 Ma) Palaeosol 0–2 123Æ24 )8Æ41 0Æ17 1Æ19 1Æ10 1Æ02

2–4 107Æ65 )6Æ59 0Æ48 0Æ66 0Æ70 0Æ22QW-3497 (21Æ83 Ma) Loess 0–2 144Æ49 )10Æ58 0Æ44 1Æ17 1Æ04 0Æ73

2–4 111Æ88 )7Æ22 0Æ23 1Æ10 1Æ11 0Æ88

*v1Hz,10K ) v1Hz,100K: loss of v1Hz from 10 to 100 K, a function of the magnetic susceptibility of the paramagneticcomponent of each sample.�v1Hz,280K ) v1Hz,100K: change in v1Hz from 100 to 280 K, a partial estimate of the magnetic susceptibility of thesuperparamagnetic component of each sample.�Difference in mean vfd: the difference in vfd either between 200 and 250 K or between 200 and 300 K (using thehigher value) and the mean vfd below 50 K, a partial estimate of the magnetic susceptibility of the main frequency-dependent component minus the finest superparamagnetic component in each sample.

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the sand fractions of many coastal sediments(Oldfield & Yu, 1994).

Taken in combination, the results of recentstudies by Katari & Bloxham (2001), Sartori et al.(2005) and Fedotov et al. (2007) point to thelikelihood that the type of link between magneticgrain-size and particle size documented here forthe finest grades may also reflect the attachmentof fine magnetic grains to clay particles, both thesize and lower density of which increase theirtendency to be concentrated in the finest sizeclasses. This type of association would also helpto explain the close link between the magneticproperties of the clay size class and evidence forpedogenesis in loess and palaeosol samples fromthe CLP (Hao et al., 2008b). Lack of evidence forcoarser magnetic grains in the <4 lm fractions

may be related to the terminal grade to whichprimary magnetite, as distinct from biogenic orpedogenic magnetite/maghemite, is comminuted(cf. Dreimanis & Vagners, 1971).

Magnetic grain-size discrimination within thefinest silt and clay fractions

The second notable feature in the data summa-rized in Tables 1 and 2 is the relative consistencywith which indicators of ferrimagnetic grain-sizesjust below the lower SD size limit and thoseindicative of grain-sizes above this, but stillwithin the SD range, fall into different sizeclasses. This feature is especially apparent inthe loess/palaeosol samples, irrespective of sitelocation or age. The distinction is less clear in the

A

B

Fig. 3. Transmission electron microscopy images of magnetic extracts from saltmarsh sediments from (A)Auchencairn Bay and (B) the Dee estuary near Kirkcudbright, Southern Scotland (Gibbs, 1997). Both sets of samplesare dominated by magnetosome chains.

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lake sediment samples and, in the case of thesaltmarsh sediments, lack of reliable vfd measure-ments makes the distinction impossible, althoughit is important to note that the indicators of SDdominance fall into the size classes above 1 lmrather than in the finest grade. In the case of thesaltmarsh clays, dominance by magnetosomespoints to the possibility that their relative paucityin the finest grade may reflect higher settlingvelocities as a result of the formation and preser-vation of chains (Fig. 3). However, in the case ofthe loess/palaeosol samples, this cannot be thecase as all the evidence, summarized, for exam-ple, by Liu et al. (2005), disposes of the possibil-ity that magnetosomes have a significant effect onthe magnetic properties of these samples.

The above observations, which are consistentwith the results of the low-temperature experi-ments considered in more detail below, point toan apparent link between a bulk sediment particle

size boundary, at 1 or 2 lm depending on the datasets used, and a rock magnetic transition ataround 0Æ025 lm; this remains a matter for spec-ulation. The greater size of SD rather than SP + fd(frequency-dependent) grains that have floccu-lated with clay particles may increase the sinkingvelocity of the clays. In addition, as notedalready, the slightly coarser grade of bulk sedi-ment characterized by magnetic properties indic-ative of SD dominance may include grains withinthe PSD size range around 0Æ2 to 2Æ5 lm.

Possible implications for pre-PleistoceneChinese loess sections

The low-temperature data (Fig. 2) are from asingle, high-susceptibility loess sample from theDongwan section dated at 4Æ26 Ma and from foursamples from the Miocene part of the Qinansection dated at between 15 and 22 Ma (Hao

WD 5 µm 15·3 6-BSE06

WD 5 µm 15·1 16SE-08A

WD 10 µm 16·3 26SE-07

WD 2 µm 16·3 14SE-05

A B

C D

Fig. 4. Scanning electron microscopy images of magnetic minerals in >8 lm particle size fraction for selectedsamples from the loess deposits in Northern China. (A) Xifeng: XF-1314 (palaeosol, 0Æ51 Ma); (B) Xifeng: XF-1385(loess, 0Æ64 Ma); (C) Qinan: QW-3481 (palaeosol, 21Æ73 Ma) and (D) Qinan: QW-3497 (loess, 21Æ83 Ma).

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B

A

Fig. 5. (A) Bilogarithmic plot ofvARM/vlf versus vARM/vfd (cf. Old-field, 1994, 2007) for particle-sizedseparates from sites on the ChineseLoess Plateau spanning the last22 Myr (Hao et al., 2008a,b). Thevalues highlighted by red solid cir-cles refer to samples used in thelow-temperature experiments. In(B), the same envelopes are super-imposed on those for sample groupsfrom the much larger and more di-verse sample sets summarized byOldfield (2007).

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et al., 2008a). The plots may be interpreted interms of several components. The steep decline insusceptibility with increasing temperature re-flects a strong paramagnetic contribution. Theextent to which this decline is reversed aboveca 50 K reflects the counter-influence of grainspassing through their blocking temperatures withconsequent changes from SD to SP response astemperature increases. The gap between the lowand high-frequency curves (Fig. 2) reflects theinfluence of frequency-dependent grains lying atthe SP/SD transition size. Several features may benoted.

1 Whereas Liu et al. (2005) found signs of aVerwey transition in the bulk samples from theYichuan section, there is no trace of a Verweytransition in any of the particle-sized samplesconsidered here. As inferred above, the fine-grained extracts lack any evidence for MDgrains.

2 Only in the case of the high-susceptibilityloess sample from Dongwan (Fig. 2A) does theincrease in susceptibility with increasing tem-perature above 50 K, which is attributable to SPgrains, reach values comparable with the initialsusceptibility at the minimum temperature. Theonly other sample with increased susceptibilitywith increased temperature after the initial para-magnetic decay is the <2 lm fraction from thehigh-susceptibility palaeosol (Fig. 2B). These twosamples have the clearest indications of SPresponses.

3 In all the other samples, the response of sus-ceptibility to temperature is dominated by para-magnetic contributions. Despite the scatterresulting from low readings close to the noiselevel of the instrument, the same samples, withthe exception of the <2 lm fraction from thehigh-susceptibility loess (Fig. 2C), also havethe minimum observed divergence between thesusceptibility traces for each frequency. Thesefeatures collectively indicate that in these low-susceptibility samples, the main ferrimagneticcomponent is in the SD size range. The dominantinfluence of paramagnetic minerals and the pau-city of either SP or fd grains in these low-susceptibility samples with ages greater than15Æ5 Ma differentiate them from both the youngerDongwan sample and the Pleistocene palaeosolsamples analysed by Liu et al. (2005).

4 The difference between the susceptibilities atthe two frequencies varies between samples, asdoes the temperature at which the peak differenceoccurs. The trends in the frequency-dependent

differences, where clearly recorded (Fig. 2A to C;Table 3), can be translated into estimates of fre-quency-dependent grain-size using the predictedcorrelation given in fig. 4 of Liu et al. (2005). Inthe case of the <2 lm fraction from the Qinanpalaeosol (Fig. 2B), the difference peaks at around250 K which points to a frequency-dependentgrain-size between 0Æ02 and 0Æ025 lm, similar tothe grain-size range inferred by Liu et al. (2005)for the palaeosol samples they studied. In the caseof the <2 lm fraction from the high-susceptibilityloess samples from Dongwan and Qinan (Fig. 2Aand C), the divergence is still increasing at 300 K,which points to a dominant frequency-dependentgrain-size in excess of 0Æ025 lm. The possibilityof a difference in frequency-dependent grain-sizebetween Miocene palaeosol and loess samplescalls for further study. Although, in most of theweak samples, the vfd values above 200 K exceedthose between 10 and 50 K, the differences arevery slight (Table 3).

5 In the case of paired loess/palaeosol samples,there is a clear distinction between the two in the<2 lm fraction from the upper Qinan samplesfrom 15Æ44 to 15Æ48 Ma (Fig. 2B and C), but thereis no clear difference in the 2 to 4 lm fraction(Fig. 2F and G). In the case of the older, pairedsamples from 21Æ73 to 21Æ83 Ma (Fig. 2D, E, H andI; Table 3), there is no clear distinction betweensamples in either fraction. The upper pairedsamples come from a zone in the Qinan Miocenerecord in which vlf and vfd are relatively high ineach palaeosol layer and there is a clear distinc-tion in the routine measurements on bulk samplesbetween the palaeosols and intervening loesslayers.

The low-temperature results highlight differ-ences in the balance between SP + fd and SDcontributions between the different sub-samplesand fractions. In each sample, except the earliestloess (Fig. 2E and I), the low-temperature data forthe <2 lm fraction indicate a higher SP compo-nent and a stronger frequency-dependent signalthan that in the 2 to 4 lm fraction (Table 3).These results can be compared with those for thewhole particle-sized sample set in the bilogarith-mic plot (Fig. 5A). The <2 lm and 2 to 4 lmfractions fall into separate discrete envelopes ofvalues. The interpretations proposed for thebilogarithmic plots (Oldfield, 1994, 2007) wouldsee the contrast in quotient values as indicative ofa higher ratio of SD to SP + fd contributions inthe 2 to 4 lm fraction. There is therefore a highdegree of consistency between the low-tempera-

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ture results and the bilogarithmic plots whichindicates that the former are reasonably represen-tative and that the latter may be interpreted asproposed by Oldfield (1994, 2007).

In the Qinan section between 8Æ5 and 22 Ma,except for the 14Æ4 to 15Æ9 Ma time interval, thereis only a weak distinction in bulk magneticproperties between the loess and palaeosol levelsand both vlf and vfd are consistently low through-out the section (Hao et al., 2008a). In the earliestMiocene samples, the low-temperature data alsoindicate minimal SP + fd contributions even inthe <2 lm fraction. If the quotient values(Fig. 5A) are compared for the <2 lm fractionsof high-susceptibility samples with those forthe low-susceptibility samples (8Æ5 to 22 Ma,excluding the interval from 14Æ4 to 15Æ9 Ma), thelow-susceptibility samples also have a distinctenvelope of values, especially for the loesssamples. For the 2 to 4 lm fractions, where thelow-temperature data point to SD dominancethroughout, there is a clear distinction on thebilogarithmic plot: the low-susceptibility set,with a single exception, sample ZW-138 with anage of 9Æ5 Ma, has higher values for vARM/vfd.These results for the 2 to 4 lm fractions point toan even lower SP + fd component in the low-susceptibility samples than in those with highersusceptibilities. The fact that this is more clearlyreflected in the vARM/vfd results than in vARM/vlf

suggests that, in the latter case, increased vARM/vlf

through a lower SP contribution may have beenbalanced by a higher contribution from larger SD,and possibly PSD grains, with lower vARM/vlf

quotients, and/or a higher contribution fromparamagnetic minerals. From the low-tempera-ture data, the ferrimagnetic component in the 2 to4 lm fractions appears to be almost exclusivelySD in response irrespective of sample type or age,but the spread of data in the bilogarithmic graphcould indicate that within the set as a whole,there may be some distinction in grain-size, withthe low-susceptibility samples containing assem-blages with, on average, larger grains.

Figure 5A and B allows further comparisonswithin the loess/palaeosol sample set, as well asbetween these samples and others previouslystudied. Oldfield (2007) suggested that MD grainswould plot outside and beyond the left-hand edgeof the soils/sources envelope on the bilogarithmicplot; this is the case with all the >8 lm fractions.These fractions form two envelopes of values,with all low-susceptibility samples between 8Æ5and 22 Ma, irrespective of whether they are fromloess or palaeosol units, with higher vARM/vfd

quotients than almost all the other samples. It isnot clear from these data whether the distinctivenature of the detrital component in the 8Æ5 to22 Ma Miocene samples reflects merely a subtledifference in modal magnetic grain-size, or adifference in magnetic mineralogy linked to dif-ferences in source, wind strength or post-deposi-tional modification.

Figure 5B compares the envelopes of valuesderived from the present research with those fromprevious studies. The >8 lm fraction comprisesthe bulk of the sediment in the particle-sizedloess/palaeosol set. It is therefore not surprisingthat the envelope of values for the <2 lm fractionsalone is markedly different from that for the bulksamples. These clays have quotients that fallwithin the lowest range for pedogenic soils/sources (Oldfield, 2007; Oldfield & Crowther,2007). Values for the 2 to 4 lm fractions also liewithin the ‘soils/sources’ envelope with theexception of those from the low-susceptibilityset (8Æ5 to 22 Ma) which lie on or just beyondits upper limit. However, the vARM/vfd valuesfor these samples lie well outside the envelopefor magnetosome-dominated sediments and thevARM/vlf values are lower than those for magneto-some-dominated sediments and those for sedi-ments in which the quotients reflect a mixture ofmagnetosomes and soil-derived ferrimagneticgrains.

From all the loess-based evidence summarizedabove, three inferences emerge that tend to shedlight on and, to some degree, modify previousinterpretations.

1 The magnetic properties of the 2 to 4 lmfractions lie mostly within the pedogenic rangeof characteristics. It is therefore quite possiblethat the optimum split for separating off thepedogenic contribution may be at 3 or 4 lmrather than 2 lm. This question is left open byHao et al. (2008b), although these authors pointout that, in the sample set under considerationhere and in their paper, the low contribution ofthe 2 to 4 lm fraction to bulk sediment massmeans that its inclusion in the pedogenic com-ponent would make relatively little difference.As indicated above, it would slightly alter thebalance between SD and finer grains in favour ofthe former.

2 In studies of Pleistocene Chinese loess, manypapers confirm that the magnetic distinctionsbetween palaeosols and loess consistently matchthe visual stratigraphy. In the Late Miocene toPliocene Red Clays, the distinctions between

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loess and palaeosols are much more difficult toestablish (Guo et al., 2001) but in the Miocenesection, from which most of the most detaileddata have been obtained, the alternations are clearand easy to recognize in the field (Guo et al.,2002, 2008; Hao & Guo, 2007; Hao et al., 2008a).However, these alternations arise not only fromthe distinctive pigmentation of the palaeosols butfrom the strong enrichment of the interveningloess layers in carbonate. Once the magneticproperties are recalculated on a carbonate-freebasis, distinctions between loess and palaeosolsremain, but they are much reduced, especially,but not exclusively, in the zones of low palaeosolsusceptibility. The present results suggest that, inmost cases, from both Pliocene and Miocene partsof the sections, there is, from the magnetic prop-erties of the <2 and 2 to 4 lm fractions alone,virtually as strong an indication of weatheringand pedogenesis in the loess layers as in the pal-aeosols. The intervals between palaeosol layers inthe Dongwan and Qinan sections are mostly lessthan 1 m, which is well within the depth range ofweathering and magnetic mineral transformationrecorded in well-drained, warm-temperate soilprofiles (e.g. Oldfield, 1991). These results pointto a strong degree of pedogenic overprinting inthe pre-Pleistocene loess layers, even in sectionsin the western part of the CLP where the Pleis-tocene section often shows minimal overprinting(Chen et al., 1995).

3 There are indications in both the low-tem-perature data and the bilogarithmic plots that themean ferrimagnetic grain-size associated withweathering and pedogenesis in the Miocenesamples with low-susceptibility values is, onaverage, greater than that in the high-suscepti-bility samples which are mostly younger in age.As indicated in Hao et al. (2008a) and notedabove, the differences in vlf are not simply afunction of age as there is a zone of higher vlf

values within the prevailing low values between8Æ5 and 22 Ma. Hao et al. note that positive evi-dence for diagenetic changes in the magneticminerals is lacking and they highlight the degreeto which the nature of the magnetic mineralassemblage in the palaeosols is consistent withthe transformation model outlined by Barron &Torrent (2002) and Torrent et al. (2006) in whichthose authors infer, on the basis of experimentalresults, the following sequence of pedogenicmineral formation: ferrihydrite > SP maghe-mite > SD maghemite > hematite. The indicationthat the palaeosol samples in the Miocene, low-susceptibility zones have a greater preponderance

of SD and possibly even PSD grains is consistentwith this sequence, with the low-susceptibilitysamples further along the transformation path-way, as the low-susceptibility palaeosols are alsoquite rich in pedogenic hematite. If this model isapplicable to the present data, the key questionthat remains is whether the transformations aretaking place within the time frame of a single,orbitally controlled loess/palaeosol couplet or ontime scales that are longer and transgressive of theclimatic shifts that control alternating loessdeposition and pedogenesis. In the first case,differences between the low and high-suscepti-bility intervals would probably be interpretable interms of shifts in climate on superorbital timescales. In the second case, such inferences wouldbe compromised by long-term diagenetic changesoverprinting signatures formed during each cycleof accumulation and weathering.

CONCLUSIONS

1 Examples abound from different depositionalenvironments in which magnetic propertiesindicative of magnetic grain-size are linked clo-sely to variations in bulk sediment particle size.

2 Stable single domain (SD) and smaller grainsare concentrated typically in the finest silt andclay grades and multi-domain (MD) grains appearto be excluded largely from these grades, althoughthis cannot be explained simply on the basis oftheir greater size.

3 Within the finest grades, there is a tendencyfor SD grains to be concentrated in a size classabove that containing the strongest indications offiner, SP or fine-viscous frequency-dependentgrains, despite the fact that the transition betweenthese types of magnetic behaviour occurs in grainswith diameters one to two orders of magnitudeless than those separating the particle size classes.

4 Association between the finest pedogenicallyderived ferrimagnetic grains and the clay fractionmay help to account for (2) above and the frequentoccurrence of SD magnetite in chains of mag-netosomes may help to explain (3) in some butnot all cases. In the loess and palaeosol samplesconsidered here, no simple explanation is offered.

5 In the particle-sized samples from the loesssections considered here, the distinction betweenthe magnetic properties of the finest and nextfinest grains is demonstrated consistently, irre-spective of whether the bulk sediment split ismade between <1 and 1 to 2 lm or between <2and 2 to 4 lm.

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6 Detailed analysis of the <2 and 2 to 4 lmfractions from the studied loess sections, usinglow-temperature experiments and bilogarithmicplots of vARM/vlf versus vARM/vfd confirms that thedistinction noted in (4) reflects a greater prepon-derance of SD compared with that of SP + fd(superparamagnetic + frequency-dependent) grainsin the larger grade.

7 The magnetic properties of the 2 to 4 lmfractions from the loess sections lie within orclose to what has previously been characterizedas the ‘soils/sources’ envelope of values on thebilogarithmic plot of vARM/vlf versus vARM/vfd. Itmay therefore be realistic to regard the 2 to 4 lmfraction as including part of the pedogenic com-ponent of the sample.

8 In most of the Miocene parts of the loess re-cord, the magnetic properties of the fine gradesdiffer only slightly between the palaeosols andintervening loess layers, which suggests that thelatter are weathered strongly and that their mag-netic properties are overprinted as a result.

9 The observed trends in magnetic properties inthe fine grades are consistent with the model ofmineral transformation in which ferrihydrite isconverted to hematite via maghemite of graduallyincreasing size. Whether this process has takenplace within the time frame of the formation ofloess/palaeosol couplets, or on longer, super-orbital time scales cannot be established from thepresent data.

10 Purely empirical and partly post hoc analy-ses of this kind, based on data from a limitedrange of routine magnetic measurements oncomplex mixtures typical of ‘natural’ samplesinevitably pose more questions than they cananswer. There is therefore a need for moreexhaustive experimental investigations usingmore controlled sample material and a wider,more sophisticated range of magnetic measure-ments.

ACKNOWLEDGEMENTS

We thank Sandra Mather and Suzanne Ye fortheir help in preparing the figures and Bob Judefor help with the magnetic measurements.J. Bloemendal and Q. Hao thank the Royal Societyfor financial support. This study was supportedby the National Science Foundation of China(projects 40730104 and 40672115) and the Chi-nese Academy of Sciences (KZCX-YW-117). Weare grateful to Andrew Roberts for his detailedcomments and useful suggestions.

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